_Article_  
**Improving Corrosion Resistance and**  
**Biocompatibility of Magnesium Alloy by Sodium**  
**Hydroxide and Hydrofluoric Acid Treatments**  
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Footnote 1  
and density are very close to human natural bone. On the other side, Mg and its alloys are good candidates for cardiovascular stents due to their good biocompatibility and biodegradation [5]. Furthermore, the addition of some elements such as Zn into magnesium alloys can also impart excellent antibacterial properties and thus make them suitable for antibacterial biomaterials [6]. Nevertheless, it is still a big challenge to control the corrosion rates while at the same time maintain good biocompatibility. The adverse effects caused by the fast in vivo adsorption after the implantation, such as excessive hydrogen production [7], local alkalization [8], enrichment of magnesium ions and the second corrosion products, etc., can result in the premature loss of the mechanical strength and thus prevent or delay the tissue healing, finally leading to implanting failure. On the other side, in most cases, the magnesium alloy surface has a lack of bioactivities because there are no any biomolecules or chemical groups on its surface; consequently, the surface biocompatibility need to be improved before clinical usage. Immobilization of biomolecules on biomaterials' surfaces represents one of the most popular and effective ways to improve the surface biocompatibility. However, direct surface biofunctionalization of the magnesium alloy may not be possible because of the fast degradation in the physiological environment; therefore, in most cases, the control of corrosion rate should be considered firstly so that the implant can maintain the sufficient mechanical properties before tissue healing. It is well known that both the corrosion and tissue-biomaterials interfacial biological reactions are surface phenomena. In most instances, surface properties play a determined role in the corrosion resistance and the biocompatibility of the magnesium alloys. Accordingly, surface modification has been one of the most important and effective methods to improve the corrosion resistance and the biocompatibility [9]. To date, numerous surface modification techniques have been developed to improve the biocompatibility and the corrosion resistance of the magnesium-based alloys, including surface alloying [10], surface chemical treatment [11], fabrication of surface coating [12], plasma treatment [13], self-assembly [14], micro-arc oxidation [15], anodization [16], plasma electrolytic oxidation [17], etc. In addition, a recent study reported that a hybrid materials of poly (ether imide) (PEI)-silica nanoparticles can be coated on the Mg surface to improve the corrosion stability and implant-tissue interfaces of magnesium substrates [18], and the results indicated that the content of silica can modulate the corrosion rate and biocompatibility and the authors stated that the hybrid systems have significant potential as a coating material of Mg for load-bearing orthopedic applications. Among these methods, formation of a chemical conversion layer via chemical surface modification is a simple and cost-effective method and has been increasingly used to improve the corrosion resistance and biocompatibility of magnesium alloy. Generally speaking, chemical surface modification includes alkaline treatment [19], fluoride treatment [20], phosphating [21], anodization [22], ion implantation [23], etc., all of which are associated with the replacement of the natively forming, and not particularly the corrosion resistant oxide layer on the surface of the Mg substrate [24]. By chemical surface modification, a dense and stable chemical conversion layer can be produced on the magnesium alloys. The layer can isolate the substrate from the corrosion medium, and thus improve the corrosion resistance and significantly reduce the adverse effects caused by the fast degradation.