Injectable immunomodulation-based porous chitosan microspheres/HPCH hydrogel composites as a controlled drug delivery system for osteochondral regeneration.

The inappropriate regenerated fibrous cartilage and subchondral bone of the injured chondral defect ultimately cause degeneration of the regenerated cartilage, which eventually leads to the failure of cartilage repair. In this study, we developed a macrophage-modulated and injectable 'building block' drug delivery system comprised of porous chitosan (CS) microspheres and hydroxypropyl chitin (HPCH) hydrogel, where the dimethyloxallyl glycine (DMOG) was encapsulated in the thermosensitive HPCH hydrogel (HD) while kartogenin (KGN) was conjugated on the porous CS microspheres (CSK-PMS). The developed HD/CSK-PMS composite scaffold effectively modulated the microenvironment at the defect site, achieved local macrophage M2 polarization and promoted cartilage regeneration. The fast-degradable HD favored hyaline cartilage regeneration, while the highly stable CSK-PMS supported the endochondral ossification and regenerated the subchondral bone. In vitro and in vivo evaluations revealed that the newly developed HD/CSK-PMS as a controlled drug delivery system could effectively create M2 macrophage microenvironment and orchestrate osteochondral (OC) regeneration. These findings indicate the importance of the immune microenvironment and subchondral bone for high-quality cartilage repair, and thus the immunomodulation-based hydrogel/PMS composite system could be a promising candidate for OC regeneration.