Enhanced Bone-Defect Regeneration Through nHA/Chitosan Nanocomposite-Facilitated Delivery of HUCB-MSCs-Derived Exosomes.

Critical-sized bone defects lack spontaneous healing capacity. While mesenchymal stem cell-derived exosomes (sEVs) are promising osteoinductive agents, their rapid in vivo clearance limits their free-form efficacy. Here, we fabricated a nano-hydroxyapatite/chitosan (nHA/CTS) composite scaffold as a protective, sustained-delivery platform for human umbilical cord blood-derived mesenchymal stem cell exosomes (HUCB-MSCs-exos) to accelerate bone repair. The 3D porous CTS/10% nHA scaffold exhibited excellent cytocompatibility and a degradation rate commensurate with new bone ingrowth. Critically, it enabled a biphasic exosome release profile-an initial burst followed by a 14-day sustained release (89.73% cumulative release). In vitro, HUCB-MSCs-exos significantly promoted the proliferation, migration, and osteogenic differentiation of bone marrow-derived MSCs, as demonstrated by enhanced alkaline phosphatase activity and matrix mineralization. In a rabbit condylar defect model (5 mm diameter), the CTS/10% nHA-exo scaffold achieved a 57.44 ± 8.42% healing rate at two months, nearly two-fold greater than the scaffold-only group (29.33 ± 6.94%). Histological and immunohistochemical analyses at two months confirmed the formation of mature, well-vascularized trabecular bone, accompanied by robust expression of late-stage osteogenic markers (OCN and OPN). These findings demonstrate that the CTS/10% nHA scaffold synergistically integrates osteoconductive structural guidance with exosome-mediated osteoinductive paracrine signaling, providing a compelling and translatable strategy for critical-sized bone-defect management.