Juan G. Santiago

All Publications

• Taylor dispersion in arbitrarily shaped axisymmetric channels JOURNAL OF FLUID MECHANICS Chang, R., Santiago, J. G. 2023; 976

  View details for DOI 10.1017/jfm.2023.504

  View details for Web of Science ID 001121111400001

• A neural network model for rapid prediction of analyte focusing in isotachophoresis. Electrophoresis Jangra, A., Shriyam, S., Santiago, J. G., Bahga, S. S. 2023

  Abstract

  We present the development and demonstration of a neural network (NN) model for fast and accurate prediction of whether or not a chosen analyte is focused by an isotachophoresis (ITP) buffer system. The NN model is useful in the rapid evaluation of possible ITP chemistries applicable to analytes of interest. We trained and tested the NN model for univalent species based on extensive data sets of over 10,000 anionic and 10,000 cationic ITP simulations. The NN model uses as inputs the mobilities and the acid dissociation constants of leading electrolyte ion, trailing electrolyte ion, counterion, and a single analyte as well as the leading-to-counterion concentration ratio of the leading zone. The output then indicates whether the chosen electrolyte system yields stable ITP focusing of the analyte. The prediction accuracy of the NN model is over 97.7%. We demonstrate the applicability of the NN by validating its predictions with reported experimental data for anionic and cationic ITP. We have packaged the NN model in a free, web-based application named IONN (isotachophoresis on neural network), which can be used to rapidly screen ITP electrolytesystems.

  View details for DOI 10.1002/elps.202300198

  View details for PubMedID 38059796

• A critical review of microfluidic systems for CRISPR assays. Lab on a chip Avaro, A. S., Santiago, J. G. 2023

  Abstract

  Reviewed are nucleic acid detection assays that incorporate clustered regularly interspaced short palindromic repeats (CRISPR)-based diagnostics and microfluidic devices and techniques. The review serves as a reference for researchers who wish to use CRISPR-Cas systems for diagnostics in microfluidic devices. The review is organized in sections reflecting a basic five-step workflow common to most CRISPR-based assays. These steps are analyte extraction, pre-amplification, target recognition, transduction, and detection. The systems described include custom microfluidic chips and custom (benchtop) chip control devices for automated assays steps. Also included are partition formats for digital assays and lateral flow biosensors as a readout modality. CRISPR-based, microfluidics-driven assays offer highly specific detection and are compatible with parallel, combinatorial implementation. They are highly reconfigurable, and assays are compatible with isothermal and even room temperature operation. A major drawback of these assays is the fact that reports of kinetic rates of these enzymes have been highly inconsistent (many demonstrably erroneous), and the low kinetic rate activity of these enzymes limits achievable sensitivity without pre-amplification. Further, the current state-of-the-art of CRISPR assays is such that nearly all systems rely on off-chip assays steps, particularly off-chip sample preparation.

  View details for DOI 10.1039/d2lc00852a

  View details for PubMedID 36601854

• Detection and Discrimination of Single Nucleotide Polymorphisms by Quantification of CRISPR-Cas Catalytic Efficiency. Analytical chemistry Blanluet, C., Huyke, D. A., Ramachandran, A., Avaro, A. S., Santiago, J. G. 2022

  Abstract

  The specificity of CRISPR-Cas12 assays is attractive for the detection of single nucleotide polymorphisms (SNPs) implicated in, e.g., cancer and SARS-CoV-2 variants. Such assays often employ endpoint measurements of SNP or wild type (WT) activated Cas12 trans-cleavage activity; however, the fundamental kinetic effects of SNP versus WT activation remain unknown. We here show that endpoint-based assays are limited by arbitrary experimental choices (like used reporter concentration and assay duration) and work best for known target concentrations. More importantly, we show that SNP (versus WT) activation results in measurable kinetic shifts in the Cas12 trans-cleavage substrate affinity (KM) and apparent catalytic efficiency (kcat*/KM). To address endpoint-based assay limitations, we then develop an assay based on the quantification of Michaelis-Menten parameters and apply this assay to a 20 base pair WT target of the SARS-CoV-2 E gene. We find that the kcat*/KM measured for WT is 130-fold greater than the lowest kcat*/KM among all 60 measured SNPs (compared to a 4.8-fold for endpoint fluorescence of the same SNP). KM also offers a strong ability to distinguish SNPs, varies 27-fold over all the cases, and, importantly, is insensitive to the target concentration. Last, we point out trends among kinetic rates and SNP base and location within the CRISPR-Cas12 targeted region.

  View details for DOI 10.1021/acs.analchem.2c03338

  View details for PubMedID 36251847

• Enzyme Kinetics and Detector Sensitivity Determine Limits of Detection of Amplification-Free CRISPR-Cas12 and CRISPR-Cas13 Diagnostics. Analytical chemistry Huyke, D. A., Ramachandran, A., Bashkirov, V. I., Kotseroglou, E. K., Kotseroglou, T., Santiago, J. G. 2022

  Abstract

  Interest in CRISPR-Cas12 and CRISPR-Cas13 detection continues to increase as these detection schemes enable the specific recognition of nucleic acids. The fundamental sensitivity limits of these schemes (and their applicability in amplification-free assays) are governed by kinetic rates. However, these kinetic rates remain poorly understood, and their reporting has been inconsistent. We quantify kinetic parameters for several enzymes (LbCas12a, AsCas12a, AapCas12b, LwaCas13a, and LbuCas13a) and their corresponding limits of detection (LoD). Collectively, we present quantification of enzyme kinetics for 14 guide RNAs (gRNAs) and nucleic acid targets for a total of 50 sets of kinetic rate parameters and 25 LoDs. We validate the self-consistency of our measurements by comparing trends and limiting behaviors with a Michaelis-Menten trans-cleavage reaction kinetics model. For our assay conditions, activated Cas12 and Cas13 enzymes exhibit trans-cleavage catalytic efficiencies between order 105 and 106 M-1 s-1. For assays that use fluorescent reporter molecules (ssDNA and ssRNA) for target detection, the kinetic rates at the current assay conditions result in an amplification-free LoD in the picomolar range. The results suggest that successful detection of target requires cleavage (by an activated CRISPR enzyme) of the order of at least 0.1% of the fluorescent reporter molecules. This fraction of reporters cleaved is required to differentiate the signal from the background, and we hypothesize that this required fraction is largely independent of the detection method (e.g., endpoint vs reaction velocity) and detector sensitivity. Our results demonstrate the fundamental nature by which kinetic rates and background signal limit LoDs and thus highlight areas of improvement for the emerging field of CRISPR diagnostics.

  View details for DOI 10.1021/acs.analchem.2c01670

  View details for PubMedID 35759403

• Isotachophoresis: Theory and Microfluidic Applications. Chemical reviews Ramachandran, A., Santiago, J. G. 2022

  Abstract

  Isotachophoresis (ITP) is a versatile electrophoretic technique that can be used for sample preconcentration, separation, purification, and mixing, and to control and accelerate chemical reactions. Although the basic technique is nearly a century old and widely used, there is a persistent need for an easily approachable, succinct, and rigorous review of ITP theory and analysis. This is important because the interest and adoption of the technique has grown over the last two decades, especially with its implementation in microfluidics and integration with on-chip chemical and biochemical assays. We here provide a review of ITP theory starting from physicochemical first-principles, including conservation of species, conservation of current, approximation of charge neutrality, pH equilibrium of weak electrolytes, and so-called regulating functions that govern transport dynamics, with a strong emphasis on steady and unsteady transport. We combine these generally applicable (to all types of ITP) theoretical discussions with applications of ITP in the field of microfluidic systems, particularly on-chip biochemical analyses. Our discussion includes principles that govern the ITP focusing of weak and strong electrolytes; ITP dynamics in peak and plateau modes; a review of simulation tools, experimental tools, and detection methods; applications of ITP for on-chip separations and trace analyte manipulation; and design considerations and challenges for microfluidic ITP systems. We conclude with remarks on possible future research directions. The intent of this review is to help make ITP analysis and design principles more accessible to the scientific and engineering communities and to provide a rigorous basis for the increased adoption of ITP in microfluidics.

  View details for DOI 10.1021/acs.chemrev.1c00640

  View details for PubMedID 35732018

• Inconsistent treatments of the kinetics of Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) impair assessment of its diagnostic potential. QRB discovery Santiago, J. G. 2022; 3: e9

  Abstract

  The scientific and technological advent of Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) is one of the most exciting developments of the past decade, particularly in the field of gene editing. The technology has two essential components, (1) a guide RNA to match a targeted gene and (2) a CRISPR-associated protein (e.g. Cas 9, Cas12 or Cas13) that acts as an endonuclease to specifically cut DNA. This specificity and reconfigurable nature of CRISPR has also spurred intense academic and commercial interest in the development of CRISPR-based molecular diagnostics. CRISPR Cas12 and Cas13 orthologs are most commonly applied to diagnostics, and these cleave and become activated by DNA and RNA targets, respectively. Despite the intense research interest, the limits of detection (LoDs) and applications of CRISP-based diagnostics remain an open question. A major reason for this is that reports of kinetic rates have been widely inconsistent, and the vast majority of these reports contain gross errors including violations of basic conservation and kinetic rate laws. It is the intent of this Perspective to bring attention to these issues and to identify potential improvements in the manner in which CRISPR kinetic rates and assay LoDs are reported and compared. The CRISPR field would benefit from verifications of self-consistency of data, providing sufficient data for reproduction of experiments, and, in the case of reports of novel assay LoDs, concurrent reporting of the associated kinetic rate constants. The early development of CRISPR-based diagnostics calls for self-reflection and urges us to proceed with caution.

  View details for DOI 10.1017/qrd.2022.7

  View details for PubMedID 37529278

• Web-Based Open-Source Tool for Isotachophoresis. Analytical chemistry Avaro, A. S., Sun, Y., Jiang, K., Bahga, S. S., Santiago, J. G. 2021

  Abstract

  We present the development of a client-side web-based simulator for complex electrophoresis phenomena, including isotachophoresis. The simulation tool is called Client-based Application for Fast Electrophoresis Simulation (CAFES). CAFES uses the broad cross-browser compatibility of JavaScript to provide a rapid and easy-to-use tool for coupled unsteady electromigration, diffusion, and equilibrium electrolyte reactions among multiple weak electrolytes. The code uses a stationary grid (for simplicity) and an adaptive time step to provide reliable estimates of ion concentration dynamics (including pH profile evolution), requiring no prior installation nor compilation. CAFES also offers a large database of commonly used species and their relevant physicochemical properties. We present a validation of predictions from CAFES by comparing them to experimental data of peak- and plateau-mode isotachophoresis experiments. The code yields accurate estimates of interface velocity, plateau length and relative intensity, and pH variations while significantly reducing the computation time compared to existing codes. The tool is open-source and available for free at https://microfluidics.stanford.edu/cafes.

  View details for DOI 10.1021/acs.analchem.1c03925

  View details for PubMedID 34788021

• CRISPR Enzyme Kinetics for Molecular Diagnostics. Analytical chemistry Ramachandran, A. n., Santiago, J. G. 2021

  Abstract

  CRISPR-diagnostic assays have gained significant interest in the last few years. This interest has grown rapidly during the current COVID-19 pandemic, where CRISPR-diagnostics have been frontline contenders for rapid testing solutions. This surge in CRISPR-diagnostic research prompts the following question: what exactly are the achievable limits of detection and associated assay times enabled by the kinetics of enzymes such as Cas12 and Cas13? To explore this question, we here present a model based on Michaelis-Menten enzyme kinetics theory applied to CRISPR enzymes. We use the model to develop analytical solutions for reaction kinetics and develop back-of-the-envelope criteria to validate and check for consistency in reported enzyme kinetic parameters. We applied our analyses to all studies known to us, which report Michaelis-Menten-type kinetic data for CRISPR-associated enzymes. These studies include all subtypes of Cas12 and Cas13 and orthologs. We found all but one study clearly violate at least two of our three rules and therefore present data that violate basic physical limits. We performed an experimental study of reaction kinetics of LbCas12a with both ssDNA and dsDNA activators and use these data to validate our model and its predicted scaling. The validated model is used to explore CRISPR reaction time scales and the degree of reaction completion for practically relevant target concentrations applicable to CRISPR-diagnostic assays. The results have broad implications for achievable limits of detection and assay times of emerging, amplification-free CRISPR-detection methods.

  View details for DOI 10.1021/acs.analchem.1c00525

  View details for PubMedID 33979119

• Electric field-driven microfluidics for rapid CRISPR-based diagnostics and its application to detection of SARS-CoV-2. Proceedings of the National Academy of Sciences of the United States of America Ramachandran, A., Huyke, D. A., Sharma, E., Sahoo, M. K., Huang, C., Banaei, N., Pinsky, B. A., Santiago, J. G. 2020

  Abstract

  The rapid spread of COVID-19 across the world has revealed major gaps in our ability to respond to new virulent pathogens. Rapid, accurate, and easily configurable molecular diagnostic tests are imperative to prevent global spread of new diseases. CRISPR-based diagnostic approaches are proving to be useful as field-deployable solutions. In one basic form of this assay, the CRISPR-Cas12 enzyme complexes with a synthetic guide RNA (gRNA). This complex becomes activated only when it specifically binds to target DNA and cleaves it. The activated complex thereafter nonspecifically cleaves single-stranded DNA reporter probes labeled with a fluorophore-quencher pair. We discovered that electric field gradients can be used to control and accelerate this CRISPR assay by cofocusing Cas12-gRNA, reporters, and target within a microfluidic chip. We achieve an appropriate electric field gradient using a selective ionic focusing technique known as isotachophoresis (ITP) implemented on a microfluidic chip. Unlike previous CRISPR diagnostic assays, we also use ITP for automated purification of target RNA from raw nasopharyngeal swab samples. We here combine this ITP purification with loop-mediated isothermal amplification and the ITP-enhanced CRISPR assay to achieve detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) RNA (from raw sample to result) in about 35 min for both contrived and clinical nasopharyngeal swab samples. This electric field control enables an alternate modality for a suite of microfluidic CRISPR-based diagnostic assays.

  View details for DOI 10.1073/pnas.2010254117

  View details for PubMedID 33148808

• Simultaneous optical and infrared thermal imaging of isotachophoresis. Analytica chimica acta Terzis, A., Ramachandran, A., Kang, J., Santiago, J. G. 2020; 1131: 9–17

  Abstract

  Joule heating in isotachophoresis (ITP) can limit minimum assay times and efforts to scale up processed sample volumes. Despite its significance, the dynamics of Joule heating on spatiotemporal temperature fields in ITP systems have not been investigated. We here present novel measurements of spatiotemporal temperature and electromigration fields in ITP. To achieve this, we obtain simultaneous and registered optical and infrared thermal images of the ITP process. We conduct a series of experiments at constant current operation and vary the leading electrolyte concentration to study and highlight the importance of buffer-dependent ionic conductivity on the resulted temperature rise. The measurements demonstrate a substantial increase of temperature in the adjusted trailing electrolyte region, and the propagation of a thermal wave in the ITP channel with a velocity equal to that of the electromigration front. We present scaling of the experimental data that indicates the dependence of front velocity and temperature rise on current density and ionic conductivity. The current study has direct application to the design and optimization of scaled-up ITP systems and the validation of numerical models of Joule heating.

  View details for DOI 10.1016/j.aca.2020.07.014

  View details for PubMedID 32928483

• On the competition between mixing rate and uniformity in a coaxial hydrodynamic focusing mixer. Analytica chimica acta Huyke, D. A., Ramachandran, A., Oyarzun, D. I., Kroll, T., DePonte, D. P., Santiago, J. G. 2020; 1103: 1–10

  Abstract

  Fast microfluidic mixers for use with line-of-sight integrating detection schemes pose unique challenges. Such detectors typically cannot discriminate signal from slow moving (e.g. near internal walls) and fast-moving portions of the fluid stream. This convolves reaction rate dynamics with fluid flow residence time dynamics. Further, the small cross sections of typical three-dimensional hydrodynamic focusing devices lead to lower detection signals. The current study focuses on achieving both small time scales of mixing and homogenous residence times. This is achieved by injecting sample through a center capillary and hydrodynamically focusing using a sheath flow within a tapered second capillary. The current design also features a third, larger coaxial capillary. The mixed stream flows into the large cross-section of this third capillary to decelerate and expand the stream by up to 14-fold to improve line-of-sight signal strength of reaction products. Hydrodynamic focusing, mixing, and expansion are studied using analytical and numerical models and also studied experimentally using a fluorescein-iodide quenching reaction. The experimentally validated models are used to explore trade-offs between mixing rate and uniformity. For the first time, this work presents detailed analysis of the Lagrangian time history of species transport during mixing inside coaxial capillaries to measure mixing nonuniformity. The mixing region enables order 100 mus mixing times and residence time widths of the same order (140 mus).

  View details for DOI 10.1016/j.aca.2020.01.013

  View details for PubMedID 32081173

• A system for the high-throughput measurement of the shear modulus distribution of human red blood cells. Lab on a chip Saadat, A. n., Huyke, D. A., Oyarzun, D. I., Escobar, P. V., Øvreeide, I. H., Shaqfeh, E. S., Santiago, J. G. 2020

  Abstract

  Reduced deformability of red blood cells (RBCs) can affect the hemodynamics of the microcirculation and reduce oxygen transport efficiency. It is also well known that reduced RBC deformability is a signature of various physical disorders, including sepsis, and that the primary determinant of RBC deformability is the membrane shear modulus. To measure the distribution of an individual's RBC shear modulus with high throughput, we a) developed a high-fidelity computational model of RBCs in confined microchannels to inform design decisions; b) created a novel experimental system combining microfluidic flow, imaging, and image analysis; and c) performed automated comparisons between measured quantities and numerical predictions to extract quantitative measures of the RBC shear modulus for each of thousands of cells. We applied our computational simulation platform to construct the appropriate deformability figure(s) of merit to quantify RBC stiffness based on an experimentally measured, steady-state cell shape in flow through a microchannel. In particular, we determined a shape parameter based on the second moment of the cell shape that is sensitive to the changes in the membrane stiffness and cell size. We then conducted microfluidic experiments and developed custom automated image processing codes to identify and track the position and shape of individual RBCs within micro-constrictions. The fabricated microchannels include a square cross-section imaging region (7 by 7 μm) and we applied order 10 kPa pressure differences to induce order 10 mm s-1 cell velocities. The combination of modeling, microfluidics, and imaging enables, for the first time, quantitative measurement of the shear moduli of thousands of RBCs in human blood samples. We demonstrate the high-throughput features by sensitive quantification of the changes in the distribution of RBC stiffness with aging. This combined measurement and computational platform is ultimately intended to diagnose blood cell disorders in patients.

  View details for DOI 10.1039/d0lc00283f

  View details for PubMedID 32648561

• Effects of Weak Electrolytes on Electric Double Layer Ion Distributions. The journal of physical chemistry letters Chamberlayne, C. F., Zare, R. N., Santiago, J. G. 2020: 8302–6

  Abstract

  Many common experimental systems have electric double layers containing weak electrolytes, including systems with buffers. The pH at the boundary of the diffuse layer is an important parameter for determining the physicochemical state of the system, including surface charge density. We show that the Boltzmann equilibrium relation can be used as an exact solution for weak electrolyte electric double layers. Using these results, we provide a closed-form relation for the maximum pH change in a buffered electric double layer, in terms of the boundary potential. Importantly, our results suggest that equilibrium electric double layer concepts developed for strong electrolytes can be expanded to include weak electrolytes.

  View details for DOI 10.1021/acs.jpclett.0c02247

  View details for PubMedID 32915583

• Performance metrics for the objective assessment of capacitive deionization systems WATER RESEARCH Hawks, S. A., Ramachandran, A., Porada, S., Campbell, P. G., Suss, M. E., Biesheuvel, P. M., Santiago, J. G., Stadermann, M. 2019; 152: 126–37

  View details for DOI 10.1016/j.watres.2018.10.074

  View details for Web of Science ID 000458223900012

• Frequency analysis and resonant operation for efficient capacitive deionization WATER RESEARCH Ramachandran, A., Hawks, S. A., Stadermann, M., Santiago, J. G. 2018; 144: 581-591

  View details for DOI 10.1016/j.watres.2018.07.066

  View details for Web of Science ID 000447569300057

• Thermodynamics of Ion Separation by Electrosorption ENVIRONMENTAL SCIENCE & TECHNOLOGY Hemmatifar, A., Ramachandran, A., Liu, K., Oyarzun, D. I., Bazant, M. Z., Santiago, J. G. 2018; 52 (17): 10196-10204

  View details for DOI 10.1021/acs.est.8b02959

  View details for Web of Science ID 000444061100069

• Self similarities in desalination dynamics and performance using capacitive deionization. Water research Ramachandran, A., Hemmatifar, A., Hawks, S. A., Stadermann, M., Santiago, J. G. 2018; 140: 323–34

  Abstract

  Charge transfer and mass transport are two underlying mechanisms which are coupled in desalination dynamics using capacitive deionization (CDI). We developed simple reduced-order models based on a mixed reactor volume principle which capture the coupled dynamics of CDI operation using closed-form semi-analytical and analytical solutions. We use the models to identify and explore self-similarities in the dynamics among flow rate, current, and voltage for CDI cell operation including both charging and discharging cycles. The similarity approach identifies the specific combination of cell (e.g. capacitance, resistance) and operational parameters (e.g. flow rate, current) which determine a unique effluent dynamic response. We here demonstrate self-similarity using a conventional flow between CDI (fbCDI) architecture, and we hypothesize that our similarity approach has potential application to a wide range of designs. We performed an experimental study of these dynamics and used well-controlled experiments of CDI cell operation to validate and explore limits of the model. For experiments, we used a CDI cell with five electrode pairs and a standard flow between (electrodes) architecture. Guided by the model, we performed a series of experiments that demonstrate natural response of the CDI system. We also identify cell parameters and operation conditions which lead to self-similar dynamics under a constant current forcing function and perform a series of experiments by varying flowrate, currents, and voltage thresholds to demonstrate self-similarity. Based on this study, we hypothesize that the average differential electric double layer (EDL) efficiency (a measure of ion adsorption rate to EDL charging rate) is mainly dependent on user-defined voltage thresholds, whereas flow efficiency (measure of how well desalinated water is recovered from inside the cell) depends on cell volumes flowed during charging, which is determined by flowrate, current and voltage thresholds. Results of experiments strongly support this hypothesis. Results show that cycle efficiency and salt removal for a given flowrate and current are maximum when average EDL and flow efficiencies are approximately equal. We further explored a range of CC operations with varying flowrates, currents, and voltage thresholds using our similarity variables to highlight trade-offs among salt removal, energy, and throughput performance.

  View details for PubMedID 29734040

• Thermodynamics of Ion Separation by Electrosorption. Environmental science & technology Hemmatifar, A., Ramachandran, A., Liu, K., Oyarzun, D. I., Bazant, M. Z., Santiago, J. G. 2018

  Abstract

  We present a simple, top-down approach for the calculation of minimum energy consumption of electrosorptive ion separation using variational form of the (Gibbs) free energy. We focus and expand on the case of electrostatic capacitive deionization (CDI). The theoretical framework is independent of details of the double-layer charge distribution and is applicable to any thermodynamically consistent model, such as the Gouy-Chapman-Stern and modified Donnan models. We demonstrate that, under certain assumptions, the minimum required electric work energy is indeed equivalent to the free energy of separation. Using the theory, we define the thermodynamic efficiency of CDI. We show that the thermodynamic efficiency of current experimental CDI systems is currently very low, around 1% for most existing systems. We applied this knowledge and constructed and operated a CDI cell to show that judicious selection of the materials, geometry, and process parameters can lead to a 9% thermodynamic efficiency and 4.6 kT per removed ion energy cost. This relatively high thermodynamic efficiency is, to our knowledge, by far the highest thermodynamic efficiency ever demonstrated for traditional CDI. We hypothesize that efficiency can be further improved by further reduction of CDI cell series resistances and optimization of operational parameters.

  View details for PubMedID 30141621

• Frequency analysis and resonant operation for efficient capacitive deionization. Water research Ramachandran, A., Hawks, S. A., Stadermann, M., Santiago, J. G. 2018; 144: 581–91

  Abstract

  Capacitive deionization (CDI) performance metrics can vary widely with operating methods. Conventional CDI operating methods such as constant current and constant voltage show advantages in either energy or salt removal performance, but not both. We here develop a theory around and experimentally demonstrate a new operation for CDI that uses sinusoidal forcing voltage (or sinusoidal current). We use a dynamic system modeling approach, and quantify the frequency response (amplitude and phase) of CDI effluent concentration. Using a wide range of operating conditions, we demonstrate that CDI can be modeled as a linear time invariant system. We validate this model with experiments, and show that a sinusoid voltage operation can simultaneously achieve high salt removal and strong energy performance, thus very likely making it superior to other conventional operating methods. Based on the underlying coupled phenomena of electrical charge (and ionic) transfer with bulk advection in CDI, we derive and validate experimentally the concept of using sinusoidal voltage forcing functions to achieve resonance-type operation for CDI. Despite the complexities of the system, we find a simple relation for the resonant time scale: the resonant time period (frequency) is proportional (inversely proportional) to the geometric mean of the flow residence time and the electrical (RC) charging time. Operation at resonance implies the optimal balance between absolute amount of salt removed (in moles) and dilution (depending on the feed volume processed), thus resulting in the maximum average concentration reduction for the desalinated water. We further develop our model to generalize the resonant time-scale operation, and provide responses for square and triangular voltage waveforms as two examples. To this end, we develop a general tool that uses Fourier analysis to construct CDI effluent dynamics for arbitrary input waveforms. Using this tool, we show that most of the salt removal (95%) for square and triangular voltage forcing waveforms is achieved by the fundamental Fourier (sinusoidal) mode. The frequency of higher Fourier modes precludes high flow efficiency for these modes, so these modes consume additional energy for minimal additional salt removed. This deficiency of higher frequency modes further highlights the advantage of DC-offset sinusoidal forcing for CDI operation.

  View details for PubMedID 30092504

• SINC-seq: correlation of transient gene expressions between nucleus and cytoplasm reflects single-cell physiology GENOME BIOLOGY Abdelmoez, M. N., Iida, K., Oguchi, Y., Nishikii, H., Yokokawa, R., Kotera, H., Uemura, S., Santiago, J. G., Shintaku, H. 2018; 19: 66

  Abstract

  We report a microfluidic system that physically separates nuclear RNA (nucRNA) and cytoplasmic RNA (cytRNA) from a single cell and enables single-cell integrated nucRNA and cytRNA-sequencing (SINC-seq). SINC-seq constructs two individual RNA-seq libraries, nucRNA and cytRNA, per cell, quantifies gene expression in the subcellular compartments, and combines them to create novel single-cell RNA-seq data. Leveraging SINC-seq, we discover distinct natures of correlation among cytRNA and nucRNA that reflect the transient physiological state of single cells. These data provide unique insights into the regulatory network of messenger RNA from the nucleus toward the cytoplasm at the single-cell level.

  View details for PubMedID 29871653

• Adsorption and capacitive regeneration of nitrate using inverted capacitive deionization with surfactant functionalized carbon electrodes SEPARATION AND PURIFICATION TECHNOLOGY Oyarzun, D. I., Hemmatifar, A., Palko, J. W., Stadermann, M., Santiago, J. G. 2018; 194: 410–15

  View details for DOI 10.1016/j.seppur.2017.11.027

  View details for Web of Science ID 000425072100048

• Extreme Two-Phase Cooling from Laser-Etched Diamond and Conformal, Template-Fabricated Microporous Copper ADVANCED FUNCTIONAL MATERIALS Palko, J. W., Lee, H., Zhang, C., Dusseault, T. J., Maitra, T., Won, Y., Agonafer, D. D., Moss, J., Houshmand, F., Rong, G., Wilbur, J. D., Rockosi, D., Mykyta, I., Resler, D., Altman, D., Asheghi, M., Santiago, J. G., Goodson, K. E. 2017; 27 (45)

  View details for DOI 10.1002/adfm.201703283

  View details for Web of Science ID 000416828500008

• Nondestructive nanostraw intracellular sampling for longitudinal cell monitoring PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Cao, Y., Hjort, M., Chen, H., Birey, F., Leal-Ortiz, S. A., Han, C. M., Santiago, J. G., Pasca, S. P., Wu, J. C., Melosh, N. A. 2017; 114 (10): E1866–E1874

  View details for DOI 10.1073/pnas.1615375114/-/DCSupplemental

  View details for Web of Science ID 000395511400013

• Nondestructive nanostraw intracellular sampling for longitudinal cell monitoring. Proceedings of the National Academy of Sciences of the United States of America Cao, Y., Hjort, M., Chen, H., Birey, F., Leal-Ortiz, S. A., Han, C. M., Santiago, J. G., Pasca, S. P., Wu, J. C., Melosh, N. A. 2017

  Abstract

  Here, we report a method for time-resolved, longitudinal extraction and quantitative measurement of intracellular proteins and mRNA from a variety of cell types. Cytosolic contents were repeatedly sampled from the same cell or population of cells for more than 5 d through a cell-culture substrate, incorporating hollow 150-nm-diameter nanostraws (NS) within a defined sampling region. Once extracted, the cellular contents were analyzed with conventional methods, including fluorescence, enzymatic assays (ELISA), and quantitative real-time PCR. This process was nondestructive with >95% cell viability after sampling, enabling long-term analysis. It is important to note that the measured quantities from the cell extract were found to constitute a statistically significant representation of the actual contents within the cells. Of 48 mRNA sequences analyzed from a population of cardiomyocytes derived from human induced pluripotent stem cells (hiPSC-CMs), 41 were accurately quantified. The NS platform samples from a select subpopulation of cells within a larger culture, allowing native cell-to-cell contact and communication even during vigorous activity such as cardiomyocyte beating. This platform was applied both to cell lines and to primary cells, including CHO cells, hiPSC-CMs, and human astrocytes derived in 3D cortical spheroids. By tracking the same cell or group of cells over time, this method offers an avenue to understand dynamic cell behavior, including processes such as induced pluripotency and differentiation.

