Professional Education


  • Doctor of Philosophy, University of Texas Austin (2009)
  • Master of Science, National Taiwan University, Robotics (1998)
  • Bachelor of Science, National Taiwan University (1996)

Stanford Advisors


  • Fan Yang, Postdoctoral Faculty Sponsor

Journal Articles


  • Photo-crosslinkable PEG-Based Microribbons for Forming 3D Macroporous Scaffolds with Decoupled Niche Properties. Advanced materials Han, L., Tong, X., Yang, F. 2014; 26 (11): 1757-1762

    Abstract

    PEG-based microribbons are designed and fabricated as building blocks for constructing a 3D cell niche with independently tunable biochemical, mechanical, and topographical cues. This platform supports direct cell encapsulation, allows spatial patterning of biochemical cues, and may provide a valuable tool for facilitating the analyses of how interactive niche signaling regulates cell fate in three dimensions.

    View details for DOI 10.1002/adma.201304805

    View details for PubMedID 24347028

  • Microribbon-Like Elastomers for Fabricating Macroporous and Highly Flexible Scaffolds that Support Cell Proliferation in 3D ADVANCED FUNCTIONAL MATERIALS Han, L., Yu, S., Wang, T., Behn, A. W., Yang, F. 2013; 23 (3): 346-358
  • A Facile Method to Fabricate Hydrogels with Microchannel-Like Porosity for Tissue Engineering TISSUE ENGINEERING PART C-METHODS Hammer, J., Han, L., Tong, X., Yang, F. 2014; 20 (2): 169-176

    Abstract

    Hydrogels are widely used as three-dimensional (3D) tissue engineering scaffolds due to their tissue-like water content, as well as their tunable physical and chemical properties. Hydrogel-based scaffolds are generally associated with nanoscale porosity, whereas macroporosity is highly desirable to facilitate nutrient transfer, vascularization, cell proliferation and matrix deposition. Diverse techniques have been developed for introducing macroporosity into hydrogel-based scaffolds. However, most of these methods involve harsh fabrication conditions that are not cell friendly, result in spherical pore structure, and are not amenable for dynamic pore formation. Human tissues contain abundant microchannel-like structures, such as microvascular network and nerve bundles, yet fabricating hydrogels containing microchannel-like pore structures remains a great challenge. To overcome these limitations, here we aim to develop a facile, cell-friendly method for engineering hydrogels with microchannel-like porosity using stimuli-responsive microfibers as porogens. Microfibers with sizes ranging 150-200 μm were fabricated using a coaxial flow of alginate and calcium chloride solution. Microfibers containing human embryonic kidney (HEK) cells were encapsulated within a 3D gelatin hydrogel, and then exposed to ethylenediaminetetraacetic acid (EDTA) solution at varying doses and duration. Scanning electron microscopy confirmed effective dissolution of alginate microfibers after EDTA treatment, leaving well-defined, interconnected microchannel structures within the 3D hydrogels. Upon release from the alginate fibers, HEK cells showed high viability and enhanced colony formation along the luminal surfaces of the microchannels. In contrast, HEK cells in non-EDTA treated control exhibited isolated cells, which remained entrapped in alginate microfibers. Together, our results showed a facile, cell-friendly process for dynamic microchannel formation within hydrogels, which may simultaneously release cells in 3D hydrogels in a spatiotemporally controlled manner. This platform may be adapted to include other cell-friendly stimuli for porogen removal, such as Matrix metalloproteinase-sensitive peptides or photodegradable gels. While we used HEK cells in this study as proof of principle, the concept described in this study may also be used for releasing clinically relevant cell types, such as smooth muscle and endothelial cells that are useful for repairing tissues involving tubular structures.

    View details for DOI 10.1089/ten.tec.2013.0176

    View details for Web of Science ID 000330310700008

    View details for PubMedID 23745610

  • Modulating polymer chemistry to enhance non-viral gene delivery inside hydrogels with tunable matrix stiffness. Biomaterials Keeney, M., Onyiah, S., Zhang, Z., Tong, X., Han, L., Yang, F. 2013; 34 (37): 9657-9665

