An injectable polymer hydrogel with hyaluronic acid as the backbone and end-grafted poly(N-isopropylacrylamide) as the thermo-sensitive moiety was synthesized. The thermo-responsive hyaluronic acid-g-poly(N-isopropylacrylamide) (HPN) copolymer was used a scaffold for cultivation of bone marrow mesenchymal stem cells (MSCs), which could be induced into articular chondrocytes in vitro for the treatment of cartilage defect. The copolymers exhibited temperature-sensitive reversibly soluble-insoluble characteristics with the lower critical solution temperature (LCST) being around 30°C, which depends on polymer compositions during grafting reactions. Results from scanning electron microscopy (SEM) confirmed a porous, inter-connected, 3-D structure of this polymer at the physiological temperature. Other temperature-dependent physical properties of the copolymer hydrogel, such as rheological properties, water content and volume change, phase transition kinetics, and reusability were also characterized in detail. For MSCs culture, 3-D cultures with HPN copolymer hydrogels were compared with 2-D cultures on TCPS for cell functionality. In addition, gelatin and transforming growth factor-β1 (TGF-β1) was incorporated into HPN by conjugating the protein to the copolymer through the carboxylic groups of HA with 1-(3-dimethylamino-propyl)-3-ethyl-carbodiimide hydrochloride (EDC) as a crosslinking agent. MSCs were seeded in HPN scaffolds with and without TGF-β1 and cultured under conditions of different periods of chondrogenic differentiation induction. Results including cell proliferation, cell viability, light microscopy and SEM observations, and glycosaminoglycans (GAG) and collagen synthesis were studied. From the experimental results obtained, we concluded that MSCs can be successfully induced into articular chondrocytes in HPN and the hydrogel shows a promising potential to be a suitable 3-D scaffold for tissue engineering of cartilage.

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