  View details for DOI 10.1073/pnas.1615375114

  View details for PubMedID 28223521

• Increasing hybridization rate and sensitivity of DNA microarrays using isotachophoresis LAB ON A CHIP Han, C. M., Katilius, E., Santiago, J. G. 2014; 14 (16): 2958-2967

  Abstract

  We present an on-chip electrokinetic method to increase the reaction kinetics and sensitivity of DNA microarray hybridization. We use isotachophoresis (ITP) to preconcentrate target molecules in solution and transport them over the immobilized probe sites of a microarray, greatly increasing the binding reaction rate. We show theoretically and experimentally that ITP-enhanced microarrays can be hybridized much faster and with higher sensitivity than conventional methods. We demonstrate our assay using a microfluidic system consisting of a PDMS microchannel superstructure bonded onto a glass slide on which 60 spots of 20-27 nt ssDNA oligonucleotide probes are immobilized. Our 30 min assay results in an 8.2 fold higher signal than the conventional overnight hybridization at 100 fM target concentration. We show rapid and quantitative detection over 4 orders of magnitude dynamic range of target concentration with no increase in the nonspecific signal. Our technique can be further multiplexed for higher density microarrays and extended for other reactions of target-surface immobilized ligands.

  View details for DOI 10.1039/c4lc00374h

  View details for Web of Science ID 000339470400010

  View details for PubMedID 24921466

• Purification of nucleic acids using isotachophoresis JOURNAL OF CHROMATOGRAPHY A Rogacs, A., Marshall, L. A., Santiago, J. G. 2014; 1335: 105-120

  Abstract

  Reviewed are methods of nucleic acid (NA) extraction and sample preparation using an electrophoretic purification and focusing method called isotachophoresis (ITP). ITP requires no special surface chemistries or geometric structures, and can be achieved in a compact system with no moving parts. ITP is also compatible with a wide range of samples and lysing methods. Described are general principles of ITP, considerations around the application of ITP to biological samples (e.g., blood, urine and saliva), ITP electrolyte design considerations for fast and selective NA purification, and examples of ITP compatible lysing methods. Several of the challenges associated with purification of NAs are presented as well as methods to address these. Lastly, specific examples of lysing methods and ITP chemistries are described for purification of NA including host and pathogenic DNA, pathogenic rRNA, and host micro-RNA from complex sample matrices.

  View details for DOI 10.1016/j.chroma.2013.12.027

  View details for Web of Science ID 000334476200013

  View details for PubMedID 24444800

• An injection molded microchip for nucleic acid purification from 25 microliter samples using isotachophoresis. Journal of chromatography. A Marshall, L. A., Rogacs, A., Meinhart, C. D., Santiago, J. G. 2014; 1331: 139-142

  Abstract

  We present a novel microchip device for purification of nucleic acids from 25μL biological samples using isotachophoresis (ITP). The device design incorporates a custom capillary barrier structure to facilitate robust sample loading. The chip uses a 2mm channel width and 0.15mm depth to reduce processing time, mitigate Joule heating, and achieve high extraction efficiency. To reduce pH changes in the device due to electrolysis, we incorporated a buffering reservoir physically separated from the sample output reservoir. To reduce dispersion of the ITP-focused zone, we used optimized turn geometries. The chip was fabricated by injection molding PMMA and COC plastics through a commercial microfluidic foundry. The extraction efficiency of nucleic acids from the device was measured using fluorescent quantification, and an average recovery efficiency of 81% was achieved for nucleic acid masses between 250pg and 250ng. The devices were also used to purify DNA from whole blood, and the extracted DNA was amplified using qPCR to show the PCR compatibility of the purified sample.

  View details for DOI 10.1016/j.chroma.2014.01.036

  View details for PubMedID 24485540

• On-chip separation and analysis of RNA and DNA from single cells. Analytical chemistry Shintaku, H., Nishikii, H., Marshall, L. A., Kotera, H., Santiago, J. G. 2014; 86 (4): 1953-1957

  Abstract

  The simultaneous analysis of RNA and DNA of single cells remains a challenge as these species have very similar physical and biochemical properties and can cross-contaminate each other. Presented is an on-chip system that enables selective lysing of single living cells, extraction, focusing, and absolute quantification of cytoplasmic RNA mass and its physical separation from DNA in the nucleus using electrical lysing and isotachophoresis (ITP). This absolute quantitation is performed without enzymatic amplification in less than 5 min. The nucleus is preserved, and its DNA fluorescence signal can be measured independently. We demonstrate the technique using single mouse lymphocyte cells, for which we extracted an average of 14.1 pg of total RNA per cell. We also demonstrate correlation analysis between the absolute amount of RNA and relative amount of DNA, showing heterogeneity associated with cell cycles. The technique is compatible with fractionation of DNA and RNA and with downstream assays of each.

  View details for DOI 10.1021/ac4040218

  View details for PubMedID 24499009

• Temperature effects on electrophoresis. Analytical chemistry Rogacs, A., Santiago, J. G. 2013; 85 (10): 5103-5113

  Abstract

  We present a model capturing the important contributors to the effects of temperature on the observable electrophoretic mobilities of small ions, and on solution conductivity and pH. Our temperature model includes relations for temperature-dependent viscosity, ionic strength corrections, degree of ionization (pK), and ion solvation effects on mobility. We incorporate thermophysical data for water viscosity, temperature-dependence of the Onsager-Fuoss model for finite ionic strength effects on mobility, temperature-dependence of the extended Debye-Huckel theory for correction of ionic activity, the Clarke-Glew approach and tabulated thermodynamic quantities of ionization reaction for acid dissociation constants as a function of temperature, and species-specific, empirically evaluated correction terms for temperature-dependence of Stokes' radii. We incorporated our model into a MATLAB-based simulation tool we named Simulation of Temperature Effects on ElectroPhoresis (STEEP). We validated our model using conductivity and pH measurements across a temperature variation of 25-70 °C for a set of electrolytes routinely used in electrophoresis. The model accurately captures electrolyte solution pH and conductivity, including important effects not captured by simple Walden-type relations.

  View details for DOI 10.1021/ac400447k

  View details for PubMedID 23627294

• Electric fields yield chaos in microflows PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Posner, J. D., Perez, C. L., Santiago, J. G. 2012; 109 (36): 14353-14356

  Abstract

  We present an investigation of chaotic dynamics of a low Reynolds number electrokinetic flow. Electrokinetic flows arise due to couplings of electric fields and electric double layers. In these flows, applied (steady) electric fields can couple with ionic conductivity gradients outside electric double layers to produce flow instabilities. The threshold of these instabilities is controlled by an electric Rayleigh number, Ra(e). As Ra(e) increases monotonically, we show here flow dynamics can transition from steady state to a time-dependent periodic state and then to an aperiodic, chaotic state. Interestingly, further monotonic increase of Ra(e) shows a transition back to a well-ordered state, followed by a second transition to a chaotic state. Temporal power spectra and time-delay phase maps of low dimensional attractors graphically depict the sequence between periodic and chaotic states. To our knowledge, this is a unique report of a low Reynolds number flow with such a sequence of periodic-to-aperiodic transitions. Also unique is a report of strange attractors triggered and sustained through electric fluid body forces.

  View details for DOI 10.1073/pnas.1204920109

  View details for Web of Science ID 000308912600021

  View details for PubMedID 22908251

  View details for PubMedCentralID PMC3437830

• Rapid hybridization of nucleic acids using isotachophoresis PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Bercovici, M., Han, C. M., Liao, J. C., Santiago, J. G. 2012; 109 (28): 11127-11132

  Abstract

  We use isotachophoresis (ITP) to control and increase the rate of nucleic acid hybridization reactions in free solution. We present a new physical model, validation experiments, and demonstrations of this assay. We studied the coupled physicochemical processes of preconcentration, mixing, and chemical reaction kinetics under ITP. Our experimentally validated model enables a closed form solution for ITP-aided reaction kinetics, and reveals a new characteristic time scale which correctly predicts order 10,000-fold speed-up of chemical reaction rate for order 100 pM reactants, and greater enhancement at lower concentrations. At 500 pM concentration, we measured a reaction time which is 14,000-fold lower than that predicted for standard second-order hybridization. The model and method are generally applicable to acceleration of reactions involving nucleic acids, and may be applicable to a wide range of reactions involving ionic reactants.

  View details for DOI 10.1073/pnas.1205004109

  View details for Web of Science ID 000306642100027

  View details for PubMedID 22733732

  View details for PubMedCentralID PMC3396536

• On-chip Isotachophoresis for Separation of Ions and Purification of Nucleic Acids JOVE-JOURNAL OF VISUALIZED EXPERIMENTS Garcia-Schwarz, G., Rogacs, A., Bahga, S. S., Santiago, J. G. 2012

  Abstract

  Electrokinetic techniques are a staple of microscale applications because of their unique ability to perform a variety of fluidic and electrophoretic processes in simple, compact systems with no moving parts. Isotachophoresis (ITP) is a simple and very robust electrokinetic technique that can achieve million-fold preconcentration and efficient separation and extraction based on ionic mobility. For example, we have demonstrated the application of ITP to separation and sensitive detection of unlabeled ionic molecules (e.g. toxins, DNA, rRNA, miRNA) with little or no sample preparation and to extraction and purification of nucleic acids from complex matrices including cell culture, urine, and blood. ITP achieves focusing and separation using an applied electric field and two buffers within a fluidic channel system. For anionic analytes, the leading electrolyte (LE) buffer is chosen such that its anions have higher effective electrophoretic mobility than the anions of the trailing electrolyte (TE) buffer (Effective mobility describes the observable drift velocity of an ion and takes into account the ionization state of the ion, as described in detail by Persat et al.). After establishing an interface between the TE and LE, an electric field is applied such that LE ions move away from the region occupied by TE ions. Sample ions of intermediate effective mobility race ahead of TE ions but cannot overtake LE ions, and so they focus at the LE-TE interface (hereafter called the "ITP interface"). Further, the TE and LE form regions of respectively low and high conductivity, which establish a steep electric field gradient at the ITP interface. This field gradient preconcentrates sample species as they focus. Proper choice of TE and LE results in focusing and purification of target species from other non-focused species and, eventually, separation and segregation of sample species. We here review the physical principles underlying ITP and discuss two standard modes of operation: "peak" and "plateau" modes. In peak mode, relatively dilute sample ions focus together within overlapping narrow peaks at the ITP interface. In plateau mode, more abundant sample ions reach a steady-state concentration and segregate into adjoining plateau-like zones ordered by their effective mobility. Peak and plateau modes arise out of the same underlying physics, but represent distinct regimes differentiated by the initial analyte concentration and/or the amount of time allotted for sample accumulation. We first describe in detail a model peak mode experiment and then demonstrate a peak mode assay for the extraction of nucleic acids from E. coli cell culture. We conclude by presenting a plateau mode assay, where we use a non-focusing tracer (NFT) species to visualize the separation and perform quantitation of amino acids.

  View details for DOI 10.3791/3890

  View details for Web of Science ID 000209222800048

  View details for PubMedCentralID PMC3399465

• Free-surface microfluidic control of surface-enhanced Raman spectroscopy for the optimized detection of airborne molecules PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Piorek, B. D., Lee, S. J., Santiago, J. G., Moskovits, M., Banerjee, S., Meinhart, C. D. 2007; 104 (48): 18898-18901

  Abstract

  We present a microfluidic technique for sensitive, real-time, optimized detection of airborne water-soluble molecules by surface-enhanced Raman spectroscopy (SERS). The method is based on a free-surface fluidic device in which a pressure-driven liquid microchannel flow is constrained by surface tension. A colloidal suspension of silver nanoparticles flowing through the microchannel that is open to the atmosphere absorbs gas-phase 4-aminobenzenethiol (4-ABT) from the surrounding environment. As surface ions adsorbed on the colloid nanoparticles are substituted by 4-ABT, the colloid aggregates, forming SERS "hot spots" whose concentrations vary predictably along the microchannel flow. 4-ABT confined in these hot spots produces SERS spectra of very great intensity. An aggregation model is used to account quantitatively for the extent of colloid aggregation as determined from the variation of the SERS intensity measured as a function of the streamwise position along the microchannel, which also corresponds to nanoparticle exposure time. This allows us to monitor simultaneously the nanoparticle aggregation process and to determine the location at which the SERS signal is optimized.

  View details for DOI 10.1073/pnas.0708596104

  View details for Web of Science ID 000251498700007

  View details for PubMedID 18025462

  View details for PubMedCentralID PMC2141879

• Analytical solutions for viscoelectric effects in electrokinetic nanochannels. Electrophoresis Ma, K., Ramachandran, A., Santiago, J. G. 2024

  Abstract

  Understanding electrokinetic transport in nanochannels and nanopores is essential for emerging biological and electrochemical applications. The viscoelectric effect is an important mechanism implicated in the increase of local viscosity due to the polarization of a solvent under a strong electric field. However, most analyses of the viscoelectric effect have been limited to numerical analyses. In this work, we present a set of analytical solutions applicable to the physical description of viscoelectric effects in nanochannel electrokinetic systems. To achieve such closed-form solutions, we employ the Debye-Hückel approximation of small diffuse charge layer potentials compared to the thermal potential. We analyze critical parameters, including electroosmotic flow profiles, electroosmotic mobility, flow rate, and channel conductance. We compare and benchmark our analytical solutions with published predictions from numerical models. Importantly, we leverage these analytical solutions to identify essential thermophysical and nondimensional parameters that govern the behavior of these systems. We identify scaling parameters and relations among surface charge density, ionic strength, and nanochannel height.

  View details for DOI 10.1002/elps.202300204

  View details for PubMedID 38350722

• Stream lamination and rapid mixing in a microfluidic jet for X-ray spectroscopy studies FLOW Huyke, D. A., Avaro, A. S., Kroll, T., Santiago, J. G. 2023; 3

  View details for DOI 10.1017/flo.2023.15

  View details for Web of Science ID 001052592500001

• Liquid Heterostructures: Generation of Liquid-Liquid Interfaces in Free-Flowing Liquid Sheets. Langmuir : the ACS journal of surfaces and colloids Hoffman, D. J., Bechtel, H. A., Huyke, D. A., Santiago, J. G., DePonte, D. P., Koralek, J. D. 2022

  Abstract

  Chemical reactions and biological processes are frequently governed by the structure and dynamics of the interface between two liquid phases, but these interfaces are often difficult to study due to the relative abundance of the bulk liquids. Here, we demonstrate a method for generating multilayer thin film stacks of liquids, which we call liquid heterostructures. These free-flowing layered liquid sheets are produced with a microfluidic nozzle that impinges two converging jets of one liquid onto opposite sides of a third jet of another liquid. The resulting sheet consists of two layers of the first liquid enveloping an inner layer of the second liquid. Infrared microscopy, white light reflectivity, and imaging ellipsometry measurements demonstrate that the buried liquid layer has a tunable thickness and displays well-defined liquid-liquid interfaces and that this inner layer can be only tens of nanometers thick. The demonstrated multilayer liquid sheets minimize the amount of bulk liquid relative to their buried interfaces, which makes them ideal targets for spectroscopy and scattering experiments.

  View details for DOI 10.1021/acs.langmuir.2c01724

  View details for PubMedID 36220141

• Uncertainty Quantification of Michaelis-Menten Kinetic Rates and Its Application to the Analysis of CRISPR-Based Diagnostics. Angewandte Chemie (International ed. in English) Avaro, A. S., Santiago, J. G. 2022

  Abstract

  Michaelis-Menten kinetics is an essential model to rationalize enzyme reactions. The quantification of Michaelis-Menten parameters can be very challenging as it is sensitive to even small experimental errors. We here present a quantification of the uncertainty inherent to the experimental determination of kinetic rate parameters for enzymatic reactions. We study the influence of several sources of uncertainty and bias, including the inner filter effect, pipetting errors, number of points in the Michaelis-Menten curve, and flat-field correction. Using Monte Carlo simulations and analyses of experimental data, we compute typical uncertainties of [[EQUATION]] , [[EQUATION]] , and catalytic efficiency [[EQUATION]] . As a salient example, we analyze the extraction of such parameters for CRISPR-Cas systems. CRISPR diagnostics have recently attracted much interest and yet reports of these enzymatic kinetic rates have been highly unreliable and inconsistent.

  View details for DOI 10.1002/anie.202209527

  View details for PubMedID 36117459

• Electrochemical Methods for Water Purification, Ion Separations, and Energy Conversion. Chemical reviews Alkhadra, M. A., Su, X., Suss, M. E., Tian, H., Guyes, E. N., Shocron, A. N., Conforti, K. M., de Souza, J. P., Kim, N., Tedesco, M., Khoiruddin, K., Wenten, I. G., Santiago, J. G., Hatton, T. A., Bazant, M. Z. 2022

  Abstract

  Agricultural development, extensive industrialization, and rapid growth of the global population have inadvertently been accompanied by environmental pollution. Water pollution is exacerbated by the decreasing ability of traditional treatment methods to comply with tightening environmental standards. This review provides a comprehensive description of the principles and applications of electrochemical methods for water purification, ion separations, and energy conversion. Electrochemical methods have attractive features such as compact size, chemical selectivity, broad applicability, and reduced generation of secondary waste. Perhaps the greatest advantage of electrochemical methods, however, is that they remove contaminants directly from the water, while other technologies extract the water from the contaminants, which enables efficient removal of trace pollutants. The review begins with an overview of conventional electrochemical methods, which drive chemical or physical transformations via Faradaic reactions at electrodes, and proceeds to a detailed examination of the two primary mechanisms by which contaminants are separated in nondestructive electrochemical processes, namely electrokinetics and electrosorption. In these sections, special attention is given to emerging methods, such as shock electrodialysis and Faradaic electrosorption. Given the importance of generating clean, renewable energy, which may sometimes be combined with water purification, the review also discusses inverse methods of electrochemical energy conversion based on reverse electrosorption, electrowetting, and electrokinetic phenomena. The review concludes with a discussion of technology comparisons, remaining challenges, and potential innovations for the field such as process intensification and technoeconomic optimization.

  View details for DOI 10.1021/acs.chemrev.1c00396

  View details for PubMedID 35904408

• Millisecond timescale reactions observed via X-ray spectroscopy in a 3D microfabricated fused silica mixer. Corrigendum. Journal of synchrotron radiation Huyke, D. A., Ramachandran, A., Ramirez-Neri, O., Guerrero-Cruz, J. A., Gee, L. B., Braun, A., Sokaras, D., Garcia-Estrada, B., Solomon, E. I., Hedman, B., Delgado-Jaime, M. U., DePonte, D. P., Kroll, T., Santiago, J. G. 2022; 29 (Pt 3): 930

  Abstract

  A figure in the article by Huyke et al. [(2021), J. Synchrotron Rad. 28, 1100-1113] is corrected.

  View details for DOI 10.1107/S1600577522002806

  View details for PubMedID 35511027

• A modular and reconfigurable open-channel gated device for the electrokinetic extraction of cell-free DNA assays. Analytica chimica acta Futai, N., Fukazawa, Y., Kashiwagi, T., Tamaki, S., Sakai, R., Hogan, C. A., Murugesan, K., Ramachandran, A., Banaei, N., Santiago, J. G. 2022; 1200: 339435

  Abstract

  The high-efficiency separation and extraction of short fragments of cell-free DNA (cfDNA) remain challenging due to their low abundance and short lengths. This study presents a method for separating short cfDNA fragments, with lengths ranging from about 100 to 200 base pairs, from liquid human plasma samples into separable and extractable bands as solid agarose gel slabs. To achieve this, a novel millimeter-scale fluidic device is used for sample handling, transient isotachophoresis, and extraction. The device features open-to-atmosphere liquid chambers that define and manually actuated (i.e., movable) agarose-made gate valve structures. The agarose gates then define discrete zones for buffers, sample injection, DNA pre-concentration via isotachophoresis, size-based gel separation, and DNA-band extraction. As a demonstration of its efficacy, the device is applied to the enrichment and purification of M.tuberculosis genomic DNA fragments spiked in human plasma samples. This purified cfDNA is analyzed using the quantitative polymerase chain reaction (qPCR) of the IS6110 repetitive sequence in the M.tuberculosis genome. The data from this study demonstrates that high sensitivity can be achieved in cfDNA detection, as shown by the comparison with a typical solid-phase extraction method and buffer spiked with cfDNA. Evidence is presented that suggests plasma peptides generated by treatment of the sample with proteinase K acts as endogenous spacer molecules, which improve the resolution and purification of DNA relative to the marker dye and other contaminants that decrease the signal level in qPCR.

  View details for DOI 10.1016/j.aca.2022.339435

  View details for PubMedID 35256135

• Species Abundance and Reaction Off-Rate Regulate Product Formation in Reactions Accelerated Using Isotachophoresis. Analytical chemistry Jiang, Q., Ramachandran, A., Santiago, J. G. 2021

  Abstract

  We present a model for second-order and pseudo-first-order reversible chemical reactions accelerated using peak-mode isotachophoresis (ITP). In such systems, ITP preconcentrates and co-locates the reactants between the leading and trailing electrolyte zones, and this significantly accelerates chemical reactions. Our model quantifies the effects of reaction rate constants and species abundance on product formation rate. We identify two key non-dimensional parameters, which are specific groupings of reaction rate constants, species concentrations, and influx rates. We then use a regular perturbation to study the effects of reverse reaction rate and relative species abundance (and relative rates of species accumulation) on production rate. We also use this perturbation method to derive an analytical expression for the quasi-steady-state production rate achievable by ITP. Our analytical models and numerical solutions are generally applicable to a wide range of systems, which use ITP to enhance reactions. The model is also an interesting case study of the complex coupling of electric field-driven species transport and reaction kinetics.

  View details for DOI 10.1021/acs.analchem.1c01805

  View details for PubMedID 34492181

• Millisecond timescale reactions observed via X-ray spectroscopy in a 3D microfabricated fused silica mixer. Journal of synchrotron radiation Huyke, D. A., Ramachandran, A., Ramirez-Neri, O., Guerrero-Cruz, J. A., Gee, L. B., Braun, A., Sokaras, D., Garcia-Estrada, B., Solomon, E. I., Hedman, B., Delgado-Jaime, M. U., DePonte, D. P., Kroll, T., Santiago, J. G. 2021; 28 (Pt 4): 1100-1113

  Abstract

  Determination of electronic structures during chemical reactions remains challenging in studies which involve reactions in the millisecond timescale, toxic chemicals, and/or anaerobic conditions. In this study, a three-dimensionally (3D) microfabricated microfluidic mixer platform that is compatible with time-resolved X-ray absorption and emission spectroscopy (XAS and XES, respectively) is presented. This platform, to initiate reactions and study their progression, mixes a high flow rate (0.50-1.5 ml min-1) sheath stream with a low-flow-rate (5-90 l min-1) sample stream within a monolithic fused silica chip. The chip geometry enables hydrodynamic focusing of the sample stream in3D and sample widths as small as 5 m. The chip is also connected to a polyimide capillary downstream to enable sample stream deceleration, expansion, and X-ray detection. In this capillary, sample widths of 50 m are demonstrated. Further, convection-diffusion-reaction models of the mixer are presented. The models are experimentally validated using confocal epifluorescence microscopy and XAS/XES measurements of a ferricyanide and ascorbic acid reaction. The models additionally enable prediction of the residence time and residence time uncertainty of reactive species as well as mixing times. Residence times (from initiation of mixing to the point of X-ray detection) during sample stream expansion as small as 2.1 ± 0.3 ms are also demonstrated. Importantly, an exploration of the mixer operational space reveals a theoretical minimum mixing time of 0.91 ms. The proposed platform is applicable to the determination of the electronic structure of conventionally inaccessible reaction intermediates.

  View details for DOI 10.1107/S1600577521003830

  View details for PubMedID 34212873

• Understanding resistances in capacitive deionization devices ENVIRONMENTAL SCIENCE-WATER RESEARCH & TECHNOLOGY Kuo, H. A., Ramachandran, A., Oyarzun, D. I., Clevenger, E. C., Santiago, J. G., Stadermann, M., Campbell, P. G., Hawks, S. A. 2020; 6 (7): 1842–54

  View details for DOI 10.1039/d0ew00169d

  View details for Web of Science ID 000545743800007

• High-Frequency Water Vapor Sorption Cycling Using Fluidization of Metal-Organic Frameworks CELL REPORTS PHYSICAL SCIENCE Terzis, A., Ramachandran, A., Wang, K., Asheghi, M., Goodson, K. E., Santiago, J. G. 2020; 1 (5)

  View details for DOI 10.1016/j.xcrp.2020.100057

  View details for Web of Science ID 000658743200010

• Energy transfer for storage or recovery in capacitive deionization using a DC-DC converter JOURNAL OF POWER SOURCES Oyarzun, D. I., Hawks, S. A., Campbell, P. G., Hemmatifar, A., Krishna, A., Santiago, J. G., Stadermann, M. 2020; 448

  View details for DOI 10.1016/j.jpowsour.2019.227409

  View details for Web of Science ID 000509632300090

• Process design tools and techno-economic analysis for capacitive deionization. Water research Hasseler, T. D., Ramachandran, A. n., Tarpeh, W. A., Stadermann, M. n., Santiago, J. G. 2020; 183: 116034

  Abstract

  Capacitive deionization (CDI) devices use cyclical electrosorption on porous electrode surfaces to achieve water desalination. Process modeling and design of CDI systems requires accurate treatment of the coupling among input electrical forcing, input flow rates, and system responses including salt removal dynamics, water recovery, energy storage, and dissipation. Techno-economic analyses of CDI further require a method to calculate and compare between a produced commodity (e.g. desalted water) versus capital and operational costs of the system. We here demonstrate a new modeling and analysis tool for CDI developed as an installable Matlab program that allows direct numerical simulation of CDI dynamics and calculation of key performance and cost parameters. The program is provided for free and is used to run open-source Simulink models. The Simulink environment sends information to the program and allows for a drag and drop design space where users can connect CDI cells to relevant periphery blocks such as grid energy, battery, solar panel, waste disposal, and maintenance/labor cost streams. The program allows for simulation of arbitrary current forcing and arbitrary flow rate forcing of one or more CDI cells. We employ validated well-mixed reactor formulations together with a non-linear circuit model formulation that can accommodate a variety of electric double layer sub-models (e.g. for charge efficiency). The program includes a graphical user interface (GUI) to specify CDI plant parameters, specify operating conditions, run individual tests or parameter batch-mode simulations, and plot relevant results. The techno-economic models convert among dimensional streams of species (e.g. feed, desalted water, and brine), energy, and cost and enable a variety of economic estimates including levelized water costs.

  View details for DOI 10.1016/j.watres.2020.116034

  View details for PubMedID 32736269

• CONCENTRATION GRADIENTS INSIDE MICRODROPLETS. Micro total analysis systems : proceedings of the ... [Mu] TAS International Conference on Miniaturized Chemical and Biochemical Analysis Systems. [Mu] TAS (Conference) Chamberlayne, C. F., Santiago, J., Zare, R. N. 2020; 2020: 212-213

  Abstract

  Small water microdroplets in microfluidic systems have a high surface charge density resulting from charged surfactants. As a result, an electric double layer forms inside the droplet. Depletion of ions from the center of the droplet to form the double layer can shift the concentration of ions dramatically from that of the microdroplet precursor solution. Here we show numerical solutions to the Gouy-Chapman model in spherical coordinates. Some notable effects include: 1) large percentages of the microdroplet volume experience very large DC electric fields; 2) many ions get forced into a Stern layer giving dramatically different conditions from the bulk.

  View details for PubMedID 34557061

  View details for PubMedCentralID PMC8457253

• Simultaneous RNA purification and size selection using on-chip isotachophoresis with an ionic spacer. Lab on a chip Han, C. M., Catoe, D., Munro, S. A., Khnouf, R., Snyder, M. P., Santiago, J. G., Salit, M. L., Cenik, C. 2019

  Abstract

  We present an on-chip method for the extraction of RNA within a specific size range from low-abundance samples. We use isotachophoresis (ITP) with an ionic spacer and a sieving matrix to enable size-selection with a high yield of RNA in the target size range. The spacer zone separates two concentrated ITP peaks, the first containing unwanted single nucleotides and the second focusing RNA of the target size range (2-35 nt). Our ITP method excludes >90% of single nucleotides and >65% of longer RNAs (>35 nt). Compared to size selection using gel electrophoresis, ITP-based size-selection yields a 2.2-fold increase in the amount of extracted RNAs within the target size range. We also demonstrate compatibility of the ITP-based size-selection with downstream next generation sequencing. On-chip ITP-prepared samples reveal higher reproducibility of transcript-specific measurements compared to samples size-selected by gel electrophoresis. Our method offers an attractive alternative to conventional sample preparation for sequencing with shorter assay time, higher extraction efficiency and reproducibility. Potential applications of ITP-based size-selection include sequencing-based analyses of small RNAs from low-abundance samples such as rare cell types, samples from fluorescence activated cell sorting (FACS), or limited clinical samples.