    Abstract

    Non-viral gene delivery holds great promise for promoting tissue regeneration, and offers a potentially safer alternative than viral vectors. Great progress has been made to develop biodegradable polymeric vectors for non-viral gene delivery in 2D culture, which generally involves isolating and modifying cells in vitro, followed by subsequent transplantation in vivo. Scaffold-mediated gene delivery may eliminate the need for the multiple-step process in vitro, and allows sustained release of nucleic acids in situ. Hydrogels are widely used tissue engineering scaffolds given their tissue-like water content, injectability and tunable biochemical and biophysical properties. However, previous attempts on developing hydrogel-mediated non-viral gene delivery have generally resulted in low levels of transgene expression inside 3D hydrogels, and increasing hydrogel stiffness further decreased such transfection efficiency. Here we report the development of biodegradable polymeric vectors that led to efficient gene delivery inside poly(ethylene glycol) (PEG)-based hydrogels with tunable matrix stiffness. Photocrosslinkable gelatin was maintained constant in the hydrogel network to allow cell adhesion. We identified a lead biodegradable polymeric vector, E6, which resulted in increased polyplex stability, DNA protection and achieved sustained high levels of transgene expression inside 3D PEG-DMA hydrogels for at least 12 days. Furthermore, we demonstrated that E6-based polyplexes allowed efficient gene delivery inside hydrogels with tunable stiffness ranging from 2 to 175 kPa, with the peak transfection efficiency observed in hydrogels with intermediate stiffness (28 kPa). The reported hydrogel-mediated gene delivery platform using biodegradable polyplexes may serve as a local depot for sustained transgene expression in situ to enhance tissue engineering across broad tissue types.

    View details for DOI 10.1016/j.biomaterials.2013.08.050

    View details for PubMedID 24011715

  • Dynamic tissue engineering scaffolds with stimuli-responsive macroporosity formation BIOMATERIALS Han, L., Lai, J. H., Yu, S., Yang, F. 2013; 34 (17): 4251-4258

    Abstract

    Macropores in tissue engineering scaffolds provide space for vascularization, cell-proliferation and cellular interactions, and is crucial for successful tissue regeneration. Modulating the size and density of macropores may promote desirable cellular processes at different stages of tissue development. Most current techniques for fabricating macroporous scaffolds produce fixed macroporosity and do not allow the control of porosity during cell culture. Most macropore-forming techniques also involve non-physiological conditions, such that cells can only be seeded in a post-fabrication process, which often leads to low cell seeding efficiency and uneven cell distribution. Here we report a process to create dynamic hydrogels as tissue engineering scaffolds with tunable macroporosity using stimuli-responsive porogens of gelatin, alginate and hyaluronic acid, which degrade in response to specific stimuli including temperature, chelating and enzymatic digestion, respectively. SEM imaging confirmed sequential pore formation in response to sequential stimulations: 37 °C on day 0, EDTA on day 7, and hyaluronidase on day 14. Bovine chondrocytes were encapsulated in the Alg porogen, which served as cell-delivery vehicles, and changes in cell viability, proliferation and tissue formation during sequential stimuli treatments were evaluated. Our results showed effective cell release from Alg porogen with high cell viability and markedly increased cell proliferation and spreading throughout the 3D hydrogels. Dynamic pore formation also led to significantly enhanced type II and X collagen production by chondrocytes. This platform provides a valuable tool to create stimuli-responsive scaffolds with dynamic macroporosity for a broad range of tissue engineering applications, and may also be used for fundamental studies to examine cell responses to dynamic niche properties.

    View details for DOI 10.1016/j.biomaterials.2013.02.051

    View details for Web of Science ID 000317700400006

    View details for PubMedID 23489920

  • The effects of interactive mechanical and biochemical niche signaling on osteogenic differentiation of adipose-derived stem cells using combinatorial hydrogels ACTA BIOMATERIALIA Nii, M., Lai, J. H., Keeney, M., Han, L., Behn, A., Imanbayev, G., Yang, F. 2013; 9 (3): 5475-5483

    Abstract

    Stem cells reside in a multi-factorial environment containing biochemical and mechanical signals. Changing biochemical signals in most scaffolds often leads to simultaneous changes in mechanical properties, which makes it difficult to elucidate the complex interplay between niche cues. Combinatorial studies on cell-material interactions have emerged as a tool to facilitate analyses of stem cell responses to various niche cues, but most studies to date have been performed on two-dimensional environments. Here we developed three-dimensional combinatorial hydrogels with independent control of biochemical and mechanical properties to facilitate analysis of interactive biochemical and mechanical signaling on adipose-derived stem cell osteogenesis in three dimensions. Our results suggest that scaffold biochemical and mechanical signals synergize only at specific combinations to promote bone differentiation. Leading compositions were identified to have intermediate stiffness (?55kPa) and low concentration of fibronectin (10?g ml(-1)), which led to an increase in osteocalcin gene expression of over 130-fold. Our results suggest that scaffolds with independently tunable niche cues could provide a powerful tool for conducting mechanistic studies to decipher how complex niche cues regulate stem cell fate in three dimensions, and facilitate rapid identification of optimal niche cues that promote desirable cellular processes or tissue regeneration.

