Preparation and Properties of Pure Magnesium Coating on Titanium Alloy Simulated by COMSOL
Because of the excellent mechanical properties and biocompatibility,titanium(Ti)alloy has been widely used as orthopedic implants in clinical.However,as a bioinert material,aseptic loosening of Ti alloy implants always takes place after surgery due to the lack of osseointegration ability,and infection caused by bacteria is always unavoidable.Therefore,the bioactivity of Ti alloy needs to be further improved.Magnesium(Mg)-based metal is biodegradable.The alkaline environment generated through degradation process and the metal ions such as Mg2+release have multiple biological activities which is beneficial to bone tissue reconstruction,such as antibacterial properties,osseointegration and vascularization.Nevertheless,the strength of Mg is relatively low,and it cannot be applied to load bearing position of human body.Therefore,the work aims to fabricate Mg coating on Ti alloy and combine the advantage of mechanical properties for Ti alloy and biological activity for Mg coating,providing a potential alternative choice for bone implant materials.COMSOL Multiphysic software was adopted to simulate the fabrication of pure Mg coating on Ti alloy by vacuum evaporation method.Combined with practical coating deposition experiments,the effect of evaporation temperature,deposition time and substrate temperature on the quality of Mg coating was investigated.The coating deposition parameters were optimized base on the simulation results.The surface morphology,wettability,degradability and antibacterial properties of the coating were studied.The simulation results showed that the pure Mg coating prepared on the surface of Ti alloy substrate was thicker near the central axis of the evaporation material.The thickness of coating gradually decreased from the central axis to periphery,and the thickest part was 13.6％thicker than the thinnest part.The thickness of the pure magnesium coating was proportional to the deposition time,and deposition rate was about 0.000 229 mm/min.Among the three factors that affected the thickness of coating,the evaporation temperature had the greatest effect.As the evaporation temperature gradually increased,the gaps between particles gradually decreased,and the particle size gradually became larger and there was a tendency to merge together.For all evaporation temperatures,the contact angles of the coating were all greater than 90°,showing hydrophobicity,which was helpful to reduce the risk of bacterial contamination and postoperative infection for biomedical metal materials.Therefore,the coating possessed good antibacterial,self-cleaning and anticoagulation functions.The in vitro degradation experimental results showed that the coated samples prepared under 700 ℃ could maintain an alkaline environment and Mg coating sustained for a certain period of time which was enough for antibacterial behavior.The antibacterial rate for E.coli and S.aureus reached more than 90％,showing good antibacterial properties and the coating could effectively resist the postoperative infection of implanted materials.The high pH degradation environment of Mg-coated samples had an inhibitory effect on E.coli and SS.aureus,which was of great significance for bone implantation materials.Especially,in the early stage of implantation that was very important for inhibiting bacterial growth,the application of Mg as antibacterial agent would reduce the use of antibiotics and improve the success rate of surgery.The application of Mg coating will bring a new hope for the solution of the infection problem faced by orthopedic implant devices.
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