MAO/SA composite coating on magnesium alloy: Corrosion resistance improvement and time-programmed Ca2 +/Mg2+ release for bone repair.

Magnesium (Mg) alloys are attractive as orthopedic implants because of their favorable biofunctions, mechanical properties, and biodegradability. However, their clinical applications are hindered by their rapid degradation rate in physiological environments, leading to inadequate functional and mechanical behavior of implants across different stages of bone repair. Here, Mg alloys were firstly modified by micro-arc oxidation (MAO), and then sealed with Ca2+-crosslinked sodium alginate (SA) hydrogel coatings through different immersion cycles, constructing the MAO/SA composite coatings to further improve the corrosion resistance and simultaneously supply Ca2+ alongside Mg2+. It was found that the coatings prepared with three immersion cycles showed markedly improved corrosion resistance, concomitant with a thicker, more uniform, and defect-scarce surface layer. Increasing immersion cycles also enhanced surface hydrophilicity and supported favorable in vitro cellular responses, including improved cell attachment and osteogenic differentiation. Results of in vivo implantation in rat femoral condyles demonstrated desirable biocompatibility, superior bone-implant integration and better new bone formation for MAO/SA-treated samples. In addition, the heterostructure of the MAO/SA-Mg composite enabled a time-programmed ratio of Ca2+/Mg2+ release profile, which may facilitate a stage-adaptive immune niche for bone repair. The MAO/SA-modified Mg alloys are considered as promising candidates for orthopedic applications.