Injectable in situ-forming hydrogel for cartilage tissue engineering

Methoxy polyethylene glycol-poly(ε-caprolactone) (MPEG-PCL; MP) diblock copolymers undergo a solution-to-gel phase transition at body temperature and serve as ideal biomaterials for drug delivery and tissue engineering. Here, we examined the potential use of a chondrocyte-loaded MP solution as an injectable, in situ-forming hydrogel for cartilage regeneration. The chondrocyte-MP solution underwent a temperature-dependent solution-to-gel phase transition in vitro, as shown by an increase in viscosity from 1 cP at 20-30 °C to 1.6 × 10⁵ cP at 37 °C. The chondrocytes readily attached to and proliferated on the MP hydrogel in vitro. The chondrocyte-MP solution transitioned to a hydrogel immediately after subcutaneous injection into mice, and formed an interconnected pore structure required to support the growth, proliferation, and differentiation of the chondrocytes. The chondrocyte-MP hydrogels formed cartilage in vivo, as shown by the histological and immunohistochemical staining of glycosaminoglycans, proteoglycans, and type II collagen, the major components of cartilage. Cartilage formation increased with hydrogel implantation time, and the expression of glycosaminoglycans, and type II collagen reached maximal levels at 6 weeks post-implantation. Collectively, these data suggest that in situ-forming chondrocyte-MP hydrogels have potential as non-invasive alternatives for tissue-engineered cartilage formation.