Instructor, Medicine - Cardiovascular Medicine
Honors & Awards
Siebel Scholar, Stanford (2011-2012)
New York Stem Cell Foundation Meeting-1st Place Award, Stanford (2011)
Gordon Conference Scholarship, Stanford (2011)
Keystone Conference Postdoc Competition Finalist, Stanford (2011)
Bio-X Travel Award, Stanford (2010)
Advanced Residency Program at Stanford (ARTS), Stanford (2007-2011)
Internationall Endovascular Fellows' Research Award, 1st Place, Stanford (2006)
Ethicon Endosurgery Fellowship, Stanford (2005-2006)
Dean's Fellowship, Stanford (2005)
Franklyn Ellenbogen Prize in Hematology/Oncology, Cornell (2002)
US-European Medical Education Exchange (US-EUMEE) Fellowship, Cornell (2002)
Max Kade Foundation Fellowship, Cornell (2002)
Pi Tau Sigma Engineering Honor Society, University of Texas (1991)
PhD, Stanford University, Bioengineering (2012)
Resident, Stanford University Medical Center, Surgery (2004)
Intern, Stanford University Medical Center, Surgery (2003)
MD, Cornell University-Weill Medical College, Medicine (2002)
BS, University of Texas at Austin, Mechanical Engineering (1992)
Current Research and Scholarly Interests
Bioengineering, biophysical control of cardiovascular development, pluripotent stem cell biology, optogenetics, electrophysiology, cell mechanics, directed cellular evolution, multiscale engineering, microfluidics, computational biology
Chemically defined generation of human cardiomyocytes.
2014; 11 (8): 855-860
Existing methods for human induced pluripotent stem cell (hiPSC) cardiac differentiation are efficient but require complex, undefined medium constituents that hinder further elucidation of the molecular mechanisms of cardiomyogenesis. Using hiPSCs derived under chemically defined conditions on synthetic matrices, we systematically developed an optimized cardiac differentiation strategy, using a chemically defined medium consisting of just three components: the basal medium RPMI 1640, L-ascorbic acid 2-phosphate and rice-derived recombinant human albumin. Along with small molecule-based induction of differentiation, this protocol produced contractile sheets of up to 95% TNNT2(+) cardiomyocytes at a yield of up to 100 cardiomyocytes for every input pluripotent cell and was effective in 11 hiPSC lines tested. This chemically defined platform for cardiac specification of hiPSCs will allow the elucidation of cardiomyocyte macromolecular and metabolic requirements and will provide a minimal system for the study of maturation and subtype specification.
View details for DOI 10.1038/nmeth.2999
View details for PubMedID 24930130
- Stem cell isolation: Differential stickiness. Nature materials 2013; 12 (6): 474-476
Prospective isolation of human embryonic stem cell-derived cardiovascular progenitors that integrate into human fetal heart tissue
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
2013; 110 (9): 3405-3410
A goal of regenerative medicine is to identify cardiovascular progenitors from human ES cells (hESCs) that can functionally integrate into the human heart. Previous studies to evaluate the developmental potential of candidate hESC-derived progenitors have delivered these cells into murine and porcine cardiac tissue, with inconclusive evidence regarding the capacity of these human cells to physiologically engraft in xenotransplantation assays. Further, the potential of hESC-derived cardiovascular lineage cells to functionally couple to human myocardium remains untested and unknown. Here, we have prospectively identified a population of hESC-derived ROR2(+)/CD13(+)/KDR(+)/PDGFR?(+) cells that give rise to cardiomyocytes, endothelial cells, and vascular smooth muscle cells in vitro at a clonal level. We observed rare clusters of ROR2(+) cells and diffuse expression of KDR and PDGFR? in first-trimester human fetal hearts. We then developed an in vivo transplantation model by transplanting second-trimester human fetal heart tissues s.c. into the ear pinna of a SCID mouse. ROR2(+)/CD13(+)/KDR(+)/PDGFR?(+) cells were delivered into these functioning fetal heart tissues: in contrast to traditional murine heart models for cell transplantation, we show structural and functional integration of hESC-derived cardiovascular progenitors into human heart.
View details for DOI 10.1073/pnas.1220832110
View details for Web of Science ID 000315841900046
View details for PubMedID 23391730
Abnormal Calcium Handling Properties Underlie Familial Hypertrophic Cardiomyopathy Pathology in Patient-Specific Induced Pluripotent Stem Cells
CELL STEM CELL
2013; 12 (1): 101-113
Familial hypertrophic cardiomyopathy (HCM) is a prevalent hereditary cardiac disorder linked to arrhythmia and sudden cardiac death. While the causes of HCM have been identified as genetic mutations in the cardiac sarcomere, the pathways by which sarcomeric mutations engender myocyte hypertrophy and electrophysiological abnormalities are not understood. To elucidate the mechanisms underlying HCM development, we generated patient-specific induced pluripotent stem cell cardiomyocytes (iPSC-CMs) from a ten-member family cohort carrying a hereditary HCM missense mutation (Arg663His) in the MYH7 gene. Diseased iPSC-CMs recapitulated numerous aspects of the HCM phenotype including cellular enlargement and contractile arrhythmia at the single-cell level. Calcium (Ca(2+)) imaging indicated dysregulation of Ca(2+) cycling and elevation in intracellular Ca(2+) ([Ca(2+)](i)) are central mechanisms for disease pathogenesis. Pharmacological restoration of Ca(2+) homeostasis prevented development of hypertrophy and electrophysiological irregularities. We anticipate that these findings will help elucidate the mechanisms underlying HCM development and identify novel therapies for the disease.
