Current Projects

Composite Biomaterials for Tissue Engineering and Regenerative Medicine

We are developing composite biomaterials consisting of the proteins collagen I and fibrin, which are simultaneously polymerized to produce interpenetrating polymer networks. By embedding cells directly into such matrices at the time of polymerization, we produce rudimentary tissues. A main focus of this project is to develop mechanistic models of how such composites lead to increased mechanical properties, by combining detailed characterization of the microstructure of these matrices with macroscopic materials testing. A second arm of this project examines how cell function is altered in such 3D matrices. We examine changes in gene and protein expression in response to changes in composite structure, and link these changes to intracellular signaling events. Our goal is to determine which key signaling pathways are involved in controlling cell function in 3D matrices, and to design environments that can regulate these pathways.

The mechanical testing techniques we use include conventional compressing and tensile testing, as well as viscoelastic properties using dynamic mechanical analysis, creep recovery and stress relaxation tests. In addition, we have initiated collaborations on multiscale computational modeling of our materials, and are investigating the addition of other polymers in the matrix, as well as the application of defined mechanical strain. We have developed initial mechanistic models of how protein composite materials behave under stress and how they fail, as well as how the mechanical properties and protein composition affect cell function. These conceptual models provide a framework from which we can generate testable hypotheses.

Directed Differentiation of Stem Cells

This project involves embedding stem cells (currently we use adult human mesenchymal stem cells, hMSC) directly in protein-based hydrogel beads of 50-200 micron diameter. This allows us to tailor the protein composition of the bead in order to promote differentiation of the embedded hMSC toward a desired lineage. The advantage of the bead format is that it requires only small amounts of protein, and the beads can be concentrated and handled as a slurry or paste, such that beads can be delivered directly to a site of injury. Our current focus is on osteogenic differentiation of hMSC for the purpose of bone repair. We have developed a variety of bead formulations using composites of collagen, vitronectin, fibronectin, gelatin, chitosan, and agarose. We also are extending this technology to the enhancement of stem cell engraftment in the heart after transplantation.

The technology of making defined 3D microenvironments from biologically-relevant proteins and polysaccharides has potentially broad application. In addition to the orthopedic and cardiovascular applications currently being pursued, this technique can be applied to other tissue types in which the cellular microenvironment is of particular importance, for example cartilage and neural tissue. This project also can be extended to other types of stem cells, in which the cellular niche is known to control cell fate. Because the bead format offers a way to deliver cells while still encased in the appropriate protein environment, we have begun development of a minimally invasive delivery system and we have initiated pilot studies on the use of protein-based beads for the suspension culture of anchorage-dependent mammalian cells.

Collagen-Carbon Nanotube Composite Biomaterials

The goal of this project is to combine the self-assembly features of the structural protein collagen with the unique physical properties of carbon nanotubes (CNT), to produce a new class of mechanically robust and electrically conductive biomaterials. CNT are attractive as additives in fiber-reinforced composites due to their very high aspect ratio, remarkable strength in tension, and unparalleled electrical conductivity. This project harnesses the biochemical process of collagen fibrillogenesis and the biological process of cell-mediated collagen remodeling to induce alignment of CNT in collagen matrices. The current focus is on gaining a fundamental understanding of protein-CNT composite materials and how their properties can be enhanced, and then using this understanding to rationally design the next generation of functional biomaterials. We currently are using such biomaterials to develop nerve guidance materials for neural repair.

This is a challenging project that promises to deliver a new class of protein-CNT composite biomaterials. A key to achieving enhanced electrical and mechanical properties is the generation of anisotropy in the matrix, and we use two types of mechanical strain bioreactor system to pursue this goal. The materials we envision would have use in studying and replacing tissues in which electrical conductivity or signaling is important, e.g. heart, nerve, and potentially bone. We also plan to create similar protein-CNT materials using other ECM proteins, and to use them as electrically conductive substrates to study and guide cell function, or as leads in biosensor development. Finally, we expect that CNT loading also will enhance the mechanical properties of these matrices, allowing them to be used in more demanding load-bearing environments than pure protein matrices.