Preparation And Properties Of MOFs/LDH Multifunctional Composite Coatings On Medical Magnesium Alloys

Magnesium（Mg）and its alloys are sought after as metal implant materials because of their biodegradability,nontoxicity,and elastic moduli that are similar to those of natural bone.Unfortunately,Mg alloys are highly susceptible to corrosion because of their high chemical electroactivity,which limits them from meeting the lifespan requirements of implant materials.Indeed,pits and cracks caused by rapid corrosion may result in premature cracking and poor biocompatibility,eventually leading to implant failure.Moreover,implants that allow bacterial adhesion increase the possibility of microbial infection during clinical implantation,which may lead to early postoperative bacterial infection.An ideal absorbable metal implant must be capable of proper degradation without inducing undesirable reactions in the host and preventing bacterial adhesion.Layered double hydroxides（LDHs）have attracted extensive attention in the preparation of functional coatings on magnesium alloys due to their ion exchange properties.Recent studies have shown that there is a strong affinity interaction between metal-organic frameworks（MOFs）and LDH structures,and the photothermal properties of some MOFs are widely used in the preparation of antibacterial functional coatings.In this study,after micro-arc oxidation（MAO）pretreatment,by adding zeolitic imidazolate framework nanoparticles（ZIF-8）to the hydrothermal reaction solution,a nano-silver（Ag）/ZIF-8/magnesium-aluminum layered double hydroxide（LDH）composite coating（Ag/ZIF-8/LDH）was prepared that combines ion-releasing antibacterial activity and photothermal therapy.The effects of ZIF-8 addition on the corrosion resistance,antibacterial properties and biocompatibility of the coating were investigated.Then,the Prussian blue（PB）/Mg-Zn-Fe LDH（PB-LDH）coating was prepared on the surface of MAO pretreated magnesium alloy by adding PB.The reaction temperature and reaction time were optimized by the control variable method,and their effects on the morphology,composition and corrosion resistance of the coating were studied.Finally,the antibacterial and biocompatibility of the optimized PB-LDH coating were evaluated.The main research contents are as follows:（1）The Ag/ZIF-8/LDH coating was successfully prepared by adding ZIF-8.The SEM results showed that the surface morphology was denser than the MAO coating,and the corrosion current density was reduced by an order of magnitude.According to the long-term corrosion results,the annual corrosion rate of Ag/ZIF-8/LDH coating（0.338±0.028 mm/year）was significantly lower than that of Ag/LDH（0.384±0.037mm/year）,which proved that the addition of ZIF-8 greatly improved the corrosion resistance of the substrate,and the photothermal performance of Ag/ZIF-8/LDH coating was significantly improved（its temperature increased by 46.7°C）.The release of Ag⁺and Zn²⁺and the synergistic effect of photothermal therapy make the sample have better antibacterial properties.Its antibacterial rate against S.aureus and E.coli was as high as 99.80%and 99.89%.（2）The effects of reaction temperature and reaction time on the composition and morphology of PB-LDH coating were analyzed by control variable method.The SEM results showed that the surface of the coating was rougher with the increase of reaction time,and the coating had the densest surface morphology at 24 h.When the reaction temperature was 75°C,the coating had the densest surface morphology.Combined with in vitro electrochemical test and hydrogen evolution test to detect the in vitro degradation performance of the coating samples,the optimal reaction conditions were as follows:the reaction temperature was 75°C and the reaction time was 24 h.（3）The biocompatibility and photothermal antibacterial properties of the above optimized PB-LDH coatings were evaluated by cck8 method and plate counting method,respectively.The cell viability of PB-LDH coating measured by cck8 was more than90%after 1,3 and 5 d of culture,which proved that it had excellent biocompatibility.The results of osteogenic gene expression showed that the osteogenic expression of PB-LDH was much higher than that of MAO samples and magnesium alloy samples,which indicated that PB-LDH coating had good osteogenic ability and biocompatibility in vitro.The photothermal results showed that PB-LDH samples had good photothermal properties,and the synergistic effect of Zn²⁺and Fe³⁺release and photothermal therapy made the samples have better antibacterial properties.Its antibacterial rate against S.aureus and E.coli was as high as 99.71%and 99.93%.