    View details for DOI 10.1016/j.actbio.2012.11.002

    View details for Web of Science ID 000315536000007

    View details for PubMedID 23153761

  • Tissue Engineering: Focus on musculoskeletal system Biomaterials Science: An Integrated Clinical and Engineering Approach, CRC Press Michael Keeney, Li-Hsin Han; Sheila Onyiah; Fan Yang 2012: 191-222
  • Solid freeform fabrication of designer scaffolds of hyaluronic acid for nerve tissue engineering BIOMEDICAL MICRODEVICES Suri, S., Han, L., Zhang, W., Singh, A., Chen, S., Schmidt, C. E. 2011; 13 (6): 983-993

    Abstract

    The field of tissue engineering and regenerative medicine will tremendously benefit from the development of three dimensional scaffolds with defined micro- and macro-architecture that replicate the geometry and chemical composition of native tissues. The current report describes a freeform fabrication technique that permits the development of nerve regeneration scaffolds with precisely engineered architecture that mimics that of native nerve, using the native extracellular matrix component hyaluronic acid (HA). To demonstrate the flexibility of the fabrication system, scaffolds exhibiting different geometries with varying pore shapes, sizes and controlled degradability were fabricated in a layer-by-layer fashion. To promote cell adhesion, scaffolds were covalently functionalized with laminin. This approach offers tremendous spatio-temporal flexibility to create architecturally complex structures such as scaffolds with branched tubes to mimic branched nerves at a plexus. We further demonstrate the ability to create bidirectional gradients within the microfabricated nerve conduits. We believe that combining the biological properties of HA with precise three dimensional micro-architecture could offer a useful platform for the development of a wide range of bioartificial organs.

    View details for DOI 10.1007/s10544-011-9568-9

    View details for Web of Science ID 000297121700004

    View details for PubMedID 21773726

  • Three-Dimensional Polymer Constructs Exhibiting a Tunable Negative Poisson's Ratio ADVANCED FUNCTIONAL MATERIALS Fozdar, D. Y., Soman, P., Lee, J. W., Han, L., Chen, S. 2011; 21 (14): 2712-2720

    Abstract

    Young's modulus and Poisson's ratio of a porous polymeric construct (scaffold) quantitatively describe how it supports and transmits external stresses to its surroundings. While Young's modulus is always non-negative and highly tunable in magnitude, Poisson's ratio can, indeed, take on negative values despite the fact that it is non-negative for virtually every naturally occurring and artificial material. In some applications, a construct having a tunable negative Poisson's ratio (an auxetic construct) may be more suitable for supporting the external forces imposed upon it by its environment. Here, three-dimensional polyethylene glycol scaffolds with tunable negative Poisson's ratios are fabricated. Digital micromirror device projection printing (DMD-PP) is used to print single-layer constructs composed of cellular structures (pores) with special geometries, arrangements, and deformation mechanisms. The presence of the unit-cellular structures tunes the magnitude and polarity (positive or negative) of Poisson's ratio. Multilayer constructs are fabricated with DMD-PP by stacking the single-layer constructs with alternating layers of vertical connecting posts. The Poisson's ratios of the single- and multilayer constructs are determined from strain experiments, which show (1) that the Poisson's ratios of the constructs are accurately predicted by analytical deformation models and (2) that no slipping occurrs between layers in the multilayer constructs and the addition of new layers does not affect Poisson's ratio.

    View details for DOI 10.1002/adfm.201002022

    View details for Web of Science ID 000293715800013

    View details for PubMedID 21841943

  • Light-Powered Micromotor: Design, Fabrication, and Mathematical Modeling JOURNAL OF MICROELECTROMECHANICAL SYSTEMS Han, L., Wu, S., Condit, J. C., Kemp, N. J., Milner, T. E., Feldman, M. D., Chen, S. 2011; 20 (2): 487-496
  • Fabrication of three-dimensional scaffolds for heterogeneous tissue engineering BIOMEDICAL MICRODEVICES Han, L., Suri, S., Schmidt, C. E., Chen, S. 2010; 12 (4): 721-725

    Abstract

    The development of biomedical scaffolds mimicking a heterogeneous cellular microenvironment for a specified regulation of cell-fates is very promising for tissue engineering. In this study, three-dimensional scaffolds with heterogeneous microstructure were developed using a DMD-PP apparatus. During the fabrication process, this apparatus can efficiently switch monomers to form microstructures with localized, different material properties; the resolution in the arrangement of material properties is comparable to the characteristic size of functional subunits in living organs, namely, a hundred microns. The effectiveness of this DMD-PP apparatus is demonstrated by a woodpile microstructure with heterogeneous fluorescence and also by a microporous cell-culturing scaffold with selected sites for protein adhesion. Cell-cultivation experiment was performed with the microporous scaffold, in which selective cell adhesion was observed.

