| As a new type of biodegradable implant material, magnesium and its alloys haveattracted more and more attention owing to their excellent mechanical properties suchas elastic modulus close to that of natural bones, good machinability, biocompatibilityand biodegradability. However, problems such as excessive hydrogen release, localalkalization and earlier failure of implants have hindered their clinical applications.Such a rapid degradation caused by their poor corrosion resistance in the chlorineions contained physiological environment will bring an adverse effect to human body.In this work, parameters including voltage and electrolyte concentration ratio wereoptimized to prepare micro-arc oxidation (MAO) coatings containing hydroxyapatiteon the surface of biodegradable ZK60magnesium alloy. The microstructure, chemicalcomposition and corrosion resistance of the coatings were characterized by SEM,EDS, XRD, XPS, electrochemical experiment and in vitro immersion test for thepurpose of selecting a coating with good combination of corrosion resistance andphase composition. At last, the MAO coating was sealed by sol-gel deposition toimprove both the corrosion resistance and the bioactivity. In addition, thebiocompatibilities of these coatings were studied by hemolysis test and cytotoxicitytest. The main conclusions are summarized as follows:When Ca10(PO4)6(OH)2was at a concentration of1g/L in the electrolyte,micro-arc oxidation coatings obtained at different voltages were mainly composed ofO, F, Mg, P and Ca. XRD analysis indicated that these porous coatings werecomposed of MgO, MgF2and Ca10(PO4)6(OH)2. XPS analysis indicated thatCa3(PO4)2, Ca2P2O7, CaHPO4and Ca(H2PO4)2were also in the coatings. Withincrease of the applied voltage, the thickness and compactness of the coatings wereincreased. The coating obtained at420V of applied voltage behaved the bestcorrosion resistance in Hank’s solution through electrochemical and immersion tests.Besides, compared with coatings without Ca10(PO4)6(OH)2, coatings withCa10(PO4)6(OH)2not only possessed better compactness and corrosion resistance, butalso displayed better biocompatibility for their lower cell toxicity in cytotoxicity test. When prepared at420V, with different content of Ca10(PO4)6(OH)2in theelectrolyte, the surface morphologies of the coatings appeared to be similar, showingthat the majority of micro-holes was filled up by compound particles. However thecoating with higher compactness and thickness was obtained when Ca10(PO4)6(OH)2was at the concentration of1g/L. For a long time, this coating had the best stability inthe corrosive solution, indicating its superior protection for the magnesium alloy.The corrosion resistance of the coating prepared by micro-arc oxidation withsol-gel sealing was further improved. In vitro immersion tests indicated that the pHvalues of Hank’s solution always maintained at the lowest levels and considerableamount of calcium phosphate was detected on the composite coating after immersion.Meanwhile in comparison with bare ZK60magnesium alloy, the corrosion currentdensity of the sample coated by composite coating decreased significantly. Thus therapid corrosion of the magnesium substrate was effectively restricted. Hemolysis testand cytotoxicity test showed that the CaHPO4·2H2O contained composite coating hadlower hemolysis ratio and better cell compatibility than the MAO coating. Thebiocompatibility of the uncoated alloy was obviously enhanced by the two surfacetreatments, which meets the requirement of the medical implants. |
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