Microstructure Of Calcification Coating On Micro-arc Oxidized Magnesium And Biodegradation Behavior

As biodegradable biomedical materials magnesium and its alloy have receivedmore and more attentions. However, high corrosion rate of magnesium inhibits itsclinical application. To deal with the fast corrosion problem of magnesium, acalcium phosphate coating was prepared on the surface of micro-arc oxidizedmagnesium by chemical method. By controlling the microstructure of the calciumphosphate coating, the corrosion resistance and bioactivity of magnesium wasimproved.The microstructure of calcification coating was characterized in detail. Somemajor factors which affected the coating’s formation were discussed, the formationmechanism of calcium phosphate coating （Ca-P coating） was proposed. Then thecorrosion behavior and the microstructure evolution of the calcified micro-arcoxidized magnesium were investigated, where the interaction of microstructure andanticorrosion property was built up. The Ca-P coating played an important role inthe inducing bonelike apatite formation and improving corrosion behavior ofmicro-arc oxidized magnesium in vitro.The results indicated that, solution pH, calcification temperature andcalcification process affected the formation of Ca-P coating. During calcification inacidic solution, spherical HA was firstly formed on the substrate surface, whichdeveloped into porous spherical structures, after that DCPD was deposited on thesubstrate surface. There was a local alkaline zone near the substrate surface aftersoaking in acidic calcified solution. This alkaline zone promoted HPO₄^2-totransform into PO₄^3-. HA crystal began to form on the surface of Mg（OH）2byconsuming PO₄^3-and Ca²⁺. More and more HA crystals formed on the substratesurface, which inhibited the corrosion of substrtate. Local alkaline zone then becameweak, so DCPD began to deposit on the substrate surface. Local alkaline zone wasvery important for Ca-P coating formation. The coating can’t be formed duringneutral and alkali solution.Under27oC and47oC normal calcification, the Ca-P coating was composed ofHA/DCPD. When temperature was67oC, only HA was formed the substrate surface.Under27oC and47oC inverse calcification, the Ca-P coating was composed ofDCPD. Under67oC, some HA were found in the coating. High concentration of[Ca²⁺] improved HA formation, while high HPO₄^2-and low Ca²⁺promoted DCPDdeposition. Moreover, with an increase of temperature, more HA were formed on thesubstrate surface. Although high temperature improved DCPD plate to grow up, the deposition of DCPD on the substrate was inhibited.The corrosion behavior of calcified substrate in SBF were similar. The coatingresistance, oxidation layer resistance and electrical transfer resistance were includedin EIS spectra. The Ca-P coating protected the oxidation layer well during SBFimmersion, which then improved the corrosion resistance of magnesium.Microstructure of Ca-P coating proceeded a self-adjustment during SBF immersion,resulting in higher corrosion resistance. Calcified substrate behaved good stabilityduring static and dynamic SBF. The influence of flow rate on the corrosion rate ofCa-P coated specimen in SBF was little. Lower corrosion rate gave the human bodyenough time to deal with the corrosion product.New bone like apatite including Mg²⁺and CO₃^2-substitution was formed on theCa-P coating. The apatite layer was continuously getting thick showing fine crystals.The new formed apatite weakened the difference of Ca-P coatings with an increaseof immersion time. The substrate was covered by bonelike apatite, which couldfurther improve the ability of inducing more apatite formation.The biocompatibility evaluation showed that hemolytic rate of Mg was52.34%.The high concentration of Mg²⁺and high pH were responsible for high hemolytic.The hemolytic rate of MAO and Ca-P coated substrate were0.32%and0.14%. Thetreated magnesium decreased hemolytic rate, improving the cell attachment andproliferation. The Ca-P coated substrate behaved good compatibility, showing apromising future of clinical application.