Effects of composition and hierarchical structures of calcium phosphate coating on the corrosion resistance and osteoblast compatibility of Mg alloys

Highlights: • Hierarchically structured HA coatings are developed on Mg by hydrothermal conversion. • Local dissolution and reprecipitation mechanism from brushite precursor are elucidated. • The HA coating greatly impedes Mg corrosion owing to its compact structure and alkalinity. • The HA coating provides favorable microenvironment to improve in vitro osteoinductivity. Magnesium and its alloys have been recently used in biomedical applications such as orthopedic implants, whereas the weak corrosion resistance undermines their clinical efficacy. Herein, to address this critical challenge, the preparation of hierarchically structured hydroxyapatite-based coatings was proposed. Compact coatings were fabricated on a Mg alloy through a facile two-step method of chemical deposition of brushite precursor and subsequent hydrothermal conversion. A series of HA-based coatings were obtained with kinetic conversion process with formation mechanism revealed. The hydroxyapatite coating demonstrated the greatest corrosion resistance for Mg in electrochemical and long-term immersion tests, especially against pitting corrosion, attributable to its compact structure, alkaline degradation environment and self-induced growth capacity. The in vitro cytocompatibility and osteoinductivity were dictated. Additionally, anti-corrosion mechanisms were compared among different coating compositions and structures, along with their correlation with cellular response. Our study brings hints for a tailored surface design for resorbable biomedical device applications.