Development of thermally-crosslinked hydrogels as injectable cell carriers for orthopaedic tissue engineering
Temenoff, Johnna Sue
Mikos, Antonios G.
Doctor of Philosophy
Synthetic hydrogel materials based on oligo(poly(ethylene glycol) fumarate) (OPF) were developed and characterized as injectable cell carriers for orthopaedic tissue engineering. Through alteration of the poly(ethylene glycol) molecular weight used in the synthesis of the OPF macromer, swelling and mechanical properties of the resulting crosslinked hydrogels could be controlled. These hydrogels were characterized, in both the swollen and dry states, leading to calculation of their mesh sizes, which varied depending on the OPF type used in crosslinking. In addition, it was found that these gels could be laminated during crosslinking, with each layer having distinct mechanical properties. Before their use as injectable cell carriers, the cytotoxicity of all OPF hydrogel precursor molecules, including radical initiators and their derivatives, was evaluated using rat marrow stromal cells as a model cell type. Results indicated that the overall pH of the formulation, as well as length of exposure to the components, had significant effects on cell viability. Using this information, an initiator was identified which remained near neutral pH in cell culture media and resulted in crosslinking of two types of OPF hydrogels in 8 min at 37°C. The optimized OPF formulations were then used to investigate effects of changes in hydrogel swelling properties and media supplements on osteogenic differentiation of encapsulated rat marrow stromal cells. After 28 days of in vitro culture, evidence of cellular differentiation was found in all sample types, indicating that the encapsulation procedure did not have a detrimental effect on the ability of the marrow stromal cells to form bone-like tissue. In the presence of osteogenic supplements, OPF hydrogels with greater swelling promoted embedded MSC differentiation over those that swelled less. In all specimens examined, areas of mineralized matrix were obvious many microns away from the cells, indicating that the hydrogel mesh size was large enough to allow diffusion of matrix components throughout the material. These results demonstrate the great potential of OPF hydrogels as injectable carriers for delivery of cells to a variety of complex orthopaedic defects.