In vivo Study on the Corrosion Behavior of Magnesium Alloy Surface Treated with Micro-arc Oxidation and Hydrothermal Deposition

Objective: To study the corrosion behavior of magnesium alloy surface treated with micro-arc oxidation and hydrothermal deposition in living animals. Methods: A magnesium oxide (MgO) layer was prepared on Mg alloy using micro-arc oxidation technology, and then a composite coating composed of magnesium hydroxide, hydroxyapatite, and MgO was coated on the MgO layer using the hydrothermal deposition method for 2 h and 24 h. Male 3-month-old white New Zealand rabbits (n = 48) weighting 2200-2300 g, were divided into four groups randomly. The prepared Mg alloy samples with composite coatings were implanted into the femoral medullary cavity of rabbits. For the Mg group, bare Mg samples without any treatment were implanted; for the MgO group, bare Mg samples undergoing MAO treatment were implanted; for the HT2h group, samples of the MgO group undergoing hydrothermal treatment (HT) for 2 h were implanted; and for the HT24h group, samples of group MgO undergoing HT for 24 h were implanted. Then the in vivo corrosion behaviors of implants were evaluated by X-ray observation, micro-CT analysis and serum Mg2+ examination. Results: The X-ray showed that samples implanted in animals were decreased as time went by. The micro-CT showed that on the fourth week, the residual volume percentages (RVP) of samples of the Mg, MgO, HT2h, and HT24h groups were 72.81% +/- 2.10%, 71.68% +/- 1.49%, 81.14% +/- 1.54%, and 82.04% +/- 0.89%, respectively; on the eighth week, the RVP of four groups were 29.45% +/- 1.06%, 41.82% +/- 1.13%, 53.92% +/- 0.37%, and 62.53% +/- 2.06%, respectively; while on the 12th week, RVP were 8.45% +/- 0.49%, 9.97% +/- 0.75%, 37.09% +/- 0.89%, 46.71% +/- 1.87%. The RVP of the HT2h group and the HT24h group were higher than for the Mg group and the MgO group for all three time points (P < 0.05); the RVP for HT24h was higher than for HT2h at 8 and 12 weeks, and the differences were significant, indicating that the degradation of Mg alloy slowed down after composite coating. In addition, the composite-coated Mg alloy by 24-h hydrothermal treatment exhibited a slower degradation than that treated by 2 h. Serum Mg2+ concentration results showed that on the second week, the Mg2+ concentrations of the Mg, MgO, HT2h, and HT24h groups were 2.24 +/- 0.10 mmol/L, 2.12 +/- 0.07 mmol/L, 2.06 +/- 0.11 mmol/L, and 2.15 +/- 0.12 mmol/L, respectively. On the fourth week, these concentrations were 1.99 +/- 0.33 mmol/L, 2.18 +/- 0.06 mmol/L, 2.17 +/- 0.09 mmol/L, and 2.13 +/- 0.14 mmol/L, respectively. On the eighth week, the concentrations were 2.22 +/- 0.09 mmol/L, 2.20 +/- 0.17 mmol/L, 2.06 +/- 0.11 mmol/L, and 2.14 +/- 0.07 mmol/L, respectively. On the 12th week, the concentrations were 2.18 +/- 0.04 mmol/L, 2.20 +/- 0.08 mmol/L, 2.09 +/- 0.02 mmol/L, and 2.16 +/- 0.11 mmol/L. Conclusion: The combination of micro-arc oxidation and hydrothermal deposition can greatly improve the anti-corrosion behavior of Mg alloy, and Mg alloy coated with this composite coating is a promising biomaterial with a satisfactory degradation rate.