  View details for DOI 10.1039/c9lc00311h

  View details for PubMedID 31328753

• Comments on "Comparison of energy consumption in desalination by capacitive deionization and reverse osmosis" DESALINATION Ramachandran, A., Oyarzun, D. I., Hawks, S. A., Campbell, P. G., Stadermann, M., Santiago, J. G. 2019; 461: 30–36

  View details for DOI 10.1016/j.desal.2019.03.010

  View details for Web of Science ID 000466623000004

• Using Ultramicroporous Carbon for the Selective Removal of Nitrate with Capacitive Deionization. Environmental science & technology Hawks, S. A., Ceron, M. R., Oyarzun, D. I., Pham, T. A., Zhan, C., Loeb, C. K., Mew, D., Deinhart, A., Wood, B. C., Santiago, J. G., Stadermann, M., Campbell, P. G. 2019

  Abstract

  The contamination of water resources with nitrate is a growing and significant problem. Here we report the use of ultramicroporous carbon as a capacitive deionization (CDI) electrode for selectively removing nitrate from an anion mixture. Through moderate activation, we achieve a micropore-size distribution consisting almost exclusively of narrow (<1 nm) pores that are well suited for adsorbing the planar, weakly hydrated nitrate molecule. Cyclic voltammetry measurements reveal an enhanced capacitance for nitrate when compared to chloride as well as significant ion sieving effects when sulfate is the only anion present. We measure high selectivities ( S) of both nitrate over sulfate ( SNO3/SO4 = 17.8 ± 3.6 at 0.6 V) and nitrate over chloride ( SNO3/Cl = 6.1 ± 0.4 at 0.6 V) when performing a constant voltage CDI separation on 3.33 mM/3.33 mM/1.67 mM Cl/NO3/SO4 feedwater. These results are particularly encouraging considering that a divalent interferant was present in the feed. Using molecular dynamics simulations, we examine the solvation characteristics of these ions to better understand why nitrate is preferentially electrosorbed over sulfate and chloride.

  View details for DOI 10.1021/acs.est.9b01374

  View details for PubMedID 31244071

• High water recovery and improved thermodynamic efficiency for capacitive deionization using variable flowrate operation WATER RESEARCH Ramachandran, A., Oyarzun, D. I., Hawks, S. A., Stadermann, M., Santiago, J. G. 2019; 155: 76–85

  View details for DOI 10.1016/j.watres.2019.02.007

  View details for Web of Science ID 000464488500007

• High water recovery and improved thermodynamic efficiency for capacitive deionization using variable flowrate operation. Water research Ramachandran, A., Oyarzun, D. I., Hawks, S. A., Stadermann, M., Santiago, J. G. 2019; 155: 76–85

  Abstract

  Water recovery is a measure of the amount of treated water produced relative to the total amount of water processed through the system, and is an important performance metric for any desalination method. Conventional operating methods for desalination using capacitive deionization (CDI) have so far limited water recovery to be about 50%. To improve water recovery for CDI, we here introduce a new operating scheme based on a variable (in time) flow rate wherein a low flow rate during discharge is used to produce a brine volume which is significantly less than the volume of diluent produced. We demonstrate experimentally and study systematically this novel variable flowrate operating scheme in the framework of both constant current and constant voltage charge-discharge modes. We show that the variable flowrate operation can increase water recovery for CDI to very high values of 90% and can improve thermodynamic efficiency by about 2- to 3-fold compared to conventional constant flowrate operation. Importantly, this is achieved with minimal performance reductions in salt removal, energy consumption, and volume throughput. Our work highlights that water recovery can be readily improved for CDI at very minimal additional cost using simple flow control schemes.

  View details for PubMedID 30831426

• Ion selectivity in capacitive deionization with functionalized electrode: Theory and experimental validation. Water research X Oyarzun, D. I., Hemmatifar, A., Palko, J. W., Stadermann, M., Santiago, J. G. 2018; 1: 100008

  Abstract

  Capacitive deionization (CDI) is a promising technique for salt removal and may have potential for highly selective removal of ion species. In this work, we take advantage of functional groups usually used with ionic exchange resins and apply these to CDI. To this end, we functionalize activated carbon with a quaternary amines surfactant and use this surface to selectively and passively (no applied field) trap nitrate ions. We then set the cell voltage to a constant value to regenerate these electrodes, resulting in an inverted capacitive deionization (i-CDI) operation. Unlike resins, we avoid use of concentrated chemicals for regeneration. We measure the selectivity of nitrate versus chloride ions as a function of regeneration voltage and initial chloride concentration. We experimentally demonstrate up to about 6.5-fold (observable) selectivity in a cycle with a regeneration voltage of 0.4 V. We also demonstrate a novel multi-pass, air-flush i-CDI operation to selectively enrich nitrate with high water recovery. We further present a dynamic, multi-species electrosorption and equilibrium solution-to-surface chemical reaction model and validate the model with detailed measurements. Our i-CDI system exhibits higher nitrate selectivity at lower voltages; making it possible to reduce NaNO3 concentrations from ∼170 ppm to below the limit of maximum allowed values for nitrate in drinking water of about 50 ppm NaNO3.

  View details for DOI 10.1016/j.wroa.2018.100008

  View details for PubMedID 31194024

  View details for PubMedCentralID PMC6549908

• Ion selectivity in capacitive deionization with functionalized electrode: Theory and experimental validation WATER RESEARCH X Oyarzun, D., Hemmatifar, A., Palko, J. W., Stadermann, M., Santiago, J. G. 2018; 1

  View details for DOI 10.1016/j.wroa.2018.100008

  View details for Web of Science ID 000461243100008

• Device design and flow scaling for liquid sheet jets PHYSICAL REVIEW FLUIDS Ha, B., DePonte, D. P., Santiago, J. G. 2018; 3 (11)

  View details for DOI 10.1103/PhysRevFluids.3.114202

  View details for Web of Science ID 000451013700003

• A method for quantifying in plane permeability of porous thin films JOURNAL OF COLLOID AND INTERFACE SCIENCE Rong, G., Palko, J. W., Oyarzun, D. I., Zhang, C., Hammerle, J., Asheghi, M., Goodson, K. E., Santiago, J. G. 2018; 530: 667–74

  View details for DOI 10.1016/j.jcis.2018.05.062

  View details for Web of Science ID 000442700000072

• Performance metrics for the objective assessment of capacitive deionization systems. Water research Hawks, S. A., Ramachandran, A., Porada, S., Campbell, P. G., Suss, M. E., Biesheuvel, P. M., Santiago, J. G., Stadermann, M. 2018; 152: 126–37

  Abstract

  In the growing field of capacitive deionization (CDI), a number of performance metrics have emerged to describe the desalination process. Unfortunately, the separation conditions under which these metrics are measured are often not specified, resulting in optimal performance at minimal removal. Here we outline a system of performance metrics and reporting conditions that resolves this issue. Our proposed system is based on volumetric energy consumption (Wh/m3) and throughput productivity (L/h/m2) reported for a specific average concentration reduction, water recovery, and feed salinity. To facilitate and rationalize comparisons between devices, materials, and operation modes, we propose a nominal standard separation of removing 5 mM from a 20 mM NaCl feed solution at 50% water recovery. We propose this particular separation as a standard, but emphasize that the rationale presented here applies irrespective of separation details. Using our proposed separation, we compare the desalination performance of a flow-through electrode (fte-CDI) cell and a flow between membrane (fb-MCDI) device, showing how significantly different systems can be compared in terms of generally desirable desalination characteristics. In general, we find that performance analysis must be considered carefully so to not allow for ambiguous separation conditions or the maximization of one metric at the expense of another. Additionally, for context and clarity, we discuss a number of important underlying performance indicators and cell characteristics that are not performance measures in and of themselves but can be examined to better understand differences in performance.

  View details for PubMedID 30665159

• Efficient Production of On-Target Reads for Small RNA Sequencing of Single Cells Using Modified Adapters ANALYTICAL CHEMISTRY Khnouf, R., Shore, S., Han, C. M., Henderson, J. M., Munro, S. A., McCaffrey, A. P., Shintaku, H., Santiago, J. G. 2018; 90 (21): 12609-12615

  View details for DOI 10.1021/acs.analchem.8b02773

  View details for Web of Science ID 000449722500040

• Enhanced Capillary-Fed Boiling in Copper Inverse Opals via Template Sintering ADVANCED FUNCTIONAL MATERIALS Zhang, C., Palko, J. W., Barako, M. T., Asheghi, M., Santiago, J. G., Goodson, K. E. 2018; 28 (41)

  View details for DOI 10.1002/adfm.201803689

  View details for Web of Science ID 000446550700009

• Efficient Production of On-Target Reads for Small RNA Sequencing of Single Cells Using Modified Adapters. Analytical chemistry Khnouf, R., Shore, S., Han, C. M., Henderson, J. M., Munro, S. A., McCaffrey, A. P., Shintaku, H., Santiago, J. G. 2018

  Abstract

  Although single-cell mRNA sequencing has been a powerful tool to explore cellular heterogeneity, the sequencing of small RNA at the single-cell level (sc-sRNA-seq) remains a challenge, as these have no consensus sequence, are relatively low abundant, and are difficult to amplify in a bias-free fashion. We present two methods of single-cell-lysis that enable sc-sRNA-seq. The first method is a chemical-based technique with overnight freezing while the second method leverages on-chip electrical lysis of plasma membrane and physical extraction and separation of cytoplasmic RNA via isotachophoresis. We coupled these two methods with off-chip small RNA library preparation using CleanTag modified adapters to prevent the formation of adapter dimers. We then demonstrated sc-sRNA-seq with single K562 human leukemic cells. Our approaches offer a relatively short hands-on time of 6 h and efficient generation of on-target reads. The sc-sRNA-seq with our approaches showed detection of miRNA with various abundances ranging from 16 000 copies/cell to about 10 copies/cell. We anticipate this approach will create a new opportunity to explore cellular heterogeneity through small RNA expression.

  View details for PubMedID 30260208

• Tailoring Permeability of Microporous Copper Structures through Template Sintering ACS APPLIED MATERIALS & INTERFACES Zhang, C., Palko, J. W., Rong, G., Pringle, K. S., Barako, M. T., Dusseault, T. J., Asheghi, M., Santiago, J. G., Goodson, K. E. 2018; 10 (36): 30487–94

  View details for DOI 10.1021/acsami.8b03774

  View details for Web of Science ID 000444793000053

• Tailored porous electrode resistance for controlling electrolyte depletion and improving charging response in electrochemical systems JOURNAL OF POWER SOURCES Palko, J. W., Hemmatifar, A., Santiago, J. G. 2018; 397: 252–61

  View details for DOI 10.1016/j.jpowsour.2018.06.102

  View details for Web of Science ID 000445317900031

• Self similarities in desalination dynamics and performance using capacitive deionization WATER RESEARCH Ramachandran, A., Hemmatifar, A., Hawks, S. A., Stadermann, M., Santiago, J. G. 2018; 140: 323-334

  View details for DOI 10.1016/j.watres.2018.04.042

  View details for Web of Science ID 000447359000031

• Tailoring Permeability of Microporous Copper Structures through Template Sintering. ACS applied materials & interfaces Zhang, C., Palko, J. W., Rong, G., Pringle, K. S., Barako, M. T., Dusseault, T. J., Asheghi, M., Santiago, J. G., Goodson, K. E. 2018

  Abstract

  Microporous metals are used extensively for applications that combine convective and conductive transport and benefit from low resistance to both modes of transport. Conventional fabrication methods, such as direct sintering of metallic particles, however, often produce structures with limited fluid transport properties due to the lack of control over pore morphologies such as the pore size and porosity. Here, we demonstrate control and improvement of hydraulic permeability of microporous copper structures fabricated using template-assisted electrodeposition. Template sintering is shown to modify the fluid transport network in a manner that increases permeability by nearly an order of magnitude with a less significant decrease (38%) in thermal conductivity. The measured permeabilities range from 4.8 * 10-14 to 1.3 * 10-12 m2 with 5 mum pores, with the peak value being roughly 5 times larger than the published values for sintered copper particles of comparable feature sizes. Analysis indicates that the enhancement of permeability is limited by constrictions, i.e., bottlenecks between connecting pores, whose dimensions are highly sensitive to the sintering conditions. We further show contrasting trends in permeability versus conductivity of the electrodeposited microporous copper and conventional sintered copper particles and suggest these differing trends to be the result of their inverse structural relationship.

  View details for PubMedID 30096232

• Self-Cleaning Porous Surfaces for Dry Condensation ACS APPLIED MATERIALS & INTERFACES Liu, K., Huang, Z., Hemmatifar, A., Oyarzun, D. I., Zhou, J., Santiago, J. G. 2018; 10 (31): 26759–64

  Abstract

  Efficient water removal from a cool surface during condensation is critically important to the enhancement of a variety of heat transfer applications. Previous work has focused on the fabrication of superhydrophobic surfaces which promote water droplets and removal via droplet shedding or jumping. Here, we report a novel strategy with a droplet self-cleaning surface which spontaneously transports all of the droplets from the condensation surface to the back side. We fabricate the self-cleaning surface by simply tailoring the wettability of the two sides of a porous membrane and demonstrate that the hydrophobic side is effective in clearing off droplets of a wide range of diameters. Even during rapid impingement of droplets smaller than 10 μm, this surface remains dry. We further demonstrate a "dry condensation" process wherein a surface undergoing rapid condensation is maintained free of droplets. This minimizes the essential thermal resistance of the process, and we estimate a twofold increase in condensation rate compared with a simple copper surface under the same conditions. Our method is tailorable, extendable to a wide range of materials and geometries, and shows great potential for a broad range of condensation processes.

  View details for PubMedID 30059209

• Modelling and optimization applied to the design of fast hydrodynamic focusing microfluidic mixer for protein folding JOURNAL OF MATHEMATICS IN INDUSTRY Ivorra, B., Ferrandez, M. R., Crespo, M., Redondo, J. L., Ortigosa, P. M., Santiago, J. G., Ramos, A. M. 2018; 8

  View details for DOI 10.1186/s13362-018-0046-3

  View details for Web of Science ID 000436333400001

• A method for quantifying in plane permeability of porous thin films. Journal of colloid and interface science Rong, G., Palko, J. W., Oyarzun, D. I., Zhang, C., Hammerle, J., Asheghi, M., Goodson, K. E., Santiago, J. G. 2018; 530: 667–74

  Abstract

  The in-plane permeability of porous thin films is an important fluid mechanical property that determines wicking and pressure-driven flow behavior in such materials. This property has so far been challenging to measure directly due to the small sidewall cross-sectional area of thin films available for flow. In this work, we propose and experimentally demonstrate a novel technique for directly measuring in-plane permeability of porous thin films of arbitrary thicknesses, in situ, using a manifold pressed to the top surface of the film. We both measure and simulate the influence of the two dimensional flow field produced in a film by the manifold and extract the permeability from measurements of pressure drop at fixed flow rates. Permeability values measured using the technique for a periodic array of channels are comparable to theoretical predictions. We also determine in-plane permeability of arrays of pillars and electrodeposited porous copper films. This technique is a robust tool to characterize permeability of thin films of arbitrary thicknesses on a variety of substrates. In Supplementary material, we provide a solid model, which is useful in three-dimensional printer reproductions of our device.

  View details for PubMedID 30007196

• Nitrate removal from water using electrostatic regeneration of functionalized adsorbent CHEMICAL ENGINEERING JOURNAL Palko, J. W., Oyarzun, D. I., Ha, B., Stadermann, M., Santiago, J. G. 2018; 334: 1289–96

  View details for DOI 10.1016/j.cej.2017.10.161

  View details for Web of Science ID 000418533400129

• Quantifying the flow efficiency in constant-current capacitive deionization WATER RESEARCH Hawks, S. A., Knipe, J. M., Campbell, P. G., Loeb, C. K., Hubert, M. A., Santiago, J. G., Stadermann, M. 2018; 129: 327–36

  Abstract

  Here we detail a previously unappreciated loss mechanism inherent to capacitive deionization (CDI) cycling operation that has a substantial role determining performance. This mechanism reflects the fact that desalinated water inside a cell is partially lost to re-salination if desorption is carried out immediately after adsorption. We describe such effects by a parameter called the flow efficiency, and show that this efficiency is distinct from and yet multiplicative with other highly-studied adsorption efficiencies. Flow losses can be minimized by flowing more feed solution through the cell during desalination; however, this also results in less effluent concentration reduction. While the rationale outlined here is applicable to all CDI cell architectures that rely on cycling, we validate our model with a flow-through electrode CDI device operated in constant-current mode. We find excellent agreement between flow efficiency model predictions and experimental results, thus giving researchers simple equations by which they can estimate this distinct loss process for their operation.

  View details for PubMedID 29161663

• Charging and Transport Dynamics of a Flow-Through Electrode Capacitive Deionization System JOURNAL OF PHYSICAL CHEMISTRY B Qu, Y., Campbell, P. G., Hemmatifar, A., Knipe, J. M., Loeb, C. K., Reidy, J. J., Hubert, M. A., Stadermann, M., Santiago, J. G. 2018; 122 (1): 240–49

  Abstract

  We present a study of the interplay among electric charging rate, capacitance, salt removal, and mass transport in "flow-through electrode" capacitive deionization (CDI) systems. We develop two models describing coupled transport and electro-adsorption/desorption which capture salt removal dynamics. The first model is a simplified, unsteady zero-dimensional volume-averaged model which identifies dimensionless parameters and figures of merits associated with cell performance. The second model is a higher fidelity area-averaged model which captures both spatial and temporal responses of charging. We further conducted an experimental study of these dynamics and considered two salt transport regimes: (1) advection-limited regime and (2) dispersion-limited regime. We use these data to validate models. The study shows that, in the advection-limited regime, differential charge efficiency determines the salt adsorption at the early stage of the deionization process. Subsequently, charging transitions to a quasi-steady state where salt removal rate is proportional to applied current scaled by the inlet flow rate. In the dispersion-dominated regime, differential charge efficiency, cell volume, and diffusion rates govern adsorption dynamics and flow rate has little effect. In both regimes, the interplay among mass transport rate, differential charge efficiency, cell capacitance, and (electric) charging current governs salt removal in flow-through electrode CDI.

  View details for PubMedID 29292999

• Equilibria model for pH variations and ion adsorption in capacitive deionization electrodes. Water research Hemmatifar, A., Oyarzun, D. I., Palko, J. W., Hawks, S. A., Stadermann, M., Santiago, J. G. 2017; 122: 387-397

  Abstract

  Ion adsorption and equilibrium between electrolyte and microstructure of porous electrodes are at the heart of capacitive deionization (CDI) research. Surface functional groups are among the factors which fundamentally affect adsorption characteristics of the material and hence CDI system performance in general. Current CDI-based models for surface charge are mainly based on a fixed (constant) charge density, and do not treat acid-base equilibria of electrode microstructure including so-called micropores. We here expand current models by coupling the modified Donnan (mD) model with weak electrolyte acid-base equilibria theory. In our model, surface charge density can vary based on equilibrium constants (pK's) of individual surface groups as well as micropore and electrolyte pH environments. In this initial paper, we consider this equilibrium in the absence of Faradaic reactions. The model shows the preferential adsorption of cations versus anions to surfaces with respectively acidic or basic surface functional groups. We introduce a new parameter we term "chemical charge efficiency" to quantify efficiency of salt removal due to surface functional groups. We validate our model using well controlled titration experiments for an activated carbon cloth (ACC), and quantify initial and final pH of solution after adding the ACC sample. We also leverage inductively coupled plasma mass spectrometry (ICP-MS) and ion chromatography (IC) to quantify the final background concentrations of individual ionic species. Our results show a very good agreement between experiments and model. The model is extendable to a wide variety of porous electrode systems and CDI systems with applied potential.

  View details for DOI 10.1016/j.watres.2017.05.036

  View details for PubMedID 28622631

• Rapid Hydrogen-Deuterium Exchange in Liquid Droplets. Journal of the American Chemical Society Jansson, E. T., Lai, Y., Santiago, J. G., Zare, R. N. 2017; 139 (20): 6851-6854

  Abstract

  The rate of hydrogen-deuterium exchange (HDX) in aqueous droplets of phenethylamine has been determined with submillisecond temporal resolution by mass spectrometry using nanoelectrospray ionization with a theta-capillary. The average speed of the microdroplets is measured using microparticle image velocimetry. The droplet travel time is varied from 20 to 320 μs by changing the distance between the emitter and the heated inlet to the mass spectrometer and the voltage applied to the emitter source. The droplets were found to accelerate by ∼30% during their observable travel time. Our droplet imaging shows that the theta-capillary produces two Taylor cone-jets (one per channel), causing mixing to take place from droplet fusion in the Taylor spray zone. Phenethylamine (ϕCH2CH2NH2) was chosen to study because it has only one functional group (-NH2) that undergoes rapid HDX. We model the HDX with a system of ordinary differential equations. The rate constant for the formation of -NH2D(+) from -NH3(+) is 3660 ± 290 s(-1), and the rate constant for the formation of -NHD2(+) from -NH2D(+) is 3330 ± 270 s(-1). The observed rates are about 3 times faster than what has been reported for rapidly exchangeable peptide side-chain groups in bulk measurements using stopped-flow kinetics and NMR spectroscopy. We also applied this technique to determine the HDX rates for a small 10-residue peptide, angiotensin I, in aqueous droplets, from which we found a 7-fold acceleration of HDX in the droplet compared to that in bulk solution.

  View details for DOI 10.1021/jacs.7b03541

  View details for PubMedID 28481522

• Assay for Listeria monocytogenes cells in whole blood using isotachophoresis and recombinase polymerase amplification ANALYST Eid, C., Santiago, J. G. 2017; 142 (1): 48-54

  Abstract

  We present a new approach which enables lysis, extraction, and detection of inactivated Listeria monocytogenes cells from blood using isotachophoresis (ITP) and recombinase polymerase amplification (RPA). We use an ITP-compatible alkaline and proteinase K approach for rapid and effective lysis. We then perform ITP purification to separate bacterial DNA from whole blood contaminants using a microfluidic device that processes 25 μL sample volume. Lysis, mixing, dispensing, and on-chip ITP purification are completed in a total of less than 50 min. We transfer extracted DNA directly into RPA master mix for isothermal incubation and detection, an additional 25 min. We first validate our assay in the detection of purified genomic DNA spiked into whole blood, and demonstrate a limit of detection of 16.7 fg μL(-1) genomic DNA, the equivalent of 5 × 10(3) cells per mL. We then show detection of chemically-inactivated L. monocytogenes cells spiked into whole blood, and demonstrate a limit of detection of 2 × 10(4) cells per mL. Lastly, we show preliminary experimental data demonstrating the feasibility of the integration of ITP purification with RPA detection on a microfluidic chip. Our results suggest that ITP purification is compatible with RPA detection, and has potential to extend the applicability of RPA to whole blood.

  View details for DOI 10.1039/c6an02119k

  View details for Web of Science ID 000391448300004

• Equilibria model for pH variations and ion adsorption in capacitive deionization electrodes Water Research Hemmatifar, A., Oyarzun, D. I., Palko, J. W., Hawks, S. A., Stadermann, M., Santiago, J. G. 2017: 387-397

  Abstract

  Ion adsorption and equilibrium between electrolyte and microstructure of porous electrodes are at the heart of capacitive deionization (CDI) research. Surface functional groups are among the factors which fundamentally affect adsorption characteristics of the material and hence CDI system performance in general. Current CDI-based models for surface charge are mainly based on a fixed (constant) charge density, and do not treat acid-base equilibria of electrode microstructure including so-called micropores. We here expand current models by coupling the modified Donnan (mD) model with weak electrolyte acid-base equilibria theory. In our model, surface charge density can vary based on equilibrium constants (pK's) of individual surface groups as well as micropore and electrolyte pH environments. In this initial paper, we consider this equilibrium in the absence of Faradaic reactions. The model shows the preferential adsorption of cations versus anions to surfaces with respectively acidic or basic surface functional groups. We introduce a new parameter we term "chemical charge efficiency" to quantify efficiency of salt removal due to surface functional groups. We validate our model using well controlled titration experiments for an activated carbon cloth (ACC), and quantify initial and final pH of solution after adding the ACC sample. We also leverage inductively coupled plasma mass spectrometry (ICP-MS) and ion chromatography (IC) to quantify the final background concentrations of individual ionic species. Our results show a very good agreement between experiments and model. The model is extendable to a wide variety of porous electrode systems and CDI systems with applied potential.

  View details for DOI 10.1016/j.watres.2017.05.036

• Assay for Listeria monocytogenes cells in whole blood using isotachophoresis and recombinase polymerase amplification. Analyst Eid, C., Santiago, J. G. 2016; 142 (1): 48-54

  Abstract

  We present a new approach which enables lysis, extraction, and detection of inactivated Listeria monocytogenes cells from blood using isotachophoresis (ITP) and recombinase polymerase amplification (RPA). We use an ITP-compatible alkaline and proteinase K approach for rapid and effective lysis. We then perform ITP purification to separate bacterial DNA from whole blood contaminants using a microfluidic device that processes 25 μL sample volume. Lysis, mixing, dispensing, and on-chip ITP purification are completed in a total of less than 50 min. We transfer extracted DNA directly into RPA master mix for isothermal incubation and detection, an additional 25 min. We first validate our assay in the detection of purified genomic DNA spiked into whole blood, and demonstrate a limit of detection of 16.7 fg μL(-1) genomic DNA, the equivalent of 5 × 10(3) cells per mL. We then show detection of chemically-inactivated L. monocytogenes cells spiked into whole blood, and demonstrate a limit of detection of 2 × 10(4) cells per mL. Lastly, we show preliminary experimental data demonstrating the feasibility of the integration of ITP purification with RPA detection on a microfluidic chip. Our results suggest that ITP purification is compatible with RPA detection, and has potential to extend the applicability of RPA to whole blood.

  View details for PubMedID 27904893

• Energy consumption analysis of constant voltage and constant current operations in capacitive deionization DESALINATION Qu, Y., Campbell, P. G., Gu, L., Knipe, J. M., Dzenitis, E., Santiago, J. G., Stadermann, M. 2016; 400: 18-24

  View details for DOI 10.1016/j.desal.2016.09.014

  View details for Web of Science ID 000386738800003

• Influx and Production Rates in Peak-Mode Isotachophoresis ANALYTICAL CHEMISTRY Eid, C., Santiago, J. G. 2016; 88 (23): 11352-11357

  Abstract

  We present an analytical model useful in the design of peak-mode isotachophoresis (ITP) experiments. The model quantifies sample influx and production rates, the latter in applications where ITP is used to accelerate chemical reactions. We include analysis of the effect of initial sample placement location. We derive and identify key nondimensional parameters for the general case of weak electrolyte buffer ions in terms of sample placement (injection mode), initial concentrations, fully ionized mobilities, and reaction kinetic constants. We then discuss how to use these parameters in the optimal design of peak-mode ITP assays and highlight regimes of particular interest. We clearly identify a quasi-equilibrium regime wherein production rates increase until they equal the influx rate of the low abundance sample species. The model and analysis are generally applicable to both cationic and anionic ITP assays and likely to a wide range of sample species.

  View details for DOI 10.1021/acs.analchem.6b03467

  View details for Web of Science ID 000389556900017

  View details for PubMedID 27934117

• Energy breakdown in capacitive deionization. Water research Hemmatifar, A., Palko, J. W., Stadermann, M., Santiago, J. G. 2016; 104: 303-311

  Abstract

  We explored the energy loss mechanisms in capacitive deionization (CDI). We hypothesize that resistive and parasitic losses are two main sources of energy losses. We measured contribution from each loss mechanism in water desalination with constant current (CC) charge/discharge cycling. Resistive energy loss is expected to dominate in high current charging cases, as it increases approximately linearly with current for fixed charge transfer (resistive power loss scales as square of current and charging time scales as inverse of current). On the other hand, parasitic loss is dominant in low current cases, as the electrodes spend more time at higher voltages. We built a CDI cell with five electrode pairs and standard flow between architecture. We performed a series of experiments with various cycling currents and cut-off voltages (voltage at which current is reversed) and studied these energy losses. To this end, we measured series resistance of the cell (contact resistances, resistance of wires, and resistance of solution in spacers) during charging and discharging from voltage response of a small amplitude AC current signal added to the underlying cycling current. We performed a separate set of experiments to quantify parasitic (or leakage) current of the cell versus cell voltage. We then used these data to estimate parasitic losses under the assumption that leakage current is primarily voltage (and not current) dependent. Our results confirmed that resistive and parasitic losses respectively dominate in the limit of high and low currents. We also measured salt adsorption and report energy-normalized adsorbed salt (ENAS, energy loss per ion removed) and average salt adsorption rate (ASAR). We show a clear tradeoff between ASAR and ENAS and show that balancing these losses leads to optimal energy efficiency.

  View details for DOI 10.1016/j.watres.2016.08.020

  View details for PubMedID 27565115

• An Ohmic model for electrokinetic flows of binary asymmetric electrolytes CURRENT OPINION IN COLLOID & INTERFACE SCIENCE Persat, A., Santiago, J. G. 2016; 24: 52-63

  View details for DOI 10.1016/j.cocis.2016.06.001

  View details for Web of Science ID 000384778800007

• Design sensitivity and mixing uniformity of a micro-fluidic mixer PHYSICS OF FLUIDS Ivorra, B., Lopez Redondo, J., Ramos, A. M., Santiago, J. G. 2016; 28 (1)

  View details for DOI 10.1063/1.4939006

  View details for Web of Science ID 000374595500012

• High Heat Flux Two-Phase Cooling of Electronics with Integrated Diamond/Porous Copper Heat Sinks and Microfluidic Coolant Supply Palko, J. W., Lee, H., Agonafer, D. D., Zhang, C., Jung, K., Moss, J., Wilbur, J. D., Dusseault, T. J., Barako, M. T., Houshmand, F., Rong, G., Maitra, T., Gorle, C., Won, Y., Rockosi, D., Mykyta, I., Resler, D., Altman, D., Asheghi, M., Santiago, J. G., Goodson, K. E., IEEE IEEE. 2016: 1511–17

  View details for Web of Science ID 000390436000208

• Approaching the limits of two-phase boiling heat transfer: High heat flux and low superheat APPLIED PHYSICS LETTERS Palko, J. W., Zhang, C., Wilbur, J. D., Dusseault, T. J., Asheghi, M., Goodson, K. E., Santiago, J. G. 2015; 107 (25)

  View details for DOI 10.1063/1.4938202

  View details for Web of Science ID 000368442100048

• Two-Dimensional Porous Electrode Model for Capacitive Deionization JOURNAL OF PHYSICAL CHEMISTRY C Hemmatifar, A., Stadermann, M., Santiago, J. G. 2015; 119 (44): 24681-24694

  View details for DOI 10.1021/acs.jpcc.5b05847

  View details for Web of Science ID 000364435200001

• Characterization of Resistances of a Capacitive Deionization System ENVIRONMENTAL SCIENCE & TECHNOLOGY Qu, Y., Baumann, T. F., Santiago, J. G., Stadermann, M. 2015; 49 (16): 9699-9706

  Abstract

  Capacitive deionization (CDI) is a promising desalination technology, which operates at low pressure, low temperature, requires little infrastructure, and has the potential to consume less energy for brackish water desalination. However, CDI devices consume significantly more energy than the theoretical thermodynamic minimum, and this is at least partly due to resistive power dissipation. We here report our efforts to characterize electric resistances in a CDI system, with a focus on the resistance associated with the contact between current collectors and porous electrodes. We present an equivalent circuit model to describe resistive components in a CDI cell. We propose measurable figures of merit to characterize cell resistance. We also show that contact pressure between porous electrodes and current collectors can significantly reduce contact resistance. Lastly, we propose and test an alternative electrical contact configuration which uses a pore-filling conductive adhesive (silver epoxy) and achieves significant reductions in contact resistance.

