Magnesium-based alloys exhibit biodegradable, biocompatible and excellent mechanical properties which enable them to serve as ideal candidate biomedical materials. In particular, their biodegradable ability helps patients to avoid a second surgery. The corrosion rate, however, is too rapid to sustain the healing process. Alloying is an effective method to slow down the corrosion rate. However, currently magnesium alloys used as biomaterials are mostly commercial alloys without considering cytotoxicity from the perspective of biosafety. This article comprehensively reviews the status of various existing and newly developed degradable magnesium-based alloys specially designed for biomedical application. The effects of critical alloying elements, compositions, heat treatment and processing technology on the microstructure, mechanical properties and corrosion resistance of magnesium alloys are discussed in detail. This article covers Mg–Ca based, Mg–Zn based, Mg–Sr based, Mg–RE based and Mg–Cu-based alloy systems. The novel methods of fabricating Mg-based biomaterials and surface treatment on Mg based alloys for potential biomedical applications are summarized.

Magnesium and its alloys are fascinating candidates for biodegradable implants due to their suitable mechanical properties, biodegradability and biocompatibility. However, the rapid corrosion rate, generation of a large amount of hydrogen gas and increase in local pH of the body fluid limit their potential biomedical applications. Various techniques, including alloying, mechanical processing and surface treatment, have been used to reduce the corrosion rate of materials. Surface treatment is one of the widely used techniques to improve the corrosion resistance of the Mg alloys. Compared to other surface modification processes, micro-arc oxidation can provide good adherent, hard, scratch-resistant, wear-resistant, and corrosion-resistant coatings on magnesium alloys. This article mainly reviews the influence of key relatively process parameters, such as electrolytic composition and presence of additives. Normally, electrolytic system and additives can improve the corrosion behaviour of magnesium alloys using appropriate concentration. The future prospects are summarised as well.

Magnesium alloys are the most promising implant materials due to their excellent biodegradability. However, their high degradation rate limits their practical application. In this study, we produced a calcium–phosphate (Ca–P) coating and a calcium–phosphate–silicon (Ca–P–Si) coating via one-step and two-step micro-arc oxidation processes, respectively. The microstructure and chemical composition of the MAO coatings were characterized using SEM, XRD and EDS. The degradation behaviors of the MAO coatings and the substrate were investigated using electrochemical techniques and immersion tests in simulated body fluid (SBF). The results show that the silicate was successfully incorporated into the Ca–P coating in the second MAO step, and this also increased the thickness of the coating. The Ca–P–Si coatings remarkably reduced the corrosion rate of the Mg alloy and Ca–P coating during 18 days of immersion in SBF. In addition, the bone-like apatite layer on the sample surface demonstrated the good biomineralization ability of the Ca–P–Si coating. Potentiodynamic polarization results showed that the MAO coating could clearly enhance the corrosion resistance of the Mg alloy. Moreover, we propose the growth mechanism of the MAO coating in the second step.

The rapid degradation rate restricts the application of magnesium-based biomaterials. Therefore, we constructed Ag-containing micro-arc oxidation (MAO) coatings on Mg–3Zn–0.5Sr alloys to improve the properties of magnesium alloys. Different concentrations of CH₃COOAg (0, 1, 2, 3 g L⁻¹) were added into the electrolyte to investigate the effects of Ag⁺ content on properties of MAO coatings. The addition of CH₃COOAg led to the formation of Ag₂O and Ag₂CO₃ phases, which sealed the micro-pores and improved the corrosion resistance of the samples significantly. The samples with addition of 2 and 3 g L⁻¹ CH₃COOAg exhibited strong antibacterial ability against E.coli due to the gradual release of Ag⁺ in the degradation process. It was verified that the samples with addition of 2 g L⁻¹ CH₃COOAg obtained the most high bonding strength, dense construction, excellent corrosion resistance, cytocompatibility and antibacterial properties. The bacteria-killing methods of Ag-containing MAO coatings were discussed in the article.

Four additives (Na₂WO₄, nano-hydroxyapatite, K₂TiF₆ and NaF) were added into the Na₅P₃O₁₀ + NaOH + C₃H₈O₃ base electrolyte according to the orthogonal design of four factors three levels (L₉ (3⁴)). Nine different micro-arc oxidation (MAO) coatings were fabricated on Mg–2Zn–0.5Ca alloys through orthogonal experiments. The effects of four additives on the microstructure, mechanical properties, corrosion resistance and biocompatibility of MAO coatings were investigated through X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS), electrochemical corrosion test and in vitro degradation test. The addition of nano-hydroxyapatite and K₂TiF₆ showed self-sealing effects and contributed to the corrosion resistance of the samples significantly. The addition of 0.5 g L⁻¹ Na₂WO₄ markedly elevated the bonding strength of the coatings with the substrate. The optimal combination of factors and levels considering both mechanical properties and corrosion resistance was: 0.5 g L⁻¹ Na₂WO₄, 0 g L⁻¹ NaF, 5 g L⁻¹ n-HAp, 5 g L⁻¹ K₂TiF₆. The growth mechanism of MAO coatings combining with the visual phenomenon was discussed as well.