Preparation And Properties Of Multifuctional Coatings Based On Micro-arc Oxidation On Biomedical Magnesium

Magnesium（Mg）and its alloys have a promising future in bone repair domain owing to its excellent biodegradability in physiological environment,low elastic modulus that comparable to human bone and excellent biocompatibility,thus is considered as the next-generation orthopedic implant material.However,a critical limitation in clinical application is that Mg exhibits poor corrosion resistance in physiological environment,which may lead to loss of its mechanical integrity before fulfillment of bone reparation.Thus improving the corrosion resistance of Mg and decreasing the degradation rate are the key to address above issue.In addition,bone regeneration and repair is a complex physiological activity,which involves to behavior of various cells and is regulated by various growth factor.Thus,better cytocompatibility and biological function are also needed for Mg based implants to up-regulate cell function and accelerate the bone regeneration and repair.Surface modification is a common method,which can endow Mg implants both improved corrosion resistance and biological function.During various surface modification technologies,Micro-arc oxidation（MAO）is attractive though which a porous MgO coating can be prepared on Mg,which presents metallurgical connection with the matrix and endows Mg considerable corrosion protection in physiological environment.However,the porous character of MAO coating make it difficult to accomplish long-term service in vitro.Furthermore,the osteoinduction of MAO coating is verified to be poor,thus it can’t up-regulate the function of osteogenesis-related cells to accelerate bone regeneration and repair.In this study,we prepared four different multifunction coatings on micro-arc oxidized Mg to improve its corrosion resistance and endow it biological function needed for bone repair and accomplish the perfect matching between degradation of Mg implants and bone repair as the ultimate goal.The main studies is as following:（1）A porous MgO coating was obtained on pure Mg though MAO process.Then the octacalcium phosphate（OCP）coatings were prepared on MgO coated Mg through chemical deposition.The results show that after 0.5 h deposition,OCP layer grows and covers the MgO coating completely.OCP layers grow in a layer-by-layer manner with increasing the deposition time.The OCP layer obtained in 0.5 h deposition presents a net-like morphology and improve cell adhesion.Prolonging the deposition to 1 h coarsens the OCP morphology and increases its thickness,which leads to improving of corrosion resistance and osteoblast function.Prolonging the deposition time to 2h,OCP layer presents flake-like morphology with a thickness of 10μm,which shows the best corrosion resistance and in vitro bio-activity.（2）A Sr-doped porous MgO coating was obtained on pure Mg though MAO process carried out in Sr（OH）₂contained electrolyte.Then chemical deposition was carried out to prepare OCP layer on Sr-doped porous MgO coating.The results show that Sr content in MAO coating was 3.99%when 3 g/L Sr（OH）₂was added in the electrolyte.Sr element exists in the form of Sr（PO₃）₂in MAO coating,which decreases the density of micro-pores thus improves the corrosion resistance of MAO coating.After chemical deposition,the corrosion resistance of Sr-doped MAO coating is further elevated owing to the pore-sealing effect of OCP.During immersion in physiological environment,Sr²⁺was released from the composite coating and improve the function of osteoblast and endothelial cells.（3）A porous MgO coating was obtained on pure Mg though MAO process.Then hydrothermal treatment（HT）was carried out to produce pore-sealed interlayer on porous MgO coating.The HT samples were immersed in dopamine solution to prepare polydopamine（PDA）coating on the surface of interlayer through layer-by-layer method.The results confirm that a Mg（OH）₂coating with bulk-like morphology is developed on the MgO coating of Mg after HT,which seals the micro-pores of the MAO coating completely and improves the corrosion resistance of MgO-coated Mg.Grain-like PDA grows on HT samples after immersion in dopamine solution,which further elevate the corrosion resistance of HT samples.In vivo biological tests show that the PDA-loaded Mg（OH）₂modified MAO coating promotes spread,proliferation and differentiation of osteoblast.（4）A porous MgO coating was obtained on AZ31 though MAO process.Thenβ-tricalcium phosphate（β-TCP）layers with grain-like structure were grown on MgO coating of AZ31 through HT.The results show that theβ-TCP layers grow uniformly on MgO coating,the morphology of which depends on the p H value of Ca（NO₃）₂solution.Specifically,the fusiformis-likeβ-TCP with the length of about 300～900 nm is observed when the p H value of solution is 5 and globe-likeβ-TCP with the diameter about 50～300nm can be obtained if the solution p H values are 7 and 11.In vivo biological tests show that the composite coating with micro/nano structure promotes osteoblast cells adhesion and differentiation observably,which also up-regulates the expression of M2 makers and osteogenic-related factor in macrophage,while down-regulates the expression of inflammatory factor.In vitro tests confirm thatβ-TCP modified porous MgO coating accelerates the bone regeneration.