Biomineralization-inspired scaffolds using citrate-based polymers to stabilize amorphous calcium phosphate promote osteogenesis and angiogenesis for bone defect repair

Bone defect repair requires bone mineralization, during which amorphous calcium phosphate (ACP) plays a critical role in the formation and phase transformation of bone apatite. ACP-based biomaterials continuously release calcium and phosphate, promoting the deposition and maturation of bone minerals. This effectively overcomes the limitations of insufficient osteoinductivity of crystalline calcium phosphate (CaP) phases such as hydroxyapatite (HA). However, the instability of ACP leads to its spontaneous conversion into stable CaP phases, reducing its inherent osteogenic potential. Therefore, stabilizing ACP to maintain its bioactivity is crucial for bone repair biomaterials. Inspired by the bone biomineralization and the natural stabilization of ACP by citrate in bone, we developed a porous biomimetic mineralized scaffold (POC-ACP) using citrate-based poly (octamethylene citrate) (POC) to stabilize ACP for bone defect repair. The stabilized ACP acted as a mineralization seed, initiating the bone mineralization process and promoting new bone formation. Meanwhile, its excellent mechanical and porous structure supported cell and tissue ingrowth. Compared to the POC-HA scaffold, the POC-ACP scaffold significantly enhanced osteogenesis and angiogenesis both in vitro and in vivo . Mechanistically, RNA-sequencing elucidated that the POC-ACP scaffold promoted osteogenic differentiation by activating the AMPK and TGF-β signaling pathways. Our study provides a novel biomimetic mineralized scaffold with ACP stabilization, offering a promising alternative for clinical bone defect repair. The schematic diagram of POC-ACP scaffold preparation, its biomimetic mineralization process, and the mechanism underlying bone defect regeneration. In the citrate-based POC-ACP scaffold, POC stabilizes ACP, which acts as a mineralization seed by continuously releasing calcium and phosphate ions to promote apatite deposition, while itself gradually matures into crystalline HA that integrates with the surrounding new bone. Mechanistically, the scaffold promotes osteogenic differentiation of BMSCs by activating the AMPK and TGF-β signaling pathways. POC: poly (octamethylene citrate); ACP: amorphous calcium phosphate; HA: hydroxylapatite; BMSC: bone marrow mesenchymal stem cell. • A biomimetic POC-ACP scaffold is developed for bone defect regeneration. • Poly (octamethylene citrate) stabilizes ACP to maintain its bioactivity. • POC-ACP scaffold enhances osteogenesis and angiogenesis outperforms crystalline POC-HA. • POC-ACP scaffold promotes osteogenesis by activating AMPK and TGF-β pathways.