  View details for DOI 10.1021/acs.est.5b02542

  View details for Web of Science ID 000359891700037

• Characterization of Resistances of a Capacitive Deionization System. Environmental science & technology Qu, Y., Baumann, T. F., Santiago, J. G., Stadermann, M. 2015; 49 (16): 9699-9706

  Abstract

  Capacitive deionization (CDI) is a promising desalination technology, which operates at low pressure, low temperature, requires little infrastructure, and has the potential to consume less energy for brackish water desalination. However, CDI devices consume significantly more energy than the theoretical thermodynamic minimum, and this is at least partly due to resistive power dissipation. We here report our efforts to characterize electric resistances in a CDI system, with a focus on the resistance associated with the contact between current collectors and porous electrodes. We present an equivalent circuit model to describe resistive components in a CDI cell. We propose measurable figures of merit to characterize cell resistance. We also show that contact pressure between porous electrodes and current collectors can significantly reduce contact resistance. Lastly, we propose and test an alternative electrical contact configuration which uses a pore-filling conductive adhesive (silver epoxy) and achieves significant reductions in contact resistance.

  View details for DOI 10.1021/acs.est.5b02542

  View details for PubMedID 26214554

• Rapid Slow Off-Rate Modified Aptamer (SOMAmer)-Based Detection of C-Reactive Protein Using Isotachophoresis and an Ionic Spacer. Analytical chemistry Eid, C., Palko, J. W., Katilius, E., Santiago, J. G. 2015; 87 (13): 6736-6743

  Abstract

  We present an on-chip electrophoretic assay for rapid protein detection with a SOMAmer (Slow Off-Rate Modified Aptamer) reagent. We used isotachophoresis (ITP) coupled with an ionic spacer to both react and separate SOMAmer-protein complex from free SOMAmer reagent. ITP accelerates the reaction kinetics as the ionic spacer concurrently separates the reaction products. We developed a numerical and analytical model to describe ITP spacer assays, which involve low-mobility, nonfocusing targets that are recruited into the ITP zone by higher-mobility, ITP-focused probes. We demonstrated a proof-of-concept of this assay using C-reactive protein (CRP) in buffer, and achieved a 2 nM limit of detection (LOD) with a combined 20 min assay time (10 min off-chip preparation of reagents and 10 min on-chip run). Our findings suggest that this approach has potential as a simple and rapid alternative to other homogeneous immunoassays. We also explore the extension of this assay to a diluted serum sample spiked with CRP, where we observe decreased sensitivity (an LOD of 25 nM in 20-fold diluted serum). We describe the challenges in extending this assay to complex samples and achieving higher sensitivity and specificity for clinical applications.

  View details for DOI 10.1021/acs.analchem.5b00886

  View details for PubMedID 26024067

• Isotachophoresis for fractionation and recovery of cytoplasmic RNA and nucleus from single cells. Electrophoresis Kuriyama, K., Shintaku, H., Santiago, J. G. 2015; 36 (14): 1658-1662

  Abstract

  There is a substantial need for simultaneous analyses of RNA and DNA from individual single cells. Such analysis provides unique evidence of cell-to-cell differences and the correlation between gene expression and genomic mutation in highly heterogeneous cell populations. We present a novel microfluidic system that leverages isotachophoresis to fractionate and isolate cytoplasmic RNA and genomic DNA (gDNA) from single cells. The system uniquely enables independent, sequence-specific analyses of these critical markers. Our system uses a microfluidic chip with a simple geometry and four end-channel electrodes, and completes the entire process in <5 min, including lysis, purification, fractionation, and delivery to DNA and RNA output reservoirs, each containing high quality and purity aliquots with no measurable cross-contamination of cytoplasmic RNA versus gDNA. We demonstrate our system with simultaneous, sequence-specific quantitation using off-chip RT-qPCR and qPCR for simultaneous cytoplasmic RNA and gDNA analyses, respectively.

  View details for DOI 10.1002/elps.201500040

  View details for PubMedID 25820552

• Transient delivery of modified mRNA encoding TERT rapidly extends telomeres in human cells FASEB JOURNAL Ramunas, J., Yakubov, E., Brady, J. J., Corbel, S. Y., Holbrook, C., Brandt, M., Stein, J., Santiago, J. G., Cooke, J. P., Blau, H. M. 2015; 29 (5): 1930-1939

  Abstract

  Telomere extension has been proposed as a means to improve cell culture and tissue engineering and to treat disease. However, telomere extension by nonviral, nonintegrating methods remains inefficient. Here we report that delivery of modified mRNA encoding TERT to human fibroblasts and myoblasts increases telomerase activity transiently (24-48 h) and rapidly extends telomeres, after which telomeres resume shortening. Three successive transfections over a 4 d period extended telomeres up to 0.9 kb in a cell type-specific manner in fibroblasts and myoblasts and conferred an additional 28 ± 1.5 and 3.4 ± 0.4 population doublings (PD), respectively. Proliferative capacity increased in a dose-dependent manner. The second and third transfections had less effect on proliferative capacity than the first, revealing a refractory period. However, the refractory period was transient as a later fourth transfection increased fibroblast proliferative capacity by an additional 15.2 ± 1.1 PD, similar to the first transfection. Overall, these treatments led to an increase in absolute cell number of more than 10(12)-fold. Notably, unlike immortalized cells, all treated cell populations eventually stopped increasing in number and expressed senescence markers to the same extent as untreated cells. This rapid method of extending telomeres and increasing cell proliferative capacity without risk of insertional mutagenesis should have broad utility in disease modeling, drug screening, and regenerative medicine.-Ramunas, J., Yakubov, E., Brady, J. J., Corbel, S. Y., Holbrook, C., Brandt, M., Stein, J., Santiago, J. G., Cooke, J. P., Blau, H. M. Transient delivery of modified mRNA encoding TERT rapidly extends telomeres in human cells.

  View details for DOI 10.1096/fj.14-259531

  View details for Web of Science ID 000354114600027

  View details for PubMedID 25614443

• Tailoring of permeability in copper inverse opal for electronic cooling applications Zhang, C., Rong, G., Palko, J. W., Dusseault, T. J., Asheghi, M., Santiago, J. G., Goodson, K. E., ASME AMER SOC MECHANICAL ENGINEERS. 2015

  View details for Web of Science ID 000373517300069

• Extraction and fractionation of RNA and DNA from single cells using selective lysing and isotachophoresis Shintaku, H., Santiago, J. G., Gray, B. L., Becker, H. SPIE-INT SOC OPTICAL ENGINEERING. 2015

  View details for DOI 10.1117/12.2084981

  View details for Web of Science ID 000354516300018

• Increasing Hybridization Rate and Sensitivity of Bead-Based Assays Using Isotachophoresis ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Shintaku, H., Palko, J. W., Sanders, G. M., Santiago, J. G. 2014; 53 (50): 13813-13816

  Abstract

  We present an electrokinetic technique to increase the reaction rate and sensitivity of bead-based assays. We use isotachophoresis (ITP) to preconcentrate and co-focus target molecules and beads into a single ITP zone. The process achieves rapid mixing, stirring, and strongly increases the binding reaction rate. We demonstrate our assay with quantitative detection of 24 nt single-stranded DNA over a dynamic range of three orders of magnitude and multiplexed detection of ten target species per sample. We show that ITP can achieve approximately the same sensitivity as a well-stirred standard reaction in 60-fold reduced reaction time (20 min versus 20 h). Alternately, compared to standard reaction times of 30 min, we show that 20 min ITP hybridization can achieve 5.3-fold higher sensitivity.

  View details for DOI 10.1002/anie.201408403

  View details for Web of Science ID 000345833100030

  View details for PubMedID 25303671

• Simultaneous purification and fractionation of nucleic acids and proteins from complex samples using bidirectional isotachophoresis. Analytical chemistry Qu, Y., Marshall, L. A., Santiago, J. G. 2014; 86 (15): 7264-7268

  Abstract

  We report on our efforts to create an on-chip system to simultaneously purify and fractionate nucleic acids and proteins from complex samples using isotachophoresis (ITP). We have developed this technique to simultaneously extract extracellular DNA and proteins from human blood serum samples and deliver these to two separate output reservoirs on a chip. The purified DNA is compatible with quantitative polymerase chain reaction (qPCR), and proteins can be extracted so as to exclude albumin, the most abundant protein in serum. We describe significant remaining challenges in making this bidirectional method a robust and efficient technique. These challenges include managing channel surface adsorption of proteins, identifying the cause of observed reductions in low molecular weight proteins, and dealing with nonspecific binding of proteins and DNA.

  View details for DOI 10.1021/ac501299a

  View details for PubMedID 24945850

• Coupling Isotachophoresis with Affinity Chromatography for Rapid and Selective Purification with High Column Utilization, Part 2: Experimental Study ANALYTICAL CHEMISTRY Shkolnikov, V., Santiago, J. G. 2014; 86 (13): 6229-6236

  Abstract

  We present an experimental study of coupling of isotachophoresis (ITP) and affinity chromatography (AC) to effect rapid, selective purification with high column utilization and high resolution. We provide a detailed protocol for performing ITP-AC and describe the design of a buffer system to perform sequence specific separation of nucleic acids. We describe the synthesis and functionalization of our affinity substrate, poly(glycidyl methacrylate-co-ethylene dimethacrylate) porous polymer monolith (GMA-EDMA PPM). This substrate allows easy immobilization of affinity probes, is nonsieving (even to macromolecules), and exhibits negligible nonspecific binding. We demonstrate ITP-AC with 25 nt, Cy5 labeled DNA target and a DNA probe and study the spatiotemporal dynamics using epifluorescence imaging. We make qualitative and quantitative comparisons between these data and the model presented in the first part of this two-paper series. We vary the target concentration from 1 pg μL(-1) to 100 pg μL(-1) and ITP velocity over the range of 10-50 μm s(-1), and thereby explore over 4 orders of magnitude of scaled target amount. We observe very good agreement between predictions and experimental data for the spatiotemporal behavior of the coupled ITP and affinity process, and for key figures of merit, including scaled capture length and maximum capture efficiency. Lastly, we demonstrate that the resolution of ITP-AC increases linearly with time and purify 25 nt target DNA from 10,000-fold higher abundance background (contaminating) genomic fish sperm DNA. We perform this capture from 200 μL of sample in under 1 mm column length and within <10 min.

  View details for DOI 10.1021/ac5011074

  View details for Web of Science ID 000338488800012

  View details for PubMedCentralID PMC4079321

• Coupling isotachophoresis with affinity chromatography for rapid and selective purification with high column utilization, part 1: theory. Analytical chemistry Shkolnikov, V., Santiago, J. G. 2014; 86 (13): 6220-6228

  Abstract

  We present a novel technique that couples isotachophoresis (ITP) with affinity chromatography (AC) to achieve rapid, selective purification with high column utilization. ITP simultaneously preconcentrates an analyte and purifies it, based on differences in mobility of sample components, excluding species that may foul or compete with the target at the affinity substrate. ITP preconcentration accelerates the affinity reaction, reducing assay time, improving column utilization, and allowing for capture of targets with higher dissociation constants. Furthermore, ITP-AC separates the target and contaminants into nondiffusing zones, thus achieving high resolution in a short distance and time. We present an analytical model for spatiotemporal dynamics of ITP-AC. We identify and explore the effect of key process parameters, including target distribution width and height, ITP zone velocity, forward and reverse reaction constants, and probe concentration on necessary affinity region length, assay time, and capture efficiency. Our analytical approach shows collapse of these variables to three nondimensional parameters. The analysis yields simple analytical relations for capture length and capture time in relevant ITP-AC regimes, and it demonstrates how ITP greatly reduces assay time and improves column utilization. In the second part of this two-part series, we will present experimental validation of our model and demonstrate ITP-AC separation of the target from 10,000-fold more-abundant contaminants.

  View details for DOI 10.1021/ac5011052

  View details for PubMedID 24937679

• Coupling isotachophoresis with affinity chromatography for rapid and selective purification with high column utilization, part 2: experimental study. Analytical chemistry Shkolnikov, V., Santiago, J. G. 2014; 86 (13): 6229-6236

  Abstract

  We present an experimental study of coupling of isotachophoresis (ITP) and affinity chromatography (AC) to effect rapid, selective purification with high column utilization and high resolution. We provide a detailed protocol for performing ITP-AC and describe the design of a buffer system to perform sequence specific separation of nucleic acids. We describe the synthesis and functionalization of our affinity substrate, poly(glycidyl methacrylate-co-ethylene dimethacrylate) porous polymer monolith (GMA-EDMA PPM). This substrate allows easy immobilization of affinity probes, is nonsieving (even to macromolecules), and exhibits negligible nonspecific binding. We demonstrate ITP-AC with 25 nt, Cy5 labeled DNA target and a DNA probe and study the spatiotemporal dynamics using epifluorescence imaging. We make qualitative and quantitative comparisons between these data and the model presented in the first part of this two-paper series. We vary the target concentration from 1 pg μL(-1) to 100 pg μL(-1) and ITP velocity over the range of 10-50 μm s(-1), and thereby explore over 4 orders of magnitude of scaled target amount. We observe very good agreement between predictions and experimental data for the spatiotemporal behavior of the coupled ITP and affinity process, and for key figures of merit, including scaled capture length and maximum capture efficiency. Lastly, we demonstrate that the resolution of ITP-AC increases linearly with time and purify 25 nt target DNA from 10,000-fold higher abundance background (contaminating) genomic fish sperm DNA. We perform this capture from 200 μL of sample in under 1 mm column length and within <10 min.

  View details for DOI 10.1021/ac5011074

  View details for PubMedID 24937777

• Coupling Isotachophoresis with Affinity Chromatography for Rapid and Selective Purification with High Column Utilization, Part 1: Theory ANALYTICAL CHEMISTRY Shkolnikov, V., Santiago, J. G. 2014; 86 (13): 6220-6228

  Abstract

  We present a novel technique that couples isotachophoresis (ITP) with affinity chromatography (AC) to achieve rapid, selective purification with high column utilization. ITP simultaneously preconcentrates an analyte and purifies it, based on differences in mobility of sample components, excluding species that may foul or compete with the target at the affinity substrate. ITP preconcentration accelerates the affinity reaction, reducing assay time, improving column utilization, and allowing for capture of targets with higher dissociation constants. Furthermore, ITP-AC separates the target and contaminants into nondiffusing zones, thus achieving high resolution in a short distance and time. We present an analytical model for spatiotemporal dynamics of ITP-AC. We identify and explore the effect of key process parameters, including target distribution width and height, ITP zone velocity, forward and reverse reaction constants, and probe concentration on necessary affinity region length, assay time, and capture efficiency. Our analytical approach shows collapse of these variables to three nondimensional parameters. The analysis yields simple analytical relations for capture length and capture time in relevant ITP-AC regimes, and it demonstrates how ITP greatly reduces assay time and improves column utilization. In the second part of this two-part series, we will present experimental validation of our model and demonstrate ITP-AC separation of the target from 10,000-fold more-abundant contaminants.

  View details for DOI 10.1021/ac5011052

  View details for Web of Science ID 000338488800011

  View details for PubMedCentralID PMC4079320

• In Situ Spatially and Temporally Resolved Measurements of Salt Concentration between Charging Porous Electrodes for Desalination by Capacitive Deionization ENVIRONMENTAL SCIENCE & TECHNOLOGY Suss, M. E., Biesheuvel, P. M., Baumann, T. F., Stadermann, M., Santiago, J. G. 2014; 48 (3): 2008-2015

  Abstract

  Capacitive deionization (CDI) is an emerging water desalination technique. In CDI, pairs of porous electrode capacitors are electrically charged to remove salt from brackish water present between the electrodes. We here present a novel experimental technique allowing measurement of spatially and temporally resolved salt concentration between the CDI electrodes. Our technique measures the local fluorescence intensity of a neutrally charged fluorescent probe which is collisionally quenched by chloride ions. To our knowledge, our system is the first to measure in situ and spatially resolved chloride concentration in a laboratory CDI cell. We here demonstrate good agreement between our dynamic measurements of salt concentration in a charging, millimeter-scale CDI system to the results of a modified Donnan porous electrode transport model. Further, we utilize our dynamic measurements to demonstrate that salt removal between our charging CDI electrodes occurs on a longer time scale than the capacitive charging time scales of our CDI cell. Compared to typical measurements of CDI system performance (namely, measurements of outflow ionic conductivity), our technique can enable more advanced and better-controlled studies of ion transport in CDI systems, which can potentially catalyze future performance improvements.

  View details for DOI 10.1021/es403682n

  View details for Web of Science ID 000331015100078

  View details for PubMedID 24433022

• Particle Tracking and Multispectral Collocation Method for Particle-to-Particle Binding Assays ANALYTICAL CHEMISTRY Rogacs, A., Santiago, J. G. 2014; 86 (1): 608-614

  Abstract

  We present a simple-to-implement method for analyzing images of randomly distributed particles transported through a fluidic channel. We term this method particle imaging, tracking and collocation (PITC). Our method uses off-the-shelf optics including a CCD camera, epifluorescence microscope, and a dual-view color separator to image freely suspended particles in a wide variety of microchannels (with optical access for image collection). The particles can be transported via electrophoresis and/or pressure driven flow to increase throughput of analysis. We here describe the implementation of the algorithm and demonstrate and validate three of its capabilities: (1) identification of particle coordinates, (2) tracking of particle motion, and (3) monitoring of particle interaction via collocation analysis. We use Monte Carlo simulations for validation and optimization of the input parameters. We also present an experimental demonstration of the analysis on challenging image data, including a flow of two, interacting Brownian particle populations. In the latter example, we use PITC to detect the presence of target DNA by monitoring the hybridization-induced binding between the two populations of beads, each functionalized with DNA probes complementary to the target molecule.

  View details for DOI 10.1021/ac402830q

  View details for Web of Science ID 000329548700063

  View details for PubMedID 24266609

• Progress on Phase Separation Microfluidics Agonafer, D. D., Palko, J., Won, Y., Lopez, K., Dusseault, T., Gires, J., Asheghi, M., Santiago, J. G., Goodson, K. E., IEEE IEEE. 2014

  View details for Web of Science ID 000369763800053

• Inverse Opals for Fluid Delivery in Electronics Cooling Systems Dusseault, T. J., Gires, J., Barako, M. T., Won, Y., Agonafer, D. D., Asheghi, M., Santiago, J. G., Goodson, K. E., IEEE IEEE. 2014: 750–55

  View details for Web of Science ID 000366567000099

• Phase-Separation of Wetting Fluids Using Nanoporous Alumina Membranes and Micro-glass Capillaries Agonafer, D. D., Lopez, K., Won, Y., Palko, J., Asheghi, M., Santiago, J. G., Goodson, K. E., IEEE IEEE. 2014: 306–16

  View details for Web of Science ID 000366567000041

• Impedance-based study of capacitive porous carbon electrodes with hierarchical and bimodal porosity JOURNAL OF POWER SOURCES Suss, M. E., Baumann, T. F., Worsley, M. A., Rose, K. A., Jaramillo, T. F., Stadermann, M., Santiago, J. G. 2013; 241: 266-273

  View details for DOI 10.1016/j.jpowsour.2013.03.178

  View details for Web of Science ID 000323093700034

• Rapid High-Specificity microRNA Detection Using a Two-stage Isotachophoresis Assay. Angewandte Chemie (International ed. in English) Garcia-Schwarz, G., Santiago, J. G. 2013; 52 (44): 11534-11537

  Abstract

  Focusing in on the small: A two-stage microRNA detection assay uses electrokinetic focusing to speed up hybridization and a functionalized hydrogel for affinity purification. This method detects microRNAs in 15 minutes with single-nucleotide specificity, and processes only 5 ng of total RNA.

  View details for DOI 10.1002/anie.201305875

  View details for PubMedID 24038732

• Isotachophoresis with ionic spacer and two-stage separation for high sensitivity DNA hybridization assay. Analyst Eid, C., Garcia-Schwarz, G., Santiago, J. G. 2013; 138 (11): 3117-3120

  Abstract

  We present an on-chip electrophoretic assay for rapid and high sensitivity nucleic acid (NA) detection. The assay uses isotachophoresis (ITP) to enhance NA hybridization and an ionic spacer molecule to subsequently separate reaction products. In the first stage, the probe and target focus and mix rapidly in free solution under ITP. The reaction mixture then enters a region containing a sieving matrix, which allows the spacer ion to overtake and separate the slower probe-target complex from free, unhybridized probes. This results in the formation of two focused ITP peaks corresponding to probe and probe-target complex signals. For a 149 nt DNA target, we achieve a 220 fM limit of detection (LOD) within 10 min, with a 3.5 decade dynamic range. This LOD constitutes a 12× improvement over previous ITP-based hybridization assays. The technique offers an alternative to traditional DNA hybridization assays, and can be multiplexed and extended to detect other biomolecules.

  View details for DOI 10.1039/c3an00374d

  View details for PubMedID 23615527

• A method for non-invasive full-field imaging and quantification of chemical species. Lab on a chip Shkolnikov, V., Santiago, J. G. 2013; 13 (8): 1632-1643

  Abstract

  We present a novel method for full-field scalar visualization and quantification of species concentration fields. We term this method species-altered fluorescence imaging (SAFI). The method employs electrically neutral fluorescent dyes whose quantum yields are selectively quenched or enhanced by species of interest. SAFI enables simultaneous imaging of material interfaces and provides non-invasive, scalar-field quantitation of two-dimensional species concentration fields. We describe criteria for choosing SAFI dyes and tabulate 35 promising SAFI dyes and their relevant properties. Next, we describe species concentration quantification with SAFI via Stern-Volmer quenching and discuss the sensitivity and resolution of our method. We demonstrate this method with two dyes, 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ) and 10-(3-sulfopropyl)acridinium betaine (SAB). We demonstrate our method in full-field visualization of several challenging electrokinetic flows: isotachophoresis (ITP) in both cationic and anionic modes, and in a convective electrokinetic instability (EKI) flow. Through these experiments we collectively quantify ion concentration shock velocities, simultaneously measure concentrations of five species, and quantify the development of an unsteady, chaotic, 2D flow.

  View details for DOI 10.1039/c3lc41293h

  View details for PubMedID 23463253

• Two- and three-dimensional modeling and optimization applied to the design of a fast hydrodynamic focusing microfluidic mixer for protein folding PHYSICS OF FLUIDS Ivorra, B., Redondo, J. L., Santiago, J. G., Ortigosa, P. M., Ramos, A. M. 2013; 25 (3)

  View details for DOI 10.1063/1.4793612

  View details for Web of Science ID 000316951900006

• Coupling isotachophoresis and capillary electrophoresis: a review and comparison of methods ANALYST Bahga, S. S., Santiago, J. G. 2013; 138 (3): 735-754

  Abstract

  We present a comprehensive review and comparison of the methodologies for increasing sensitivity and resolution of capillary electrophoresis (CE) using online transient isotachophoresis (tITP). We categorize the diverse set of coupled tITP and CE (tITP-CE) methods based on their fundamental principles for disrupting isotachophoretic preconcentration and triggering electrophoretic separation. Based on this classification, we discuss important features, advantages, limitations, and optimization principles of various tITP-CE methods. We substantiate our discussion with original simulations, instructive examples, and published experimental results.

  View details for DOI 10.1039/c2an36150g

  View details for Web of Science ID 000312944400002

  View details for PubMedID 23232502

• A method for non-invasive full-field imaging and quantification of chemical species LAB ON A CHIP Shkolnikov, V., Santiago, J. G. 2013; 13 (8): 1632-1643

  Abstract

  We present a novel method for full-field scalar visualization and quantification of species concentration fields. We term this method species-altered fluorescence imaging (SAFI). The method employs electrically neutral fluorescent dyes whose quantum yields are selectively quenched or enhanced by species of interest. SAFI enables simultaneous imaging of material interfaces and provides non-invasive, scalar-field quantitation of two-dimensional species concentration fields. We describe criteria for choosing SAFI dyes and tabulate 35 promising SAFI dyes and their relevant properties. Next, we describe species concentration quantification with SAFI via Stern-Volmer quenching and discuss the sensitivity and resolution of our method. We demonstrate this method with two dyes, 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ) and 10-(3-sulfopropyl)acridinium betaine (SAB). We demonstrate our method in full-field visualization of several challenging electrokinetic flows: isotachophoresis (ITP) in both cationic and anionic modes, and in a convective electrokinetic instability (EKI) flow. Through these experiments we collectively quantify ion concentration shock velocities, simultaneously measure concentrations of five species, and quantify the development of an unsteady, chaotic, 2D flow.

  View details for DOI 10.1039/c3lc41293h

  View details for Web of Science ID 000316275200026

  View details for PubMedID 23463253

• Particle Tracking and Multispectral Collocation Method for Cytometry-Like and Particle-to-Particle Binding Assays Particle Tracking and Multispectral Collocation Method for Particle-to-Particle Binding Assays, Analytical Chemistry Rogacs, A., Santiago, J., G. 2013; 1 (86): 608-614
• Integration of rapid DNA hybridization and capillary zone electrophoresis using bidirectional isotachophoresis ANALYST Bahga, S. S., Han, C. M., Santiago, J. G. 2013; 138 (1): 87-90

  Abstract

  We present a method for rapid, sequence-specific detection of multiple DNA fragments by integrating isotachophoresis (ITP) based DNA hybridization and capillary zone electrophoresis (CZE) using bidirectional ITP. Our method leverages the high preconcentration ability of ITP to accelerate slow, second-order DNA hybridization kinetics, and the high resolving power of CZE to separate and identify reaction products. We demonstrate the speed and sensitivity of our assay by detecting 5 pM, 39 nt ssDNA target within 3 min, using a molecular beacon probe. We also demonstrate the feasibility of our assay for multiplexed detection of multiple-length ssDNA targets by simultaneously detecting 39 and 90 nt ssDNA targets.

  View details for DOI 10.1039/c2an36249j

  View details for Web of Science ID 000311823200007

  View details for PubMedID 23103998

• Unraveling the potential and pore-size dependent capacitance of slit-shaped graphitic carbon pores in aqueous electrolytes PHYSICAL CHEMISTRY CHEMICAL PHYSICS Kalluri, R. K., Biener, M. M., Suss, M. E., Merrill, M. D., Stadermann, M., Santiago, J. G., Baumann, T. F., Biener, J., Striolo, A. 2013; 15 (7): 2309-2320

  Abstract

  Understanding and leveraging physicochemical processes at the pore scale are believed to be essential to future performance improvements of supercapacitors and capacitive desalination (CD) cells. Here, we report on a combination of electrochemical experiments and fully atomistic simulations to study the effect of pore size and surface charge density on the capacitance of graphitic nanoporous carbon electrodes. Specifically, we used cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) to study the effect of potential and pore size on the capacitance of nanoporous carbon foams. Molecular dynamics simulations were performed to study the pore-size dependent accumulation of aqueous electrolytes in slit-shaped graphitic carbon pores of different widths (0.65 to 1.6 nm). Experimentally, we observe a pronounced increase of the capacitance of sub-nm pores as the applied potential window gets wider, from a few F g(-1) for narrow potential ranges (-0.3 to 0.3 V vs. Ag/AgCl) to ~40 F g(-1) for wider potential windows (-0.9 V to 0.9 V vs. Ag/AgCl). By contrast, the capacitance of wider pores does not depend significantly on the applied potential window. Molecular dynamics simulations confirm that the penetration of ions into pores becomes more difficult with decreasing pore width and increasing strength of the hydration shell. Consistent with our experimental results, we observe a pore- and ion-size dependent threshold-like charging behavior when the pore width becomes comparable to the size of the hydrated ion (0.65 nm pores for Na(+) and 0.79 nm pores for Cl(-) ions). The observed pore-size and potential dependent accumulation of ions in slit-shaped carbon pores can be explained by the hydration structure of the ions entering the charged pores. The results are discussed in view of their effect on energy-storage and desalination efficiency.

  View details for DOI 10.1039/c2cp43361c

  View details for Web of Science ID 000313891400008

  View details for PubMedID 23295944

• Integrated Printed Circuit Board Device for Cell Lysis and Nucleic Acid Extraction ANALYTICAL CHEMISTRY Marshall, L. A., Wu, L. L., Babikian, S., Bachman, M., Santiago, J. G. 2012; 84 (21): 9640-9645

  Abstract

  Preparation of raw, untreated biological samples remains a major challenge in microfluidics. We present a novel microfluidic device based on the integration of printed circuit boards and an isotachophoresis assay for sample preparation of nucleic acids from biological samples. The device has integrated resistive heaters and temperature sensors as well as a 70 μm × 300 μm × 3.7 cm microfluidic channel connecting two 15 μL reservoirs. We demonstrated this device by extracting pathogenic nucleic acids from 1 μL dispensed volume of whole blood spiked with Plasmodium falciparum. We dispensed whole blood directly onto an on-chip reservoir, and the system's integrated heaters simultaneously lysed and mixed the sample. We used isotachophoresis to extract the nucleic acids into a secondary buffer via isotachophoresis. We analyzed the convective mixing action with micro particle image velocimetry (micro-PIV) and verified the purity and amount of extracted nucleic acids using off-chip quantitative polymerase chain reaction (PCR). We achieved a clinically relevant limit of detection of 500 parasites per microliter. The system has no moving parts, and the process is potentially compatible with a wide range of on-chip hybridization or amplification assays.

