Biodegradable Hydrogel Composites for Growth Factor and Stem Cell Delivery in Osteochondral Tissue Engineering
Mikos, Antonios G
Doctor of Philosophy
Cartilage has a limited endogenous ability for self-repair and current clinical treatments for damaged or diseased cartilage tissue are insufficient. Additionally, there is a biological and mechanical interplay between cartilage and the underlying subchondral bone, linking the pathogenesis/regeneration of both tissues. Thus, this thesis seeks to develop hydrogel composites as growth factor and cell delivery vehicles to study the regeneration of osteochondral tissue. First, we investigated the release of growth factors from acellular hydrogel composites containing gelatin microparticles (GMPs) to stimulate the repair of cartilage tissue in an in vivo osteochondral defect model. Transforming growth factor-β3 (TGF-β3) with varying release kinetics and/or insulin-like growth factor-1 (IGF-1) were delivered from the chondral layer of bilayered hydrogel composites while the subchondral layer remained growth factor-free. Results demonstrated that dual delivery of TGF-β3 and IGF-1 did not synergistically enhance cartilage repair, regardless of release kinetics, and the delivery of IGF-1 alone positively stimulated osteochondral tissue repair. Subsequently, we focused on improving the repair of the subchondral bone. The second part of this thesis investigated the delivery of IGF-1 and bone morphogenetic protein-2 (BMP-2) from the chondral and subchondral layers, respectively, of bilayered scaffolds in vivo. Results showed that BMP-2 enhanced subchondral bone repair, and that while the dual delivery of both growth factors did not improve cartilage repair, they synergistically enhanced subchondral bone formation over the delivery of IGF-1 alone. Using the results from this study, we also investigated relationships between specific cartilage and bone repair metrics to provide a fuller understanding of the osteochondral repair process. Correlation analysis revealed an intrinsic association between the degree of subchondral bone formation and cartilage surface regularity. Lastly, the third part of this thesis investigated the hydrogel composites as stem cell delivery vehicles. Degradable GMPs were used as temporary adherent substrates for anchorage-dependent mesenchymal stem cells (MSCs). MSCs were seeded onto GMPs and subsequently encapsulated in hydrogels to investigate their role on influencing MSC differentiation and aggregation. Non-seeded MSCs co-encapsulated with GMPs in the hydrogels were used as a control for comparison. Results revealed that MSC-seeded GMPs exhibited more cell-cell contacts, greater chondrogenic potential, and a down-regulation of osteogenic markers compared to the controls. Overall, these hydrogel composites demonstrate potential as growth factor and cell delivery vehicles for the stimulation and study of osteochondral tissue regeneration.