    View details for DOI 10.1007/s10544-010-9425-2

    View details for Web of Science ID 000279500200016

    View details for PubMedID 20393801

  • Fluorinated Colloidal Emulsion of Photochangeable Rheological Behavior as a Sacrificial Agent to Fabricate Organic, Three-Dimensional Microstructures LANGMUIR Han, L., Easley, J. A., Ellison, C. J., Chen, S. 2010; 26 (9): 6108-6110

    Abstract

    Three-dimensional organic microfabrication, an emerging technology, faces the challenge of lacking a sacrificial agent (SA) to temporarily support the formation of microscale geometries, which can be removed after a microstructure is constructed. In this study, an ultradense oil-in-organofluorine colloidal emulsion with photopolymerizable submicrometer droplets (diameter approximately 500 nm) was prepared and used as the required SA. Upon exposure to light, the colloidal emulsion undergoes a significant rheological change, which hardens the emulsion and presents the molding/protecting function that an SA must have. Importantly, the emulsion includes a synthesized fluorophilic/fluorophobic block copolymer surfactant to stabilize the droplet compartments, facilitating the dissolution of the postexposure SA. Two successfully built, complex, organic 3D microstructures show the effectiveness of using this novel SA material.

    View details for DOI 10.1021/la100014K

    View details for Web of Science ID 000276969700007

    View details for PubMedID 20349967

  • Integrated Two-Photon Polymerization With Nanoimprinting for Direct Digital Nanomanufacturing Journal of Manufacturing Science and Engineering Wande Zhang, Li-Hsin Han; Shaochen Chen 2010; 132 (3): 030907
  • Light-powered micromotor driven by geometry-assisted, asymmetric photon-heating and subsequent gas convection Applied Physics Letters Li-Hsin Han, Shaomin Wu; J. Christopher Condit; Nate J. Kemp; Thomas E. Milner; Marc D. Feldman;Shaochen Chen 2010; 96 (21): 213509
  • Three-dimensional selective growth of nanoparticles on a polymer microstructure NANOTECHNOLOGY Wu, S., Han, L., Chen, S. 2009; 20 (28)

    Abstract

    We demonstrate a new technique for selectively growing gold nanoparticles on a patterned three-dimensional (3D) polymer microstructure. The technique integrates 3D direct writing of heterogeneous microstructures with nanoparticle synthesis. A digital micromirror device is employed as a dynamic mask in the digital projection photopolymerization process to build the heterogeneous microstructure layer by layer. An amine-bearing polyelectrolyte, branched poly(ethylenimine), is selectively attached to the microstructure and acts as both a reducing and a protective agent in the nanoparticle synthesis. Scanning electron microscopy, energy dispersive x-ray spectroscopy and x-ray photoelectron spectroscopy are utilized to analyze the microstructure and the 3D selectivity of the nanoparticle growth.

    View details for DOI 10.1088/0957-4484/20/28/285312

    View details for Web of Science ID 000267612600016

    View details for PubMedID 19546503

  • Analytical and Experimental Investigations of Electromagnetic Field Enhancement Among Nanospheres With Varying Spacing Journal of Heat Transfer Li-Hsin Han, Wei Wang; Yalin Lu; R. J. Knize; Kitt Reinhardt; John Howell; Shaochen Chen 2009; 131 (3): 033110
  • Projection Microfabrication of Three-Dimensional Scaffolds for Tissue Engineering Journal of Manufacturing Science and Engineering Li-Hsin Han, Gazell Mapili; Shaochen Chen; Krishnendu Roy 2008; 130 (2): 021005
  • Large-area patterning of a solution-processable organic semiconductor to reduce parasitic leakage and off currents in thin-film transistors Applied Physics Letters Kimberly C. Dickey, Sankar Subramanian; John E. Anthony; Li-Hsin Han; Shaochen Chen; Yueh-Lin Loo 2007; 90 (24): 244103
  • Tuning the absorptions of Au nanospheres on a microshell by photo-deformation NANOTECHNOLOGY Han, L., Tang, T., Chen, S. 2006; 17 (18): 4600-4605

    Abstract

    We report the design, modelling, fabrication, and testing of a photo-tunable particulate medium that comprises photo-responsible polymeric microshells covered by Au nanospheres. The microshells were formed by a layer-by-layer method. The charged Au nanospheres were coated on outer surfaces of the microshells. The polymeric shells contain azobenzene moieties and shrink upon ultraviolet irradiation; therefore, the interparticle spaces between the nanospheres are tunable by UV light. The absorption spectrum of the Au nanosphere modified microshells changed drastically after the shrinkage. A decreased absorption peak of Au nanospheres and an enhanced absorption in the near-infrared region were clearly observed. The results of theoretical modelling suggest that the overall spectral changes stem mainly from the enhanced particle interactions and also from the field enhancements in the space among the nanospheres.

    View details for DOI 10.1088/0957-4484/17/18/012

    View details for Web of Science ID 000241157000012

    View details for PubMedID 21727583

  • Wireless bimorph micro-actuators by pulsed laser heating Sensors and Actuators A Li-Hsin Han, Shaochen Chen 2005; 121 (1): 35-43