Functionalized Thermosensitive Hydrogel Regulates the Immune Microenvironment and Stem Cell Differentiation to Enhance Cartilage Regeneration

Osteoarthritis (OA) is a heterogeneous degenerative joint disease involving cartilage degradation, subchondral bone remodeling, and chronic inflammation, which collectively lead to pain and disability. Current therapeutic strategies remain insufficient to effectively halt disease progression. To address this, we engineered a multifunctional injectable hydrogel based on hydroxybutyl chitosan (HBC) coloaded with strontium acetate (Sr) and nanosized chondroprogenitor cell-derived exosomes (CPC-EXOs) (HBC@Sr@CPC-EXOs) to modulate the inflammatory joint microenvironment and drive cartilage regeneration. The hydrogel underwent a sol–gel transition at ∼17 °C (gelling at body temperature) and swelled to equilibrium in ∼16 h, forming a water-rich matrix. Release profiling showed ∼50% cumulative Sr release at 36 h and exosome release at 60 h, yielding a functionally sequential (time-staggered) exposure, where Sr mediates early attenuation of inflammatory mediators, while CPC-EXOs sustain late-phase immunoprogramming. Under inflammatory conditions in vitro, HBC@Sr@CPC-EXOs enhanced chondrocyte viability, increased extracellular matrix anabolism, and polarized macrophages toward M2. Time-resolved assays revealed early suppression of pro-inflammatory cytokines (0–12 h), followed by later enhancement of immunoregulatory cytokines (24–48 h) ( p < 0.05). Mechanistically, HBC@Sr@CPC-EXOs activated TGF-β1/Smad2/3 while suppressing Notch, driving bone marrow stromal cell (BMSC) chondrogenic differentiation. In a Sprague–Dawley (S–D) rat osteochondral defect model, HBC@Sr@CPC-EXOs achieved superior osteochondral repair, including higher International Cartilage Repair Society (ICRS) scores and greater subchondral bone regeneration than the Control ( p < 0.05). Collectively, HBC@Sr@CPC-EXOs integrate immunomodulatory and regenerative cues, offering a promising strategy for attenuating OA progression and fostering cartilage repair.