View details for DOI 10.1016/j.stem.2012.10.010
View details for Web of Science ID 000313839500014
- Robust pluripotent stem cell expansion and cardiomyocyte differentiation via geometric patterning INTEGRATIVE BIOLOGY 2013; 5 (12): 1495-1506
Multiscale Computational Models for Optogenetic Control of Cardiac Function
2011; 101 (6): 1326-1334
The ability to stimulate mammalian cells with light has significantly changed our understanding of electrically excitable tissues in health and disease, paving the way toward various novel therapeutic applications. Here, we demonstrate the potential of optogenetic control in cardiac cells using a hybrid experimental/computational technique. Experimentally, we introduced channelrhodopsin-2 into undifferentiated human embryonic stem cells via a lentiviral vector, and sorted and expanded the genetically engineered cells. Via directed differentiation, we created channelrhodopsin-expressing cardiomyocytes, which we subjected to optical stimulation. To quantify the impact of photostimulation, we assessed electrical, biochemical, and mechanical signals using patch-clamping, multielectrode array recordings, and video microscopy. Computationally, we introduced channelrhodopsin-2 into a classic autorhythmic cardiac cell model via an additional photocurrent governed by a light-sensitive gating variable. Upon optical stimulation, the channel opens and allows sodium ions to enter the cell, inducing a fast upstroke of the transmembrane potential. We calibrated the channelrhodopsin-expressing cell model using single action potential readings for different photostimulation amplitudes, pulse widths, and frequencies. To illustrate the potential of the proposed approach, we virtually injected channelrhodopsin-expressing cells into different locations of a human heart, and explored its activation sequences upon optical stimulation. Our experimentally calibrated computational toolbox allows us to virtually probe landscapes of process parameters, and identify optimal photostimulation sequences toward pacing hearts with light.
View details for DOI 10.1016/j.bpj.2011.08.004
View details for Web of Science ID 000295197300006
View details for PubMedID 21943413
Effect of human donor cell source on differentiation and function of cardiac induced pluripotent stem cells.
Journal of the American College of Cardiology
2014; 64 (5): 436-448
Human-induced pluripotent stem cells (iPSCs) are a potentially unlimited source for generation of cardiomyocytes (iPSC-CMs). However, current protocols for iPSC-CM derivation face several challenges, including variability in somatic cell sources and inconsistencies in cardiac differentiation efficiency.This study aimed to assess the effect of epigenetic memory on differentiation and function of iPSC-CMs generated from somatic cell sources of cardiac versus noncardiac origins.Cardiac progenitor cells (CPCs) and skin fibroblasts from the same donors were reprogrammed into iPSCs and differentiated into iPSC-CMs via embryoid body and monolayer-based differentiation protocols.Differentiation efficiency was found to be higher in CPC-derived iPSC-CMs (CPC-iPSC-CMs) than in fibroblast-derived iPSC-CMs (Fib-iPSC-CMs). Gene expression analysis during cardiac differentiation demonstrated up-regulation of cardiac transcription factors in CPC-iPSC-CMs, including NKX2-5, MESP1, ISL1, HAND2, MYOCD, MEF2C, and GATA4. Epigenetic assessment revealed higher methylation in the promoter region of NKX2-5 in Fib-iPSC-CMs compared with CPC-iPSC-CMs. Epigenetic differences were found to dissipate with increased cell passaging, and a battery of in vitro assays revealed no significant differences in their morphological and electrophysiological properties at early passage. Finally, cell delivery into a small animal myocardial infarction model indicated that CPC-iPSC-CMs and Fib-iPSC-CMs possess comparable therapeutic capabilities in improving functional recovery in vivo.This is the first study to compare differentiation of iPSC-CMs from human CPCs versus human fibroblasts from the same donors. The authors demonstrate that although epigenetic memory improves differentiation efficiency of cardiac versus noncardiac somatic cell sources in vitro, it does not contribute to improved functional outcome in vivo.
View details for DOI 10.1016/j.jacc.2014.04.056
View details for PubMedID 25082575
Multi-cellular interactions sustain long-term contractility of human pluripotent stem cell-derived cardiomyocytes.
American journal of translational research
2014; 6 (6): 724-735
Therapeutic delivery of cardiomyocytes derived from human pluripotent stem cells (hPSC-CMs) represents a novel clinical approach to regenerate the injured myocardium. However, poor survival and contractility of these cells are a significant bottleneck to their clinical use. To better understand the role of cell-cell communication in enhancing the phenotype and contractile properties of hPSC-CMs, we developed a three-dimensional (3D) hydrogel composed of hPSC-CMs, human pluripotent stem cell-derived endothelial cells (hPSC-ECs), and/or human amniotic mesenchymal stem cells (hAMSCs). The objective of this study was to examine the role of multi-cellular interactions among hPSC-ECs and hAMSCs on the survival and long-term contractile phenotype of hPSC-CMs in a 3D hydrogel. Quantification of spontaneous contractility of hPSC-CMs in tri-culture demonstrated a 6-fold increase in the area of contractile motion after 6 weeks with characteristic rhythmic contraction frequency, when compared to hPSC-CMs alone (P < 0.05). This finding was supported by a statistically significant increase in cardiac troponin T protein expression in the tri-culture hydrogel construct at 6 weeks, when compared to hPSC-CMs alone (P < 0.001). The sustained hPSC-CM survival and contractility in tri-culture was associated with a significant upregulation in the gene expression of L-type Ca(2+) ion channel, Cav1.2, and the inward-rectifier potassium channel, Kir2.1 (P < 0.05), suggesting a role of ion channels in mediating these processes. These findings demonstrate that multi-cellular interactions modulate hPSC-CM phenotype, function, and survival, and they will have important implications in engineering cardiac tissues for treatment of cardiovascular diseases.
View details for PubMedID 25628783
Human pluripotent stem cell tools for cardiac optogenetics
Conf Proc IEEE Eng Med Biol Soc
View details for DOI 10.1109/EMBC.2014.6945038
Optogenetic LED array for perturbing cardiac electrophysiology.
