Injectable biodegradable hydrogel composites for rabbit marrow mesenchymal stem cell and growth factor delivery for cartilage tissue engineering

We investigated the development of an injectable, biodegradable hydrogel composite of oligo(poly(ethylene glycol) fumarate) (OPF) with encapsulated rabbit marrow mesenchymal stem cells (MSCs) and gelatin microparticles (MPs) loaded with transforming growth factor-β1 (TGF-β1) for cartilage tissue engineering applications. Rabbit MSCs and TGF-β1-loaded MPs were mixed with OPF, a poly(ethylene glycol)-diacrylate crosslinker and the radical initiators ammonium persulfate and N,N,N′,N′-tetramethylethylenediamine, and then crosslinked at 37 °C for 8 min to form hydrogel composites. Three studies were conducted over 14 days in order to examine the effects of: (1) the composite formulation, (2) the MSC seeding density, and (3) the TGF-β1 concentration on the chondrogenic differentiation of encapsulated rabbit MSCs. Bioassay results showed no significant difference in DNA amount between groups, however, groups with MPs had a significant increase in glycosaminoglycan content per DNA starting at day 7 as compared to controls at day 0. Chondrocyte-specific gene expression of type II collagen and aggrecan were only evident in groups containing TGF-β1-loaded MPs and varied with TGF-β1 concentration in a dose-dependent manner. Specifically, type II collagen gene expression exhibited a 161±49-fold increase and aggrecan gene expression a 221±151-fold increase after 14 days with the highest dose of TGF-β1 (16 ng/ml). These results indicate that encapsulated rabbit MSCs remained viable over the culture period and differentiated into chondrocyte-like cells, thus suggesting the potential of OPF composite hydrogels as part of a novel strategy for localized delivery of stem cells and bioactive molecules.