Covalently cross-linked hydroxyapatite-citric acid-based biomimetic polymeric composites for bone applications

Composite materials based on bioceramics and polymers offer excellent opportunities in the quest for developing optimal bone grafts for bone tissue engineering. Herein, we have functionalized nano hydroxyapatite with citric acid and subsequently cross-linked with poly(propylene fumarate) and poly(ethylene glycol) to afford a composite with better interfacial bonding properties. This study involved two biomimetic composites, 3CP-VP and 5CP-VP, prepared by varying the concentration of hydroxyapatite. Uniform homogenous distribution of hydroxyapatite was identified through Raman spectral imaging in both the composite matrices. The compressive moduli of the biomimetic composites after 4-week immersion in phosphate-buffered saline ranged between 100 and 300MPa, which falls well within the accepted values reported for human trabecular bone. Moreover, biodegradation studies revealed only an average weight loss of 10%-17% during the 7-week time period. Furthermore, apatite mineralization was evaluated using scanning electron microscopy and energy dispersive X-ray analysis, and contact angle measurements revealed hydrophobic surfaces with preferential adsorption to albumin. More importantly, blood compatibility studies demonstrated no significant hemolysis and no visible red blood cell aggregation, while cytotoxicity evaluation via direct contact, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide, and live-dead assays on human osteoblast sarcoma cell line exhibited good biocompatibility with negligible cytotoxicity. In addition, in vitro drug release studies with gentamycin-loaded composites demonstrated a controlled and sustained release profile with about 35% of drug released over a period of 2weeks. These findings show that these composites could be developed into stand-alone bone substitutes for bone tissue engineering coupled with drug delivery applications.