Encapsulating chondrocytes in copolymer gels: bimodal degradation kinetics influence cell phenotype and extracellular matrix development.

Hydrogels provide an ideal environment for encapsulating chondrocytes and facilitating the production of cartilaginous tissue. However, the deposition of extracellular matrix (ECM) and ultimate tissue function are significantly affected by degradation of gel scaffolds. It was hypothesized that a bimodal degradation process would capture the critical features necessary for neotissue development. Specifically, most of the initial crosslinks would degrade quickly and enable ECM deposition, whereas a critical amount would remain or degrade much more slowly to provide structural integrity over a longer time period. In this study, chondrocytes were encapsulated in copolymer gels of nondegradable [poly(ethylene glycol) dimethacrylate] and degradable [poly(lactic acid)-b-poly(ethylene glycol)-b-poly(lactic acid) dimethacrylate] macromers to investigate the effects of gel degradation on ECM evolution. All gels were synthesized from 10 wt % total macromer solutions consisting of 0, 19, 21, 23, 25, or 100 mol % nondegradable units. The copolymer constructs were found to have lower DNA content than completely degradable constructs after 8 weeks. However, total biochemical content was very similar among the various copolymer constructs. Histological analysis gave more interesting insight, showing a more uniform spatial distribution of ECM components in copolymer samples than in constructs with 100 mol % nondegradable units. In addition, a number of major structural defects were present in constructs with 0 mol % nondegradable units that became less apparent as the amount of nondegradable units was increased. Overall, the copolymer gels had a higher compressive modulus during neotissue development and also showed no evidence of chondrocyte dedifferentiation. With their bimodal degradation profile, copolymer gels with carefully selected ratios of degrading to slow or nondegrading crosslinks provide distinct advantages for ECM development in tissue-engineered cartilage.