Degradation Behavior And Biocompatibility Of Biomedical Magnesium Alloys Modified By Hydrotalcite-based Coatings

Compared to traditional inert biomedical metallic materials,magnesium(Mg)and its alloys do not need to be removed through a secondary surgery due to their degradability and their elastic modulus similar to that of human bone can effectively avoid the stress shielding effect.Therefore,degradable Mg-based orthopedic implants have received extensive attention.However,Mg and its alloys have low standard electrode potentials which leads to the natural corrosion reaction and a rapid degradation rate in the corrosive solution.To ensure bone tissue healing,implants generally need to maintain mechanical integrity for 12 weeks.The rapid degradation of biomedical Mg and its alloys in vivo leads to premature loss of sufficient mechanical strength before achieving their service mission.Therefore,it is urgent to control the degradation rate of Mg and its alloys and surface modification technology is a feasible strategy to achieve controllable degradation of Mgbased implants.Layered double hydroxides(LDH)also called hydrotalcite-like compounds have attracted extensive attention in the biomedical field due to their good biocompatibility and anion exchange between layers.A LDH coating intercalated with CO32-is prepared on the surface of AZ31B Mg alloy by coprecipitation and hydrothermal treatment and an outer layer of poly lactic acid(PLLA)coating is then spin coated with the 3-ammonia propyl triethoxy silane(APTES)as an interlayer.The composite coating(LDH-APTES-PLLA)plays a positive role in the corrosion protection.Compared to common physical barrier coatings,the self-healing coatings can actively repair the coating defects in the presence of external stimuli and prolong the service time of the coatings.Thus,a self-healing coating loaded with corrosion inhibitor is prepared in this paper.The LDH coating intercalated NO3-(LDH-NO3)is firstly fabricated on the anodized Mg alloy and the selfhealing LDH coating intercalated SiO32-(LDH-SiO3)is then acquired by anion exchange.Surface morphology and compositions of the composite coatings,corrosion resistance,and biocompatibility are investigated systematically by scanning electron microscope,energy dispersive spectroscopy,X-ray diffractometer,Fourier transform infrared spectroscopy,electrochemical tests,immersion studies,and CCK-8 cytotoxicity tests.The mechanisms of coating formation and corrosion protection are disclosed.The main conclusions are as follows:The scanning electron microscope results reveal that LDH-APTES-PLLA coating is uniform and dense,is closely connected to the Mg alloy substrate,and has a thickness of approximately 3.45±0.06 μm.The scratch test shows that the composite coating has good adhesion which is attributed to the APTES intermediate layer improving the adhesion of the PLLA layer.APTES first forms firm Si-O-Mg covalent bonds with LDH.In the meantime,-NH2 on the surface of APTES can provide more hydrogen bond donors in the formation of hydrogen bonds with-OH in PLLA,and the formation of more hydrogen bonds leads to the enhancement of bonding strength of the coating.The electrochemical results show that the corrosion current density of the Mg alloy modified by the composite coating in the simulated body fluid decreases by 400 times and charge transfer resistance increases by 942 times.Immersion experiments show that the uniform and dense composite coating retards the penetration of the corrosive medium and avoids the risk of the coating peeling.In vitro cell attachment and CCK-8 cytotoxicity tests show that the Mg alloy modified by the LDH-APTES-PLLA coating benefits the adhesion and proliferation of MC3T3E1 pre-osteoblasts.The self-healing LDH-SiO3 coating prepared on the anodized Mg alloy exhibits a doublelayer structure composed of a dense inner layer and a porous outer layer.The X-ray diffractometer and infrared spectroscopy results prove that SiO32-is intercalated into the gallery of LDH and the thickness of the LDH-SiO3 coating is 4.03±0.48 μm.The electrochemical results demonstrate that the LDH-SiO3 shows better protective ability.Compared to those of the substrate,the corrosion current density of the modified Mg alloy in the simulated body fluid is reduced by 300 times and the charge transfer resistance increases by 453 times.The 15-day soaking studies reveal that the self-healing coating significantly inhibits the formation and propagation of corrosion pits,which is attributed to the ion exchange effect of LDH to capture Cl-in the corrosive medium.Moreover,corrosion inhibitor ions are released to form precipitate and plug the coating defects,which prolongs the time for the corrosive Cl-to reach the substrate surface.The adhesion and proliferation experiments of MC3T3-E1 pre-osteoblasts show that the cells exhibit good adhesion,spreading,and proliferation behavior on the surface of the Mg alloy modified by the self-healing coating and in the corresponding extract,indicating that the self-healing coating significantly improves the cytocompatibility of the Mg alloy.