Professional Education

  • Doctor of Philosophy, Beijing Institute Of Technology (2009)

Stanford Advisors

  • Fan Yang, Postdoctoral Faculty Sponsor


All Publications

  • The effects of varying poly(ethylene glycol) hydrogel crosslinking density and the crosslinking mechanism on protein accumulation in three-dimensional hydrogels ACTA BIOMATERIALIA Lee, S., Tong, X., Yang, F. 2014; 10 (10): 4167-4174
  • Chondrogenic Differentiation of Adipose-Derived Stromal Cells in Combinatorial Hydrogels Containing Cartilage Matrix Proteins with Decoupled Mechanical Stiffness TISSUE ENGINEERING PART A Wang, T., Lai, J. H., Han, L., Tong, X., Yang, F. 2014; 20 (15-16): 2131-2139


    Adipose-derived stromal cells (ADSCs) are attractive autologous cell sources for cartilage repair given their relative abundance and ease of isolation. Previous studies have demonstrated the potential of extracellular matrix (ECM) molecules as three-dimensional (3D) scaffolds for promoting chondrogenesis. However, few studies have compared the effects of varying types or doses of ECM molecules on chondrogenesis of ADSCs in 3D. Furthermore, increasing ECM molecule concentrations often result in simultaneous changes in the matrix stiffness, which makes it difficult to elucidate the relative contribution of biochemical cues or matrix stiffness on stem cell fate. Here we report the development of an ECM-containing hydrogel platform with largely decoupled biochemical and mechanical cues by modulating the degree of methacrylation of ECM molecules. Specifically, we incorporated three types of ECM molecules that are commonly found in the cartilage matrix, including chondroitin sulfate (CS), hyaluronic acid (HA), and heparan sulfate (HS). To elucidate the effects of interactive biochemical and mechanical signaling on chondrogenesis, ADSCs were encapsulated in 39 combinatorial hydrogel compositions with independently tunable ECM types (CS, HA, and HS), concentrations (0.5%, 1.25%, 2.5%, and 5% [w/v]), and matrix stiffness (3, 30, and 90 kPa). Our results show that the effect of ECM composition on chondrogenesis is dependent on the matrix stiffness of hydrogels, suggesting that matrix stiffness and biochemical cues interact in a nonlinear manner to regulate chondrogenesis of ADSCs in 3D. In soft hydrogels (∼3kPa), increasing HA concentrations resulted in substantial upregulation of aggrecan and collagen type II expression in a dose-dependent manner. This trend was reversed in HA-containing hydrogels with higher stiffness (∼90 kPa). The platform reported herein could provide a useful tool for elucidating how ECM biochemical cues and matrix stiffness interact together to regulate stem cell fate, and for rapidly optimizing ECM-containing scaffolds to support stem cell differentiation and tissue regeneration.

    View details for DOI 10.1089/ten.tea.2013.0531

    View details for Web of Science ID 000340846500009

  • Bioengineered 3D Brain Tumor Model To Elucidate the Effects of Matrix Stiffness on Glioblastoma Cell Behavior Using PEG-Based Hydrogels. Molecular pharmaceutics Wang, C., Tong, X., Yang, F. 2014; 11 (7): 2115-2125


    Glioblastoma (GBM) is the most common and aggressive form of primary brain tumor with a median survival of 12-15 months, and the mechanisms underlying GBM tumor progression remain largely elusive. Given the importance of tumor niche signaling in driving GBM progression, there is a strong need to develop in vitro models to facilitate analysis of brain tumor cell-niche interactions in a physiologically relevant and controllable manner. Here we report the development of a bioengineered 3D brain tumor model to help elucidate the effects of matrix stiffness on GBM cell fate using poly(ethylene-glycol) (PEG)-based hydrogels with brain-mimicking biochemical and mechanical properties. We have chosen PEG given its bioinert nature and tunable physical property, and the resulting hydrogels allow tunable matrix stiffness without changing the biochemical contents. To facilitate cell proliferation and migration, CRGDS and a MMP-cleavable peptide were chemically incorporated. Hyaluronic acid (HA) was also incorporated to mimic the concentration in the brain extracellular matrix. Using U87 cells as a model GBM cell line, we demonstrate that such biomimetic hydrogels support U87 cell growth, spreading, and migration in 3D over the course of 3 weeks in culture. Gene expression analyses showed U87 cells actively deposited extracellular matrix and continued to upregulate matrix remodeling genes. To examine the effects of matrix stiffness on GBM cell fate in 3D, we encapsulated U87 cells in soft (1 kPa) or stiff (26 kPa) hydrogels, which respectively mimics the matrix stiffness of normal brain or GBM tumor tissues. Our results suggest that changes in matrix stiffness induce differential GBM cell proliferation, morphology, and migration modes in 3D. Increasing matrix stiffness led to delayed U87 cell proliferation inside hydrogels, but cells formed denser spheroids with extended cell protrusions. Cells cultured in stiff hydrogels also showed upregulation of HA synthase 1 and matrix metalloproteinase-1 (MMP-1), while simultaneously downregulating HA synthase 2 and MMP-9. This suggests that varying matrix stiffness can induce differential ECM deposition and remodeling by employing different HA synthases or MMPs. Furthermore, increasing matrix stiffness led to simultaneous upregulation of Hras, RhoA, and ROCK1, suggesting a potential link between the mechanosensing pathways and the observed differential cell responses to changes in matrix stiffness. The bioengineered 3D hydrogel platform reported here may provide a useful 3D in vitro brain tumor model for elucidating the mechanisms underlying GBM progression, as well as for evaluating the efficacy of potential drug candidates for treating GBM.

