| Vascular stent intervention is an effective means to treat vascular diseases,and magnesium-based stents have good mechanical properties,excellent biocompatibility and biodegradability,and can meet the clinical use requirements of vascular stents.Therefore,they are being focused on as the third generation of degradable stents.However,the standard electrode potential of magnesium alloy is low,and it is very easy to be rapidly corroded in the body fluid environment to make its support function fail,while the excessive hydrogen released can cause serious damage to tissues,and the excessive Mg2+ can also cause hazards such as lower blood pressure and respiratory system diseases.In addition,magnesium-based stents are also prone to endovascular hyperplasia,inflammation and other pathologies after implantation.Therefore,direct surface modification of magnesium alloy to retard its corrosion rate and control the concentration of degradation products are one of the effective ways to solve the above problems.In this paper,the surface treatment method of fluorinating magnesium alloy and compounding organic coating is used to improve the corrosion rate of magnesium alloy and enhance the stent performance.In this paper,fluoride coatings were prepared on the surface of magnesium alloy by fluoride conversion method,and then L-polylactic-acid/poly（1,3-trimethylene carbonate）coatings were prepared by organic spin coating method.The coatings were systematically characterized by X-ray diffraction analysis（XRD）,Fourier transform infrared spectroscopy（FTIR）,swept surface microscopy（FE-SEM）,laser confocal microscopy（LSCM）and other test methods,focusing on the effects of fluoride concentration and fluorination time on the structure and properties of fluoride coatings.The effects of PTMC concentration and spin coating frequency on the structure and properties of PLLA+PTMC coatings were investigated.In order to further enhance the performance of magnesium alloy vascular stents,the fluoride/PLLA+PTMC composite coating was also constructed on the prepared fluorinated coatings,and the physical phase composition,microstructure and corrosion resistance of the multi-component coating were systematically analyzed.It was shown that the fluoride concentration and fluorination time significantly affected the structure and properties of the fluoride coatings on the surface of substrate.The increase of fluorination solution concentration and fluorination time promoted the growth of fluorinated coatings.Specifically,the MgF2 grains were refined,and the grain size and surface defects of the fluorinated coatings were reduced,with the integrity of the coatings increasing.When the fluoride concentration was 40%and the fluorination time was 27 h,the surface flatness and uniform denseness of the fluorinated coatings prepared on the magnesium alloy substrate were optimized.Meanwhile,the surface roughness（Sq）was 0.274 μm,and the hydrophilicity of the fluorinated coatings was better,while the thickness of the coating was about 3 μm.Compared with the corrosion resistance of magnesium alloy substrate,the Icorr of the fluorinated coatings was reduced to 7.68×10-8 A/cm2,and Rp increased to 550×103 Ω·cm2,while Pe reached more than 98%,which indicated that the corrosion resistance of magnesium alloy was significantly improved after fluorination treatment.For the PLLA+PTMC coating obtained after spin coating treatment of magnesium alloy substrate,the concentration of PTMC and the times of spin coating had a greater influence on the structure and properties of the PLLA+PTMC coatings.The increase in PTMC concentration and times of spin coats led to a decrease in defects such as scratches and polymer particle deposition on the coating surface and a smooth and flat central part of the surface,along with a slight increase in film thickness.Under the conditions of PLLA+PTMC concentration ratio of 1:1 and 4 times of spin coating.the prepared magnesium-based PLLA+PTMC coating had the best smooth,flat surface and uniform denseness.In addition,the coating was less hydrophilic,and the coating thickness was about 1 μm.Compared with the magnesium alloy substrate,the Icorr of PLLA+PTMC coatings decreased to 6.52×10-8 A/cm2,and Rp increased to 656×103 Ω·cm2,while Pe reached more than 88%,which indicated that the corrosion resistance of the magnesium-based coating was significantly improved.Based on the optimization of the two coatings,the fluorinated/PLLA+PTMC composite coating was prepared on the surface of magnesium alloy.The results showed that the composite coating was structurally similar to the PLLA+PTMC coating,with a flat and smooth surface center part,no defects such as scratches and particle deposition,and a thicker film layer.Compared with the magnesium alloy substrate,the Icorr of the composite coating reduced to 0.789×10-8 A/cm2,and the Rp enhanced to 5627.1×103Ω·cm2,while the Pe reached 99.84%.After soaking in Hank’s solution for 48 h,the pH value of the composite coating was 8.53.which was lowest pH value among four samples.After 7 days of immersion,the uncoated magnesium alloy substrate surface was covered with corrosion cracks and corrosion pits,while a large amount of Ca-P compounds were deposited.Meanwhile,the fluorinated coatings and PLLA+PTMC coatings of magnesium alloy were covered with cracks and were severely broken.However,the surface of composite coating remained intact.This indicated that the composite coating significantly improved the corrosion resistance and hemocompatibility of the substrate. |
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