# Microstructural, Electrochemical and In Vitro Analysis of Mg-0.5 Ca-xGd Biodegradable Alloys
## 3 Results and Discussions
### Cytocompatibility Study  
In the current study, the MTT test results for the cytocompatibility assessment of Mg-0.5 Ca-xGd experimental alloys are shown in Figure 7, calculated as percentages from the viability of the negative control (i.e., control wells). The results of Day 1 of co-incubation revealed that the level of cell viability varied in an increasing way and proportional to the increase in the percentage of Gd as the alloying element, being similar to the case of alloys with 2% Gd and 3% Gd (\(p>0.05\)), but significantly higher compared with the viability level results in the case of the other experimental samples (\(p<0.05\)). The cytocompatibility profile recorded after 3 days of co-incubation with the studied alloys reached a similar level and showed that there were no statistically significant differences among all studied samples (\(p>0.05\)) at 3 days. It was observed that with an increase in co-incubation time for up to 5 days, the cell viability profile registered an increase in the case of all alloys but remained significantly lower (\(p<0.05\)) for alloys with 0.5% Gd and 1% Gd. In addition, the results showed no significant differences in cell viability after 5 days for samples with a Gd quantity of 1.5%, 2% and 3% (\(p>0.05\)), similar to the Mg-Ca control alloy. The MTT results seem to suggest the fact that increasing the percentage of gadolinium used for alloying did not affect the viability level in a dose-dependent manner (i.e., no significant differences among the 1.5, 2 and 3% Gd-alloyed Mg-Ca alloys) and this could be due to an increase in the stability of alloys in a humid environment (as was detailed in the previous section; i.e., Electrochemical Corrosion Resistance Analysis) with a complex composition (such as culture medium or simulated biological fluids). The decreased level of viability after 1 day of co-incubation of the cells with the alloy samples could be attributed to the degradation \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Mg–0.5 Ca**} & Chemical elements & **Mg** & **Ca** & **Gd** & **O** & **Cl** & **Na** \\  & & wt.\% & wt.\% & wt.\% & wt.\% & wt.\% & wt.\% \\  & Surface with oxides & 81 & 0.8 & - & 18.2 & - & - \\ \hline \multirow{2}{*}{**Mg–0.5 Ca–**0.5 Gd**} & Chemical elements & **Mg** & **Ca** & **Gd** & **O** & **Cl** & **Na** \\  & & wt.\% & wt.\% & wt.\% & wt.\% & wt.\% & wt.\% \\  & Surface with oxides & 70.5 & 0.8 & 0.8 & 27.9 & - & - \\ \hline \multirow{2}{*}{**Mg–0.5 Ca– 1 Gd**} & Chemical elements & **Mg** & **Ca** & **Gd** & **O** & **Cl** & **Na** \\  & Surface with oxides & 77.7 & 1.1 & 1.2 & 18.7 & - & 1.3 \\ \hline \multirow{2}{*}{**Mg–0.5 Ca– 2 Gd**} & Chemical elements & **Mg** & **Ca** & **Gd** & **O** & **Cl** & **Na** \\  & Surface with oxides & 72.2 & 0.8 & 0.8 & 23.3 & 2.2 & 0.6 \\ \hline \multirow{2}{*}{**Mg–0.5 Ca– 2 Gd**} & Chemical elements & **Mg** & **Ca** & **Gd** & **O** & **Cl** & **Na** \\  & Surface with oxides & 65.3 & 0.9 & 0.6 & 32.2 & 0.3 & 0.7 \\ \hline \multirow{2}{*}{**Mg–0.5 Ca– 3 Gd**} & Chemical elements & **Mg** & **Ca** & **Gd** & **O** & **Cl** & **Na** \\  & & wt.\% & wt.\% & wt.\% & wt.\% & wt.\% & wt.\% \\ \hline \multirow{2}{*}{SD (determined for the main elements after 20 chemical composition analyses on the same area) are: Mg: \(\pm\)1.1; Ca: \(\pm\)0.2; Gd: \(\pm\)0.1; O: \(\pm\)1.5; Cl: \(\pm\)0.1; Na: \(\pm\)0.1.} & & & & \\ \end{tabular}
\end{table} 
Table 6: Elements identified on the Mg–0.5 Ca–xGd alloys’ surface after the electro-corrosion resistance tests. of the alloys immersed in the experimental culture medium [53], which has the immediate consequence of releasing hydrogen with a change in pH (i.e., subsequent alkalinization, as a consequence of an Mg\({}^{2+}\)-related anodic reaction by which H\({}_{2}\)O is reduced to H\({}_{2}\) and OH\({}^{-}\)).