Conference proceedings : ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference
2013; 2013: 1619-1622
Optogenetics is the targeted genetic introduction of light-sensitive channels, such as Channelrhodopsin, and pumps, such as Halorhodopsin, into electrically-excitable cells that enables high spatiotemporal electrical stimulation and inhibition by optical actuation. Technologies for inducing optogenetically-based electrical stimulation for investigating in vitro and in vivo neural perturbations have been described. However, modification of existing technologies or creation of new ones has not been described for chronic cardiac applications. Here, an LED array system for optogenetically perturbing cardiac electrophysiology is described. The overall layout of the system consists of an LED holder containing six LED's that deliver pulsed ∼470 nm light to pluripotent stem cell-derived cardiomyocytes cultured in a 6-well tissue culture plate. The response of the cardiomyocytes is monitored by microscopy and the system is enclosed within a standard incubator. This system is relatively simple to create and uses mostly off-the-shelf components. The overall function of the system is to deliver chronic light stimulation over days to weeks to differentiating stem cell-derived cardiomyocytes in order to investigate perturbations in their electrophysiology.
View details for DOI 10.1109/EMBC.2013.6609826
View details for PubMedID 24110013
Label-free electrophysiological cytometry for stem cell-derived cardiomyocyte clusters
LAB ON A CHIP
2013; 13 (2): 220-228
Stem cell therapies hold great promise for repairing tissues damaged due to disease or injury. However, a major obstacle facing this field is the difficulty in identifying cells of a desired phenotype from the heterogeneous population that arises during stem cell differentiation. Conventional fluorescence flow cytometry and magnetic cell purification require exogenous labeling of cell surface markers which can interfere with the performance of the cells of interest. Here, we describe a non-genetic, label-free cell cytometry method based on electrophysiological response to stimulus. As many of the cell types relevant for regenerative medicine are electrically-excitable (e.g. cardiomyocytes, neurons, smooth muscle cells), this technology is well-suited for identifying cells from heterogeneous stem cell progeny without the risk and expense associated with molecular labeling or genetic modification. Our label-free cell cytometer is capable of distinguishing clusters of undifferentiated human induced pluripotent stem cells (iPSC) from iPSC-derived cardiomyocyte (iPSC-CM) clusters. The system utilizes a microfluidic device with integrated electrodes for both electrical stimulation and recording of extracellular field potential (FP) signals from suspended cells in flow. The unique electrode configuration provides excellent rejection of field stimulus artifact while enabling sensitive detection of FPs with a noise floor of 2 ?V(rms). Cells are self-aligned to the recording electrodes via hydrodynamic flow focusing. Based on automated analysis of these extracellular signals, the system distinguishes cardiomyocytes from non-cardiomyocytes. This is an entirely new approach to cell cytometry, in which a cell's functionality is assessed rather than its expression profile or physical characteristics.
View details for DOI 10.1039/c2lc40905d
View details for Web of Science ID 000312219100006
View details for PubMedID 23207961
Stretching Skeletal Muscle: Chronic Muscle Lengthening through Sarcomerogenesis
2012; 7 (10)
Skeletal muscle responds to passive overstretch through sarcomerogenesis, the creation and serial deposition of new sarcomere units. Sarcomerogenesis is critical to muscle function: It gradually re-positions the muscle back into its optimal operating regime. Animal models of immobilization, limb lengthening, and tendon transfer have provided significant insight into muscle adaptation in vivo. Yet, to date, there is no mathematical model that allows us to predict how skeletal muscle adapts to mechanical stretch in silico. Here we propose a novel mechanistic model for chronic longitudinal muscle growth in response to passive mechanical stretch. We characterize growth through a single scalar-valued internal variable, the serial sarcomere number. Sarcomerogenesis, the evolution of this variable, is driven by the elastic mechanical stretch. To analyze realistic three-dimensional muscle geometries, we embed our model into a nonlinear finite element framework. In a chronic limb lengthening study with a muscle stretch of 1.14, the model predicts an acute sarcomere lengthening from 3.09[Formula: see text]m to 3.51[Formula: see text]m, and a chronic gradual return to the initial sarcomere length within two weeks. Compared to the experiment, the acute model error was 0.00% by design of the model; the chronic model error was 2.13%, which lies within the rage of the experimental standard deviation. Our model explains, from a mechanistic point of view, why gradual multi-step muscle lengthening is less invasive than single-step lengthening. It also explains regional variations in sarcomere length, shorter close to and longer away from the muscle-tendon interface. Once calibrated with a richer data set, our model may help surgeons to prevent muscle overstretch and make informed decisions about optimal stretch increments, stretch timing, and stretch amplitudes. We anticipate our study to open new avenues in orthopedic and reconstructive surgery and enhance treatment for patients with ill proportioned limbs, tendon lengthening, tendon transfer, tendon tear, and chronically retracted muscles.
View details for DOI 10.1371/journal.pone.0045661
View details for Web of Science ID 000309388500010
View details for PubMedID 23049683
Computational optogenetics: A novel continuum framework for the photoelectrochemistry of living systems
JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS
2012; 60 (6): 1158-1178
Electrical stimulation is currently the gold standard treatment for heart rhythm disorders. However, electrical pacing is associated with technical limitations and unavoidable potential complications. Recent developments now enable the stimulation of mammalian cells with light using a novel technology known as optogenetics. The optical stimulation of genetically engineered cells has significantly changed our understanding of electrically excitable tissues, paving the way towards controlling heart rhythm disorders by means of photostimulation. Controlling these disorders, in turn, restores coordinated force generation to avoid sudden cardiac death. Here, we report a novel continuum framework for the photoelectrochemistry of living systems that allows us to decipher the mechanisms by which this technology regulates the electrical and mechanical function of the heart. Using a modular multiscale approach, we introduce a non-selective cation channel, channelrhodopsin-2, into a conventional cardiac muscle cell model via an additional photocurrent governed by a light-sensitive gating variable. Upon optical stimulation, this channel opens and allows sodium ions to enter the cell, inducing electrical activation. In side-by-side comparisons with conventional heart muscle cells, we show that photostimulation directly increases the sodium concentration, which indirectly decreases the potassium concentration in the cell, while all other characteristics of the cell remain virtually unchanged. We integrate our model cells into a continuum model for excitable tissue using a nonlinear parabolic second order partial differential equation, which we discretize in time using finite differences and in space using finite elements. To illustrate the potential of this computational model, we virtually inject our photosensitive cells into different locations of a human heart, and explore its activation sequences upon photostimulation. Our computational optogenetics tool box allows us to virtually probe landscapes of process parameters, and to identify optimal photostimulation sequences with the goal to pace human hearts with light and, ultimately, to restore mechanical function.
