## 3 Results
### Wettability, Polarization Curves, and SBF Corrosion Tests  
Surface wettability of AZ31 Mg alloy was investigated by contact angle measurements. Water contact angles are reported in Table 2 for _not hydrothermally treated_ and _hydrothermally treated_ samples with and without PHB coating. The wettability tests confirmed the presence of a continuous surface layer of Mg(OH)\({}_{2}\) on the hydrothermally treated samples. As reported in Table 2, in fact, the not hydrothermally treated MG alloy showed significantly higher contact angle value and variance (46.1\({}^{\circ}\) and \(\pm\)2.5\({}^{\circ}\) respectively) compared to those of the hydrothermally treated ones (42.2\({}^{\circ}\) and \(\pm\)0.4\({}^{\circ}\), respectively). It can be inferred, then, that the significantly lower data variance of the treated samples could be attributed to more homogeneous surface properties guaranteed by the formation of a homogeneous layer of Mg(OH)\({}_{2}\). In particular, the contact angle of the not hydrothermally treated AZ31 alloy was about 46.1\({}^{\circ}\), indicating a mild hydrophilic character. However, after the hydrothermal treatment, the measured contact angle lowered to 42.2\({}^{\circ}\), indicating that the hydrothermally formed magnesium hydroxide increased the sample surface's hydrophilicity. Conversely, the presence of a PHB polymer coating on the hydrothermally treated AZ31 metal surface significantly increased the contact angle up to 62.3\({}^{\circ}\), considerably lowering its wettability compared to that of the only hydrothermally treated one. The more hydrophobic the outer layer, the lower the surface wettability and the lower the water penetration rate [53; 54; 55]. Surface wettability, then, could play a relevant role in the corrosion and biological responses of implanted devices because it influences water and solution electrolyte diffusion, the chemistry of corrosion, protein absorption, and cell activity at the implant interface. This could be particularly significant in the case of magnesium alloy prostheses. When implanted in the body, the magnesium alloy is in contact with body fluids that wet its surface, starting a corrosion and resorption process that may become detrimental to its functionality if it occurs at too high rates [57]. Our wettability test indicates that the presence of a Mg(OH)\({}_{2}\) layer increases magnesium alloy wettability and, therefore, any associated detrimental effects, while the use of \begin{table}
\begin{tabular}{c c c} \hline
**AZ31 Mg Alloy** & **AZ31 Mg Alloy + Mg(OH)\({}_{2}\)** & **AZ31 Mg Alloy + Mg(OH)\({}_{2}\) + PHB** \\ \hline
46.1\({}^{\circ}\) & * 42.2\({}^{\circ}\) & **62.3\({}^{\circ}\)** \\ \(\pm\)2.5\({}^{\circ}\) & \(\pm\)0.4\({}^{\circ}\) & \(\pm\)0.3\({}^{\circ}\) \\ \hline \end{tabular} \({}^{*}\) significant difference (\(p\) < 0.05); ** highly significant difference (\(p\) < 0.01).  
\end{table} 
Table 2: Water contact angles for AZ Mg alloy not hydrothermally treated (first column), hydrothermally treated with the formation of a Mg(OH)\({}_{2}\) surface coating (second column), and hydrothermally treated with an additional PHB coating (third column). the PHB coating, which significantly reduces surface wettability and protects the metal surface, could allow for better control of erosion processes, lengthening its functionality. From the previous discussion, it can be inferred that for implanted prostheses, magnesium erosion and ion release in the biological surrounding environment could be directly related to the electrochemical character of the reduction and oxidation reactions occurring at the metal interface and the external electrolytic solution. Information on the corrosion rate, passivity, pitting susceptibility, and cathodic and anodic behavior of an electrochemical system can be obtained by interpreting the polarization curves reported in Figure 2c for bare and PHB-coated Mg alloy.