Study On Microstructure,Properties And Biocompatibility Of Biodegradable Mg-2Sr-xZn Alloy

Magnesium alloy has good biocompatibility,mechanical properties matching with bone tissue,it can be degraded and absorbed in human body.Magnesium has become a new bone implant material with great clinical application prospects.However,magnesium alloy as a bone implant material still has the drawbacks of poor mechanical properties and rapid corrosion rate.In this paper,using human nutritional elements Sr and Zn as alloy elements,it investigated the microstructure,mechanical properties,and corrosion resistance of cast and extruded Mg-2Sr-x Zn alloys.In order to further improve the corrosion resistance of the alloy,MAO+Ta₂O₅composite coatings were prepared on the magnesium alloy surface by micro arc oxidation（MAO）and sol-gel method.By evaluating the biocompatibility of Mg-2Sr alloy,Mg-2Sr-Zn alloy,MAO coating and MAO+Ta₂O₅composite coating,the effects of different materials on cell adhesion,proliferation,differentiation,hemolysis and other behaviors were analyzed.In this paper,the human nutrient Zn was added to the Mg-2Sr alloy matrix,and the Mg-2Sr-x Zn alloys（Mg-2Sr,Mg-2Sr-Zn,Mg-2Sr-2Zn,Mg-2Sr-4Zn）with different Zn contents were designed and prepared.The effect of Zn content on the microstructure,mechanical properties and corrosion resistance of as-cast magnesium alloy was studied.The results showed that Sr element mainly exists at the grain boundary in the form of the second phase Mg₁₇Sr₂,and an appropriate amount of Zn element was dissolved inα-Mg matrix,excessive Zn element would form Mg Zn phase and existed at grain boundary.The addition of appropriate amount of Zn significantly refined the alloy grain,improved the mechanical properties and corrosion resistance of the alloy,but the addition of excessive amount of Zn reduced the plasticity and corrosion resistance of the alloy（Mg-2Sr-2Zn had the best comprehensive properties）.Through the slow strain tensile test in simulated body fluid（SBF）solution,it was found that the addition of Zn could effectively reduce the stress corrosion sensitivity coefficient of as-cast Mg-2Sr alloy.There were a large number of secondary phases at the grain boundary of the as-cast Mg-2Sr-x Zn alloy.During the stress corrosion process in SBF solution,the alloy caused intergranular corrosion fracture due to micro-electric corrosion.The effect of plastic deformation on the microstructure,mechanical properties and corrosion resistance of the magnesium alloy was studied by using hot extrusion with high extrusion ratio.It was found that the grains of the extruded alloy had been significantly refined and the second phase of the cast alloy mesh has been fully broken.The second phase in the extruded alloy existed in the form of island and rod,but the composition of the second phase had not changed.The mechanical properties of the alloy after extrusion had been significantly improved.The addition of Zn effectively weakens the base texture produced by extrusion deformation and reduced the small-angle grain boundary of the alloy to improved the plasticity of the alloy.The corrosion resistance of the alloy after extrusion deformation had also been improved compared with the as-cast state,mainly due to the obvious refinement of the grain after plastic deformation and the full fragmentation of the second phase,that the corrosion of the alloy had changed from pitting to uniform corrosion.Hot extrusion deformation reduced the stress sensitivity coefficient of as-cast magnesium alloy in SBF solution,and improves the corrosion mode of Mg-2Sr-x Zn alloys in SBF solution.Through constant stress corrosion experiments,the electrochemical corrosion behavior and immersion corrosion behavior of different alloys under stress conditions were studied.It was found that Zn can improve the constant stress corrosion behavior of the alloy.The greater the loading stress,the smaller the corrosion potential of the alloy in SBF solution,and the greater the corrosion current density.Through the preparation of MAO coating on the surface of extruded Mg-2Sr-Zn alloy,it was found that the coating prepared with the method of low voltage first and then high voltage had fewer holes on the surface,and the coating thickness was the thickest,which had the best corrosion resistance.The MAO+Ta₂O₅composite coating was prepared by the sol-gel method,and the Ta₂O₅coating was used to seal the holes in the micro arc oxidation layer,effectively improving the corrosion resistance of the alloy.The composite coating obtained after 10 layers of immersion coating had less surface cracks and had the best corrosion resistance.The static hydrogen evolution experiment of alloy MAO coating and MAO+Ta₂O₅composite coating shows that the hydrogen evolution amount of MAO+Ta₂O₅composite coating sample was about 1/6 of that of MAO coating sample.After soaking for 7 days,the MAO+Ta₂O₅coating had no cracks and the sample has no obvious corrosion.The samples of bare metal,MAO coating and MAO+Ta₂O₅composite coating were respectively loaded with 20MPa tensile stress for constant stress corrosion test.The coating on the surface of MAO+Ta₂O₅composite coating began to fall off after 28 days of stress corrosion,and the tensile strength of the samples did not significantly decrease,indicating that MAO+Ta₂O₅composite coating had good resistance to stress corrosion.By evaluating the biocompatibility of Mg-2Sr alloy,Mg-2Sr-Zn alloy,MAO coating and MAO+Ta₂O₅composite coating,the effects of different materials on cell adhesion,proliferation,differentiation,hemolysis and other behaviors were analyzed.The surface hydrophilicity of MAO coating and MAO+Ta₂O₅composite coating had been significantly improved.Through the early spreading morphology of cells,it was found that the cells were in good spreading state in the culture medium of four kinds of material extracts,and the cells grew filamentous pseudopods and spread around,showing good biocompatibility.The cell proliferation experiment showed excellent proliferation performance.Through the analysis of cell bone differentiation performance,it was found that the MAO+Ta₂O₅composite coating showed better osteogenic differentiation ability in vitro,regardless of the qualitative and quantitative analysis of alkaline phosphatase,collagen secretion and mineralized calcium deposition.Through the test of platelet adhesion and hemolysis rate,it was found that the four materials showed good anticoagulant properties without obvious pseudopodia formation.The hemolysis rate of MAO+Ta₂O₅composite coating was less than 5%,and MAO+Ta₂O₅showed good blood compatibility.