    View details for DOI 10.1021/mp5000828

    View details for PubMedID 24712441

  • 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


    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 Web of Science ID 000332912100016

  • Engineering interpenetrating network hydrogels as biomimetic cell niche with independently tunable biochemical and mechanical properties BIOMATERIALS Tong, X., Yang, F. 2014; 35 (6): 1807-1815


    Hydrogels have been widely used as artificial cell niche to mimic extracellular matrix with tunable properties. However, changing biochemical cues in hydrogels developed-to-date would often induce simultaneous changes in mechanical properties, which do not support mechanistic studies on stem cell-niche interactions. Here we report the development of a PEG-based interpenetrating network (IPN), which is composed of two polymer networks that can independently and simultaneously crosslink to form hydrogels in a cell-friendly manner. The resulting IPN hydrogel allows independently tunable biochemical and mechanical properties, as well as stable and more homogeneous presentation of biochemical ligands in 3D than currently available methods. We demonstrate the potential of our IPN platform for elucidating stem cell-niche interactions by modulating osteogenic differentiation of human adipose-derived stem cells. The versatility of such IPN hydrogels is further demonstrated using three distinct and widely used polymers to form the mechanical network while keeping the biochemical network constant.

    View details for DOI 10.1016/j.biomaterials.2013.11.064

    View details for Web of Science ID 000331018700004

  • 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


    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


    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 Web of Science ID 000326901200042

    View details for PubMedID 24011715

  • Effects of Polymer End-Group Chemistry and Order of Deposition on Controlled Protein Delivery from Layer-by-Layer Assembly BIOMACROMOLECULES Keeney, M., Mathur, M., Cheng, E., Tong, X., Yang, F. 2013; 14 (3): 794-800


    Layer-by-layer (LBL) assembly is an attractive platform for controlled release of biologics given its mild fabrication process and versatility in coating substrates of any shape. Proteins can be incorporated into LBL coatings by sequentially depositing oppositely charged polyelectrolytes, which self-assemble into nanoscale films on medical devices or tissue engineering scaffolds. However, previously reported LBL platforms often require the use of a few hundred layers to avoid burst release, which hinders their broad translation due to the lengthy fabrication process, cost, and batch-to-batch variability. Here we report a biodegradable LBL platform composed of only 10 layers with tunable protein release kinetics, which is an order of magnitude less than previously reported LBL platforms. We performed a combinatorial study to examine the effects of polymer chemistry and order of deposition of poly(β-amino) esters on protein release kinetics under 81 LBL assembly conditions. Using the optimal "polyelectrolyte couples" for constructing the LBL film, basic fibroblast growth factor (bFGF) was released gradually over 14 days with retained biological activity to stimulate cell proliferation. The method reported herein is applicable for coating various substrates including metals, polymers, and ceramics and may be used for a broad range of biomedical and tissue engineering applications.

    View details for DOI 10.1021/bm3018559

    View details for Web of Science ID 000316044700024

    View details for PubMedID 23360295

  • Evaluation of an in situ chemically crosslinked hydrogel as a long-term vitreous substitute material ACTA BIOMATERIALIA Tao, Y., Tong, X., Zhang, Y., Lai, J., Huang, Y., Jiang, Y., Guo, B. 2013; 9 (2): 5022-5030


    Currently there is no material that can be used as a long-term vitreous substitute, and this remains an unmet clinical need in ophthalmology. In this study, we developed an injectable, in situ chemically crosslinked hydrogel system and evaluated it in a rabbit model. The system consisted of two components, both based on multi-functional poly(ethylene glycol) (PEG) but with complementarily reactive end groups of thiol and active vinyl groups, respectively. The two components are mixed and injected as a solution mixture, react in vivo via the Michael addition route and form a chemically crosslinked hydrogel in situ. The linkages between the end groups and the backbone PEG chains are specially designed to ensure that the final network structure is hydrolysis-resistant. In the rabbit study and with an optimized operation protocol, we demonstrated that the hydrogel indeed formed in situ after injection, and remained transparent and stable during the study period of 9 months without significant adverse reactions. In addition, the hydrogel formed in situ showed rheological properties very similar to the natural vitreous. Therefore, our study demonstrated that this in situ chemically crosslinked PEG gel system is suitable as a potential long-term vitreous substitute.

