Hydroxyapatite-whitlockite composite coating as a biomimetic material for bone integration

Mg deficiency contributes to disrupting mineral metabolism and impairing bone growth. Continuous supply of Mg2+ from orthopedic implant has emerged as a therapeutic strategy for fracture healing. However, therapeutic efficiency of Mg2+ is concentration-dependent and the controlled Mg2+ delivery is challenging. Mg whitlockite (WH) is Mg2+ reservoir in bone mineral which manipulates Mg2+ release and enhances mechanical strength of natural bone. In this aspect, WH was incorporated into the hydroxyapatite (HAp) coating to endow the coating with Mg2+-release properties and enhance bonding strength to the metal substrate. The HAp/WH composite coating with 30 wt% WH exhibited a two-fold increase in bonding strength than the HAp coating mainly due to the better thermal adaptation of WH to the Ti substrate. The Mg2+-concentration-dependent osteoblastic behaviors were evaluated by using the WH, annealed-treated WH (WH-800) and HAp/WH composite coatings. Compared to HAp and WH-800 coatings, WH and HAp/WH coatings with higher Mg2+ release (4.1–11.5 mM within 7 days) and more negative zeta potential (−36.5 mV and −27.3 mV) exerted greater effects on fibronectin adsorption and unfolding the exposure of RGD motif which facilitated cytoskeleton spreading and enhanced osteoblastic differentiation and mineralization. Considering the difference in Mg2+ release among the WH-containing coatings, our study suggested that 3–12 mM Mg2+ in vitro may be optimized. Taken together, the first-time reported HAp/WH composite coating as a biomimetic material shows promise in enhancing bone integration.