Co-delivery of Growth Factor-Loaded Microspheres and Adipose-Derived Stem Cells in A Gel Matrix for Cartilage Repair
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Co-delivery of the embedded growth factor-loaded microspheres and adult stem cells in a hydrogel matrix was studied for its potential as a cell-based therapeutic strategy for cartilage regeneration in partial thickness chondral defects. A photopolymerizable N-methacrylate glycol chitosan (MGC) was employed to form an in situ gel that was embedded with two formulations of growth factor-loaded microspheres and human adipose-derived stem cells (ASC). The polymeric microspheres were used as a delivery vehicle for the controlled release of growth factors to stimulate differentiation of the ASC towards the chondrocyte lineage. The microspheres were made of amphiphilic low molecular weight (Mn < 10,000 Da) poly(1,3-trimethylene carbonate-co--caprolactone)-b-poly(ethylene glycol)-b-poly(1,3-trimethylene carbonate-co--caprolactone) (P(TMC-CL)2-PEG)). This triblock copolymer is solid below 100C, but liquid with a low degree of crystallinity at physiological temperature and degrades slowly, and so acidic degradation products do not accumulate locally. Bone morphogenetic protein-6 (BMP-6) and transforming growth factor-3 (TGF-3) were delivered at 5 ng/day with initial bursts of 14.3 and 23.6%, respectively. Both growth factors were highly bioactive when released, retaining greater than 95% bioactivity for 33 days as measured by cell-based assays. To improve ASC viability within the MGC vehicle, an RGD-containing ligand was grafted to the MGC backbone. Prior to chondrogenic induction within the MGC gel, ASC viability was assessed and greater than 90% of ASC were viable in the gel grafted with cell-adhesive RGD peptides as compared to that in non-RGD grafted gels. For ASC chondrogenesis induced by the sustained release of BMP-6 and TGF-3 in MGC gels, the ASC cellularity and glycosaminosglycan production were similar for 28 days. The ratio of collagen type II to I per cell (normalized to deoxyribonucleic acid content) in the microsphere delivery group was significantly higher than that of non-induced ASC or with soluble growth factor administration in the culture media, and increased with time. Thus, the co-delivery of growth factor-loaded microspheres and ASC in MGC gels successfully induced ASC chondrogenesis and is a promising strategy for cartilage repair.