Enhancement of osteogenic differentiation and bone regeneration by poly (amino acid)/cobalt-doped hydroxyapatite

The materials with good biocompatibility, osteogenic activity, and mechanical properties are necessary in the field of bone tissue engineering for enhancing the repair capacity and restoring bone integrity. Cobalt-doped hydroxyapatite (Co-HA) plays an important role in guiding the osteogenic and angiogenic differentiation of bone marrow stromal cells (BMSCs). We speculate that both osteogenic inducibility in vitro and bone regeneration in vivo can be achieved by incorporating Co-HA into poly (amino acids) (PAAs) to form PAA/Co-HA composites. To test this, PAA/Co-HA composites with different Co-HA loading (30 wt% labeled as PAA/Co-HA30) were prepared using an in-situ melting polycondensation, and the physical and chemical properties in vitro were characterized. The result shows that the PAA/Co-HA composites contained characteristic PAA and Co-HA structures. The crystallization temperature (Tc), melting temperature (Tm), and maximum weight loss temperature of the PAA/Co-HA30 composite were 163.0 °C, 204.7 °C, and 442.88 °C, respectively. Co-HA particles could be uniformly dispersed in PAA matrix. The bending strength, compressive strength, and yield strength of the PAA/Co-HA30 composite were around 85, 125, and 110 MPa, respectively. The leaching components from PAA/Co-HA30 could stimulate osteogenic differentiation of BMSCs (assessed using alizarin red staining). Western blotting and RT-PCR analysis showed that the leaching components of the PAA/Co-HA30 composite up-regulated the gene and protein expression levels of Runt-Related Transcription Factor-2 (Runx2), Integrin Binding Sialoprotein (IBSP), Osteoblast-Specific Factor (Osterix, OSX), and osteopontin (OPN), indicating great promotion of osteogenic differentiation of BMSCs. In an intramuscular and intraskeletal model, the PAA/Co-HA30 composite possessed good biocompatibility and could enhance bone formation, showing good bone conductivity. All results suggested that the PAA/Co-HA30 composite is a promising material for bone substitute in clinical applications.