| As the candidate of biodegradable metallic materials for orthopaedic application,magnesium and its alloys exhibit good properties such as similar mechanical properties with natural bone,good biocompatibility,biodegradability and osteoinduction property.However,due to the poor corrosion resistance,magnesium alloys may lead to the implant failure before the fracture site has been healed.Hence,To improve the corrosion resistance of magnesium alloys is the key problem to be solved urgently for the biodegradable bone implantation of magnesium alloys.In this study,MAO/?Zn-CaP?composite coatings were firstly synthesized on Mg-Zn-Ca alloys by combined micro-arc oxidation and pulse-reverse current eletrodeposition technique.The morphology,elementary and phase composition of composite coatings were analyzed by SEM,EDS,XRD,FTIR and XPS.The process parameters have been optimized and the mechanism of composite coating formation has been proposed.To investigate the corrosion resistance and degradation behavior,coatings were immersed in simulated body fluid for a certain period.In addition,the in vitro biological characteristics of coatings were analyzed via cytotoxicity test,cell adhesion,proliferation,platelet adhesion and human fibrinogen deformation test.?1?The composite coatings have been synthesized first by the combined MAO and electrodeposition technique.The parameters of PRC technique were optimized as15%of zinc amount,was 30 minutes of deposition time,and 10 mA/cm2 of current density.Dense and uniform coatings have been obtained,of which the main composition was CaZn2?PO4?2·2H2O.During the electrodeposition,crystals formed mechanical interlocking structure with micro-pores in MAO layer and the bond strength reached up to 27.7 MPa.The thickness of the composite coatings was about13±0.4?m and the wettability has been improved with the contact angle of31.49°±0.87°.Furthermore,the mechanism of coating formation has been revealed as:Zn-CaP nuclei randomly formed on the MAO coating nucleation grows into grain cluster grain cluster grows bigger and new grains formed clusters contact each other and finally seal the pores of MAO and covering the entire surface of the specimen.?2?As compared to the magnesium substrate,corrosion potential(Ecorr)of composite coatings has increased by 40 mV,while the corrosion current density decreased by about two orders of magnitude.In addition,lower rate of hydrogen evolution and weight loss was observed on composite coatings when compared with MAO coatings and substrate.After immersion in SBF,new calcium phosphorus salts deposited on the surface of composite coatings,indicating the great apatite-forming ability in the SBF.Based on the analysis of EIS curves and the equivalent circuit,the corrosion degradation model was also established as:Zn-CaP grain degraded and induced new CaP deposite?the lap crevice between Zn-CaP crystals was exposed and more new CaP deposite?the lap crevice was extend and new CaP degraded?Zn-CaP crystals completely degraded and the coating degraded through MAO coating degradation model.?3?According the standard of GB/T 16886.5-2016,the cytotoxicity grade of composite coatings was grade 1 indicating no cytotoxicity.Cell staining and adhesion experiments showed that cells grew well on the composite coatings and had more filopodia.Cell proliferation experiments showed that the composite coatings could promote cell proliferation and show good cytocompatibility.Furthermore,the platelet adhesion and human Fbg-denaturation experiments show that composite coatings could promote platelet adhesion and activation,and thus accelerate wound callus formation and bone healing.The study has successfully synthesized MAO/?Zn-CaP?coatings via MAO and PRC eletrodeposition technique,which can effectively improve bonding strength between coatings and substrates,and enhance the corrosion resistance.Moreover,the addition of Zn into coatings may promote the biological property and accelerate new bone formation as well as bone healing,which can coordinate the mismatch between the degradation rate of magnesium alloy and bone healing time,and in turn provide theoretical guidance for the design and development of bone implant coatings on magnesium alloys. |
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