Plasma Copolymerized Fluorocarbon Coating For Improving The Corrosion Resistance Of Magnesium Alloys

Magnesium and its alloys are preferred in the study of degradable biomaterials because of their good biodegradability,biocompatibility and mechanical properties.However,the excessive corrosion rate of magnesium alloys under physiological conditions challenged its clinical application.Surface modification is one of the most common strategies for enhanceing the corrosion resistance of magnesium and its alloys.Coatings act as physical barriers to reduce the contact between corrosive media and magnesium substrate,thereby slow down the corrosion rate.Although a large number of coatings had been reported and commercial polymer coatings had appeared,the protective effects are limited.Researcheres still focused on new coating materials and preparation processes for anti-corrosion of magnesium alloy.Hydrophobic plasma-polymerized fluorocarbon?C-F?coatings are similar to polytetrafluoroethylene in chemical structures which has many special physic-chemical properties,including low dielectric constant,low permeability and good biocompatibility,etc,and their properties can be regulated with plasma parameters,so they had been swidely studied for surface modification of biomaterials due to its potential application prospects,especially,their special structure and properties could be valuable in the field of corrosion protection for biomedical magnesium and its alloys.However,when fluorocarbon coatings are prepared using fluorinated alkanes,the controllability in deposition rate are limited.Herein,hexafluoroethane?C₂F₆?mixing with acetylene?C₂H₂?to prepare a plasma copolymerized C-F coating.The unsaturated triple bonds in C₂H₂ can promote polymerization,and the deposition rate can be cantrolled by adjusting the ratio of C₂H₂.The coating is uniform,dense and non-porous,and completely covered the surface of MgZnMn alloy in thickness of hundreds nanometer.Electrochemical results showed that the C-F coatings with a thickness of 100-400 nm can significantly increased the natural corrosion potential and capacitive reactance,decreased the natural corrosion currrent of MgZnMn.The results of the in vitro immersion test at 37°C also showed that the C-F coating can significantly reduce the corrosion rate of MgZnMn alloy in PBS.Coating with a chickness of 400 nm coating showed the best anti-corrosion ability while coating with a chickness of 100 nm showed slightly insufficient in protective effect.The coating thickness is an important factor in anti-corrrosion effect of the C-F coating.In order to obtain better cytocompatibility on the material surface,an ultra-thin plasma polymerized polyallylamine?PPAam?coating was further prepared on the surface of the C-F coating,and the MgZnMn was modified by using the stacked coating.The stacked coating realized the design concept that the underlying hydrophobic fluorocarbon is mainly act for anti-corrosion and surface hydrophilic polyallylamine for functionalization.The indirect and direct contact test of endothelial cells showed that excessive concentration of MgZnMn extract inhibited cell growth,while fluorocarbon coating significantly reduced corrosion rate of MgZnMn alloy and weeken the negative effects on cell growth of the excessive corrosion.However,the hydrophobic surface of C-F coating showed poor performance in cell adhesion.Amino group can be introduced by depositing a plasma polyallyl film on the surface of the fluorocarbon coating and the CF/PPAam stack coating can significantly improve cell adhesion and growth.