View details for DOI 10.1016/j.jmps.2012.02.004
View details for Web of Science ID 000303285600007
View details for PubMedID 22773861
Patient-Specific Induced Pluripotent Stem Cells as a Model for Familial Dilated Cardiomyopathy
SCIENCE TRANSLATIONAL MEDICINE
2012; 4 (130)
Characterized by ventricular dilatation, systolic dysfunction, and progressive heart failure, dilated cardiomyopathy (DCM) is the most common form of cardiomyopathy in patients. DCM is the most common diagnosis leading to heart transplantation and places a significant burden on healthcare worldwide. The advent of induced pluripotent stem cells (iPSCs) offers an exceptional opportunity for creating disease-specific cellular models, investigating underlying mechanisms, and optimizing therapy. Here, we generated cardiomyocytes from iPSCs derived from patients in a DCM family carrying a point mutation (R173W) in the gene encoding sarcomeric protein cardiac troponin T. Compared to control healthy individuals in the same family cohort, cardiomyocytes derived from iPSCs from DCM patients exhibited altered regulation of calcium ion (Ca(2+)), decreased contractility, and abnormal distribution of sarcomeric ?-actinin. When stimulated with a ?-adrenergic agonist, DCM iPSC-derived cardiomyocytes showed characteristics of cellular stress such as reduced beating rates, compromised contraction, and a greater number of cells with abnormal sarcomeric ?-actinin distribution. Treatment with ?-adrenergic blockers or overexpression of sarcoplasmic reticulum Ca(2+) adenosine triphosphatase (Serca2a) improved the function of iPSC-derived cardiomyocytes from DCM patients. Thus, iPSC-derived cardiomyocytes from DCM patients recapitulate to some extent the morphological and functional phenotypes of DCM and may serve as a useful platform for exploring disease mechanisms and for drug screening.
View details for DOI 10.1126/scitranslmed.3003552
View details for Web of Science ID 000303045900004
View details for PubMedID 22517884
IN VITRO/IN SILICO CHARACTERIZATION OF ACTIVE AND PASSIVE STRESSES IN CARDIAC MUSCLE
INTERNATIONAL JOURNAL FOR MULTISCALE COMPUTATIONAL ENGINEERING
2012; 10 (2): 171-188
View details for Web of Science ID 000302835000006
COMPUTATIONAL MODELLING OF OPTOGENETICS IN CARDIAC CELLS
PROCEEDINGS OF THE ASME SUMMER BIOENGINEERING CONFERENCE, PTS A AND B
View details for Web of Science ID 000325036600177
2012 ANNUAL INTERNATIONAL CONFERENCE OF THE IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY (EMBC)
For therapies based on human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes (CM) to be effective, arrhythmias must be avoided. Towards achieving this goal, light-activated channelrhodopsin-2 (ChR2), a cation channel activated with 480 nm light, and a first generation halorhodopsin (NpHR1.0), an anion pump activated by 580 nm light, have been introduced into hiPSC. By using in vitro approaches, hiPSC-CM are able to be optogenetically activated and inhibited. ChR2 and NpHR1.0 are stably transduced into undifferentiated hiPSC via a lentiviral vector. Via directed differentiation, both wildtype hiPSC-CM (hiPSC(WT)-CM) and hiPSC(ChR2/NpHR)-CM are produced and subjected to both electrical and optical stimulation. Both hiPSC(WT)-CM and hiPSC(ChR2/NpHR)-CM respond to traditional electrical stimulation and produce similar contractility features but only hiPSC(ChR2/NpHR)-CM can be synchronized and inhibited by optical stimulation. Here it is shown that light sensitive proteins can enable in vitro optical control of hiPSC-CM. For future therapy, in vivo optical stimulation could allow precise and specific synchronization of implanted hiPSC-CM with patient cardiac rates and rhythms.
View details for Web of Science ID 000313296501161
View details for PubMedID 23366158
Computational modeling of growth: systemic and pulmonary hypertension in the heart
BIOMECHANICS AND MODELING IN MECHANOBIOLOGY
2011; 10 (6): 799-811
We introduce a novel constitutive model for growing soft biological tissue and study its performance in two characteristic cases of mechanically induced wall thickening of the heart. We adopt the concept of an incompatible growth configuration introducing the multiplicative decomposition of the deformation gradient into an elastic and a growth part. The key feature of the model is the definition of the evolution equation for the growth tensor which we motivate by pressure-overload-induced sarcomerogenesis. In response to the deposition of sarcomere units on the molecular level, the individual heart muscle cells increase in diameter, and the wall of the heart becomes progressively thicker. We present the underlying constitutive equations and their algorithmic implementation within an implicit nonlinear finite element framework. To demonstrate the features of the proposed approach, we study two classical growth phenomena in the heart: left and right ventricular wall thickening in response to systemic and pulmonary hypertension.
