## 3 Results and Discussion
### Interactions with Endothelial Cells  
Vascular endothelium is a thin layer of simple endothelial cells, which forms an interface between circulating blood in the lumen and the rest of the vessel wall. Endothelial dysfunction, or the loss of proper endothelial function, is often regarded as a key early event in the development of atherosclerosis [34]. For the blood contacting materials or devices such as intravascular stents, the inserted devices or materials inevitably cause the damage of endothelium and thus lead to endothelial cell dysfunction. Therefore, endothelial cell behaviors on these implants surfaces play an important role in the in vivo performances. Magnesium alloy has been intensively investigated as a cardiovascular stent; therefore, endothelial cell adhesion was investigated in the present study. Figure 8 shows the typical fluorescent images of the endothelial cells adhered to the different substrates. Because the cell adhesion mainly occurs with 24 h, the amount of the adhered cells had no obvious change after six and 24 h cultures for the same sample. Nevertheless, the cells exhibited better spread shapes on the substrates after 24 h culture, suggesting that the adhered cells can sense the surroundings to extend their pseudopod to spread on the surface. As compared to the pristine magnesium, more cells can be observed on the chemical modified surfaces, demonstrating that the surface chemical modification can enhance the cell adhesion. Because cells are very sensitive to environmental fluctuations, any dramatic physical and chemical changes around them can produce deleterious and sometimes fatal effects [35; 36]. As shown by our results, the corrosion of magnesium could cause pH increase and release of a significant amount of magnesium ions, resulting in the environment changes of cells and thus leading to poor cell adhesion on the pristine Mg substrate. On the other side, proper reduction of the degradation rate of magnesium alloy not only creates a relatively stable interface for cell adhesion  
Figure 7: The typical SEM images of the adhered platelets on AZ31B (**a,d**), Mg–OH (**b,e**) and Mg–HF (**c,f**). and growth but also retards the release of corrosion products to reduce the cytotoxicity, therefore finally resulting in the enhanced cytocompatibility [37]. It was reported that passivation of the Mg surface in 1 M NaOH, which forms the Mg(OH)\({}_{2}\) passivation layer, can strongly enhance cell survival and adhesion [38]. Our results indicated that both of the NaOH and HF treatments can improve the corrosion of Mg alloys and thus promote cell adhesion. Although the number of adhered cells on Mg-OH had no difference with Mg-HF; however, it can be seen that the attached cells on Mg-OH displayed typical cobblestone morphology, while the cells on Mg-HF and the pristine magnesium alloy had relative shrinkage shape, indicating that cells on Mg-OH had better spreading. It is well known that the proliferation percentage of cells on a surface indicates the cytocompatibility of a substrate, which is useful in determining the potential biomedical applications of the substrate [39]. In the present study, endothelial cell proliferation was investigated by the CCK-8 method, and the results are summarized in Figure 9. Clearly, more cells could be found on Mg-OH and Mg-HF as compared to the pristine magnesium alloy, indicating that the chemical conversion layer can promote cell proliferation to some degree. According to the corrosion mechanism of magnesium alloy, the fast degradation can produce a larger amount of OH\({}^{-}\) and thus cause high local alkalization, which may severely affect the pH-dependent physiological processes such as cell adhesion and proliferation. After chemical surface modification, a chemical conversion layer with the improved corrosion resistance can be produced, which can reduce the corrosion rate and thus reduce the pH change, leading to better cell proliferation.