  View details for DOI 10.1021/ac302622v

  View details for Web of Science ID 000310664600107

  View details for PubMedID 23046297

• Effect of PVP on the electroosmotic mobility of wet-etched glass microchannels ELECTROPHORESIS Milanova, D., Chambers, R. D., Bahga, S. S., Santiago, J. G. 2012; 33 (21): 3259-3262

  Abstract

  We present an experimental study on the effect of polymer PVP on EOF mobility of microchannels wet etched into optical white soda lime glass, also known as Crown glass. We performed experiments to evaluate the effect of PVP concentration and pH on EOF mobility. We used on-chip capillary zone electrophoresis and a neutral fluorescent dye as a passive marker to quantify the electroosmotic flow. We performed experiments under controlled conditions by varying pH from 5.2 and 10.3 and concentration of PVP from 0 to 2.0% w/w at constant ionic strength (30 mM). Our experiments show that PVP at concentrations of 1.0% or above very effectively suppress EOF at low pH (6.6). At high pH of 10.3, PVP has a much weaker suppressing effect on EOF and increasing its concentration above about 0.5% showed negligible effect on EOF mobility. Finally, we briefly discuss the effects of pH on using PVP as an adsorbed coating. Our experiments provide useful guidelines on choosing correct pH and concentration of PVP for effective EOF suppression in glass channels.

  View details for DOI 10.1002/elps.201200336

  View details for Web of Science ID 000310476600019

  View details for PubMedID 23065690

• Capacitive desalination with flow-through electrodes ENERGY & ENVIRONMENTAL SCIENCE Suss, M. E., Baumann, T. F., Bourcier, W. L., Spadaccini, C. M., Rose, K. A., Santiago, J. G., Stadermann, M. 2012; 5 (11): 9511-9519

  View details for DOI 10.1039/c2ee21498a

  View details for Web of Science ID 000310006200035

• Robust and high-resolution simulations of nonlinear electrokinetic processes in variable cross-section channels ELECTROPHORESIS Bahga, S. S., Bercovici, M., Santiago, J. G. 2012; 33 (19-20): 3036-3051

  Abstract

  We present a model and an associated numerical scheme to simulate complex electrokinetic processes in channels with nonuniform cross-sectional area. We develop a quasi-1D model based on local cross-sectional area averaging of the equations describing unsteady, multispecies, electromigration-diffusion transport. Our approach uses techniques of lubrication theory to approximate electrokinetic flows in channels with arbitrary variations in cross-section; and we include chemical equilibrium calculations for weak electrolytes, Taylor-Aris type dispersion due of nonuniform bulk flow, and the effects of ionic strength on species mobility and on acid-base equilibrium constants. To solve the quasi-1D governing equations, we provide a dissipative finite volume scheme that adds numerical dissipation at selective locations to ensure both unconditional stability and high accuracy. We couple the numerical scheme with a novel adaptive grid refinement algorithm that further improves the accuracy of simulations by minimizing numerical dissipation. We benchmark our numerical scheme with existing numerical schemes by simulating nonlinear electrokinetic problems, including ITP and electromigration dispersion in CZE. Simulation results show that our approach yields fast, stable, and high-resolution solutions using an order of magnitude less grid points compared to the existing dissipative schemes. To highlight our model's capabilities, we demonstrate simulations that predict increase in detection sensitivity of ITP in converging cross-sectional area channels. We also show that our simulations of ITP in variable cross-sectional area channels have very good quantitative agreement with published experimental data.

  View details for DOI 10.1002/elps.201200264

  View details for Web of Science ID 000310289600013

  View details for PubMedID 22996734

• Integration of On-Chip Isotachophoresis and Functionalized Hydrogels for Enhanced-Sensitivity Nucleic Acid Detection ANALYTICAL CHEMISTRY Garcia-Schwarz, G., Santiago, J. G. 2012; 84 (15): 6366-6369

  Abstract

  We introduce an on-chip electrokinetic assay to perform high-sensitivity nucleic acid (NA) detection. This assay integrates electrokinetic sample focusing using isotachophoresis (ITP) with a background signal-removal strategy that employs photopatterened, DNA-functionalized hydrogels. In this multistage assay, ITP first enhances hybridization kinetics between target NAs and end-labeled complementary reporters. After enhanced hybridization, migration through a DNA-functionalized hydrogel region removes excess reporters through affinity interactions. We demonstrate our assay on microRNAs, an important class of low-abundance biomarkers. The assay exhibits 4 orders of magnitude dynamic range, near 1 pM detection limits starting from less than 100 fg of microRNA, and high selectivity for mature microRNA sequences, all within a 10 min run time. This new microfluidic framework provides a unique quantitative assay for NA detection.

  View details for DOI 10.1021/ac301586q

  View details for Web of Science ID 000307159200019

  View details for PubMedID 22803507

• Bacterial RNA Extraction and Purification from Whole Human Blood Using Isotachophoresis ANALYTICAL CHEMISTRY Rogacs, A., Qu, Y., Santiago, J. G. 2012; 84 (14): 5858-5863

  Abstract

  We demonstrate a novel assay for physicochemical extraction and isotachophoresis-based purification of 16S rRNA from whole human blood infected with Pseudomonas putida . This on-chip assay is unique in that the extraction can be automated using isotachophoresis in a simple device with no moving parts, it protects RNA from degradation when isolating from ribonuclease-rich matrices (such as blood), and produces a purified total nucleic acid sample that is compatible with enzymatic amplification assays. We show that the purified RNA is compatible with reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and demonstrate a clinically relevant sensitivity of 0.03 bacteria per nanoliter using RT-qPCR.

  View details for DOI 10.1021/ac301021d

  View details for Web of Science ID 000306441200006

  View details for PubMedID 22816776

• Concentration cascade of leading electrolyte using bidirectional isotachophoresis ELECTROPHORESIS Bahga, S. S., Santiago, J. G. 2012; 33 (6): 1048-1059

  Abstract

  We present a novel method of creating concentration cascade of leading electrolyte (LE) in isotachophoresis (ITP) by using bidirectional ITP. ITP establishes ion-concentration shock waves between high-mobility LE and low-mobility trailing electrolyte (TE) ions. In bidirectional ITP, we set up simultaneous shock waves between anions and cations such that these waves approach each other and interact. The shock interaction causes a sudden decrease in LE concentration ahead of the focused anions and a corresponding decrease in analyte zone concentrations. This readjustment of analyte zone concentrations is accompanied by a corresponding increase in their zone lengths, in accordance to conservation laws. The method generates in situ gradient in the LE concentration, and therefore can be achieved in a single, straight channel simply by establishing the initial electrolyte chemistry. We have developed an analytical model useful in designing the process for maximum sensitivity and estimating increase in sample zone length due to shock interaction. We also illustrate the technique and evaluate its effectiveness in increasing detection sensitivity using transient simulations of species transport equations. We validated the theoretical predictions using experimental visualizations of bidirectional ITP zones for various electrolyte chemistries. Lastly, we use our technique to demonstrate a factor of 20 increase in the sensitivity of ITP-based detection of 2,4,6-trichlorophenol.

  View details for DOI 10.1002/elps.201100510

  View details for Web of Science ID 000303155700020

  View details for PubMedID 22528425

• On-chip isotachophoresis for separation of ions and purification of nucleic acids. Journal of visualized experiments : JoVE Garcia-Schwarz, G., Rogacs, A., Bahga, S. S., Santiago, J. G. 2012: e3890-?

  Abstract

  Electrokinetic techniques are a staple of microscale applications because of their unique ability to perform a variety of fluidic and electrophoretic processes in simple, compact systems with no moving parts. Isotachophoresis (ITP) is a simple and very robust electrokinetic technique that can achieve million-fold preconcentration and efficient separation and extraction based on ionic mobility. For example, we have demonstrated the application of ITP to separation and sensitive detection of unlabeled ionic molecules (e.g. toxins, DNA, rRNA, miRNA) with little or no sample preparation and to extraction and purification of nucleic acids from complex matrices including cell culture, urine, and blood. ITP achieves focusing and separation using an applied electric field and two buffers within a fluidic channel system. For anionic analytes, the leading electrolyte (LE) buffer is chosen such that its anions have higher effective electrophoretic mobility than the anions of the trailing electrolyte (TE) buffer (Effective mobility describes the observable drift velocity of an ion and takes into account the ionization state of the ion, as described in detail by Persat et al.). After establishing an interface between the TE and LE, an electric field is applied such that LE ions move away from the region occupied by TE ions. Sample ions of intermediate effective mobility race ahead of TE ions but cannot overtake LE ions, and so they focus at the LE-TE interface (hereafter called the "ITP interface"). Further, the TE and LE form regions of respectively low and high conductivity, which establish a steep electric field gradient at the ITP interface. This field gradient preconcentrates sample species as they focus. Proper choice of TE and LE results in focusing and purification of target species from other non-focused species and, eventually, separation and segregation of sample species. We here review the physical principles underlying ITP and discuss two standard modes of operation: "peak" and "plateau" modes. In peak mode, relatively dilute sample ions focus together within overlapping narrow peaks at the ITP interface. In plateau mode, more abundant sample ions reach a steady-state concentration and segregate into adjoining plateau-like zones ordered by their effective mobility. Peak and plateau modes arise out of the same underlying physics, but represent distinct regimes differentiated by the initial analyte concentration and/or the amount of time allotted for sample accumulation. We first describe in detail a model peak mode experiment and then demonstrate a peak mode assay for the extraction of nucleic acids from E. coli cell culture. We conclude by presenting a plateau mode assay, where we use a non-focusing tracer (NFT) species to visualize the separation and perform quantitation of amino acids.

  View details for DOI 10.3791/3890

  View details for PubMedID 22415002

  View details for PubMedCentralID PMC3399465

• An Integrated Printed Circuit Board Device for Cell Lysis and Nucleic Acid Extraction Analytical Chemistry Marshall, L., A., Li, L., Babikain, S., Bachman, M., Santiago, J., G. 2012; 21 (84): 9640-9645
• Desalination and hydrogen, chlorine, and sodium hydroxide production via electrophoretic ion exchange and precipitation PHYSICAL CHEMISTRY CHEMICAL PHYSICS Shkolnikov, V., Bahga, S. S., Santiago, J. G. 2012; 14 (32): 11534-11545

  Abstract

  We demonstrate and analyze a novel desalination method which works by electrophoretically replacing sodium and chloride in feed salt water with a pair of ions, calcium and carbonate, that react and precipitate out. The resulting calcium carbonate precipitate is benign to health, and can be filtered or settled out, yielding low ionic strength product water. The ion exchange and precipitation employs self-sharpening interfaces induced by movement of multiple ions in an electric field to prevent contamination of the product water. Simultaneously, the electrolysis associated with the electromigration produces hydrogen gas, chlorine gas, and sodium hydroxide. We conducted an experimental study of this method's basic efficacy to desalinate salt water from 100 to 600 mol m(-3) sodium chloride. We also present physicochemical models of the process, and analyze replacement reagents consumption, permeate recovery ratio, and energy consumption. We hypothesize that the precipitate can be recycled back to replacement reagents using the well-known, commercially implemented Solvay process. We show that the method's permeate recovery ratio is 58% to 46%, which is on par with that of reverse osmosis. We show that the method's energy consumption requirement over and above that necessary to generate electrolysis is 3 to 10 W h l(-1), which is on par with the energy consumed by state-of-the-art desalination methods. Furthermore, the method operates at ambient temperature and pressure, and uses no specialized membranes. The process may be feasible as a part of a desalination-co-generation facility: generating fresh water, hydrogen and chlorine gas, and sodium hydroxide.

  View details for DOI 10.1039/c2cp42121f

  View details for Web of Science ID 000306708300034

  View details for PubMedID 22806549

• Extraction of DNA from Malaria-Infected Erythrocytes Using Isotachophoresis ANALYTICAL CHEMISTRY Marshall, L. A., Han, C. M., Santiago, J. G. 2011; 83 (24): 9715-9718

  Abstract

  We demonstrate a technique for purification of nucleic acids from malaria parasites infecting human erythrocytes using isotachophoresis (ITP). We release nucleic acids from malaria-infected erythrocytes by lysing with heat and proteinase K for 10 min and immediately, thereafter, load sample onto a capillary device. We study the effect of temperature on lysis efficiency. We also implement pressure-driven counterflow during ITP extraction to extend focusing time and increase nucleic acid yield. We show that the purified genomic DNA samples are compatible with polymerase chain reaction (PCR) and demonstrate a clinically relevant limit of detection of 0.5 parasites per nanoliter using quantitative PCR.

  View details for DOI 10.1021/ac202567j

  View details for Web of Science ID 000297946900077

  View details for PubMedID 22074444

• Electrophoretic mobility measurements of fluorescent dyes using on-chip capillary electrophoresis ELECTROPHORESIS Milanova, D., Chambers, R. D., Bahga, S. S., Santiago, J. G. 2011; 32 (22): 3286-3294

  Abstract

  We present an experimental study of the effect of pH, ionic strength, and concentrations of the electroosmotic flow (EOF)-suppressing polymer polyvinylpyrrolidone (PVP) on the electrophoretic mobilities of commonly used fluorescent dyes (fluorescein, Rhodamine 6G, and Alexa Fluor 488). We performed on-chip capillary zone electrophoresis experiments to directly quantify the effective electrophoretic mobility. We use Rhodamine B as a fluorescent neutral marker (to quantify EOF) and CCD detection. We also report relevant acid dissociation constants and analyte diffusivities based on our absolute estimate (as per Nernst-Einstein diffusion). We perform well-controlled experiments in a pH range of 3-11 and ionic strengths ranging from 30 to 90  mM. We account for the influence of ionic strength on the electrophoretic transport of sample analytes through the Onsager and Fuoss theory extended for finite radii ions to obtain the absolute mobility of the fluorophores. Lastly, we briefly explore the effect of PVP on adsorption-desorption dynamics of all three analytes, with particular attention to cationic R6G.

  View details for DOI 10.1002/elps.201100210

  View details for Web of Science ID 000298098700026

  View details for PubMedID 22102501

• Coupled Isotachophoretic Preconcentration and Electrophoretic Separation Using Bidirectional Isotachophoresis ANALYTICAL CHEMISTRY Bahga, S. S., Chambers, R. D., Santiago, J. G. 2011; 83 (16): 6154-6162

  Abstract

  We present a novel technique for coupling isotachophoretic preconcentration and electrophoretic separation using bidirectional isotachophoresis (ITP). Bidirectional ITP simultaneously sets up sharp ITP interfaces between relatively high- and low-mobility cations and high- and low-mobility anions. These two interfaces can migrate toward each other and be described as ion concentration shock waves. We here demonstrate a bidirectional ITP process in which we use the interaction of these anionic and cationic ITP shock waves to trigger a transformation from ITP preconcentration to electrophoretic separation. We use anionic ITP to focus anionic sample species prior to shock interaction. The interaction of the counter-propagating anionic and cationic ITP shocks then changes the local pH (and ionic strength) of the focused analyte zones. Under this new condition, the analytes no longer focus and begin to separate electrophoretically. The method provides faster and much less dispersive transition from ITP preconcentration to electrophoretic separation compared with traditional (unidirectional) transient ITP. It eliminates the need for intermediate steps between focusing and separation, such as manual buffer exchanges. We illustrate the technique with numerical simulations of species transport equations. We have validated our simulations with experimental visualization of bidirectional ITP zones. We then show the effectiveness of the technique by coupling ITP preconcentration and high-resolution separation of a 1 kbp DNA ladder via shock interaction in bidirectional ITP.

  View details for DOI 10.1021/ac200268f

  View details for Web of Science ID 000293758800007

  View details for PubMedID 21728346

• Sample dispersion in isotachophoresis JOURNAL OF FLUID MECHANICS Garcia-Schwarz, G., Bercovici, M., Marshall, L. A., Santiago, J. G. 2011; 679: 455-475

  View details for DOI 10.1017/jfm.2011.139

  View details for Web of Science ID 000292644600018

• Rapid Detection of Urinary Tract Infections Using Isotachophoresis and Molecular Beacons ANALYTICAL CHEMISTRY Bercovici, M., Kaigala, G. V., Mach, K. E., Han, C. M., Liao, J. C., Santiago, J. G. 2011; 83 (11): 4110-4117

  Abstract

  We present a novel assay for rapid detection and identification of bacterial urinary tract infections using isotachophoresis (ITP) and molecular beacons. We applied on-chip ITP to extract and focus 16S rRNA directly from bacterial lysate and used molecular beacons to achieve detection of bacteria specific sequences. We demonstrated detection of E. coli in bacteria cultures as well as in patient urine samples in the clinically relevant range 1E6-1E8 cfu/mL. For bacterial cultures we further demonstrate quantification in this range. The assay requires minimal sample preparation (a single centrifugation and dilution), and can be completed, from beginning of lysing to detection, in under 15 min. We believe that the principles presented here can be used for design of other rapid diagnostics or detection methods for pathogenic diseases.

  View details for DOI 10.1021/ac200253x

  View details for Web of Science ID 000290978500022

  View details for PubMedID 21545089

  View details for PubMedCentralID PMC3116659

• MicroRNA Profiling by Simultaneous Selective Isotachophoresis and Hybridization with Molecular Beacons ANALYTICAL CHEMISTRY Persat, A., Santiago, J. G. 2011; 83 (6): 2310-2316

  Abstract

  We present and demonstrate a novel assay for the detection and quantification of microRNA (miRNA) that leverages isotachophoresis (ITP) and molecular beacon (MB) hybridization. We use ITP to selectively preconcentrate miRNA from total RNA. We simultaneously focus MBs and use the ITP zone as a 10 pL reactor with active mixing where MBs fluoresce upon hybridization to target miRNA. To increase both sensitivity and selectivity, we leverage a multistage ITP strategy composed of three discrete regions of different concentrations of denaturant, sieving matrix, and magnesium chloride. We show that ITP hybridization is specific and selective to the miRNA target. We demonstrate ITP hybridization of miRNA in a biologically relevant case by detecting and quantifying miR-122 in human kidney and liver. ITP hybridization is a cheap, simple, high-speed, and amplification-free miRNA profiling method which requires small amounts (order 100 ng) of sample. The technique therefore represents an attractive alternative to PCR or Northern blot for miRNAs.

  View details for DOI 10.1021/ac103225c

  View details for Web of Science ID 000288182900060

  View details for PubMedID 21329391

• High-sensitivity detection using isotachophoresis with variable cross-section geometry ELECTROPHORESIS Bahga, S. S., Kaigala, G. V., Bercovici, M., Santiago, J. G. 2011; 32 (5): 563-572

  Abstract

  We present a theoretical and experimental study on increasing the sensitivity of ITP assays by varying channel cross-section. We present a simple, unsteady, diffusion-free model for plateau mode ITP in channels with axially varying cross-section. Our model takes into account detailed chemical equilibrium calculations and handles arbitrary variations in channel cross-section. We have validated our model with numerical simulations of a more comprehensive model of ITP. We show that using strongly convergent channels can lead to a large increase in sensitivity and simultaneous reduction in assay time, compared to uniform cross-section channels. We have validated our theoretical predictions with detailed experiments by varying channel geometry and analyte concentrations. We show the effectiveness of using strongly convergent channels by demonstrating indirect fluorescence detection with a sensitivity of 100 nM. We also present simple analytical relations for dependence of zone length and assay time on geometric parameters of strongly convergent channels. Our theoretical analysis and experimental validations provide useful guidelines on optimizing chip geometry for maximum sensitivity under constraints of required assay time, chip area and power supply.

  View details for DOI 10.1002/elps.201000338

  View details for Web of Science ID 000288094500010

  View details for PubMedID 21308693

• Electroosmotic pump performance is affected by concentration polarizations of both electrodes and pump. Sensors and actuators. A, Physical Suss, M. E., Mani, A., Zangle, T. A., Santiago, J. G. 2011; 165 (2): 310-315

  Abstract

  Current methods of optimizing electroosmotic (EO) pump performance include reducing pore diameter and reducing ionic strength of the pumped electrolyte. However, these approaches each increase the fraction of total ionic current carried by diffuse electric double layer (EDL) counterions. When this fraction becomes significant, concentration polarization (CP) effects become important, and traditional EO pump models are no longer valid. We here report on the first simultaneous concentration field measurements, pH visualizations, flow rate, and voltage measurements on such systems. Together, these measurements elucidate key parameters affecting EO pump performance in the CP dominated regime. Concentration field visualizations show propagating CP enrichment and depletion fronts sourced by our pump substrate and traveling at order mm/min velocities through millimeter-scale channels connected serially to our pump. The observed propagation in millimeter-scale channels is not explained by current propagating CP models. Additionally, visualizations show that CP fronts are sourced by and propagate from the electrodes of our system, and then interact with the EO pump-generated CP zones. With pH visualizations, we directly detect that electrolyte properties vary sharply across the anode enrichment front interface. Our observations lead us to hypothesize possible mechanisms for the propagation of both pump- and electrode-sourced CP zones. Lastly, our experiments show the dynamics associated with the interaction of electrode and membrane CP fronts, and we describe the effect of these phenomena on EO pump flow rates and applied voltages under galvanostatic conditions.

  View details for DOI 10.1016/j.sna.2010.10.002

  View details for PubMedID 21516230

  View details for PubMedCentralID PMC3079224

• Electroosmotic pump performance is affected by concentration polarizations of both electrodes and pump SENSORS AND ACTUATORS A-PHYSICAL Suss, M. E., Mani, A., Zangle, T. A., Santiago, J. G. 2011; 165 (2): 310-315

  Abstract

  Current methods of optimizing electroosmotic (EO) pump performance include reducing pore diameter and reducing ionic strength of the pumped electrolyte. However, these approaches each increase the fraction of total ionic current carried by diffuse electric double layer (EDL) counterions. When this fraction becomes significant, concentration polarization (CP) effects become important, and traditional EO pump models are no longer valid. We here report on the first simultaneous concentration field measurements, pH visualizations, flow rate, and voltage measurements on such systems. Together, these measurements elucidate key parameters affecting EO pump performance in the CP dominated regime. Concentration field visualizations show propagating CP enrichment and depletion fronts sourced by our pump substrate and traveling at order mm/min velocities through millimeter-scale channels connected serially to our pump. The observed propagation in millimeter-scale channels is not explained by current propagating CP models. Additionally, visualizations show that CP fronts are sourced by and propagate from the electrodes of our system, and then interact with the EO pump-generated CP zones. With pH visualizations, we directly detect that electrolyte properties vary sharply across the anode enrichment front interface. Our observations lead us to hypothesize possible mechanisms for the propagation of both pump- and electrode-sourced CP zones. Lastly, our experiments show the dynamics associated with the interaction of electrode and membrane CP fronts, and we describe the effect of these phenomena on EO pump flow rates and applied voltages under galvanostatic conditions.

  View details for DOI 10.1016/j.sna.2010.10.002

  View details for Web of Science ID 000288108500025

  View details for PubMedCentralID PMC3079224

• Toward an Electrolytic Micropump Actuator Design with Controlled Cyclic Bubble Growth and Recombination Symposium on Sensors, Actuators, and Microsystems General Session/219th Meeting of the Electrochemical-Society (ECS) Hsu, L., Ramunas, J., Gonzalez, J., Santiago, J. G., STRICKLAND, D. G. ELECTROCHEMICAL SOC INC. 2011: 3–11

  View details for DOI 10.1149/1.3653918

  View details for Web of Science ID 000300930300001

• High sensitivity detection using isotachophoresis with variable cross-section geometry Electrophoresis Bahga, S, S., Kaigala, G., V., Bercovici, M., Santiago, J., G. 2011; 32: 311-314
• Quantification of Global MicroRNA Abundance by Selective Isotachophoresis ANALYTICAL CHEMISTRY Persat, A., Chivukula, R. R., Mendell, J. T., Santiago, J. G. 2010; 82 (23): 9631-9635

  Abstract

  We here present and demonstrate a novel technique based on isotachophoresis (ITP) for the quantification of global microRNA (miRNA) abundance in total RNA. We leverage the selectivity of ITP to concentrate miRNA and exclude longer RNA molecules from the focused zone. We designed a novel ITP strategy where we initially establish three contiguous zones of sieving polymer, electrolyte, and denaturant concentrations. This allows for successive preconcentration, selection, and detection of miRNA. We optimized chemistry in each zone for high sensitivity and exquisite selectivity for miRNA. This technique allows for the measurement of the total miRNA content in a sample and its comparison between different cell types and tissues. We demonstrated and validated the efficacy of this technique by comparing global miRNA abundance in subconfluent and confluent cell cultures.

  View details for DOI 10.1021/ac102496m

  View details for Web of Science ID 000284668600009

  View details for PubMedID 21062022

• Design and fabrication of porous polymer wick structures SENSORS AND ACTUATORS B-CHEMICAL Shkolnikov, V., Strickland, D. G., Fenning, D. P., Santiago, J. G. 2010; 150 (2): 556-563

  View details for DOI 10.1016/j.snb.2010.08.040

  View details for Web of Science ID 000284339800010

• A two-liquid electroosmotic pump using low applied voltage and power SENSORS AND ACTUATORS A-PHYSICAL Litster, S., Suss, M. E., Santiago, J. G. 2010; 163 (1): 311-314

  View details for DOI 10.1016/j.sna.2010.07.008

  View details for Web of Science ID 000283671000043

• Active water management at the cathode of a planar air-breathing polymer electrolyte membrane fuel cell using an electroosmotic pump JOURNAL OF POWER SOURCES Fabian, T., O'Hayre, R., Litster, S., Prinz, F. B., Santiago, J. G. 2010; 195 (11): 3640-3644

  View details for DOI 10.1016/j.jpowsour.2009.12.025

  View details for Web of Science ID 000275386100039

• Passive water management at the cathode of a planar air-breathing proton exchange membrane fuel cell JOURNAL OF POWER SOURCES Fabian, T., O'Hayre, R., Litster, S., Prinz, F. B., Santiago, J. G. 2010; 195 (10): 3201-3206

  View details for DOI 10.1016/j.jpowsour.2009.12.030

  View details for Web of Science ID 000275074200023

• A self-priming, roller-free, miniature, peristaltic pump operable with a single, reciprocating actuator SENSORS AND ACTUATORS A-PHYSICAL Shkolnikov, V., Ramunas, J., Santiago, J. G. 2010; 160 (1-2): 141-146

  Abstract

  We present a design for a miniature self-priming peristaltic pump actuated with a single linear actuator, and which can be manufactured using conventional materials and methods. The pump is tolerant of bubbles and particles and can pump liquids, foams, and gases. We explore designs actuated by a motor (in depth) and a shape memory alloy (briefly); and briefly present a manually actuated version. The pump consists of a Delrin acetal plastic body with two integrated valves, a flexible silicone tube, and an actuator. Pumping is achieved as the forward motion of the actuator first closes the upstream valve, and then compresses a section of the tube. The increased internal pressure opens a downstream burst valve to expel the fluid. Reduced pressure in the pump tube allows the downstream valve to close, and removal of actuator force allows the upstream valve and pump tube to open, refilling the pump. The motor actuated design offers a linear dependence of flow rate on voltage in the range of 1.75-3 V. Flow rate decreases from 780 μl/min with increasing back pressure up to the maximum back pressure of 48 kPa. At 3 V and minimum back pressure, the pump consumes 90 mW. The shape memory alloy actuated design offers a 5-fold size and 4-fold weight reduction over the motor design, higher maximum back pressure, and substantial insensitivity of flow rate to back pressure at the cost of lower power efficiency and flow rate. The manually actuated version is simpler and appropriate for applications unconstrained by actuation distance.

  View details for DOI 10.1016/j.sna.2010.04.018

  View details for Web of Science ID 000279084100020

  View details for PubMedCentralID PMC3963388

• A self-priming, roller-free, miniature, peristaltic pump operable with a single, reciprocating actuator. Sensors and actuators. A, Physical Shkolnikov, V., Ramunas, J., Santiago, J. G. 2010; 160 (1-2): 141-146

  Abstract

  We present a design for a miniature self-priming peristaltic pump actuated with a single linear actuator, and which can be manufactured using conventional materials and methods. The pump is tolerant of bubbles and particles and can pump liquids, foams, and gases. We explore designs actuated by a motor (in depth) and a shape memory alloy (briefly); and briefly present a manually actuated version. The pump consists of a Delrin acetal plastic body with two integrated valves, a flexible silicone tube, and an actuator. Pumping is achieved as the forward motion of the actuator first closes the upstream valve, and then compresses a section of the tube. The increased internal pressure opens a downstream burst valve to expel the fluid. Reduced pressure in the pump tube allows the downstream valve to close, and removal of actuator force allows the upstream valve and pump tube to open, refilling the pump. The motor actuated design offers a linear dependence of flow rate on voltage in the range of 1.75-3 V. Flow rate decreases from 780 μl/min with increasing back pressure up to the maximum back pressure of 48 kPa. At 3 V and minimum back pressure, the pump consumes 90 mW. The shape memory alloy actuated design offers a 5-fold size and 4-fold weight reduction over the motor design, higher maximum back pressure, and substantial insensitivity of flow rate to back pressure at the cost of lower power efficiency and flow rate. The manually actuated version is simpler and appropriate for applications unconstrained by actuation distance.

  View details for DOI 10.1016/j.sna.2010.04.018

  View details for PubMedID 24672145

  View details for PubMedCentralID PMC3963388

• Effects of Constant Voltage on Time Evolution of Propagating Concentration Polarization ANALYTICAL CHEMISTRY Zangle, T. A., Mani, A., Santiago, J. G. 2010; 82 (8): 3114-3117

  Abstract

  We extend the analytical theory of propagating concentration polarization (CP) to describe and compare the effects of constant-voltage versus constant-current conditions on the transient development of CP enrichment and depletion zones. We support our analysis with computational and experimental results. We find that at constant voltage, enrichment and depletion regions spread as t(1/2) as opposed to the previously observed t(1) scaling for constant current conditions. At low, constant voltages, the growth and propagation of CP zones can easily be misinterpreted as nonpropagating behavior.