View details for DOI 10.1007/s10237-010-0275-x
View details for Web of Science ID 000296634000001
View details for PubMedID 21188611
Identification of Cardiovascular Progenitors From Human Embryonic Stem Cells
LIPPINCOTT WILLIAMS & WILKINS. 2011
View details for Web of Science ID 000299738707322
Vascular anastomosis using controlled phase transitions in poloxamer gels
2011; 17 (9): 1147-U160
Vascular anastomosis is the cornerstone of vascular, cardiovascular and transplant surgery. Most anastomoses are performed with sutures, which are technically challenging and can lead to failure from intimal hyperplasia and foreign body reaction. Numerous alternatives to sutures have been proposed, but none has proven superior, particularly in small or atherosclerotic vessels. We have developed a new method of sutureless and atraumatic vascular anastomosis that uses US Food and Drug Administration (FDA)-approved thermoreversible tri-block polymers to temporarily maintain an open lumen for precise approximation with commercially available glues. We performed end-to-end anastomoses five times more rapidly than we performed hand-sewn controls, and vessels that were too small (<1.0 mm) to sew were successfully reconstructed with this sutureless approach. Imaging of reconstructed rat aorta confirmed equivalent patency, flow and burst strength, and histological analysis demonstrated decreased inflammation and fibrosis at up to 2 years after the procedure. This new technology has potential for improving efficiency and outcomes in the surgical treatment of cardiovascular disease.
View details for DOI 10.1038/nm.2424
View details for Web of Science ID 000294605100038
View details for PubMedID 21873986
- Stretchable microelectrode array using room-temperature liquid alloy interconnects JOURNAL OF MICROMECHANICS AND MICROENGINEERING 2011; 21 (5)
Localized control of exsanguinating arterial hemorrhage: an experimental model.
Polski przeglad chirurgiczny
2011; 83 (1): 1-9
To develop an arterial injury model for testing hemostatic devices at well-defined high and low bleeding rates.A side-hole arterial injury was created in the carotid artery of sheep. Shed blood was collected in a jugular venous reservoir and bleeding rate at the site of arterial injury was controlled by regulating outflow resistance from the venous reservoir. Two models were studied: uncontrolled exsanguinating hemorrhage and bleeding at controlled rates with blood return to maintain hemodynamic stability. Transcutaneous Duplex ultrasound was used to characterize ultrasound signatures at various bleeding rates.A 2.5 mm arterial side-hole resulted in exsanguinating hemorrhage with an initial bleeding rate of 400 ml/min which, without resuscitation, decreased to below 100 ml/min in 5 minutes. After 17 minutes, bleeding from the injury site stopped and the animal had lost 60% of total blood volume. Reinfusion of shed blood maintained normal hemodynamics and both high and low bleeding rates could be maintained without hemorrhagic shock. Bleeding rate at the arterial injury site was held at 395±78 ml/min for 8 minutes, 110±11 ml/min for 15 minutes, and 12±1 ml/min for 12 minutes. Doppler flow signatures at the site of injury were characterized by high peak and end-diastolic flow velocities at the bleeding site which varied with the rate of hemorrhage.We have developed a hemodynamically stable model of acute arterial injury which can be used to evaluate diagnostic and treatment methods focused on control of the arterial injury site.
View details for DOI 10.2478/v10035-011-0001-0
View details for PubMedID 22166236
Stimulation and Artifact-Free Extracellular Electrophysiological Recording of Cells in Suspension
IEEE. 2011: 4030-4033
We have developed instrumentation which stimulates and records electrophysiological signals from populations of suspended cells in microfluidic channels. We are employing this instrumentation in a new approach to cell sorting and flow cytometry which distinguishes cells based on their electrophysiology. This label-free approach is ideal for applications where labeling or genetic modification of cells is undesirable, such as in purifying cells for tissue replacement therapies. Electrophysiology is a powerful indicator of phenotype for electrically-excitable cells such as myocytes and neurons. However, extracellular field potential signals are notoriously weak and large stimulus artifacts can easily obscure these signals if care is not taken to suppress them. This is particularly true for suspended cells. Here, we describe a novel microelectrode configuration and the associated instrumentation for suppressing stimulus artifacts and faithfully recovering the extracellular field potential signal. We show that the device is capable of distinguishing cardiomyocytes from non-cardiomyocytes derived from the same stem cell population. Finally, we explain the relationship between extracellular field potentials and the more familiar transmembrane action potential signal, noting the physiologically important features of these signals.
View details for Web of Science ID 000298810003093
View details for PubMedID 22255224
Power Law as a Method for Ultrasound Detection of Internal Bleeding: In Vivo Rabbit Validation
IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING
2010; 57 (12): 2870-2875
New detection methods for vascular injuries can augment the usability of an ultrasound (US) imager in trauma settings. The goal of this study was to evaluate a potential-detection strategy for internal bleeding that employs a well-established theoretical biofluid model, the power law. This law characterizes normal blood-flow rates through an arterial tree by its bifurcation geometry. By detecting flows that deviate from the model, we hypothesized that vascular abnormalities could be localized. We devised a bleed metric, flow-split deviation (FSD), that quantified the difference between patient and model blood flows at vessel bifurcations. Femoral bleeds were introduced into ten rabbits (?5 kg) using a cannula attached to a variable pump. Different bleed rates (0% as control, 5%, 10%, 15%, 20%, 25%, and 30% of descending aortic flow) were created at two physiological states (rest and elevated state with epinephrine). FSDs were found by US imaging the iliac arteries. Our bleed metric demonstrated good sensitivity and specificity at moderate bleed rates; area under receiver-operating characteristic curves were greater than 0.95 for bleed rates 20% and higher. Thus, FSD was a good indicator of bleed severity and may serve as an additional tool in the US bleed detection.
