Academic Appointments


Honors & Awards


  • SWE Professor/Lecturer of the Year Award, Stanford University (2016)
  • Tau Beta Pi Award for Excellence in Undergraduate Teaching, Stanford University (2015)
  • Participant in National Academy of Engineering's Frontiers of Engineering Education, Stanford University (2014)

Boards, Advisory Committees, Professional Organizations


  • Member, American Institute of Chemical Engineers (1999 - Present)
  • Member, American Society for Engineering Education (2013 - Present)

2018-19 Courses


All Publications


  • Comparison of Extruded and Sonicated Vesicles for Planar Bilayer Self-Assembly MATERIALS Cho, N., Hwang, L. Y., Solandt, J. J., Frank, C. W. 2013; 6 (8): 3294-3308

    Abstract

    Lipid vesicles are an important class of biomaterials that have a wide range of applications, including drug delivery, cosmetic formulations and model membrane platforms on solid supports. Depending on the application, properties of a vesicle population such as size distribution, charge and permeability need to be optimized. Preparation methods such as mechanical extrusion and sonication play a key role in controlling these properties, and yet the effects of vesicle preparation method on vesicular properties and integrity (e.g., shape, size, distribution and tension) remain incompletely understood. In this study, we prepared vesicles composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipid by either extrusion or sonication, and investigated the effects on vesicle size distribution over time as well as the concomitant effects on the self-assembly of solid-supported planar lipid bilayers. Dynamic light scattering (DLS), quartz crystal microbalance with dissipation (QCM-D) monitoring, fluorescence recovery after photobleaching (FRAP) and atomic force microscopy (AFM) experiments were performed to characterize vesicles in solution as well as their interactions with silicon oxide substrates. Collectively, the data support that sonicated vesicles offer more robust control over the self-assembly of homogenous planar lipid bilayers, whereas extruded vesicles are vulnerable to aging and must be used soon after preparation.

    View details for DOI 10.3390/ma6083294

    View details for Web of Science ID 000330293100017

    View details for PubMedCentralID PMC5521307

  • Preparation and Characterization of Glycoacrylate-Based Polymer-Tethered Lipid Bilayers on Benzophenone-Modified Substrates LANGMUIR Hwang, L. Y., Goetz, H., Knoll, W., Hawker, C. J., Frank, C. W. 2008; 24 (24): 14088-14098

    Abstract

    Polymer-tethered lipid bilayers are promising models for biological membranes as they may provide a soft, lubricating environment with sufficient spacing between the substrate and bilayer for incorporating transmembrane proteins. We present such a system that uses a glycoacrylate-based telechelic lipopolymer in combination with a lipid analogue. Characterization of the mixed monolayers of lipopolymers and free lipids at the air-water interface is used to examine the molecular organization that dictates the final assembly properties. Isotherms indicate that the source of the dominating interactions, whether polymer interactions in the subphase or alkyl chain interactions, depends on both the tethering density and area per molecule. Moreover, a critical composition exists at which the alkyl chain interactions dominate the monolayer behavior regardless of the area per molecule. Isobaric creep and hysteresis experiments suggest that permanent states due to irreversible polymer-polymer interactions are not created as the monolayer is compressed. These data, combined with theoretical polymer predictions, are used to understand the organization of the monolayers at the air-water interface and, hence, the separation distance between the bottom of the bilayer and substrate in the water-swollen state of the final bilayer assembly. Atomic force microscopy is used to confirm that the measured separation distance of 11.2 nm is on the order of what would be predicted using a theoretical analysis for a representative 5 mol % lipopolymer-tethered bilayer. Next, the homogeneity of the final bilayer is probed at multiple scales. Fluorescence microscopy is used to demonstrate that homogeneous and continuous bilayers can be formed (within the optical resolution limit of 500 nm) with all polymer tethering densities used in this study. Atomic force microscopy studies demonstrate that homogeneity comparable to that of a solid-supported lipid bilayer can be achieved for a representative 5 mol % lipopolymer-tethered bilayer. Langmuir-Blodgett transfer conditions for depositing monolayers that can be used to create homogeneous, fluid bilayers are also discussed. Finally, the distal leaflet lateral mobility is measured using fluorescence recovery after photobleaching experiments and shown to be a function of the tethering density. A possible model for the mobility data is developed in which the tethered lipids in the proximal leaflet act as immobile lipid obstacles that couple to distal leaflet lipids.