  View details for DOI 10.1021/ac100432q

  View details for Web of Science ID 000276557600004

  View details for PubMedID 20349992

• In situ-polymerized wicks for passive water management in proton exchange membrane fuel cells JOURNAL OF POWER SOURCES Strickland, D. G., Santiago, J. G. 2010; 195 (6): 1667-1675

  View details for DOI 10.1016/j.jpowsour.2009.09.034

  View details for Web of Science ID 000272580900020

• Ionic strength effects on electrophoretic focusing and separations ELECTROPHORESIS Bahga, S. S., Bercovici, M., Santiago, J. G. 2010; 31 (5): 910-919

  Abstract

  We present a numerical and experimental study of the effects of ionic strength on electrophoretic focusing and separations. We review the development of ionic strength models for electrophoretic mobility and chemical activity and highlight their differences in the context of electrophoretic separation and focusing simulations. We couple a fast numerical solver for electrophoretic transport with the Onsager-Fuoss model for actual ionic mobility and the extended Debye-Huckle theory for correction of ionic activity. Model predictions for fluorescein mobility as a function of ionic strength and pH compare well with data from CZE experiments. Simulation predictions of preconcentration factors in peak mode ITP also compare well with the published experimental data. We performed ITP experiments to study the effect of ionic strength on the simultaneous focusing and separation. Our comparisons of the latter data with simulation results at 10 and 250 mM ionic strength show the model is able to capture the observed qualitative differences in ITP analyte zone shape and order. Finally, we present simulations of CZE experiments where changes in the ionic strength result in significant change in selectivity and order of analyte peaks. Our simulations of ionic strength effects in capillary electrophoresis compare well with the published experimental data.

  View details for DOI 10.1002/elps.200900560

  View details for Web of Science ID 000275696400016

  View details for PubMedID 20191554

• Fluorescent Carrier Ampholytes Assay for Portable, Label-Free Detection of Chemical Toxins in Tap Water ANALYTICAL CHEMISTRY Bercovici, M., Kaigala, G. V., Backhouse, C. J., Santiago, J. G. 2010; 82 (5): 1858-1866

  Abstract

  We present a novel method for fluorescence-based indirect detection of analytes and demonstrate its use for label-free detection of chemical toxins in a hand-held device. We fluorescently label a mixture of low-concentration carrier ampholytes and introduce it into an isotachophoresis (ITP) separation. The carrier ampholytes provide a large number of fluorescent species with a wide range of closely spaced effective electrophoretic mobilities. Analytes focus under ITP and displace subsets of these carrier ampholytes. The analytes are detected indirectly and quantified by analyzing the gaps in the fluorescent ampholyte signal. The large number (on the order of 1000) of carrier ampholytes enables detection of a wide range of analytes, requiring little a priori knowledge of their electrophoretic properties. We discuss the principles of the technique and demonstrate its use in the detection of various analytes using a standard microscope system. We then present the integration of the technique into a self-contained hand-held device and demonstrate detection of chemical toxins (2-nitrophenol and 2,4,6-trichlorophenol) in tap water, with no sample preparation steps.

  View details for DOI 10.1021/ac902526g

  View details for Web of Science ID 000274841300044

  View details for PubMedID 20141152

• Method for Analyte Identification Using Isotachophoresis and a Fluorescent Carrier Ampholyte Assay ANALYTICAL CHEMISTRY Bercovici, M., Kaigala, G. V., Santiago, J. G. 2010; 82 (5): 2134-2138

  Abstract

  We present a novel method for identification of unlabeled analytes using fluorescent carrier ampholytes and isotachophoresis (ITP). The method is based on previous work where we showed that the ITP displacement of carrier ampholytes can be used for detection of unlabeled (nonfluorescent) analytes. We here propose a signal analysis method based on integration of the associated fluorescent signal. We define a normalized signal integral which is equivalent to an accurate measure of the amount of carrier ampholytes which are focused between the leading electrolyte and the analyte. We show that this parameter can be related directly to analyte effective mobility. Using several well characterized analytes, we construct calibration curves relating effective mobility and carrier ampholyte displacement at two different leading electrolyte (LE) buffers. On the basis of these calibration curves, we demonstrate the extraction of fully ionized mobility and dissociation constant of 2-nitrophenol and 2,4,6-trichlorophenol from ITP experiments with fluorescent carrier ampholytes. This extraction is based on no a priori assumptions or knowledge of these two toxic chemicals. This technique allows simultaneous identification of multiple analytes by their physiochemical properties in a few minutes and with no sample preparation.

  View details for DOI 10.1021/ac9025658

  View details for Web of Science ID 000274841300080

  View details for PubMedID 20141174

• Compact adaptive-grid scheme for high numerical resolution simulations of isotachophoresis JOURNAL OF CHROMATOGRAPHY A Bercovici, M., Lele, S. K., Santiago, J. G. 2010; 1217 (4): 588-599

  Abstract

  In a previous publication we demonstrated a fast simulation tool for solution of electrophoretic focusing and separation. We here describe the novel mathematical model and numerical algorithms used to create this code. These include the representation of advection-diffusion equations on an adaptive grid, high-resolution discretization of the equations (sixth order compact), a new variational-based approach for controlling the motion of grid points, and new boundary conditions which enable solution in a moving frame of reference. We discuss the advantages of combining a high-resolution discretization with an adaptive grid in accurately resolving sharp interfaces in isotachophoresis, and provide verification against known analytical solutions and comparison with prevailing exiting numerical algorithms.

  View details for DOI 10.1016/j.chroma.2009.11.072

  View details for Web of Science ID 000274479200020

  View details for PubMedID 20022605

• Evidence shows concentration polarization and its propagation can be key factors determining electroosmotic pump performance SENSORS AND ACTUATORS B-CHEMICAL Strickland, D. G., Suss, M. E., Zangle, T. A., Santiago, J. G. 2010; 143 (2): 795-798

  View details for DOI 10.1016/j.snb.2009.10.005

  View details for Web of Science ID 000274774100049

• Miniaturized system for isotachophoresis assays LAB ON A CHIP Kaigala, G. V., Bercovici, M., Behnam, M., Elliott, D., Santiago, J. G., Backhouse, C. J. 2010; 10 (17): 2242-2250

  Abstract

  We present an inexpensive hand-held device (240 g) that implements microchip isotachophoresis (ITP) with laser induced fluorescence (LIF) detection. This self-contained instrument integrates the functionality required for high voltage generation onto a microelectronic chip, includes LIF detection and is powered by a universal serial bus (USB) link connected to a laptop computer. Using this device we demonstrate focusing and detection of a fluorescent species with a limit of detection of 100 pM. We show that the response of the detector is linear with the initial analyte concentration, making this device suitable for quantitative analysis. We also demonstrate the use of our simulation tools for design and prediction of ITP assays, and validate these results with a demonstration of multiplexed indirect detection of (unlabeled) analytes performed using the device. We find good agreement between simulations and experimental results. Using a label-free isotachaphoresis assay implemented in the hand-held device we detect two explosives and an endocrine disruptor spiked in river water, with no prior sample processing.

  View details for DOI 10.1039/c004120c

  View details for Web of Science ID 000280812600010

  View details for PubMedID 20571691

• Theory and experiments of concentration polarization and ion focusing at microchannel and nanochannel interfaces CHEMICAL SOCIETY REVIEWS Zangle, T. A., Mani, A., Santiago, J. G. 2010; 39 (3): 1014-1035

  Abstract

  In this tutorial review aimed at researchers using nanofluidic devices, we summarize the current state of theoretical and experimental approaches to describing concentration polarization (CP) in hybrid microfluidic-nanofluidic systems. We also analyze experimental results for these systems and place them in the context of recent theoretical developments. We then extend the theory to explain the behavior of both positively and negatively charged, low-concentration, analyte species in systems with CP. We conclude by discussing several applications of CP in microfluidics.

  View details for DOI 10.1039/b902074h

  View details for Web of Science ID 000274920300011

  View details for PubMedID 20179822

• Purification of Nucleic Acids from Whole Blood Using Isotachophoresis ANALYTICAL CHEMISTRY Persat, A., Marshall, L. A., Santiago, J. G. 2009; 81 (22): 9507-9511

  Abstract

  We present and demonstrate a novel technique for the purification of nucleic acids from biological samples using isotachophoresis (ITP). We demonstrate a simple and rapid method to achieve ITP-based extraction, preconcentration, and purification of DNA from nanoliter volumes of whole blood. We show that ITP purification yields genomic DNA samples which can be quantitated with fluorescence measurements and are immediately compatible with polymerase chain reaction (PCR) (e.g., a PCR-friendly solution free of significant inhibitors). We hypothesize ITP purification is applicable to processing of a wide range of complex biological samples.

  View details for DOI 10.1021/ac901965v

  View details for Web of Science ID 000271662400039

  View details for PubMedID 19831356

• Engineering model for coupling wicks and electroosmotic pumps with proton exchange membrane fuel cells for active water management ELECTROCHIMICA ACTA Litster, S., Buie, C. R., Santiago, J. G. 2009; 54 (26): 6223-6233

  View details for DOI 10.1016/j.electacta.2009.05.001

  View details for Web of Science ID 000270646600015

• Two-phase hydrodynamics in a miniature direct methanol fuel cell INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER Buie, C. R., Santiago, J. G. 2009; 52 (21-22): 5158-5166

  View details for DOI 10.1016/j.ijheatmasstransfer.2009.05.003

  View details for Web of Science ID 000270164200054

• Electrokinetic control of sample splitting at a channel bifurcation using isotachophoresis NEW JOURNAL OF PHYSICS Persat, A., Santiago, J. G. 2009; 11

  View details for DOI 10.1088/1367-2630/11/7/075026

  View details for Web of Science ID 000268699200013

• Effects of carbon dioxide on peak mode isotachophoresis: Simultaneous preconcentration and separation LAB ON A CHIP Khurana, T. K., Santiago, J. G. 2009; 9 (10): 1377-1384

  Abstract

  We present a method that achieves simultaneous preconcentration and separation of analytes using peak-mode isotachophoresis with a single step injection in simple, off-the-shelf microchannels or capillaries. We leverage ions resulting from dissolved atmospheric carbon dioxide to weakly disrupt isotachophoretic preconcentration and induce separation of analyte species. We experimentally study the region between the leading and trailing electrolytes, and individually identify the carbonate and carbamate zones that result from the hydration and carbamation reaction of dissolved atmospheric carbon dioxide, respectively. The width of these zones and the gradient regions between them grow with time and create an electric field gradient that causes analytes to separate. Using this assay, we achieve focusing and separation of a 25 bp DNA ladder in a straight, 34 microm wide microchannel in a single loading step. As a demonstration of the fractionation capabilities of the assay, we show simultaneous preconcentration and separation of a DNA ladder from two proteins, GFP and allophycocyanin.

  View details for DOI 10.1039/b815460k

  View details for Web of Science ID 000268227400010

  View details for PubMedID 19417904

• Imaging and Quantification of Isotachophoresis Zones Using Nonfocusing Fluorescent Tracers ANALYTICAL CHEMISTRY Chambers, R. D., Santiago, J. G. 2009; 81 (8): 3022-3028

  Abstract

  We present a novel method for visualizing isotachophoresis (ITP) zones. We introduce negligibly small concentrations of a fluorophore that is not focused by isotachophoresis. This nonfocusing tracer (NFT) migrates through multiple isotachophoresis zones. As it enters each zone, the NFT concentration adapts to the local electric field in each zone. ITP zones can then be visualized with a point detector or camera. The method can be used to detect, identify, and quantify unknown analyte zones and can visualize complex and even transient electrophoresis processes. This visualization technique is particularly suited to microfluidic and laboratory-on-a-chip applications, as typical fluorescence microscopes and charge-coupled device (CCD) cameras can provide high-resolution spatiotemporal data. We present a theoretical description, a methodology for identifying analytes, and experimental validation. We also visualize and analyze a complex, transient DNA ITP preconcentration and separation.

  View details for DOI 10.1021/ac802698a

  View details for Web of Science ID 000265158800025

  View details for PubMedID 19290665

• On the Propagation of Concentration Polarization from Microchannel-Nanochannel Interfaces Part II: Numerical and Experimental Study LANGMUIR Zangle, T. A., Mani, A., Santiago, J. G. 2009; 25 (6): 3909-3916

  Abstract

  We present results of a combined computational and experimental study of the propagation of concentration polarization (CP) zones in a microchannel-nanochannel system. Our computational model considers the combined effects of bulk flow, electromigration, and diffusion and accurately captures the dynamics of CP. Using wall charge inside the nanochannel as a single fitting parameter, we predict experimentally observed enrichment and depletion shock velocities. Our model can also be used to compute the existence of CP with propagating enrichment and depletion shocks on the basis of measured ion mobility and wall properties. We present experiments where the background electrolyte consists of only a fluorescent ion and its counterion. These results are used to validate the computational model and to confirm predicted trends from an analytical model presented in the first of this two-paper series. We show experimentally that the enrichment region concentration is effectively independent of the applied current, while the enrichment and depletion shock velocities increase in proportion to current density.

  View details for DOI 10.1021/1a803318e

  View details for Web of Science ID 000264145000085

  View details for PubMedID 19275188

• On the Propagation of Concentration Polarization from Microchannel-Nanochannel Interfaces Part I: Analytical Model and Characteristic Analysis LANGMUIR Mani, A., Zangle, T. A., Santiago, J. G. 2009; 25 (6): 3898-3908

  Abstract

  We develop two models to describe ion transport in variable-height micro- and nanochannels. For the first model, we obtain a one-dimensional (unsteady) partial differential equation governing flow and charge transport through a shallow and wide electrokinetic channel. In this model, the effects of electric double layer (EDL) on axial transport are taken into account using exact solutions of the Poisson-Boltzmann equation. The second simpler model, which is approachable analytically, assumes that the EDLs are confined to near-wall regions. Using a characteristics analysis, we show that the latter model captures concentration polarization (CP) effects and provides useful insight into its dynamics. Two distinct CP regimes are identified: CP with propagation in which enrichment and depletion shocks propagate outward, and CP without propagation where polarization effects stay local to micro- nanochannel interfaces. The existence of each regime is found to depend on a nanochannel Dukhin number and mobility of the co-ion nondimensionalized by electroosmotic mobility. Interestingly, microchannel dimensions and axial diffusion are found to play an insignificant role in determining whether CP propagates. The steady state condition of propagating CP is shown to be controlled by channel heights, surface chemistry, and co-ion mobility instead of the reservoir condition. Both models are validated against experimental results in Part II of this two-paper series.

  View details for DOI 10.1021/1a803317p

  View details for Web of Science ID 000264145000084

  View details for PubMedID 19275187

• Electrokinetics in nanochannels. Part II. Mobility dependence on ion density and ionic current measurements (vol 325, pg 539, 2008) JOURNAL OF COLLOID AND INTERFACE SCIENCE Baldessari, F., Santiago, J. G. 2009; 331 (2): 550-550

  View details for DOI 10.1016/j.jcis.2008.12.019

  View details for Web of Science ID 000263335700044

• Electrokinetics in nanochannels. Part I. Electric double layer overlap and channel-to-well equilibrium (vol 325, pg 526, 2008) JOURNAL OF COLLOID AND INTERFACE SCIENCE Baldessari, F., Santiago, J. G. 2009; 331 (2): 549-549

  View details for DOI 10.1016/j.jcis.2008.12.013

  View details for Web of Science ID 000263335700043

• Dry gas operation of proton exchange membrane fuel cells with parallel channels: Non-porous versus porous plates JOURNAL OF POWER SOURCES Litster, S., Santiago, J. G. 2009; 188 (1): 82-88

  View details for DOI 10.1016/j.jpowsour.2008.11.069

  View details for Web of Science ID 000264391400012

• Open source simulation tool for electrophoretic stacking, focusing, and separation JOURNAL OF CHROMATOGRAPHY A Bercovici, M., Lele, S. K., Santiago, J. G. 2009; 1216 (6): 1008-1018

  Abstract

  We present the development, formulation, and performance of a new simulation tool for electrophoretic preconcentration and separation processes such as capillary electrophoresis, isotachophoresis, and field amplified sample stacking. The code solves the one-dimensional transient advection-diffusion equations for multiple multivalent weak electrolytes (including ampholytes) and includes a model for pressure-driven flow and Taylor-Aris dispersion. The code uses a new approach for the discretization of the equations, consisting of a high resolution compact scheme which is combined with an adaptive grid algorithm. We show that this combination allows for accurate resolution of sharp concentration gradients at high electric fields, while at the same time significantly reducing the computational time. We demonstrate smooth, stable, and accurate solutions at current densities as high as 5000A/m(2) using only 300 grid points, and a 75-fold reduction in computational time compared with equivalent uniform grid techniques. The code is available as an open source for free at http://microfluidics.stanford.edu.

  View details for DOI 10.1016/j.chroma.2008.12.022

  View details for Web of Science ID 000263087700017

  View details for PubMedID 19124132

• Basic principles of electrolyte chemistry for microfluidic electrokinetics. Part I: Acid-base equilibria and pH buffers LAB ON A CHIP Persat, A., Chambers, R. D., Santiago, J. G. 2009; 9 (17): 2437-2453

  Abstract

  We review fundamental and applied acid-base equilibrium chemistry useful to microfluidic electrokinetics. We present elements of acid-base equilibrium reactions and derive rules for pH calculation for simple buffers. We also present a general formulation to calculate pH of more complex, arbitrary mixtures of electrolytes, and discuss the effects of ionic strength and temperature on pH calculation. More practically, we offer advice on buffer preparation and on buffer reporting. We also discuss "real world" buffers and likely contamination sources. In particular, we discuss the effects of atmospheric carbon dioxide on buffer systems, namely, the increase in ionic strength and acidification of typical electrokinetic device buffers. In Part II of this two-paper series, we discuss the coupling of acid-base equilibria with electrolyte dynamics and electrochemistry in typical microfluidic electrokinetic systems.

  View details for DOI 10.1039/b906465f

  View details for Web of Science ID 000268975200003

  View details for PubMedID 19680570

• Corrigendum to ‘Electrokinetics in Nanochannels: Part II: Mobility Dependence on Ion Density and Ionic Current Measurements Journal of Colloid and Interface Science Baldessari, F., Santiago, J., G. 2009; 2 (331): 550-550
• Corrigendum to ‘Electrokinetics in Nanochannels. Part I: Electric Double Layer Overlap and Channel-to-Well Equilibrium Journal of Colloid and Interface Science Baldessari, F., Santiago, J., G. 2009; 2 (331): 549-549
• In-situ Polymerized Wicks for Passive Water Management and Humidification of Dry Gases 9th Proton Exchange Membrane Fuel Cell Symposium (PEMFC) Conducted Under the Auspices of the 216th Meeting of the Electrochemical-Society-Inc STRICKLAND, D. G., Santiago, J. G. ELECTROCHEMICAL SOC INC. 2009: 303–9

  View details for DOI 10.1149/1.3210581

  View details for Web of Science ID 000329585500028

• IN-SITU POLYMERIZED WICKS FOR PASSIVE WATER MANAGEMENT IN PEM FUEL CELL SYSTEMS 3rd International Conference on Energy Sustainability Strickland, D. G., Fenning, D., Litster, S., Santiago, J. G. AMER SOC MECHANICAL ENGINEERS. 2009: 325–326

  View details for Web of Science ID 000281910700038

• NANOPORE CONCENTRATION POLARIZATION ASME International Mechanical Engineering Congress and Exposition Talasaz, A. H., Zangle, T. A., Santiago, J. G. AMER SOC MECHANICAL ENGINEERS. 2009: 871–872

  View details for Web of Science ID 000266546900113

• Special issue on fundamental principles and techniques in microfluidics LAB ON A CHIP Santiago, J. G., Chen, C. 2009; 9 (17): 2423-2424

  View details for DOI 10.1039/b913538n

  View details for Web of Science ID 000268975200001

  View details for PubMedID 19680567

• Basic principles of electrolyte chemistry for microfluidic electrokinetics. Part II: Coupling between ion mobility, electrolysis, and acid-base equilibria LAB ON A CHIP Persat, A., Suss, M. E., Santiago, J. G. 2009; 9 (17): 2454-2469

  Abstract

  We present elements of electrolyte dynamics and electrochemistry relevant to microfluidic electrokinetics experiments. In Part I of this two-paper series, we presented a review and introduction to the fundamentals of acid-base chemistry. Here, we first summarize the coupling between acid-base equilibrium chemistry and electrophoretic mobilities of electrolytes, at both infinite and finite dilution. We then discuss the effects of electrode reactions on microfluidic electrokinetic experiments and derive a model for pH changes in microchip reservoirs during typical direct-current electrokinetic experiments. We present a model for the potential drop in typical microchip electrophoresis device. The latter includes finite element simulation to estimate the relative effects of channel and reservoir dimensions. Finally, we summarize effects of electrode and electrolyte characteristics on potential drop in microfluidic devices. As a whole, the discussions highlight the importance of the coupling between electromigration and electrophoresis, acid-base equilibria, and electrochemical reactions.

  View details for DOI 10.1039/b906468k

  View details for Web of Science ID 000268975200004

  View details for PubMedID 19680571

• Hydrodynamic interactions in metal rodlike-particle suspensions due to induced charge electroosmosis PHYSICAL REVIEW E Rose, K. A., Hoffman, B., Saintillan, D., Shaqfeh, E. S., Santiago, J. G. 2009; 79 (1)

  Abstract

  We present a theoretical and experimental study of the role of hydrodynamic interactions on the motion and dispersion of metal rodlike particles in the presence of an externally applied electric field. In these systems, the electric field polarizes the particles and induces an electroosmotic flow relative to the surface of each particle. The simulations include the effect of the gravitational body force, buoyancy, far-field hydrodynamic interactions, and near-field lubrication forces. The particles in the simulations and experiments were observed to experience repeated pairing interactions in which they come together axially with their ends approaching each other, slide past one another until their centers approach, and then push apart. These interactions were confirmed in measurements of particle orientations and velocities, pair distribution functions, and net dispersion of the suspension. For large electric fields, the pair distribution functions show accumulation and depletion regions consistent with many pairing events. For particle concentrations of 10;{8}particles/mL and higher, dispersion within the suspension dramatically increases with increased field strength.

  View details for DOI 10.1103/PhysRevE.79.011402

  View details for Web of Science ID 000262976600043

  View details for PubMedID 19257030

• Rapid and selective extraction, isolation, preconcentration, and quantitation of small RNAs from cell lysate using on-chip isotachophoresis LAB ON A CHIP Schoch, R. B., Ronaghi, M., Santiago, J. G. 2009; 9 (15): 2145-2152

  Abstract

  We present a technique which enables the separation of small RNAs-such as microRNAs (miRNAs), short interfering RNAs (siRNAs), and Piwi-interacting RNAs (piRNAs)-from >or=66 nucleotide RNAs and other biomolecules contained in a cell lysate. In particular, the method achieves separation of small RNAs from precursor miRNAs (pre-miRNAs) in less than 3 min. We use on-chip isotachophoresis (ITP) for the simultaneous extraction, isolation, preconcentration and quantitation of small RNAs (approximately 22 nucleotides) and employ the high-efficiency sieving matrix Pluronic F-127; a thermo-responsive triblock copolymer which allows convenient microchannel loading at low temperature. We present the isolation of small RNAs from the lysate of 293A human kidney cells, and quantitate the number of short RNA molecules per cell to be 2.9x10(7). We estimate this quantity is an aggregate of roughly 500 types of short RNA molecules per 293A cell. Currently, the minimal cell number for small RNA extraction and detection with our method is approximately 900 (from a 5 microL sample volume), and we believe that small RNA analysis from tens of cells is realizable. Techniques for rapid and sensitive extraction and isolation of small RNAs from cell lysate are much-needed to further uncover their full range and functionality, including RNA interference studies.

  View details for DOI 10.1039/b903542g

  View details for Web of Science ID 000268033900005

  View details for PubMedID 19606290

• Electrokinetics in nanochannels - Part I. Electric double layer overlap and channel-to-well equilibrium JOURNAL OF COLLOID AND INTERFACE SCIENCE Baldessari, F. 2008; 325 (2): 526-538

  Abstract

  In this paper a new model is described for calculating the electric potential field in a long, thin nanochannel with overlapped electric double layers. Electrolyte concentration in the nanochannel is predicted self-consistently via equilibrium between ionic solution in the wells and within the nanochannel. Differently than published models that require detailed iterative numerical solutions of coupled differential equations, the framework presented here is self-consistent and predictions are obtained solving a simple one-dimensional integral. The derivation clearly shows that the electric potential field depends on three new parameters: the ratio of ion density in the channel to ion density in the wells; the ratio of free-charge density to bulk ion density within the channel; and a modified Debye-Hückel thickness, which is the relevant scale for shielding of surface net charge. For completeness, three wall-surface boundary conditions are analyzed: specified zeta-potential; specified surface net charge density; and charge regulation. Predictions of experimentally observable quantities based on the model proposed here, such as depth-averaged electroosmotic flow and net ionic current, are significantly different than results from previous overlapped electric double layer models. In this first paper of a series of two, predictions are presented where channel depth is varied at constant well concentration. Results show that under conditions of electric double layer overlap, electroosmosis contributes only a small fraction of the net ionic current, and that most of the measurable current is due to ionic conduction in conditions of increased counterion density in the nanochannel. In the second of this two-paper series, predictions are presented where well-concentration is varied and the channel depth is held constant, and the model described here is employed to study the dependence of ion mobility on ionic strength, and compare predictions to measurements of ionic current as a function of channel depth and ion density.

  View details for DOI 10.1016/j.jcis.2008.06.007

  View details for Web of Science ID 000258553900033

  View details for PubMedID 18639883

• Electrokinetics in nanochannels - Part II. Mobility dependence on ion density and ionic current measurements JOURNAL OF COLLOID AND INTERFACE SCIENCE Baldessari, F. 2008; 325 (2): 539-546

  Abstract

  In the first of this two-paper series, a new model was developed for calculating the electric potential field in a long, thin nanochannel with overlapped electric double layers. The model takes into account the dependence of ion mobility on local ion densities and pH. This model is used here to study and demonstrate the effect of ion density and pH on ionic current measurements. A comparison is shown of predictions based on each of three boundary conditions, as studied in Part I. The model developed in Part I is validated by comparing simulations with measurements of ionic current as a function of sodium borate concentration. Results show that predictions based on extended Debye-Hückel theory for ion mobility significantly improve the accuracy of simulations, but that these do not predict exact scaling behavior. A simple bulk conductivity measurement used as input parameter for the simulations, in place of the predicted bulk conductivity (K(0)), guarantees agreement with data in the thin EDL region. Results also indicate that the charge regulation boundary condition, complemented with an adequate bulk electrolyte model, provides better agreement with experimental trends than the specified zeta potential or specified surface net charge boundary conditions. Further, it is shown that currents due to advection (by electroosmotic flow) are in all cases studied less than 25% of the total current in the system.

  View details for DOI 10.1016/j.jcis.2008.06.008

  View details for Web of Science ID 000258553900034

  View details for PubMedID 18639884

• Sample zone dynamics in peak mode isotachophoresis ANALYTICAL CHEMISTRY Khurana, T. K., Santiago, J. G. 2008; 80 (16): 6300-6307

  Abstract

  We present a theoretical and experimental study of analyte preconcentration via peak mode isotachophoresis (ITP). We perform perturbation analysis of the governing equations that includes electromigration, diffusion, buffer reactions, and nonlinear ionic strength effects. This analysis relaxes the inherent numerical stiffness and achieves a fast solution to the transient sample evolution problem. In this model, we have incorporated a semiempirical relation to capture dispersion phenomenon within ITP interfaces. We also present a simple, closed-form analytical model that identifies key parameters governing the preconcentration dynamics in peak mode ITP. We have validated our models through a detailed experimental study performed in constant current conditions. The relevant governing experiment parameters were varied independently; namely, the leading electrolyte concentration, trailing electrolyte concentration, and current. Through our experimental study, we have identified optimum conditions to achieve high preconcentration ratio and sample accumulation rates. Our approach to the theoretical problem and experimental study provides useful guidelines in optimizing parameters such as detector location, ITP duration, and electrolyte composition in ITP preconcentration and separation assays.

  View details for DOI 10.1021/ac800792g

  View details for Web of Science ID 000258448100021

  View details for PubMedID 18642874

• Lymphocyte electrotaxis in vitro and in vivo JOURNAL OF IMMUNOLOGY Lin, F., Baldessari, F., Gyenge, C. C., Sato, T., Chambers, R. D., Santiago, J. G., Butcher, E. C. 2008; 181 (4): 2465-2471

  Abstract

  Electric fields are generated in vivo in a variety of physiologic and pathologic settings, including penetrating injury to epithelial barriers. An applied electric field with strength within the physiologic range can induce directional cell migration (i.e., electrotaxis) of epithelial cells, endothelial cells, fibroblasts, and neutrophils suggesting a potential role in cell positioning during wound healing. In the present study, we investigated the ability of lymphocytes to respond to applied direct current (DC) electric fields. Using a modified Transwell assay and a simple microfluidic device, we show that human PBLs migrate toward the cathode in physiologically relevant DC electric fields. Additionally, electrical stimulation activates intracellular kinase signaling pathways shared with chemotactic stimuli. Finally, video microscopic tracing of GFP-tagged immunocytes in the skin of mouse ears reveals that motile cutaneous T cells actively migrate toward the cathode of an applied DC electric field. Lymphocyte positioning within tissues can thus be manipulated by externally applied electric fields, and may be influenced by endogenous electrical potential gradients as well.

