Impact of Inter- and Intra-Donor Variability by Age on the Gel-to-Tissue Transition in MMP-Sensitive PEG Hydrogels for Cartilage Regeneration.

Matrix metalloproteinase (MMP)-sensitive hydrogels are promising for cartilage tissue engineering due to cell-mediated control over hydrogel degradation. However, any variability in MMP, tissue inhibitors of matrix metalloproteinase (TIMP), and/or extracellular matrix (ECM) production among donors will impact neotissue formation in the hydrogels. The goal for this study was to investigate the impact of inter- and intra-donor variability on the hydrogel-to-tissue transition. Transforming growth factor β3 was tethered into the hydrogel to maintain the chondrogenic phenotype and support neocartilage production, allowing the use of chemically defined medium. Bovine chondrocytes were isolated from two donor groups, skeletally immature juvenile and skeletally mature adult donors (inter-donor variability) and three donors within each group (intra-donor group variability). While the hydrogel supported neocartilaginous growth by all donors, donor age impacted MMP, TIMP, and ECM synthesis rates. Of the MMPs and TIMPs studied, MMP-1 and TIMP-1 were the most abundantly produced by all donors. Adult chondrocytes secreted higher levels of MMPs, which was accompanied by higher production of TIMPs. Juvenile chondrocytes exhibited more rapid ECM growth. By day 29, juvenile chondrocytes had surpassed the gel-to-tissue transition. On the contrary, the adult donors had a percolated polymer network indicating that despite higher levels of MMPs the gel-to-transition had not yet been achieved. The intra-donor group variability of MMP, TIMP, and ECM production was higher in adult chondrocytes but did not impact the extent of the gel-to-tissue transition. In summary, age-dependent inter-donor variations in MMPs and TIMPs significantly impact the timing of the gel-to-tissue transition in MMP-sensitive hydrogels.