    View details for DOI 10.1021/la8022997

    View details for Web of Science ID 000261631700040

    View details for PubMedID 19360958

  • Glyco-acrylate copolymers for bilayer tethering on benzophenone-modified substrates COLLOIDS AND SURFACES B-BIOINTERFACES Hwang, L. Y., Goetz, H., Hawker, C. J., Frank, C. W. 2007; 54 (2): 127-135

    Abstract

    Model biological membranes are becoming increasingly important for studying fundamental biophysical phenomena and developing membrane-based devices. To address the anticipated problem of non-physiological interactions between membrane proteins and substrates seen in "solid-supported lipid bilayers" that are formed directly on hydrophilic substrates, we have developed a polymer-tethered lipid bilayer system based on a random copolymer with multiple lipid analogue anchors and a glyco-acrylate backbone. This system is targeted at applications that, most importantly, require stability and robustness since each copolymer has multiple lipid analogues that insert into the bilayer. We have combined this copolymer with a flexible photochemical coupling scheme that covalently attaches the copolymer to the substrate. The Langmuir isotherms of mixed copolymer/free lipid monolayers measured at the air-water interface indicate that the alkyl chains of the copolymer lipid analogues and the free lipids dominate the film behavior. In addition, no significant phase transitions are seen in the isotherms, while hysteresis experiments confirm that no irreversible states are formed during the monolayer compression. Isobaric creep experiments at the air-water interface and AFM experiments of the transferred monolayer are used to guide processing parameters for creating a fluid, homogeneous bilayer. Bilayer homogeneity and fluidity are monitored using fluorescence microscopy. Continuous bilayers with lateral diffusion coefficients of 0.6 microm(2)/s for both leaflets of the bilayer are observed for a 5% copolymer system.

    View details for DOI 10.1016/j.colsurfb.2006.08.010

    View details for Web of Science ID 000244473900001

    View details for PubMedID 17207977

  • Influence of mobile phase composition and thermodynamics on the normal phase chromatography of echinocandins JOURNAL OF CHROMATOGRAPHY A Roush, D. J., HWANG, L. Y., Antia, F. D. 2005; 1098 (1-2): 55-65

    Abstract

    In the normal phase preparative HPLC of fermentation derived echinocandins, resolution of key impurities from the product of interest, pneumocandin B(o), is accomplished using a ternary ethyl acetate/methanol/water mobile phase with silica gel as the sorbent. In this work, previous characterization of the system is extended to define the impact and role of water content on the separation efficiency and retention of pneumocandin B(o). Experimental results indicate that column efficiency, measured using both the product of interest and small molecule tracers (compounds used for pulse tests), is good despite the use of an irregular silica and unusually high levels (greater than 6%) of water in the mobile phase. In contrast to column efficiency measurements using small molecules (MEK and toluene), measurements performed with the product itself indicate improved efficiency with increasing water content of the mobile phase. Building on these results, a scale-up/scale-down protocol was developed based on measurements of column efficiency using theoretical plate counts determined with pneumocandin B(o). Since the solubility of pneumocandin B(o) in the ternary mobile phase is relatively low, a higher strength solvent with higher levels of methanol and water is employed for dissolution of the crude product at concentrations of up to 40g/L. The mismatch between the high strength solvent used for the feed introduction and the mobile phase has the potential to affect column performance. The impact of this mismatch using plate count measurements with the product at both analytical and semi-preparative scales was found not to be significant. Finally, a van't Hoff analysis was performed to characterize the thermodynamics of adsorption of pneumocandin B(o) on silica. The analysis shows that the adsorption process for pneumocandin B(o) on silica in the ternary solvent system is endothermic (DeltaH(ads)>0), implying that the adsorption is entropically driven. Results from an overall water balance across the column indicate significant enrichment of adsorbed water on the silica surface. These results further emphasize the importance of selective partitioning of water between the bulk mobile phase and the silica as a dominant factor in controlling retention.

    View details for DOI 10.1016/j.chroma.2005.08.042

    View details for Web of Science ID 000233954200008

    View details for PubMedID 16314161