Optimization of the in vitro biodegradability, cytocompatibility, and wear resistance of the AZ31B alloy by micro-arc oxidation coatings doped with zinc phosphate

As implanted bone fixation materials, magnesium (Mg) alloys have significant advantages because the density and elastic modulus are closest to those of the human bone and they can bio-degrade in the physiological environment. However, Mg alloys degrade too rapidly and uncontrollably thus hampering clinical adoption. In this study, a highly corrosion-resistant zinc-phosphate-doped micro-arc oxidation (MAO) coating is prepared on the AZ31B alloy, and the degradation process is assessed in vitro. With increasing zinc phosphate concentrations, both the corrosion potentials and charge transfer resistance of the AZ31B alloy coated with MAO coatings increase gradually, while the corrosion current densities diminish gradually. Immersion tests in the simulated body fluid (SBF) reveal that the increased zinc phosphate concentration in MAO coating decreases the degradation rate, consequently reducing the release rates of Mg2+ and OH– in the physiological micro-environment, which obtains the lowest weight loss of only 5.22% after immersion for 56 days. Effective regulation of degradation provides a weak alkaline environment that is suitable for long-term cell growth and subsequent promotion of bone proliferation, differentiation, mineralization, and cytocompatibility. In addition, the zinc-phosphate-doped MAO coatings show an improved wear resistance as manifested by a wear rate of only 3.81 × 10–5 mm3 N–1 m–1. The results reveal a suitable strategy to improve the properties of biodegradable Mg alloys to balance tissue healing with mechanical degradation.