View details for DOI 10.1109/TBME.2010.2058803
View details for Web of Science ID 000284360100009
View details for PubMedID 20639172
A matrix micropatterning platform for cell localization and stem cell fate determination
2010; 6 (12): 4614-4621
To study the role of cell-extracellular matrix (ECM) interactions, microscale approaches provide the potential to perform high throughput assessment of the effect of the ECM microenvironment on cellular function and phenotype. Using a microscale direct writing (MDW) technique, we characterized the generation of multicomponent ECM microarrays for cellular micropatterning, localization and stem cell fate determination. ECMs and other biomolecules of various geometries and sizes were printed onto epoxide-modified glass substrates to evaluate cell attachment by human endothelial cells. The endothelial cells displayed strong preferential attachment to the ECM patterned regions and aligned their cytoskeleton along the direction of the micropatterns. We next generated ECM microarrays that contained one or more ECM components (namely gelatin, collagen IV and fibronectin) and then cultured murine embryonic stem cell (ESCs) on the microarrays. The ESCs selectively attached to the micropatterned features and expressed markers associated with a pluripotent phenotype, such as E-cadherin and alkaline phosphatase, when maintained in growth medium containing leukemia inhibitory factor. In the presence of the soluble factors retinoic acid and bone morphogenetic protein-4 the ESCs differentiated towards the ectodermal lineage on the ECM microarray with differential ECM effects. The ESCs cultured on gelatin showed significantly higher levels of pan cytokeratin expression, when compared with cells cultured on collagen IV or fibronectin, suggesting that gelatin preferentially promotes ectodermal differentiation. In summary, our results demonstrate that MDW is a versatile approach to print ECMs of diverse geometries and compositions onto surfaces, and it is amenable to the generation of multicomponent ECM microarrays for stem cell fate determination.
View details for DOI 10.1016/j.actbio.2010.06.033
View details for Web of Science ID 000284385300018
View details for PubMedID 20601236
Dynamic MicroRNA Expression Programs During Cardiac Differentiation of Human Embryonic Stem Cells Role for miR-499
2010; 3 (5): 426-U97
MicroRNAs (miRNAs) are a newly discovered endogenous class of small, noncoding RNAs that play important posttranscriptional regulatory roles by targeting messenger RNAs for cleavage or translational repression. Human embryonic stem cells are known to express miRNAs that are often undetectable in adult organs, and a growing body of evidence has implicated miRNAs as important arbiters of heart development and disease.To better understand the transition between the human embryonic and cardiac "miRNA-omes," we report here the first miRNA profiling study of cardiomyocytes derived from human embryonic stem cells. Analyzing 711 unique miRNAs, we have identified several interesting miRNAs, including miR-1, -133, and -208, that have been previously reported to be involved in cardiac development and disease and that show surprising patterns of expression across our samples. We also identified novel miRNAs, such as miR-499, that are strongly associated with cardiac differentiation and that share many predicted targets with miR-208. Overexpression of miR-499 and -1 resulted in upregulation of important cardiac myosin heavy-chain genes in embryoid bodies; miR-499 overexpression also caused upregulation of the cardiac transcription factor MEF2C.Taken together, our data give significant insight into the regulatory networks that govern human embryonic stem cell differentiation and highlight the ability of miRNAs to perturb, and even control, the genes that are involved in cardiac specification of human embryonic stem cells.
View details for DOI 10.1161/CIRCGENETICS.109.934281
View details for Web of Science ID 000283163100006
View details for PubMedID 20733065
- A generic approach towards finite growth with examples of athlete's heart, cardiac dilation, and cardiac wall thickening JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS 2010; 58 (10): 1661-1680
A multiscale model for eccentric and concentric cardiac growth through sarcomerogenesis
JOURNAL OF THEORETICAL BIOLOGY
2010; 265 (3): 433-442
We present a novel computational model for maladaptive cardiac growth in which kinematic changes of the cardiac chambers are attributed to alterations in cytoskeletal architecture and in cellular morphology. We adopt the concept of finite volume growth characterized through the multiplicative decomposition of the deformation gradient into an elastic part and a growth part. The functional form of its growth tensor is correlated to sarcomerogenesis, the creation and deposition of new sarcomere units. In response to chronic volume-overload, an increased diastolic wall strain leads to the addition of sarcomeres in series, resulting in a relative increase in cardiomyocyte length, associated with eccentric hypertrophy and ventricular dilation. In response to chronic pressure-overload, an increased systolic wall stress leads to the addition of sacromeres in parallel, resulting in a relative increase in myocyte cross sectional area, associated with concentric hypertrophy and ventricular wall thickening. The continuum equations for both forms of maladaptive growth are discretized in space using a nonlinear finite element approach, and discretized in time using the implicit Euler backward scheme. We explore a generic bi-ventricular heart model in response to volume- and pressure-overload to demonstrate how local changes in cellular morphology translate into global alterations in cardiac form and function.
View details for DOI 10.1016/j.jtbi.2010.04.023
View details for Web of Science ID 000280374100023
View details for PubMedID 20447409
IN VITRO ASSESSMENT OF RAT HEART FORCE GENERATION: A QUANTITATIVE APPROACH FOR PREDICTING OUTCOMES FROM PLURIPOTENT STEM CELL-DERIVED THERAPY FOR MYOCARDIAL INFARCTION
PROCEEDINGS OF THE ASME SUMMER BIOENGINEERING CONFERENCE, 2010
View details for Web of Science ID 000290705300359
Outcome of open versus endovascular revascularization for chronic mesenteric ischemia: review of comparative studies
JOURNAL OF CARDIOVASCULAR SURGERY
2009; 50 (4): 509-514
Chronic mesenteric ischemia is a rare disorder that has traditionally been treated with open surgical revascularization (OR). Endovascular revascularization (ER) has recently gained popularity as an alternative modality of treatment; however, OR is still predominantly used. This study aimed at comparing the outcomes of these two treatment modalities. The literature was searched using the MEDLINE database through the PubMed search engine for relevant articles that compared the outcomes after OR and ER for chronic mesenteric ischemia. Review of the selected articles revealed that patients had lower postoperative mortality and morbidity, and shorter intensive care unit and hospital stay after ER. However, early and long-term symptomatic relief and significantly lower restenosis rate were characteristic of OR. Although no level 1 evidence governs the treatment of chronic mesenteric ischemia, the durability and efficacy of OR is such that this modality should remain the procedure of choice for patients who are fit or whose fitness could be improved before surgery. For unfit patients, or those with short life expectancy, ER is preferable owing to its minimally invasive nature and reduced postoperative mortality and morbidity. Randomized controlled studies are needed to compare the long-term durability and efficacy of ER to those of OR.
