Fabrication And Biocharacter Of Composite Coating On Mg-zn Based Alloy

With the social development and progress in medical science,biodegradable implant materials gradually become one of the hotspots of current research in biological materials filed.Magnesium alloy has the potential to be novel degradable human hard tissue replacement implants which is more and more popular,for its excellent biomechanical compatibility,bio-security as well as degradable characteristic.However,magnesium alloy degraded too fast in the the physical environment with Cl^- existence which hindered its application in the biological body. Now surface modification is applied to improve corrosion resistance and biocompatibility of magnesium alloy,and it has been extensively studied.But there still are some problems such as the band strength is low and corrosion resistance is not good enough on some surface modification methods.So it is signification to look for the new bio-coating material which is good in biological activity,anti-stripping ability and corrosion-resistance and its preparation method.In this paper,a composite coating was obtained on a degradable magnesium alloy (Mg-Zn-Ca) implant by combined micro-arc oxidation(MAO) and electrochemical deposition(ED) for improving its corrosion resistance,the bonding strength and biocompatibility.The preparation and biological characteristics of the composite coating were studied respectively.The process parameters of micro-arc oxidation and electro-deposition were optimized.The microstructure and composition of the composite coating was characterized by XRD,EDS,SEM and FTIR analysis.At the same time the bond strength and wettability of the composite coating were tested.The corrosion resistance,biodegradation behaviour,and biocompatibility of the composite coating in vitro and in vivo were investigated.The composite coating which was consisted of Hydroxyapatite(HA), OCP(Ca₈H₂(PO₄)₆·5H₂O),Mg₃(PO₄)₂,MgO was prepared on the surface of Mg-Zn-Ca alloy successfully by the optimized process parameters of micro-arc oxidation and electro-deposition.The flake calcium phosphate with about 2.5μm in width was deposited on the surface and micro-pore of the micro-arc oxidation coating, and it improved the biocompatibility and corrosion resistance of the coating by chemically half-sealing the pores in the MAO layer.The main chemical groups in the coating were PO₄^3- and OH^- and it corresponded to the results of XRD.The thickness of the composite coating was about 26μm which was more than the micro-arc oxidation layer of about 10μm.The scratch testing and shear testing were employed to measure the bond strength of the coating and substrate.The result of scratch testing showed that the grade of the coating could be 0 while shearing test demonstrated that the bonding strength between the substrate and composite coating was more than 30MPa which was meeting the requirement of implantation.The wettability of the coating obtained by two step method was better than substrate,so the interfacial energy between solid and liquid was very low that will be beneficial to form ideal biological interface between the implants and bone tissue.The coating's property of anti-corrosion was characterized by electrochemical system.It showed that the corrosion resistance the coating by two step method was better than one step method,and the corrosion potential of the Mg substrate was improved by 150 mV by composite coating,at the same time the corrosion current reduced to 9.131×10^-7A/cm² from 1.174×10^-4A/cm².The result demonstrated that the composite coating could protect magnesium alloy from corrosion efficiently.The result of immersion corrosion testing showed that the pH value of the simulated body fluid raised after the Mg alloys were immersed,and the composite coating group raised slowest.The mass loss result consisted with the pH value result, and the average degradation rate of the composite coating group was 1.2mg/(cm²·d) after 15 days immersion while the average degradation rate of the composite coating in vivo was 0.16mg/(cm²·d) in first 8 weeks.The SEM and EDS results of the samples which were immersed in SBF and the implants in vivo showed that the calcium phosphate was induced well by the composite coating.In vitro hemolysis tests showed that the composite coating could protect the substrate in the dilution blood effectively compared with serious corrosion of uncoated specimen.The hemolysis ratio of substrate group reached 56.27 percent, while that of the coated group was 2.49 percent,less than the standard allowed of 5 percent,could be used for implantation.In vivo compatibility research included X-ray observation,macro-observation,ions level in serum and urine of rabbits implanted with substrate and coated implants testing,histopathology analysis on bone tissue and important viscera.Experiments proved that the composite coating was effective in improving the interface between the implants and the surrounding bone tissue.It could be testified that coating could accelerate the hurl of the bone tissue and the inflammation reaction was not so fiercely as Mg substrate,and osteoblasts neatly arranged on the new bone.But substrate alloy degraded in vivo significantly,and the surrounding bone tissue was disorder.Foreign body giant cells and inflammatory was also found in the tissue around the substrate implants.The ions level in serum and urine were in the normal range.The Mg²⁺ coming from the degradation of the implants could be brought out the body by metabolism.The important viscera such as heart,liver,nephro and lieno were healthy.