## 4 Discussion  
The polymer coating was prepared by the PHB dip-coating method, while a magnesium hydroxide film, as confirmed by FTIR spectra, was formed on the alloy metal surface by hydrothermal treatment. The hydroxyl of magnesium hydroxide promoted the formation of hydrogen bonds with the PHB hydroxyls, improving adhesion strength between the polymer coating and the magnesium alloy surface. The corrosion natural current of uncoated magnesium AZ31 alloy was two orders of magnitude higher than that of the PHB-coated one, demonstrating that charge transport was drastically reduced by the presence of the polymeric coating. It can be inferred that the reduced degradation rate of the AZ31 PHB-coated system is to be associated to the presence of the polymeric coating film, which acts as a membrane steadily controlling the diffusion flux of the ionic species (especially Cl\({}^{-}\) ions) from the SBF solution to the metal surface and the counter-diffusion of Mg\({}^{2+}\) metal ions toward the external solution. The results of static corrosion experiments on PHB-coated and uncoated AZ31 samples showed that under the same experimental conditions, weight losses were drastically reduced by the presence of the polymer coating, and Mg ion concentration and the pH value of the corrosion SBF medium were lower than those of the uncoated AZ31. The corrosion rate for the PHB-coated Mg alloy was more stable and almost four times slower than that of the uncoated system at 1.25 and 5.5 mg/cm\({}^{2}\)day, respectively, which, according to ASTM-G31-72, correspond to surface ablation rates of 7.2 and 31.6 \(\upmu\)m/day, respectively. These findings showed that the polymer coating can not only significantly enhance the corrosion resistance of magnesium alloy materials but, due to the diffusive nature of the processes that driving the corrosion, it can also be used to program targeted resorption times. Control of resorption rates after implantation of and, hence, of Mg\({}^{2+}\) ion concentration in body fluids could enlarge the field of application of these systems. However, after implantation in the human body, a metal prosthesis's biocompatibility must fulfill a complex chain process [48; 49; 50]. First, blood and cell biocompatibility must be  
Figure 7: Total antioxidant capacity (T-AOC) contents of different groups. * significant difference (\(p<0.05\)); ** highly significant difference (\(p<0.01\)). evaluated. The hemolysis rate of PHB-AZ31 was lower than 5%, meeting the requirements of medical materials. Conversely, the hemolysis rate of bare AZ31 was 37.61%, which might induce more severe hemolytic phenomena because of the higher dissolution of magnesium ions of the uncoated AZ31. When red blood cells contacted with materials, a hemolysis phenomenon appeared because of the different osmotic pressures on both sides of the cell walls [51]. The hemolysis rate test proved that coated magnesium alloy materials showed good blood compatibility. Cell compatibility tests were also investigated; the results showed that corrosion extraction fluids with different concentrations infected endothelial cells differently. The higher extract concentration of the uncoated AZ31 induced greater endothelial injuries. The PHB-AZ31 extracts had no significant detrimental effect on cell proliferation because of the protection and diffusive control exerted by the polymer coating film, which maintained low, physiological levels of Mg\({}^{2+}\) ions concentrations. Cellular compatibility evaluation plays an important role in the evaluation of biological materials [52]. According to CCK-8 kit testing, PHB as a protective layer promoted cell proliferation. Angiogenesis means the formation of vascular lumen structures by endothelial cell proliferation and a migration process based on the existing network of blood vessels. Vascular endothelial cells are directly related to angiogenesis. On the other hand, NO is one of the most important control signals of angiogenesis. Before and after coating, NO content was not affected.