    View details for DOI 10.1016/j.actbio.2012.09.026

    View details for Web of Science ID 000315170800009

    View details for PubMedID 23022890

  • A New End Group Structure of Poly(ethylene glycol) for Hydrolysis-Resistant Biomaterials JOURNAL OF POLYMER SCIENCE PART A-POLYMER CHEMISTRY Tong, X., Lai, J., Guo, B., Huang, Y. 2011; 49 (6): 1513-1516

    View details for DOI 10.1002/pola.24562

    View details for Web of Science ID 000288465600024

  • Toward the synthesis of sequence-controlled vinyl copolymers CHEMICAL COMMUNICATIONS Tong, X., Guo, B., Huang, Y. 2011; 47 (5): 1455-1457


    An ATRA based strategy to synthesize vinyl copolymers with monomer-level sequence control is proposed. In each cycle, one allyl alcohol is added to the ATRP chain end, and then the hydroxymethyl residue is oxidized to carboxylic acid and a side group is introduced via esterification, making the new chain end active for the next cycle.

    View details for DOI 10.1039/c0cc04807k

    View details for Web of Science ID 000286389500015

    View details for PubMedID 21125120

  • End-capping double-chain stranded polypseudorotaxanes using lengthily tunable poly(2-hydroxyethyl methacrylate) blocks via atom transfer radical polymerization POLYMER INTERNATIONAL Tong, X., Gao, P., Zhang, X., Ye, L., Zhang, A., Feng, Z. 2010; 59 (7): 917-922

    View details for DOI 10.1002/pi.2806

    View details for Web of Science ID 000279476200008

  • Synthesis and characterization of block copolymers comprising a polyrotaxane middle block flanked by two brush-like PCL blocks SOFT MATTER Tong, X., Zhang, X., Ye, L., Zhang, A., Feng, Z. 2009; 5 (9): 1848-1855

    View details for DOI 10.1039/b818819j

    View details for Web of Science ID 000265413500013

  • Novel main-chain polyrotaxanes synthesized via ATRP of HEMA initiated with polypseudorotaxanes comprising BriB-PEG-iBBr and alpha-CDs POLYMER Tong, X., Zhang, X., Ye, L., Zhang, A., Feng, Z. 2008; 49 (21): 4489-4493
  • Novel main-chain polyrotaxanes synthesized via ATRP of HPMA in aqueous media JOURNAL OF POLYMER SCIENCE PART A-POLYMER CHEMISTRY Zhang, X., Zhu, X., Tong, X., Ye, L., Zhang, A., Feng, Z. 2008; 46 (15): 5283-5293

    View details for DOI 10.1002/pola.22856

    View details for Web of Science ID 000257915000031

  • A kind of novel biodegradable hydrogel made from copolymerization of gelatin with polypseudorotaxanes based on alpha-CDs BIOMEDICAL MATERIALS Hou, D., Tong, X., Yu, H., Zhang, A., Feng, Z. 2007; 2 (3): S147-S152


    A kind of novel biodegradable supramolecular hydrogel was synthesized via copolymerization of gelatin methacrylamide with photocurable and biodegradable polypseudorotaxanes under UV irradiation. These polypseudorotaxanes were prepared by supramolecular self-assemblies of alpha-cyclodextrins threaded onto amphiphilic LA-PEG-LA copolymers end-capped with methacryloyl groups. The hydrogels are injectable, and their structure was characterized in detail with FTIR, (1)H NMR, XRD, TG and DSC techniques. Their swelling behaviour and morphologies were also examined. The analytical results demonstrated that the channel-type crystalline structure of the polypseudorotaxanes remains in the as-obtained hydrogels. Moreover, the SEM pictures showed that the hydrogels having gelatin methacrylamide are more suitable for cell seeding and proliferation than those without gelatin added.

    View details for DOI 10.1088/1748-6041/2/3/S12

    View details for Web of Science ID 000249597800013

    View details for PubMedID 18458460

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