  View details for Web of Science ID 000258345300026

  View details for PubMedID 18684937

• A depth-averaged electrokinetic flow model for shallow microchannels JOURNAL OF FLUID MECHANICS Lin, H., Storey, B. D., Santiago, J. G. 2008; 608: 43-70

  View details for DOI 10.1017/S0022112008001869

  View details for Web of Science ID 000258201900003

• Ballistic dispersion in temperature gradient focusing PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES Huber, D. E., Santiago, J. G. 2008; 464 (2091): 595-612

  View details for DOI 10.1098/rspa.2007.0161

  View details for Web of Science ID 000252763200005

• High flow rate per power electroosmotic pumping using low ion density solvents SENSORS AND ACTUATORS A-PHYSICAL Kim, D., Posner, J. D., Santiago, J. G. 2008; 141 (1): 201-212

  View details for DOI 10.1016/j.sna.2007.07.023

  View details for Web of Science ID 000253390400027

• Preconcentration, separation, and indirect detection of nonfluorescent analytes using fluorescent mobility markers ANALYTICAL CHEMISTRY Khurana, T. K., Santiago, J. G. 2008; 80 (1): 279-286

  Abstract

  We present a method to achieve separation and indirect detection of nonfluorescent species using fluorescent mobility markers. This technique leverages isotachophoresis (ITP) for both preconcentration and separation. We employ a leading electrolyte (LE), trailing electrolyte (TE), and a set of fluorescent markers of mobilities designed to bound those of nonfluorescent analytes of interest. Fluorescent markers and nonfluorescent analytes are initially mixed homogenously and ITP is initiated. The dynamics of isotachophoresis cause the analyte and fluorescent marker mixture to segregate into respective zones between the LE and TE in the order of reducing mobility. Unlabeled analytes are detected as gaps (regions with local minimums in intensity) in the fluorescent signals of mobility markers. We have successfully demonstrated preconcentration, separation, and detection of unlabeled amino acids serine, glycine, and phenylalanine; and of acetic acid, aspartic acid, and 3-phenylpropionic acid. We show detection of 12 microM concentration of analytes with signal-to-noise ratio of 4.0 and with a high degree of repeatability. We discuss methods for encoding mobility marker identity using marker fluorescence intensity level and alternating fluorescence emission wavelengths. We present example experimental results of fluorescence intensity level encoding.

  View details for DOI 10.1021/ac701706h

  View details for Web of Science ID 000252026900042

  View details for PubMedID 18031059

• Quick Measurement of Electroosmotic Flow Velocity Chips & Tips, Lab on a Chip Nohmi, M., Santiago, J., G. 2008
• Physics of pumping methanol/water solutions for fuel cell applications ASME International Mechanical Engineering Congress and Exposition Buie, C. R., Litster, S., Santiago, J. G. AMER SOC MECHANICAL ENGINEERS. 2008: 637–642

  View details for Web of Science ID 000254474800072

• Taylor Dispersion in Sample Pre-Concentration Methods CRC Handbook of Electrophoresis Bharadwaj, R., Huber, D., E., Khurana, T., Santiago, Juan, G. edited by Landers, J. CRC Press. 2008; 3rd: 1085–1120
• Model and Experimental Study of Hydrodynamic Coupling between a Fuel Pump and a Direct Methanol Fuel Cell 8th Symposium on Proton Exchange Membrane Fuel Cells Buie, C. R., Santiago, J. G. ELECTROCHEMICAL SOCIETY INC. 2008: 1525–38

  View details for DOI 10.1149/1.2981993

  View details for Web of Science ID 000271859300156

• On-chip isothermal polymerase chain reaction ASME International Mechanical Engineering Congress and Exposition Morita, T., Persat, A., Santiago, J. G. AMER SOC MECHANICAL ENGINEERS. 2008: 1003–1004

  View details for Web of Science ID 000254834900134

• INDIRECT FLUORESCENCE DETECTION OF NON FLUORESCENT ANALYTES USING ISOTACHOPHORETIC MOBILITY MARKERS 6th International Conference on Nanochannels, Microchannels and Minichannels Khurana, T. K., Bercovici, M., Santiago, J. G. AMER SOC MECHANICAL ENGINEERS. 2008: 1701–1706

  View details for Web of Science ID 000262925200215

• On-chip indirect detection of non-fluorescent analytes using fluorescent spacers ASME International Mechanical Engineering Congress and Exposition Khurana, T. K., Santiago, J. G. AMER SOC MECHANICAL ENGINEERS. 2008: 901–904

  View details for Web of Science ID 000254834900118

• Two-liquid electroosmotic pump for portable drug delivery systems ASME International Mechanical Engineering Congress and Exposition Litster, S., Ha, B., Kim, D., Santiago, J. A. AMER SOC MECHANICAL ENGINEERS. 2008: 963–964

  View details for Web of Science ID 000254834900128

• Experimental study of concentration polarization at a microchannel-nanochannel interface ASME International Mechanical Engineering Congress and Exposition Mani, A., Zangle, T. A., Santiago, J. G. AMER SOC MECHANICAL ENGINEERS. 2008: 911–912

  View details for Web of Science ID 000254834900120

• On-chip preconcentration and separation of simple and complex analytes using isotachophoresis ASME International Mechanical Engineering Congress and Exposition Khurana, T. K., Persat, A., Santiago, J. G. AMER SOC MECHANICAL ENGINEERS. 2008: 857–861

  View details for Web of Science ID 000254834900111

• Toward orientation-independent design for gas recombination in closed-loop electroosmotic pumps SENSORS AND ACTUATORS B-CHEMICAL Lin, C., Yao, S., Posner, J. D., Myers, A. M., Santiago, J. G. 2007; 128 (1): 334-339

  View details for DOI 10.1016/j.snb.2007.05.029

  View details for Web of Science ID 000251465000046

• Investigation of internal pressure gradients generated in electrokinetic flows with axial conductivity gradients EXPERIMENTS IN FLUIDS Devasenathipathy, S., Bharadwaj, R., Santiago, J. G. 2007; 43 (6): 959-967

  View details for DOI 10.1007/s00348-007-0366-3

  View details for Web of Science ID 000251010400011

• Current distribution in polymer electrolyte membrane fuel cell with active water management JOURNAL OF POWER SOURCES Strickland, D. G., Litster, S., Santiago, J. G. 2007; 174 (1): 272-281

  View details for DOI 10.1016/j.jpowsour.2007.08.059

  View details for Web of Science ID 000251591000033

• Free-solution oligonucleotide separation in nanoscale channels ANALYTICAL CHEMISTRY Pennathur, S., Baldessari, F., Santiago, J. G., Kattah, M. G., Steinman, J. B., Utz, P. J. 2007; 79 (21): 8316-8322

  Abstract

  In this paper, we report an experimental study of electrokinetic transport and separation of double-stranded deoxyribonucleic acid (dsDNA) oligonucleotides in custom-fabricated fused-silica nanochannels filled with a gel-free sodium borate aqueous buffer. Mixtures of fluorescently labeled dsDNA molecules in the range of 10-100 base pair (bp), fluorescein, and fluorescein-12-UTP (UTP) were separated in less than 120 s in channels of depth ranging from 40 to 1560 nm. We varied the channel depth and background buffer concentration to achieve a 0.006-0.2 range of Debye length-to-channel-half-depth ratio (lambdaD/h), and a 0.004-1.7 range of the ratio of length of dsDNA molecule to channel half-depth (l/h). We find observed oligonucleotide migration times depend on both l/h and lambdaD/h. Electrophoretic mobility estimates agree well with published (micrometer-scale channel) values for background electrolyte (BGE) concentrations greater than approximately 10 mM. At BGE concentrations of 1 and 5 mM, mobility estimates in our nanochannels are higher than published values. Of the cases studied, the highest separation sensitivities were achieved in 100 nm channels with 1-10 mM ion density buffers. Potential applications of this technology include rapid small-scale sequencing and other fluorescence-based oligonucleotide separation and detection assays.

  View details for DOI 10.1021/ac0710580

  View details for Web of Science ID 000250584800053

  View details for PubMedID 17883279

• Taylor-Aris dispersion in temperature gradient focusing ELECTROPHORESIS Huber, D. E., Santiago, J. G. 2007; 28 (14): 2333-2344

  Abstract

  Microfluidic temperature gradient focusing (TGF) uses an axial temperature gradient to induce a gradient in electrophoretic flux within a microchannel. When balanced with an opposing fluid flow, charged analytes simultaneously focus and separate according to their electrophoretic mobilities. We present a theoretical and experimental study of dispersion in TGF. We model the system using generalized dispersion analysis that yields a 1-D convection-diffusion equation that contains dispersion terms particular to TGF. We consider analytical solutions for the model under uniform temperature gradient conditions. Using a custom TGF experimental setup, we compare focusing measurements with the theoretical predictions. We find that the theory well represents the focusing behavior for flows within the Taylor-Aris dispersion regime.

  View details for DOI 10.1002/elps.200600830

  View details for Web of Science ID 000248390900001

  View details for PubMedID 17578841

• Comments on the conditions for similitude in electroosmotic flows JOURNAL OF COLLOID AND INTERFACE SCIENCE Santiago, J. G. 2007; 310 (2): 675-677

  Abstract

  This note provides a few comments on the conditions required for similitude between velocity and electric field in electroosmotic flows. The velocity fields of certain electroosmotic flows with relatively thin electric double layers (EDLs) are known to be irrotational in regions outside of the EDL. Under restricted conditions, the velocity field, V , can be expressed in terms of the electric field, E , as V =cE , where c is a scalar constant. The irrotationality solution is certainly unique and exact for Stokes flow, but may not be stable (or unique) for flows with Reynolds numbers significantly greater than unity.

  View details for DOI 10.1016/j.jcis.2007.01.088

  View details for Web of Science ID 000246459700040

  View details for PubMedID 17350645

• Engineering model of a passive planar air breathing fuel cell cathode JOURNAL OF POWER SOURCES O'Hayre, R., Fabian, T., Litster, S., Prinz, F. B., Santiago, J. G. 2007; 167 (1): 118-129

  View details for DOI 10.1016/j.jpowsour.2007.01.073

  View details for Web of Science ID 000246256300019

• Measurement of temperature and reaction species in the cathode diffusion layer of a free-convection fuel cell JOURNAL OF THE ELECTROCHEMICAL SOCIETY Fabian, T., O'Hayre, R., Prinz, F. B., Santiago, J. G. 2007; 154 (9): B910-B918

  View details for DOI 10.1149/1.2752971

  View details for Web of Science ID 000248984600013

• Microfluidic Control of Nanoparticle Aggregation for Surfaced Enhanced Raman Spectroscopy Piorek, B., Lee, S., J., Moskovits, M., Banerjee, S., Santiago, J., G., Meinhart, C. 2007
• Ballistic Dispersion in Temperature Gradient Focusing Huber, D., E., Santiago, J., G. 2007
• On-Chip Electrophoresis Devices: Do’s, Don’ts, and Dooms Chips & Tips, Lab on a Chip Persat, A., Zangle, T., A., Posner, J., D., Santiago, J., G. 2007
• Detection of 100 aM fluorophores using a high-sensitivity on-chip CE system and transient isotachophoresis ANALYTICAL CHEMISTRY Jung, B., Zhu, Y., Santiago, J. G. 2007; 79 (1): 345-349

  Abstract

  We present a highly sensitive capillary electrophoresis (CE) assay that combines transient, single-interface on-chip isotachophoresis (ITP) and a laser-induced confocal fluorescence detection setup. We performed experimental parametric studies to show the effects of microscope objective specifications and intensity of excitation laser on optimization of a high-sensitivity on-chip CE detection system. Using the optimized detection system, single-molecule detection of Alexa Fluor 488 was demonstrated, and signal data were validated with autocorrelation analysis. We also demonstrated a separation and detection of 100 aM fluorophores (Alexa Fluor 488 and bodipy) in a fast assay using a high-sensitivity on-chip CE detection system and an ITP/CE protocol with no manual buffer exchange steps. This is, to the knowledge of the authors, the highest electrophoretic separation sensitivity ever reported.

  View details for DOI 10.1021/ac060949p

  View details for Web of Science ID 000243143300050

  View details for PubMedID 17194159

• An electro-osmotic fuel pump for direct methanol fuel cells ELECTROCHEMICAL AND SOLID STATE LETTERS Buie, C. R., Kim, D., Litster, S., Santiago, J. G. 2007; 10 (11): B196-B200

  View details for DOI 10.1149/1.2772083

  View details for Web of Science ID 000249323200007

• Active water management for PEM fuel cells JOURNAL OF THE ELECTROCHEMICAL SOCIETY Litster, S., Buie, C. R., Fabian, T., Eaton, J. K., Santiago, J. G. 2007; 154 (10): B1049-B1058

  View details for DOI 10.1149/1.2766650

  View details for Web of Science ID 000248984700021

• Rotational electrophoresis of striped metallic microrods PHYSICAL REVIEW E Rose, K. A., Meier, J. A., Dougherty, G. M., Santiago, J. G. 2007; 75 (1)

  Abstract

  Analytical models are developed for the translation and rotation of metallic rods in a uniform electric field. The limits of thin and thick electric double layers are considered. These models include the effect of stripes of different metals along the length of the particle. Modeling results are compared to experimental measurements for metallic rods. Experiments demonstrate the increased alignment of particles with increasing field strength and the increase in degree of alignment of thin versus thick electric double layers. The metal rods polarize in the applied field and align parallel to its direction due to torques on the polarized charge. The torque due to polarization has a second-order dependence on the electric field strength. The particles are also shown to have an additional alignment torque component due to nonuniform densities along their length. The orientation distributions of dilute suspensions of particles are also shown to agree well with results predicted by a rotational convective-diffusion equation.

  View details for DOI 10.1103/PhysRevE.75.011503

  View details for Web of Science ID 000243893400055

  View details for PubMedID 17358156

• The role of ambient conditions on the performance of a planar, air-breathing hydrogen PEM fuel cell JOURNAL OF POWER SOURCES Fabian, T., Posner, J. D., O'Hayre, R., Cha, S., Eaton, J. K., Prinz, F. B., Santiago, J. G. 2006; 161 (1): 168-182

  View details for DOI 10.1016/j.jpowsour.2006.03.054

  View details for Web of Science ID 000241412000024

• Water management in proton exchange membrane fuel cells using integrated electroosmotic pumping JOURNAL OF POWER SOURCES Buie, C. R., Posner, J. D., Fabian, T., Cha, S., Kim, D., Prinz, F. B., Eaton, J. K., Santiago, J. G. 2006; 161 (1): 191-202

  View details for DOI 10.1016/j.jpowsour.2006.03.021

  View details for Web of Science ID 000241412000026

• Optimization of a microfluidic mixer for studying protein folding kinetics ANALYTICAL CHEMISTRY Hertzog, D. E., Ivorra, B., Mohammadi, B., Bakajin, O., Santiago, J. G. 2006; 78 (13): 4299-4306

  Abstract

  We have applied an optimization method in conjunction with numerical simulations to minimize the mixing time of a microfluidic mixer developed for protein folding studies. The optimization method uses a semideterministic algorithm to find the global minimum of the mixing time by varying the mixer geometry and flow conditions. We describe the minimization problem and constraints and give a brief overview of the optimization algorithm. We present results of the optimization, including the optimized geometry and parameter sensitivities, and we demonstrate the improvement in mixing performance with experiments using microfabricated mixers. The dye-quenching experiments of the original and optimized mixer designs show respective mixing times of 7 and 4 mus, a 40% reduction. The new design also provides more uniform mixing across streamlines that enter the mixer. The optimized mixer is the fastest reported continuous flow mixer for protein folding.

  View details for DOI 10.1021/ac051903j

  View details for Web of Science ID 000238665200014

  View details for PubMedID 16808436

• Electroosmotic pumps fabricated from porous silicon membranes JOURNAL OF MICROELECTROMECHANICAL SYSTEMS Yao, S., Myers, A. M., Posner, J. D., Rose, K. A., Santiago, J. G. 2006; 15 (3): 717-728

  View details for DOI 10.1109/JMEMS.2006.876796

  View details for Web of Science ID 000238311000030

• A hybrid method for bubble geometry reconstruction in two-phase microchannels EXPERIMENTS IN FLUIDS Wang, E. N., Devasenathipathy, S., Lin, H., Hidrovo, C. H., Santiago, J. G., Goodson, K. E., Kenny, T. W. 2006; 40 (6): 847-858

  View details for DOI 10.1007/s00348-006-0123-z

  View details for Web of Science ID 000239549100003

• Convective instability of electrokinetic flows in a cross-shaped microchannel JOURNAL OF FLUID MECHANICS Posner, J. D., Santiago, J. G. 2006; 555: 1-42

  View details for DOI 10.1017/S0022112005008542

  View details for Web of Science ID 000238236200001

• Semi-deterministic and genetic algorithms for global optimization of microfluidic protein-folding devices INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING Ivorra, B., Hertzog, D. E., Mohammadi, B., Santiago, J. G. 2006; 66 (2): 319-333

  View details for DOI 10.1002/nme.1562

  View details for Web of Science ID 000236623800006

• On-chip millionfold sample stacking using transient isotachophoresis ANALYTICAL CHEMISTRY Jung, B., Bharadwaj, R., Santiago, J. G. 2006; 78 (7): 2319-2327

  Abstract

  We present a simple and robust isotachophoresis (ITP) method that can be integrated with microchip-based capillary electrophoresis (CE) devices to achieve millionfold sample stacking. We performed an experimental parametric study to show the effects of initial sample ion concentration, leading ion concentration, and trailing ion concentration on ITP stacking. We also discuss the usefulness and limitations of a simple one-dimensional nondispersive model and a scaling analysis for dispersion rate. We found that a single-column ITP configuration together with electroosmotic flow suppression and high leading ion concentration provide high-performance ITP and can be integrated readily with CE separation. We demonstrated detection of trace of 100 fM Alexa Fluor 488 (signal-to-noise ratio of 11) with a concentration increase of a factor of 2 x 10(6). Application of our ITP/CE protocol to the stacking and separation of negatively charged fluorescent tracers (Alexa Fluor 488 and bodipy) resulted in a concentration increase of 6.4 x 10(4) and a signal increase of 4.5 x 10(5). The ITP/CE protocol can be performed with a standard microchannel cross design or simple flow control. The method can be implemented with available off-the-shelf chip systems using off-the-shelf voltage control systems and buffer chemistries.

  View details for DOI 10.1021/ac051659w

  View details for Web of Science ID 000236686600037

  View details for PubMedID 16579615

• ELECTROPHORESIS IN NANOCHANNELS 2nd US-European Fluids Engineering Division Summer Meeting/14th International Conference on Nuclear Engineering Pennathur, S., Baldessari, F., Kattah, M., Utz, P. J., Santiago, J. G. AMER SOC MECHANICAL ENGINEERS. 2006: 589–593

  View details for Web of Science ID 000272191800082

• Advanced cooling technologies for microprocessors Workshop on Frontiers in Electronics (WOFE-04) Kenny, T. W., Goodson, K. E., Santiago, J. G., Wang, E., Koo, J., Jiang, L., Pop, E., Sinha, S., Zhang, L., Fogg, D., Yao, S., Flynn, R., Chang, C., Hidrovo, C. H. WORLD SCIENTIFIC PUBL CO PTE LTD. 2006: 301–313

  View details for Web of Science ID 000241022300016

• Direct water removal in gas diffusion layer of porton exchnage membrane fuel cells by a flexible electroosmotic pump 4th International Conference on Fuel Cell Science, Engineering and Technology Cha, S. W., Fabian, T., Posner, J., BUIE, C., Kim, D. J., Prinz, F. B., Eaton, J. K., Santiago, J. AMER SOC MECHANICAL ENGINEERS. 2006: 1169–1171

  View details for Web of Science ID 000249884000138

• A microfabricated direct methanol fuel cell with integrated electroosmotic pump 19th IEEE International Conference on Micro Electro Mechanical Systems (MEMS 2006) Buie, C. R., BANIN, Y., Tang, C. Y., Santiago, J. G., Prinz, F. B., Pruitt, B. L. IEEE. 2006: 938–941

  View details for Web of Science ID 000236994500235

• Electrophoresis in nanochannels: brief review and speculation. Journal of nanobiotechnology Baldessari, F., Santiago, J. G. 2006; 4: 12-?

  Abstract

  The relevant physical phenomena that dominate electrophoretic transport of ions and macromolecules within long, thin nanochannels are reviewed, and a few papers relevant to the discussion are cited. Sample ion transport through nanochannels is largely a function of their interaction with electric double layer. For small ions, this coupling includes the net effect of the external applied field, the internal field of the double layer, and the non-uniform velocity of the liquid. Adsorption/desorption kinetics and the effects of surface roughness may also be important in nanochannel electrophoresis. For macromolecules, the resulting motion is more complex as there is further coupling via steric interactions and perhaps polarization effects. These complex interactions and coupled physics represent a valuable opportunity for novel electrophoretic and chromatographic separations.

  View details for PubMedID 17116262

• Dynamics of field-amplified sample stacking JOURNAL OF FLUID MECHANICS Bharadwaj, R., Santiago, J. G. 2005; 543: 57-92

  View details for DOI 10.1017/S0022112005005975

  View details for Web of Science ID 000233542200004

• Electrokinetic transport in nanochannels. 1. Theory ANALYTICAL CHEMISTRY Pennathur, S., Santiago, J. G. 2005; 77 (21): 6772-6781

  Abstract

  Electrokinetic transport in fluidic channels facilitates control and separation of ionic species. In nanometer-scale electrokinetic systems, the electric double layer thickness is comparable to characteristic channel dimensions, and this results in nonuniform velocity profiles and strong electric fields transverse to the flow. In such channels, streamwise and transverse electromigration fluxes contribute to the separation and dispersion of analyte ions. In this paper, we report on analytical and numerical models for nanochannel electrophoretic transport and separation of neutral and charged analytes. We present continuum-based theoretical studies in nanoscale channels with characteristic depths on the order of the Debye length. Our model yields analytical expressions for electroosmotic flow, species transport velocity, streamwise-transverse concentration field distribution, and ratio of apparent electrophoretic mobility for a nanochannel to (standard) ion mobility. The model demonstrates that the effective mobility governing electrophoretic transport of charged species in nanochannels depends not only on electrolyte mobility values but also on zeta potential, ion valence, and background electrolyte concentration. We also present a method we term electrokinetic separation by ion valence (EKSIV) whereby both ion valence and ion mobility may be determined independently from a comparison of micro- and nanoscale transport measurements. In the second of this two-paper series, we present experimental validation of our models.

  View details for Web of Science ID 000233125400014

  View details for PubMedID 16255573

• Electrokinetic transport in nanochannels. 2. Experiments ANALYTICAL CHEMISTRY Pennathur, S., Santiago, J. G. 2005; 77 (21): 6782-6789

  Abstract

  We present an experimental study of nanoscale electrokinetic transport in custom-fabricated quartz nanochannels using quantitative epifluorescence imaging and current monitoring techniques. One aim is to yield insight into electrical double layer physics and study the applicability of continuum theory to nanoscale electrokinetic systems. A second aim is to explore a new separation modality offered by nanoscale electrophoretic separations. We perform parametric variations of applied electric field, channel depth, background buffer concentration, and species valence to impose variations on zeta potential, effective mobility, and Debye length among other parameters. These measurements were used to validate a continuum theory-based analytical model presented in the first of this two-paper series. Our results confirm the usefulness of continuum theory in predicting electrokinetic transport and electrophoretic separations in nanochannels. Our model leverages independent measurements of zeta potential performed in a microchannel system at electrolyte concentrations of interest. These data yield a zeta potential versus concentration relation that is used as a boundary condition for the nanochannel electrokinetic transport model. The data and model comparisons together show that the effective mobility governing electrophoretic transport of charged species in nanochannels depends not only on ion mobility values but also on the shape of the electric double layer and analyte ion valence. We demonstrate a method we term electrokinetic separation by ion valence, whereby both ion valence and mobility may be determined independently from a comparison of micro- and nanoscale transport measurements.

  View details for DOI 10.1021/ac0508346

  View details for Web of Science ID 000233125400015

  View details for PubMedID 16255574

• Temperature gradient focusing in a microfluidic device 9th Heat Transfer Photogallery Huber, D., Santiago, J. G. ASME-AMER SOC MECHANICAL ENG. 2005: 806–

  View details for Web of Science ID 000231839600010

• Multiple-species model for electrokinetic instability PHYSICS OF FLUIDS Oddy, M. H., Santiago, J. G. 2005; 17 (6)

  View details for DOI 10.1063/1.1931727

  View details for Web of Science ID 000229749500026

• Convective and absolute electrokinetic instability with conductivity gradients JOURNAL OF FLUID MECHANICS Chen, C. H., Lin, H., Lele, S. K., Santiago, J. G. 2005; 524: 263-303

  View details for DOI 10.1017/S0022112004002381

  View details for Web of Science ID 000227786800013

• Microsecond mixer for kinetic studies of protein folding 49th Annual Meeting of the Biophysical-Society Hertzog, D., Michalet, X., Jager, M., Kong, X. X., Santiago, J., Weiss, S., Bakajin, O. CELL PRESS. 2005: 376A–376A

  View details for Web of Science ID 000226378501830

• A Multiple-Species Model for Electrokinetic Instability Physics of Fluids Oddy, M., H., Santiago, J., G. 2005; 6 (17): 064108(1)- 064108(17)
• A high fidelity electrokinetic flow model for the prediction of electrophoregrams in on-chip electrophoresis applications ASME International Mechanical Engineering Congress and Exposition Lin, H., Bharadwaj, R., Santiago, J. G., Mohammadi, B. AMER SOC MECHANICAL ENGINEERS. 2005: 197–199

  View details for Web of Science ID 000243038600025

• Nonlinear dynamics of electrokinetic instabilities ASME International Mechanical Engineering Congress and Exposition Posner, J. D., Santiago, J. G. AMER SOC MECHANICAL ENGINEERS. 2005: 209–212

  View details for Web of Science ID 000243038600027

• Convective electrokinetic flow instabilities in a cross-shaped microchannel 8th International Conference on Miniaturized Systems for Chemistry and Life Sciences Posner, J. D., Lin, H., Santiago, J. G. SPRINGER. 2005: 623–25

  View details for Web of Science ID 000228997000209

• Non-linear stacking effects in microfluidic temperature gradient focusing ASME International Mechanical Engineering Congress and Exposition Huber, D. E., Santiago, J. G. AMER SOC MECHANICAL ENGINEERS. 2005: 341–344

  View details for Web of Science ID 000243038600046

• Electrokinetic flow instabilities in microfluidic systems 21st International Congress of Theoretical and Applied Mechanics Lin, H., Oddy, M. H., Santiago, J. G. SPRINGER. 2005: 343–354

  View details for Web of Science ID 000231853400023

• Active water management for proton exchange membrane fuel cells using an integrated electroosmotic pump ASME International Mechanical Engineering Congress and Exposition Buie, C. R., Posner, J. D., Fabian, T., Cha, S., Prinz, F. B., Eaton, J. K., Santiago, J. G. AMER SOC MECHANICAL ENGINEERS. 2005: 243–247

  View details for Web of Science ID 000243098600027

• High flow rate per power pumping of aqueous solutions and organic solvents with electroosmotic pumps ASME International Mechanical Engineering Congress and Exposition Kim, D., Posner, J. D., Santiago, J. G. AMER SOC MECHANICAL ENGINEERS. 2005: 311–314

  View details for Web of Science ID 000243038600041

• Electrokinetic transport and dispersion in nanoscale channels 8th International Conference on Miniaturized Systems for Chemistry and Life Sciences Pennathur, S., Santiago, J. G. SPRINGER. 2005: 402–4

  View details for Web of Science ID 000228997000134

• Electrokinetic instabilities in thin microchannels PHYSICS OF FLUIDS Storey, B. D., Tilley, B. S., Lin, H., Santiago, J. G. 2005; 17 (1)

  View details for DOI 10.1063/1.1823911

  View details for Web of Science ID 000226358000033

• Microfluidic mixers for UV studies of unlabeled proteins 8th International Conference on Miniaturized Systems for Chemistry and Life Sciences Hertzog, D., Santiago, J., Bakajin, O. SPRINGER. 2005: 539–41

  View details for Web of Science ID 000228997000181

• Femtomole mixer for microsecond kinetic studies of protein folding ANALYTICAL CHEMISTRY Hertzog, D. E., Michalet, X., Jager, M., Kong, X. X., Santiago, J. G., Weiss, S., Bakajin, O. 2004; 76 (24): 7169-7178

  Abstract

  We have developed a microfluidic mixer for studying protein folding and other reactions with a mixing time of 8 mus and sample consumption of femtomoles. This device enables us to access conformational changes under conditions far from equilibrium and at previously inaccessible time scales. In this paper, we discuss the design and optimization of the mixer using modeling of convective diffusion phenomena and a characterization of the mixer performance using microparticle image velocimetry, dye quenching, and Forster resonance energy-transfer (FRET) measurements of single-stranded DNA. We also demonstrate the feasibility of measuring fast protein folding kinetics using FRET with acyl-CoA binding protein.

