Preparing Ca-P coating on biodegradable magnesium alloy by hydrothermal method: In vitro degradation behavior

Magnesium alloys are potentially attractive biodegradable materials. However, their rapid corrosion rate limits their biomedical application. To slow down the rate of biodegradation, a protective calcium-phosphate coating was formed on a magnesium alloy substrate by a hydrothermal method. Scanning electron microscope results showed that the coating consisted of two layers with different crystalline characteristics. The loose outer layer showed a prism-like crystal structure, while the compact inner layer is a dense ultra-fine regular di-pyramid-like structure with an average grain dimension of similar to 200 nm. The compositions of the inner layer and outer layer were calcium-deficient hydroxyapatite (Ca-def HA) and dicalcium phosphate (DCPa), respectively. The coating adhered well to the substrate with a thickness of about 15 mu m. Immersion in Hank's solution indicated that the coating could significantly improve the degradation properties of magnesium alloy. The pH of the solution containing the coated samples increased much more slowly than the untreated control. After 8 d immersion, the uncoated sample had corroded seriously while the coated sample was much less corroded. The Ca/P atom ratio in both the layers of the coating increased and the coating was still protecting the substrate. The two layers of the coating corroded differently because of differences in solubility. The outer layer was more severely attacked and many holes were formed on the surface, the inner layer suffered less attack. In addition, a growth of precipitate on the inner layer was observed, indicating that surface bioactivity was improved by the coating. Thus, magnesium alloys coated with a Ca-P coating prepared by a hydrothermal method are promising candidate biodegradable biomaterials, and further investigation of in vivo degradation behavior is suggested.