## 3 Results and Discussion
### Degradation Properties  
The polarization curves of ZK60 and ZK60-La immersed in SBF are presented in Figure 6a. The \(E_{\mathrm{corr}}\) and \(I_{\mathrm{corr}}\) estimated using Tafel extrapolation are shown in Table 1. The \(E_{\mathrm{corr}}\) and \(I_{\mathrm{corr}}\) of ZK60 were \(-\)1.61 V and 50.2 uA/cm\({}^{2}\). It should be noted that ZK60-1.0 La had the highest \(E_{\mathrm{corr}}\) (\(-\)1.41 V) and the lowest \(I_{\mathrm{corr}}\) (26.3 uA/cm\({}^{2}\)), which suggested that it had the best degradation resistance.  
The degradation rate of ZK60 and ZK60-La calculated from the weight loss are exhibited in Figure 6b. The degradation rate increased with the immersion time increasing from 3 to 7 days, while it decreased with the increasing immersion time from 7 to 14 days. It could be attributed to the reduction of surface area for the samples in the degradation process. When the samples immersed in the solution, the surface of the sample was exposed to the solution causing several thermopositive reactions, thus increasing the degradation rate. While, protective films would form on the samples after prolonged immersion in SBF, which prohibited the contact between the samples and corrosive medium, thus decreasing the degradation rate. The degradation rate of ZK60 was 2.13 mm/year after exposure to SBF for 14 days. When the content of La increased to 1.0 wt %, the degradation rate decreased to  
Figure 5: Characterization of the extracts: (**A**) pH value; (**B**) La\({}^{3+}\) concentration; (**C**) Mechanism of ZK60-La inducing tumor cell death: (**a**) La\({}^{3+}\) and OH\({}^{-}\) transported into cell; (**b**) the mitochondrial permeability transition pore (mPTP) opening; (**c**) ROS accumulated in mitochondrion; (**d**) regulated the gene expression; (**e**) regulated the expression levels of protein; (**f**) ROS accumulation caused by OH\({}^{-}\); and (**g**) regulation of protein expression led cell death.  
1.23 mm/year. Clearly, various La content exerted an effect on the pH of extract, as shown in Figure 5A. With La content increasing to 1.0 wt %, the degradation rate was reduced. Thus, the released OH\({}^{-}\) was decreased. With La content further increased, the degradation was accelerated, and the pH value was consequently increased.  
The morphology of ZK60 and ZK60-La after immersing in SBF solution for 14 days is shown in Figure 6c. They were all covered with degradation product layer. Many cracks were observed for ZK60, while the surface film of ZK60-1.0 La maintained integrity. The EDS spectra of the degradation products are shown in Figure 6c. Corresponding EDS data were listed in Table 2. It is shown that the degradation products were primarily constituted of O, C, and Mg. The La and Zn, along with Ca, P, were also detected in the degradation products. The atomic ratio of Ca to P was about 1.34, which is between the Ca/P ratios of octacac  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & \(\mathbf{E_{\text{corr}}}\) (V) & \(\mathbf{I_{\text{corr}}}\) (\(\mathbf{\mu}\)A/cm\({}^{2}\)) \\ \hline ZK60 & \(-\)1.61 \(\pm\) 0.01 & 50.2 \(\pm\) 7 \\ ZK60-0.5 La & \(-\)1.51 \(\pm\) 0.01 & 39.4 \(\pm\) 5 \\ ZK60-1.0 La & \(-\)1.41 \(\pm\) 0.01 & 26.3 \(\pm\) 3 \\ ZK60-1.5 La & \(-\)1.45 \(\pm\) 0.01 & 35.1 \(\pm\) 5 \\ ZK60-2.0 La & \(-\)1.48 \(\pm\) 0.01 & 43.7 \(\pm\) 5 \\ \hline \hline \end{tabular}
\end{table}
Table 1: The corrosion potential and current density of ZK60 and ZK60-La estimated using Tafel extrapolation.  
Figure 6: (**a**) Polarization curves of ZK60 and ZK60-La immersed in SBF solution; (**b**) Degradation rates of ZK60 and ZK60-La calculated from the weight loss; (**c**) Morphology of the ZK60 and ZK60-La after immersing in SBF solution for 14 days; (**d**) Energy dispersive spectroscopy (EDS) spectra of the degradation products indicated by the red point in Figure 6c.