Micro-arc oxidation-assisted sol-gel preparation of calcium metaphosphate coatings on magnesium alloys for bone repair

Calcium phosphate coating is an attractive surface modification strategy for magnesium alloys, since it can increase their corrosion resistance and endow them with osteogenic function simultaneously. Herein, a calcium metaphosphate (CMP) coating was fabricated on magnesium alloy by using sol-gel approach assisted with micro-arc oxidation pre-treatment. Scanning electron microscopy showed that the micro-pores and cracks in micro-arc oxidation inner layer generated during the pre-treatment process were sealed by the grainy sol-gel outer layer. Energy dispersive spectrometry and X-ray diffraction results demonstrated the identity of the coating as CMP. The cross-cut test showed that the adhesion of CMP coating was strong. Applying bare magnesium alloy substrate as a control, the CMP coating surface was rougher and more hydrophilic. The potentiodynamic polarization test demonstrated that the corrosion resistance was significantly improved by using CMP coating. Hydrogen evolution in immersion test further confirmed that the degradation rate was decelerated within 14 days. Moreover, CMP coating facilitated the adhesion speed, spreading area, and focal adhesion formation of bone marrow stem cells. The number of cells in the active proliferating state and proliferated cells present on the CMP coating also increased. Additionally, CMP coating upregulated alkaline phosphatase activity and osteogenic gene expression in cells. In summary, the micro-arc oxidation assisted sol-gel CMP coatings increased the corrosion resistance and promoted the interfacial cell behavior for magnesium alloy implants, which might inform the further development of surface modifications on magnesium alloys for bone related applications.