View details for Web of Science ID 000271955000010
View details for PubMedID 19455085
First report of an ilio-popliteal bypass through the greater sciatic foramen - Case report
JOURNAL OF CARDIOVASCULAR SURGERY
2008; 49 (3): 341-343
A 47 year-old man, who had a history of pelvic radiotherapy for the treatment of testicular tumour 30 years ago, was referred with minor tissue loss of the right lower extremity, grade III, category 5 of the Rutherford classification. His groin region presented with severe radiation damage. Arteriography demonstrated the occlusion of external iliac and femoral arteries. Revascula-risation was performed in the lateral decubitus position, with a ringed polytetrafluoroethylene (PTFE) graft bypass between normal common iliac and popliteal arteries through the greater sciatic foramen. Quick healing was observed. Patient is well 6 months postoperatively. Immediate and 6 month postoperative imaging demonstrated the good patency of the graft. A duplex ultrasound performed 6 month postoperatively showed no significant compression while the patient was in the sitting position.
View details for Web of Science ID 000257038800005
View details for PubMedID 18446119
Lateral movement of endografts within the aneurysm sac is an indicator of stent-graft instability
JOURNAL OF ENDOVASCULAR THERAPY
2008; 15 (3): 335-343
To determine if lateral movement of an aortic endograft 1 year following endovascular abdominal aortic aneurysm (AAA) repair is an indicator of endograft instability and can serve as a predictor of late adverse events.The records of 60 high-risk AAA patients (52 men, 8 women; mean age 74 years) who were treated with infrarenal (n = 38) or suprarenal (n = 22) endografts and had serial computed tomograms (CT) over > or =12 months were analyzed. Postimplantation and 1-year CT scans were compared, and changes in endograft position within the aneurysm sac [lateral movement (LM) versus no lateral movement (NM)] were measured using a vertebral body reference point. Longitudinal endograft movement was measured with respect to the superior mesenteric artery along the aortic centerline axis. Long-term adverse event rates (endoleaks, secondary procedures, conversion, rupture, and death) were assessed.One year after endograft implantation, LM > or =5 mm was present in 16 (27%) patients; 44 (73%) endografts demonstrated no lateral movement. LM patients had larger aneurysms (6.5+/-1.5 versus 5.6+/-0.9 cm, p = 0.02) and a longer endograft-to-hypogastric artery length (p = 0.01) than NM patients. There were no significant differences between patients treated with infrarenal and suprarenal endografts. At 1 year, longitudinal migration > or =10 mm occurred in 5 (31%) of the LM patients versus 2 (5%) in the NM cohort (p<0.0001). There were no significant differences in adverse event rates between LM and NM at 1 year. However, during long-term follow-up (mean 54+/-26 months, range 12-102), 8 (50%) LM patients developed a type I endoleak versus 8 (18%) NM patients (p = 0.02), and 12 (75%) LM patients required a secondary procedure versus 9 (20%) NM patients (p = 0.0002). One (6%) LM patient experienced aneurysm rupture and 2 (13%) other LM patients underwent conversion to open repair.Lateral endograft movement within the aneurysm sac at 1 year is associated with increased risk of late adverse events and was at least as good a predictor of these complications as was longitudinal migration.
View details for Web of Science ID 000257093100012
View details for PubMedID 18540708
In vivo imaging and evaluation of different biomatrices for improvement of stem cell survival
JOURNAL OF TISSUE ENGINEERING AND REGENERATIVE MEDICINE
2007; 1 (6): 465-468
Therapeutic effects from injection of stem cells are often hampered by acute donor cell death as well as migration away from damaged areas. This is likely due to the fact that injected cells do not have the physical and biochemical cues for ordered engrafment. Here we evaluate 3 common biomatrices (Matrigel, Collagen I, Purmatrix) that has the potential of providing suitable scaffolds needed to enhance stem cell survival. The longitudinal fate of transplanted stem cells was monitored by reporter imaging techniques.
View details for DOI 10.1002/term.55
View details for Web of Science ID 000256520300008
View details for PubMedID 18163533
Iliac fixation inhibits migration of both suprarenal and infrarenal aortic endografts
JOURNAL OF VASCULAR SURGERY
2007; 45 (2): 250-257
To evaluate the role of iliac fixation in preventing migration of suprarenal and infrarenal aortic endografts.Quantitative image analysis was performed in 92 patients with infrarenal aortic aneurysms (76 men and 16 women) treated with suprarenal (n = 36) or infrarenal (n = 56) aortic endografts from 2000 to 2004. The longitudinal centerline distance from the superior mesenteric artery to the top of the stent graft was measured on preoperative, postimplantation, and 1-year three-dimensional computed tomographic scans, with movement more than 5 mm considered to be significant. Aortic diameters were measured perpendicular to the centerline axis. Proximal and distal fixation lengths were defined as the lengths of stent-graft apposition to the aortic neck and the common iliac arteries, respectively.There were no significant differences in age, comorbidities, or preoperative aneurysm size (suprarenal, 6.0 cm; infrarenal, 5.7 cm) between the suprarenal and infrarenal groups. However, the suprarenal group had less favorable aortic necks with a shorter length (13 vs 25 mm; P < .0001), a larger diameter (27 vs 24 mm; P < .0001), and greater angulation (19 degrees vs 11 degrees ; P = .007) compared with the infrarenal group. The proximal aortic fixation length was greater in the suprarenal than in the infrarenal group (22 vs 16 mm; P < .0001), with the top of the device closer to the superior mesenteric artery (8 vs 21 mm; P < .0001) as a result of the 15-mm uncovered suprarenal stent. There was no difference in iliac fixation length between the suprarenal and infrarenal groups (26 vs 25 mm; P = .8). Longitudinal centerline stent graft movement at 1 year was similar in the suprarenal and infrarenal groups (4.3 +/- 4.4 mm vs 4.8 +/- 4.3 mm; P = .6). Patients with longitudinal centerline movement of more than 5 mm at 1 year or clinical evidence of migration at any time during the follow-up period comprised the respective migrator groups. Suprarenal migrators had a shorter iliac fixation length (17 vs 29 mm; P = .006) and a similar aortic fixation length (23 vs 22 mm; P > .999) compared with suprarenal nonmigrators. Infrarenal migrators had a shorter iliac fixation length (18 vs 30 mm; P < .0001) and a similar aortic fixation length (14 vs 17 mm; P = .1) compared with infrarenal nonmigrators. Nonmigrators had closer device proximity to the hypogastric arteries in both the suprarenal (7 vs 17 mm; P = .009) and infrarenal (8 vs 24 mm; P < .0001) groups. No migration occurred in either group in patients with good iliac fixation. Multivariate logistic regression analysis revealed that iliac fixation, as evidenced by iliac fixation length (P = .004) and the device to hypogastric artery distance (P = .002), was a significant independent predictor of migration, whereas suprarenal or infrarenal treatment was not a significant predictor of migration. During a clinical follow-up period of 45 +/- 22 months (range, 12-70 months), there have been no aneurysm ruptures, abdominal aortic aneurysm-related deaths, or surgical conversions in either group.Distal iliac fixation is important in preventing migration of both suprarenal and infrarenal aortic endografts that have longitudinal columnar support. Secure iliac fixation minimizes the risk of migration despite suboptimal proximal aortic neck anatomy. Extension of both iliac limbs to cover the entire common iliac artery to the iliac bifurcation seems to prevent endograft migration.
