Biofunctional carboxymethyl chitosan hydrogels with nano-hydroxyapatite gradients accelerate bone defect healing

Large bone defects remain a clinical challenge due to limited regenerative capacity. In this study, carboxymethyl chitosan (CMCS) hydrogels incorporating gradient nano-hydroxyapatite (nHA) were developed to enhance bone repair. The composite hydrogels exhibited improved mechanical properties, controlled degradation, and good biocompatibility. In vitro, nHA-containing hydrogels promoted bone marrow mesenchymal stem cell (BMSC) proliferation and osteogenic differentiation, as shown by increased alkaline phosphatase (ALP) activity, mineralization, and upregulation of runt-related transcription factor 2 (RUNX2), osteocalcin (OCN), and bone morphogenetic protein-2 (BMP-2). In vivo, implantation in a rat distal femoral condyle defect model showed enhanced new bone formation and improved trabecular structure, particularly in the 1% nHA group. These results suggest that gradient nHA-loaded CMCS hydrogels can effectively enhance osteogenesis and support bone regeneration.