Functionalization treatment of micro-arc oxidation coatings on magnesium alloys: a review

Magnesium (Mg) alloys have become promising biomaterials due to their attractive biological and mechanical properties. However, the high corrosion rate has severely limited their wide applications in the clinical field. Micro-arc oxidation (MAO) is recognized as an effective surface treatment to enhance the corrosion resistance of Mg alloys. However, protective coatings will impair the antibacterial activity of Mg which is caused by the increase in pH value throughout its degradation. Besides, there exist abundant micro-pores and micro-cracks in the MAO coatings, which provide channels for aggressive medium to Mg substrates and thus accelerate their corrosion. By introducing antibacterial metallic elements such as silver (Ag), copper (Cu) and zinc (Zn) into the electrolyte, MAO coatings with enhanced antibacterial properties can be fabricated on Mg alloys. In addition, by covering the MAO coatings with biopolymer or calcium phosphate coatings, biofunctional MAO coatings with reinforced corrosion resistance can be obtained. This review mainly discussed the functionalization treatment of the MAO coatings on Mg alloys, including endowing MAO coatings with antibacterial properties, corrosion resistance, biocompatibility and bioactivity.