View details for DOI 10.1016/j.jvs.2006.09.061
View details for Web of Science ID 000243987200006
View details for PubMedID 17263997
Biomems platform for electromechanical stimulation of cell culture
PROCEEDING OF THE ASME SUMMER BIOENGINEERING CONFERENCE - 2007
View details for Web of Science ID 000252105700032
Pulsatile pressure system for cellular mechanical stimulation
PROCEEDING OF THE ASME SUMMER BIOENGINEERING CONFERENCE - 2007
View details for Web of Science ID 000252105700505
P19 progenitor cells progress to organized contracting myocytes after chemical and electrical stimulation: Implications for vascular tissue engineering
JOURNAL OF ENDOVASCULAR THERAPY
2006; 13 (3): 377-388
To test the hypothesis that a level of chemical and electrical stimulation exists that allows differentiation of progenitor cells into organized contracting myocytes.A custom-made bioreactor with the capability of delivering electrical pulses of varying field strengths, widths, and frequencies was constructed. Individual chambers of the bioreactor allowed continuous electrical stimulation of cultured cells under microscopic observation. On day 0, 1% dimethylsulfoxide (DMSO), known to differentiate cells into myocytes, was added to P19 progenitor cells. Additionally, for the next 22 days, electrical pulses of varying field strengths (0-3 V/cm), widths (2-40 ms), and frequencies (10-25 Hz) were continuously applied. On day 5, the medium containing DMSO was exchanged with regular medium, and the electrical stimulation was continued. From days 6-22, the cells were visually assessed for signs of viability, contractility, and organization.P19 cells remained viable with pulsed electrical fields <3 V/cm, pulse widths <40 ms, and pulse frequencies from 10 to 25 Hz. On day 12, the first spontaneous contractions were observed. For individual colonies, local synchronization and organization occurred; multiple colonies were synchronized with externally applied electrical fields.P19 progenitor cells progress to organized contracting myocytes after chemical and electrical stimulation. Incorporation of such cells into existing methods of producing endothelial cells, fibroblasts, and scaffolds may allow production of improved tissue-engineered vascular grafts.
View details for Web of Science ID 000238334300015
View details for PubMedID 16784327
A novel culture system shows that stem cells can be grown in 3D and under physiologic pulsatile conditions for tissue engineering of vascular grafts
JOURNAL OF SURGICAL RESEARCH
2006; 132 (2): 170-178
Currently available vascular grafts have been limited by variable patency rates, material availability, and immunological rejection. The creation of a tissue-engineered vascular graft (TEVG) from autologous stem cells would potentially overcome these limitations. As a first step in creating a completely autologous TEVG, our objective was to develop a novel system for culturing undifferentiated mouse embryonic stem cells (mESC) in a three-dimensional (3D) configuration and under physiological pulsatile flow and pressure conditions.A bioreactor was created to provide pulsatile conditions to a specially modified four-well Labtek Chamber-Slide culture system. Undifferentiated mESC were either suspended in a 3D Matrigel matrix or suspended only in cell-culture media within the culture system. Pulsatile conditions were applied to the suspended cells and visualized by video microscopy.Undifferentiated mESC were successfully embedded in a 3D Matrigel matrix and could withstand physiological pulsatile conditions. Video microscopy demonstrated that the mESC in the 3D matrix were constrained to the wells of the culture system, moved in unison with the applied flows, and were not washed downstream; this was in contrast to the mESC suspended in media alone.Undifferentiated mESC can be grown in 3D and under pulsatile conditions. We will use these results to study the effects of long-term pulsatile conditions on the differentiation of mESC into endothelial cells, smooth muscle cells, and fibroblast cells with the long-term goal of creating a completely autologous TEVG.
View details for DOI 10.1016/j.jss.2006.2.017
View details for Web of Science ID 000237337000005
View details for PubMedID 16542683
Superficial femoral artery transposition repair for isolated superior mesenteric artery dissection
JOURNAL OF VASCULAR SURGERY
2005; 42 (4): 788-791
Isolated dissection of the superior mesenteric artery is an uncommon event, but many new cases have been reported recently, reflecting the progress of imaging and suggesting that this pathology is not as rare as previously thought. Here we report a case of superior mesenteric artery dissection where we performed, after failure of conservative medical management, an original surgical technique for mesenteric revascularization using a superficial femoral artery transposition. To the best of our knowledge, this is the first report of the use of this technique for complex mesenteric revascularization.
View details for DOI 10.1016/j.jvs.2005.05.048
View details for Web of Science ID 000232609300033
View details for PubMedID 16242570