## 3 Results and Discussion
### Anticoagulation  
Red blood cells (erythrocyte) are the major components of human blood. When the human blood comes into contact with implants, the erythrocytes may be destroyed and activated, finally resulting in the occurrence of hemolysis and clotting [27]. Therefore, it is generally expected that the blood-contacting biomaterials have a low hemolysis rate. According to the Chinese standard, a material is acceptable and can be used for blood-contacting biomaterials when the hemolysis rate is below 5%. The hemolysis rates of the different samples are shown in Figure 6. As compared to the  
Figure 5: The typical scanning electron microscope (SEM) images of the different samples immersed into SBF solution for one day, five days and seven days, respectively. modified samples, the pristine magnesium alloy exhibited the largest hemolysis rate and the value was as high as 59%, indicating that a serious hemolysis occurred. It was considered that the high hemolysis rate is closely related to the local alkalization induced by the fast corrosion of the pristine magnesium alloy. A recent piece of literature reported that the high pH value caused by the fast degradation of magnesium alloy can lead to a high hemolysis ratio [28]. On the other side, a high pH value can increase the binding ability of hemoglobin to the membrane and subsequently increase the hemolysis rate, and the red blood cells can fuse with Ca\({}^{2+}\) of the medium, thus resulting in a rupture of erythrocytes, finally leading to serious hemolysis [29]. According to our previous study, the pH value was as high as 8.5 when the pristine magnesium alloy was immersed into SBF for two days [26], and the higher pH value can be obtained when the sample was immersed into SBF for more time. In addition, rapid release of magnesium ions from its fast corrosion is also related to severe hemolysis reaction [30]. Consequently, serious hemolysis can occur on the pristine magnesium alloy due to its fast degradation. After the chemical surface modification, it can be clearly seen that the hemolysis rate decreased significantly; however, there was no significant difference between Mg-HF and Mg-OH. As discussed above, both chemical surface modification methods can obviously enhance the corrosion resistance and reduce the corrosion rate. Our previous study indicated that the pH value of Mg-OH was significantly smaller than that of the pristine magnesium alloy [24] when they were immersed into a corrosion medium for the same time, which contributed to the lower hemolysis rates of the modified samples. However, it was noticeable that the hemolysis rate was larger than 5%, indicating that the modified samples may not be suitable for the blood-contacting biomaterials, although the chemical surface modification can significantly reduce the hemolysis rate. Therefore, it is necessary to further modify the surface properties with other approaches, such as the immobilization of biomolecules, to reduce the hemolysis rate. Generally speaking, human plasma proteins firstly adsorb in seconds to the biomaterials surface when a biomaterial makes contact with blood [31]. Subsequently, the platelets will be attached on the surface followed by complement activation and other blood cell responses, finally leading to thrombus formation [32]. Consequently, the platelets have been considered as a major cause of thrombosis, and platelet adhesion represents the popular approach to characterize the blood compatibility of biomaterials [33]. The typical SEM images of the platelets adhered to the different samples were shown in Figure 7. Obviously, more platelets can be observed on the pristine magnesium alloy surface (Figure 7a). At the same time, some aggregated platelets were also observed and these aggregated platelets may be further activated, indicating that the pristine magnesium alloy had relative poor anticoagulation. The platelets on the pristine magnesium alloy surface extended their pseudopods  
Figure 6: The hemolysis rate for different samples.