  View details for DOI 10.1021/ac048661s

  View details for Web of Science ID 000225781700003

  View details for PubMedID 15595857

  View details for PubMedCentralID PMC1413504

• Nucleation and growth of vapor bubbles in a heated silicon microchannel JOURNAL OF HEAT TRANSFER-TRANSACTIONS OF THE ASME Wang, E. N., Devasenathipathy, S., Santiago, J. G., Goodson, K. E., Kenny, T. W. 2004; 126 (4): 497-497

  View details for Web of Science ID 000223989100004

• A laser induced cavitation pump JOURNAL OF MICROMECHANICS AND MICROENGINEERING Wang, G. R., Santiago, J. G., Mungal, M. G., Young, B., Papademetriou, S. 2004; 14 (7): 1037-1046

  View details for DOI 10.1088/0960-1317/14/7/026

  View details for Web of Science ID 000223090000026

• A review of micropumps JOURNAL OF MICROMECHANICS AND MICROENGINEERING Laser, D. J., Santiago, J. G. 2004; 14 (6): R35-R64

  View details for Web of Science ID 000222318500001

• Instability of electrokinetic microchannel flows with conductivity gradients PHYSICS OF FLUIDS Lin, H., Storey, B. D., Oddy, M. H., Chen, C. H., Santiago, J. G. 2004; 16 (6): 1922-1935

  View details for DOI 10.1063/1.1710898

  View details for Web of Science ID 000221345600007

• High-pressure electroosmotic pumps based on porous polymer monoliths SENSORS AND ACTUATORS B-CHEMICAL Tripp, J. A., Svec, F., Frechet, J. M., Zeng, S. L., Mikkelsen, J. C., Santiago, J. G. 2004; 99 (1): 66-73

  View details for DOI 10.1016/j.snb.2003.10.031

  View details for Web of Science ID 000221398700011

• Computational study of band-crossing reactions JOURNAL OF MICROELECTROMECHANICAL SYSTEMS Matta, A., Knio, O. M., Ghanem, R. G., Chen, C. H., Santiago, J. G., Debusschere, B., Najm, H. N. 2004; 13 (2): 310-322

  View details for DOI 10.1109/JMEMS.2004.825315

  View details for Web of Science ID 000220759300018

• Optimized field amplified sample stacking for on-chip capillary electrophoresis. 227th National Meeting of the American-Chemical Society Bharadwaj, R., Jung, Y. S., Santiago, J. G. AMER CHEMICAL SOC. 2004: U116–U116

  View details for Web of Science ID 000223655600535

• A method for determining electrophoretic and electroosmotic mobilities using AC and DC electric field particle displacements JOURNAL OF COLLOID AND INTERFACE SCIENCE Oddy, M. H., Santiago, J. G. 2004; 269 (1): 192-204

  Abstract

  We have developed a method for measuring the electrophoretic mobility of submicrometer, fluorescently labeled particles and the electroosmotic mobility of a microchannel. We derive explicit expressions for the unknown electrophoretic and the electroosmotic mobilities as a function of particle displacements resulting from alternating current (AC) and direct current (DC) applied electric fields. Images of particle displacements are captured using an epifluorescent microscope and a CCD camera. A custom image-processing code was developed to determine image streak lengths associated with AC measurements, and a custom particle tracking velocimetry (PTV) code was devised to determine DC particle displacements. Statistical analysis was applied to relate mobility estimates to measured particle displacement distributions.

  View details for DOI 10.1016/S0021-9797(03)00601-5

  View details for Web of Science ID 000187574400026

  View details for PubMedID 14651913

• Electrokinetic Flow Diagnostics Micro- and Nano-Scale Diagnostic Techniques Devasenathipathy, S., Santiago, J., G. edited by Breuer, K. New York, Springer Verlag. 2004: 1
• Porous glass electroosmotic pumps: design and experiments JOURNAL OF COLLOID AND INTERFACE SCIENCE Yao, S. H., Hertzog, D. E., Zeng, S. L., Mikkelsen, J. C., Santiago, J. G. 2003; 268 (1): 143-153

  Abstract

  An analytical model for electroosmotic flow rate, total pump current, and thermodynamic efficiency reported in a previous paper has been applied as a design guideline to fabricate porous-structure EO pumps. We have fabricated sintered-glass EO pumps that provide maximum flow rates and pressure capacities of 33 ml/min and 1.3 atm, respectively, at applied potential 100 V. These pumps are designed to be integrated with two-phase microchannel heat exchangers with load capacities of order 100 W and greater. Experiments were conducted with pumps of various geometries and using a relevant, practical range of working electrolyte ionic concentration. Characterization of the pumping performance are discussed in the terms of porosity, tortuosity, pore size, and the dependence of zeta potential on bulk ion density of the working solution. The effects of pressure and flow rate on pump current and thermodynamic efficiency are analyzed and compared to the model prediction. In particular, we explore the important tradeoff between increasing flow rate capacity and obtaining adequate thermodynamic efficiency. This research aims to demonstrate the performance of EOF pump systems and to investigate optimal and practical pump designs. We also present a gas recombination device that makes possible the implementation of this pumping technology into a closed-flow loop where electrolytic gases are converted into water and reclaimed by the system.

  View details for DOI 10.1016/S0021-9797(03)00730-6

  View details for Web of Science ID 000186639800019

  View details for PubMedID 14611783

• Porous glass electroosmotic pumps: theory JOURNAL OF COLLOID AND INTERFACE SCIENCE Yao, S. H., Santiago, J. G. 2003; 268 (1): 133-142

  Abstract

  This paper presents an analytical study of electroosmotic (EO) pumps with porous pumping structures. We have developed an analytical model to solve for electroosmotic flow rate, pump current, and thermodynamic efficiency as a function of pump pressure load for porous-structure EO pumps. The model uses a symmetric electrolyte approximation valid for the high-zeta-potential regime and numerically solves the Poisson-Boltzmann equation for charge distribution in the idealized pore geometry. Generalized scaling of pumping performance is discussed in the context of a parameterization that includes porosity, tortuosity, pore size, bulk ionic density, and the nonuniform conductivity distribution over charge layers. The model also incorporates an approximate ionic-strength-dependent zeta potential formulation.

  View details for DOI 10.1016/S0021-9797(03)00731-8

  View details for Web of Science ID 000186639800018

  View details for PubMedID 14611782

• Thousandfold signal increase using field-amplified sample stacking for on-chip electrophoresis 19th Annual Conference of the American-Electrophoresis-Society Jung, B., Bharadwaj, R., Santiago, J. G. WILEY-V C H VERLAG GMBH. 2003: 3476–83

  Abstract

  Field-amplified sample stacking (FASS) leverages conductivity gradients between a volume of injected sample and the background buffer to increase sample concentration. A major challenge in applying FASS to on-chip assays is the initial setup of high-conductivity gradient boundaries in the region of the injected sample volume. We have designed, fabricated, and characterized a novel FASS-capillary electrophoresis (CE) chip design that uses a photoinitiated porous polymer structure to facilitate sample injection and flow control for high-gradient FASS. This polymer structure provides a region of high flow resistance that allows the electromigration of sample ions. We have demonstrated an electropherogram signal increase by a factor of 1100 in electrophoretic separations of fluorescein and Bodipy with, respectively, 2 microM and 1 microM initial concentrations.

  View details for DOI 10.1002/elps.200305611

  View details for Web of Science ID 000186457900020

  View details for PubMedID 14595694

• Particle imaging techniques for microfabricated fluidic systems EXPERIMENTS IN FLUIDS Devasenathipathy, S., Santiago, J. G., Wereley, S. T., Meinhart, C. D., Takehara, K. 2003; 34 (4): 504-514

  View details for DOI 10.1007/S00348-003-0588-y

  View details for Web of Science ID 000183198800007

• On-chip coupling of isoelectric focusing and free solution electrophoresis for multidimensional separations ANALYTICAL CHEMISTRY Herr, A. E., Molho, J. I., Drouvalakis, K. A., Mikkelsen, J. C., Utz, P. J., Santiago, J. G., Kenny, T. W. 2003; 75 (5): 1180-1187

  Abstract

  We have developed an acrylic microfluidic device that sequentially couples liquid-phase isoelectric focusing (IEF) and free solution capillary electrophoresis (CE). Rapid separation (<1 min) and preconcentration (73x) of species were achieved in the initial IEF dimension. Using full-field fluorescence imaging, we observed nondispersive mobilization velocities on the order of 20 microm/s during characterization of the IEF step. This transport behavior allowed controlled electrokinetic mobilization of focused sample bands to a channel junction, where voltage switching was used to repeatedly inject effluent from the IEF dimension into an ampholyte-based CE separation. This second dimension was capable of analyzing all fluid volumes of interest from the IEF dimension, as IEF was 'parked' during each CE analysis and refocused prior to additional CE analyses. Investigation of each dimension of the integrated system showed time-dependent species displacement and band-broadening behavior consistent with IEF and CE, respectively. The peak capacity of the 2D system was approximately 1300. A comprehensive 2D analysis of a fluid volume spanning 15% of the total IEF channel length was completed in less than 5 min.

  View details for DOI 10.1021/ac026239a

  View details for Web of Science ID 000181259300027

  View details for PubMedID 12641239

• Microfluidic flow simulation: Stacking one-dimensional study HOUILLE BLANCHE-REVUE INTERNATIONALE DE L EAU Alexis-Alexandre, G., Mohammadi, B., Santiago, J. G., Bharadwaj, R. 2003: 18-23

  View details for Web of Science ID 000187073700001

• Thermodynamic efficiency of porous glass electroosmotic pumps International Electronic Packaging Technical Conference Yao, S. H., Zeng, S. L., Santiago, J. G. AMER SOC MECHANICAL ENGINEERS. 2003: 383–390

  View details for Web of Science ID 000222624900058

• Particle Tracking Techniques for Microfabricated Fluidic Systems Experiments in Fluids Devasenathipathy, S., Santiago, J., G., Wereley, S., T., Meinhart, C., D. 2003; 4 (34): 504-513
• Incomplete Sensitivities in Design and Control of Fluidic Channels Computer Assisted Mechanics and Engineering Sciences Mohammadi, B., Santiago, J., G. 2003; 10: 201-210
• Numerical simulation of field amplified sample stacking in microfluidic system Nanotechnology Conference and Trade Show (Nanotech 2003) Feng, J. J., Krishnamoorthy, S., Sundaram, S., Bharadwaj, R., Santiago, J. G. COMPUTATIONAL PUBLICATIONS. 2003: 234–237

  View details for Web of Science ID 000223045100062

• Experimental study on two-phase heat transfer in microchannel heat sinks with hotspots 19th Annual IEEE Semiconductor Thermal Measurement and Management Symposium Cho, E. S., Koo, J. M., Jiang, L., Prasher, R. S., Kim, M. S., Santiago, J. G., Kenny, T. W., Goodson, K. E. IEEE. 2003: 242–246

  View details for Web of Science ID 000182024200038

• Silicon electroosmotic micropumps for integrated circuit thermal management 12th International Conference on Solid-State Sensors, Actuators and Microsystems (TRANSDUCERS 03) Laser, D. J., Myers, A. M., Yao, S. H., BELL, K. F., Goodson, K. E., Santiago, J. G., Kenny, T. W. IEEE. 2003: 151–154

  View details for Web of Science ID 000184567300038

• Incomplete sensitivities for the design of minimal dispersion fluidic channels COMPUTER METHODS IN APPLIED MECHANICS AND ENGINEERING Mohammadi, B., Molho, J. I., Santiago, J. G. 2003; 192 (37-38): 4131-4145

  View details for DOI 10.1016/S0045-7825(03)00380-3

  View details for Web of Science ID 000185304100005

• A planar electroosmotic micropump JOURNAL OF MICROELECTROMECHANICAL SYSTEMS Chen, C. H., Santiago, J. G. 2002; 11 (6): 672-683

  View details for DOI 10.1109/JMEMS.2002.805055

  View details for Web of Science ID 000179740900007

• Photobleached-fluorescence imaging of microflows EXPERIMENTS IN FLUIDS Mosier, B. P., Molho, J. I., Santiago, J. G. 2002; 33 (4): 545-554

  View details for DOI 10.1007/s00348-002-0486-8

  View details for Web of Science ID 000179003400007

• Closed-loop electroosmotic microchannel cooling system for VLSI circuits 17th Annual IEEE Semiconductor Thermal Measurement and Management Symposium Jiang, L. N., Mikkelsen, J., Koo, J. M., Huber, D., Yao, S. H., Zhang, L., Zhou, P., Maveety, J. G., Prasher, R., Santiago, J. G., Kenny, T. W., Goodson, K. E. IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. 2002: 347–55

  View details for DOI 10.1109/TCAPT.2002.800599

  View details for Web of Science ID 000180333200003

• Design and optimization of on-chip capillary electrophoresis ELECTROPHORESIS Bharadwaj, R., Santiago, J. G., Mohammadi, B. 2002; 23 (16): 2729-2744

  Abstract

  We present a systematic, experimentally validated method of designing electrokinetic injections for on-chip capillary electrophoresis applications. This method can be used to predict point-wise and charge-coupled device (CCD)-imaged electropherograms using estimates of species mobilities, diffusivities and initial sample plug parameters. A simple Taylor dispersion model is used to characterize electrophoretic separations in terms of resolution and signal-to-noise ratio (SNR). Detection convolutions using Gaussian and Boxcar detector response functions are used to relate optimal conditions for resolution and signal as a function of relevant system parameters including electroosmotic mobility, sample injection length, detector length scale, and the length-to-detector. Analytical solutions show a tradeoff between signal-to-noise ratio and resolution with respect to dimensionless injection width and length to the detector. In contrast, there is no tradeoff with respect to the Peclet number as increases in Peclet number favor both SNR and separation solution (R). We validate our model with quantitative epifluorescence visualizations of electrophoretic separation experiments in a simple cross channel microchip. For the pure advection regime of dispersion, we use numerical simulations of the transient convective diffusion processes associated with electrokinetics together with an optimization algorithm to design a voltage control scheme which produces an injection plug that has minimal advective dispersion. We also validate this optimal injection scheme using fluorescence visualizations. These validations show that optimized voltage scheme produces injections with a standard deviation less than one-fifth of the width of the microchannel.

  View details for Web of Science ID 000177888500022

  View details for PubMedID 12210178

• Particle tracking techniques for electrokinetic microchannel flows ANALYTICAL CHEMISTRY Devasenathipathy, S., Santiago, J. G., Takehara, K. 2002; 74 (15): 3704-3713

  Abstract

  We have applied particle tracking techniques to obtain spatially resolved velocity measurements in electrokinetic flow devices. Both micrometer-resolution particle image velocimetry (micro-PMV) and particle tracking velocimetry (PTV) techniques have been used to quantify and study flow phenomena in electrokinetic systems applicable to microfluidic bioanalytical devices. To make the flow measurements quantitative, we performed a series of seed particle calibration experiments. First, we measure the electroosmotic wall mobility of a borosilicate rectangular capillary (40 by 400 microm) using current monitoring. In addition to this wall mobility characterization, we apply PTV to determine the electrophoretic mobilities of more than 1,000 fluorescent microsphere particles in aqueous buffer solutions. Particles from this calibrated particle/ buffer mixture are then introduced into two electrokinetic flow systems for particle tracking flow experiments. In these experiments, we use micro-PIV, together with an electric field prediction, to obtain electroosmotic flow bulk fluid velocity measurements. The first example flow system is a microchannel intersection where we demonstrate a detailed documentation of the similitude between the electrical fields and the velocity fields in an electrokinetic system with uniform zeta potential, zeta. In the second system, we apply micro-PIV to a microchannel system with nonuniform zeta. The latter experiment provides a simultaneous measurement of two distinct wall mobilities within the microchannel.

  View details for DOI 10.1021/ac011243s

  View details for Web of Science ID 000177239200027

  View details for PubMedID 12175157

• Electroosmotic flow pumps with polymer frits SENSORS AND ACTUATORS B-CHEMICAL Zeng, S. L., Chen, C. H., Santiago, J. G., Chen, J. R., Zare, R. N., Tripp, J. A., Svec, F., Frechet, J. M. 2002; 82 (2-3): 209-212

  View details for Web of Science ID 000174483000010

• Measurements and modeling of two-phase flow in microchannels with nearly constant heat flux boundary conditions JOURNAL OF MICROELECTROMECHANICAL SYSTEMS Zhang, L., Koo, J. M., Jiang, L., Asheghi, M., Goodson, K. E., Santiago, J. G., Kenny, T. W. 2002; 11 (1): 12-19

  View details for Web of Science ID 000173751000002

• Bleached-Fluorescence Imaging of Microflows Experiments in Fluids Mosier, B., P., Molho, J., I., Santiago, J., G. 2002; 4 (33): 545-554
• Enhanced nucleate boiling in microchannels 15th IEEE International Conference on Micro Electro Mechanical Systems (MEMS 2002) Zhang, L., Wang, E. N., Koo, J. M., Jiang, L., Goodson, K. E., Santiago, J. G., Kenny, T. W. IEEE. 2002: 89–92

  View details for Web of Science ID 000174908700022

• Transient and sub-atmospheric performance of a closed-loop electroosmotic microchannel cooling system THERMES 2002 International Conference Jiang, L., Mikkelsen, J., Koo, J. M., Zhang, L., Huber, D., Yao, S., Bari, A., Zhou, P., Santiago, J., Kenny, T., Goodson, K. E., Maveety, J., Prasher, R., Benning, S. MILLPRESS SCIENCE PUBLISHERS. 2002: 133–139

  View details for Web of Science ID 000181956000016

• Electroosmotic microchannel cooling system for microprocessors Electronics Cooling Goodson, K., E., Santiago, J., G., Kenny, T., Jiang, L., Zeng, S., Koo, J., M. 2002; 8: 46-47
• Electrokinetic instability micromixing ANALYTICAL CHEMISTRY Oddy, M. H., Santiago, J. G., Mikkelsen, J. C. 2001; 73 (24): 5822-5832

  Abstract

  We have developed an electrokinetic process to rapidly stir micro- and nanoliter volume solutions for microfluidic bioanalytical applications. We rapidly stir microflow streams by initiating a flow instability, which we have observed in sinusoidally oscillating, electroosmotic channel flows. As the effect occurs within an oscillating electroosmotic flow, we refer to it here as an electrokinetic instability (EKI). The rapid stretching and folding of material lines associated with this instability can be used to stir fluid streams with Reynolds numbers of order unity, based on channel depth and rms electroosmotic velocity. This paper presents a preliminary description of the EKI and the design and fabrication of two micromixing devices capable of rapidly stirring two fluid streams using this flow phenomenon. A high-resolution CCD camera is used to record the stirring and diffusion of fluorescein from an initially unmixed configuration. Integration of fluorescence intensity over measurement volumes (voxels) provides a measure of the degree to which two streams are mixed to within the length scales of the voxels. Ensemble-averaged probability density functions and power spectra of the instantaneous spatial intensity profiles are used to quantify the mixing processes. Two-dimensional spectral bandwidths of the mixing images are initially anisotropic for the unmixed configuration, broaden as the stirring associated with the EKI rapidly stretches and folds material lines (adding high spatial frequencies to the concentration field), and then narrow to a relatively isotropic spectrum at the well-mixed conditions.

  View details for DOI 10.1021/ac0155411

  View details for Web of Science ID 000172884900007

  View details for PubMedID 11791550

• Fabrication and characterization of electroosmotic micropumps SENSORS AND ACTUATORS B-CHEMICAL Zeng, S. L., Chen, C. H., Mikkelsen, J. C., Santiago, J. G. 2001; 79 (2-3): 107-114

  View details for Web of Science ID 000171679600004

• Electroosmotic flows in microchannels with finite inertial and pressure forces ANALYTICAL CHEMISTRY Santiago, J. G. 2001; 73 (10): 2353-2365

  Abstract

  Emerging microfluidic systems have spurred an interest in the study of electrokinetic flow phenomena in complex geometries and a variety of flow conditions. This paper presents an analysis of the effects of fluid inertia and pressure on the velocity and vorticity field of electroosmotic flows. In typical on-chip electrokinetics applications, the flow field can be separated into an inner flow region dominated by viscous and electrostatic forces and an outer flow region dominated by inertial and pressure forces. These two regions are separated by a slip velocity condition determined by the Helmholtz-Smoulochowski equation. The validity of this assumption is investigated by analyzing the velocity field in a pressure-driven, two-dimensional flow channel with an impulsively started electric field. The regime for which the inner/outer flow model is valid is described in terms of nondimensional parameters derived from this example problem. Next, the inertial forces, surface conditions, and pressure-gradient conditions for a full-field similarity between the electric and velocity fields in electroosmotic flows are discussed. A sufficient set of conditions for this similarity to hold in arbitrarily shaped, insulating wall microchannels is the following: uniform surface charge, low Reynolds number, low Reynolds and Strouhal number product, uniform fluid properties, and zero pressure differences between inlets and outlets. Last, simple relations describing the generation of vorticity in electroosmotic flow are derived using a wall-local, streamline coordinate system.

  View details for Web of Science ID 000168708200030

  View details for PubMedID 11393863

• Simulation and design of extraction and separation fluidic devices ESAIM-MATHEMATICAL MODELLING AND NUMERICAL ANALYSIS-MODELISATION MATHEMATIQUE ET ANALYSE NUMERIQUE Mohammadi, B., Santiago, J. G. 2001; 35 (3): 513-523

  View details for Web of Science ID 000169197700006

• Optimization of turn geometries for microchip electrophoresis ANALYTICAL CHEMISTRY Molho, J. I., Herr, A. E., Mosier, B. P., Santiago, J. G., Kenny, T. W., Brennen, R. A., Gordon, G. B., Mohammadi, B. 2001; 73 (6): 1350-1360

  View details for Web of Science ID 000167546400042

• Two-phase microchannel heat sinks for an electrokinetic VLSI chip cooling system 17th Annual IEEE Semiconductor Thermal Measurement and Management Symposium Jiang, L. N., Koo, J. M., Zeng, S. L., Mikkelsen, J. C., Zhang, L., Zhou, P., Santiago, J. G., Kenny, T. W., Goodson, K. E., Maveety, J. G., Tran, Q. A. IEEE. 2001: 153–157

  View details for Web of Science ID 000168411800021

• Liquid Flows in Microchannels CRC Handbook of MEMS Sharp, K., V., Adrian, R., J., Santiago, J., G., Molho, J., I. edited by Gad-el-Hak, M. CRC Press, New York. 2001: 6–1 to 6–38
• Modeling of two-phase microchannel heat sinks for VLSI chips 14th IEEE International Conference on Micro Electro Mechanical Systems (MEMS 2001) Koo, J. M., Jiang, L. N., Zhang, L., Zhou, P., Banerjee, S. S., Kenny, T. W., Santiago, J. G., Goodson, K. E. IEEE. 2001: 422–426

  View details for Web of Science ID 000166833600102

• A micromachined silicon low-voltage parallel-plate electrokinetic pump 11th International Conference on Solid-State Sensors and Actuators Laser, D., Yao, S. H., Chen, C. H., Mikkelsen, J., Goodson, K., Santiago, J., Kenny, T. SPRINGER-VERLAG BERLIN. 2001: 920–923

  View details for Web of Science ID 000172547800211

• Passive mixing in a three-dimensional serpentine microchannel JOURNAL OF MICROELECTROMECHANICAL SYSTEMS Liu, R. H., Stremler, M. A., Sharp, K. V., Olsen, M. G., Santiago, J. G., Adrian, R. J., Aref, H., Beebe, D. J. 2000; 9 (2): 190-197

  View details for Web of Science ID 000087842600004

• A PIV algorithm for estimating time-averaged velocity fields JOURNAL OF FLUIDS ENGINEERING-TRANSACTIONS OF THE ASME Meinhart, C. D., Wereley, S. T., Santiago, J. G. 2000; 122 (2): 285-289

  View details for Web of Science ID 000167106000008

• Particle-image velocimetry measurements in electrokinetic flow. Devasenathipathy, S., Santiago, J. G. AMER CHEMICAL SOC. 2000: U578–U578

  View details for Web of Science ID 000087246103122

• Electroosmotic capillary flow with nonuniform zeta potential ANALYTICAL CHEMISTRY Herr, A. E., Molho, J. I., Santiago, J. G., Mungal, M. G., Kenny, T. W., Garguilo, M. G. 2000; 72 (5): 1053-1057

  Abstract

  The present work is an analytical and experimental study of electroosmotic flow (EOF) in cylindrical capillaries with nonuniform wall surface charge (zeta-potential) distributions. In particular, this study investigates perturbations of electroosmotic flow in open capillaries that are due to induced pressure gradients resulting from axial variations in the wall zeta-potential. The experimental inquiry focuses on electroosmotic flow under a uniform applied field in capillaries with an EOF-suppressing polymer adsorbed onto various fractions of the total capillary length. This fractional EOF suppression was achieved by coupling capillaries with substantially different zeta-potentials. The resulting flow fields were imaged with a nonintrusive, caged-fluorescence imaging technique. Simple analytical models for the velocity field and rate of sample dispersion in capillaries with axial zeta-potential variations are presented. The resulting induced pressure gradients and the associated band-broadening effects are of particular importance to the performance of chemical and biochemical analysis systems such as capillary electrokinetic chromatography and capillary zone electrophoresis.

  View details for Web of Science ID 000085605100043

• Electroosmotic capillary flow with nonuniform zeta potential Analytical chemistry Herr, A. E., Molho, J. I., Santiago, J. G., Mungal, M. G., Kenny, T. W., Garguilo, M. G. 2000; 72 (5): 1053-7

  Abstract

  The present work is an analytical and experimental study of electroosmotic flow (EOF) in cylindrical capillaries with nonuniform wall surface charge (zeta-potential) distributions. In particular, this study investigates perturbations of electroosmotic flow in open capillaries that are due to induced pressure gradients resulting from axial variations in the wall zeta-potential. The experimental inquiry focuses on electroosmotic flow under a uniform applied field in capillaries with an EOF-suppressing polymer adsorbed onto various fractions of the total capillary length. This fractional EOF suppression was achieved by coupling capillaries with substantially different zeta-potentials. The resulting flow fields were imaged with a nonintrusive, caged-fluorescence imaging technique. Simple analytical models for the velocity field and rate of sample dispersion in capillaries with axial zeta-potential variations are presented. The resulting induced pressure gradients and the associated band-broadening effects are of particular importance to the performance of chemical and biochemical analysis systems such as capillary electrokinetic chromatography and capillary zone electrophoresis.

  View details for PubMedID 10739211

• Mixing of a sonic transverse jet injected into a supersonic flow AIAA JOURNAL VanLerberghe, W. M., Santiago, J. G., Dutton, J. C., Lucht, R. P. 2000; 38 (3): 470-479

  View details for Web of Science ID 000085771400012

• Micron-resolution velocimetry techniques 9th International Symposium on Applications of Laser Techniques to Fluid Mechanics Meinhart, C. D., Wereley, S. T., Santiago, J. G. SPRINGER-VERLAG BERLIN. 2000: 57–70

  View details for Web of Science ID 000087171900004

• Designing corner compensation for electrophoresis in compact geometries 4th International Symposium on Micro Total Analysis Systems ((mu)TAS 2000) Molho, J. I., Herr, A. E., Mosier, B. P., Santiago, J. G., Kenny, T. W., Brennen, R. A., Gordon, G. B. SPRINGER. 2000: 287–290

  View details for Web of Science ID 000167335400065

• Fabrication and characterization of electrokinetic micro pumps 7th Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems Zeng, S. L., Chen, C. H., Mikkelsen, J. C., Santiago, J. G. IEEE. 2000: 31–36

  View details for DOI 10.1109/ITHERM.2000.866167

  View details for Web of Science ID 000089141400006

• Experimental investigation of flow transition in microchannels using micron-resolution particle image velocimetry 7th Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems Zeighami, R., LASER, D., Zhou, P., Asheghi, M., Devasenathipathy, S., Kenny, T., Santiago, J., Goodson, K. IEEE. 2000: 148–153

  View details for Web of Science ID 000089141400023

• Miniaturized capillary isoelectric focusing (cIEF): Towards a portable high-speed separation method 4th International Symposium on Micro Total Analysis Systems ((mu)TAS 2000) Herr, A. E., Molho, J. I., Santiago, J. G., Kenny, T. W., Borkholder, D. A., Kintz, G. J., Belgrader, P., Northrup, M. A. SPRINGER. 2000: 367–370

  View details for Web of Science ID 000167335400085

• PIV measurements of a microchannel flow EXPERIMENTS IN FLUIDS Meinhart, C. D., Wereley, S. T., Santiago, J. G. 1999; 27 (5): 414-419

  View details for Web of Science ID 000083322200005

• Diagnostic Techniques for Microfluidics Research Developments in Laser Techniques and Applications to Fluid Mechanics Meinhart, C., D., Wereley, S., T., Santiago, J., G. edited by Adrian, R., J., Durao, D., F.G., Durst, F. Springer-Verlag, Berlin. 1999: 1
• A particle image velocimetry system for microfluidics EXPERIMENTS IN FLUIDS Santiago, J. G., Wereley, S. T., Meinhart, C. D., Beebe, D. J., Adrian, R. J. 1998; 25 (4): 316-319

  View details for Web of Science ID 000076240500004

• Micro-resolution particle image velocimetry Conference on Microfabricated and Nanofabricated Structures and Devices for Biomedical Environmental Applications Wereley, S. T., Santiago, J. G., Chiu, R., Meinhart, C. D., Adrian, R. J. SPIE - INT SOC OPTICAL ENGINEERING. 1998: 122–133

  View details for Web of Science ID 000073452100015

• Crossflow vortices of a jet injected into a supersonic crossflow AIAA JOURNAL Santiago, J. G., Dutton, J. C. 1997; 35 (5): 915-917

  View details for Web of Science ID A1997WY03000024

• Velocity measurements of a jet injected into a supersonic crossflow JOURNAL OF PROPULSION AND POWER Santiago, J. G., Dutton, J. C. 1997; 13 (2): 264-273

  View details for Web of Science ID A1997WN15600015

• Crossflow Vortices of a Jet Injected into a Supersonic Crossflow AIAA Journal Santiago, J., G., Dutton, J., C. 1997; 5 (35): 915-917
• Velocity Measurements of a Jet Injected into a Supersonic Crossflow Journal of Propulsion and Power Santiago, J., G., Dutton, J., C. 1997; 2 (13): 264-273
• An application of satellite-derived sea surface temperature data to the slipjack (Katsawonus pelamis Linnaeus, 1758) and albacore tuna (Thunnus alalunga Bonaterre, 1788) fisheries in the north-east Atlantic INTERNATIONAL JOURNAL OF REMOTE SENSING Ramos, A. G., Santiago, J., Sangra, P., Canton, M. 1996; 17 (4): 749-759

  View details for Web of Science ID A1996UC01200008