| As the most potential biomedical orthopedic implant metal materials,magnesium（Mg）alloys have attracted high attention of scientific researchers,and implantable medical devices such as bone plates,bone screws and stents made of Mg alloys have been widely studied.Compared with traditional inert metals,Mg alloys have similar mechanical properties to human bone and good degradability.As orthopedic implant materials,Mg alloys will not cause stress shielding,but slowly degrade and gradually return the stress irritation to the bone tissue.Ideally,the Mg alloys can provide effective mechanical support at the initial stage of implantation and completely degrade after the bone tissue healing.Mg ions,as important cations in human body,are not only non-toxic,but also have good biocompatibility and bone inductivity.However,the rapid degradation rate,hydrogen release and local alkalinity of Mg alloys are the main reasons restricting their development.Therefore,current research mainly focus on how to control the degradation rate of biodegradable Mg alloys,and constructing composite coatings is one of the effective means to improve their corrosion resistance.Calcium phosphate（CaP）has similar chemical composition with human bone,which can improve the corrosion resistance and biocompatibility of Mg alloys when used as a coating.In particular,dicalcium phosphate dihydrate（DCPD）has good solubility,which can induce the growth of bone-like apatite and promote bone repair.However,the lifting effect of single film layer is always limited,so the construction of organic and inorganic composite coating is the current research hotspot.Siloxane is a kind of organic coating material,which is easy to prepare and friendly to environment.As natural organic polymer materials,collagen（Col）and chitosan（CS）have the dual functions of improving corrosion resistance and biocompatibility.They can be combined with DCPD to form composite coating for effective surface modification of Mg alloys.Thence,in this paper,the DCPD coating was constructed on AZ31 Mg alloy matrix at first,and then it was sealed with siloxane or Col-CS to construct DCPD/X composite coating.The microstructures and chemical compositions of the composite coating were explored by SEM,XRD,FT-IR and XPS test.The corrosion resistance of the coating was characterized by short-term and long-term corrosion tests.Cell experiments were used to evaluate the biocompatibility of the coating in vitro.Meanwhile,the formation mechanism and corrosion protection mechanism of the composite coatings were studied.The main research contents are as follows:（1）The double layer DCPD sandwiched siloxane composite coating was prepared on the surface of Mg alloy by chemical conversion method,immersion treatment method and bionic deposition method.The addition of the inducer in biomimetic deposition accelerated the nucleation and deposition of DCPD,and the final thickness of the coating was about 20.00μm.The chemically transformed DCPD of the first layer improved the corrosion resistance and the binding force.The siloxane further improved the corrosion resistance by closing the holes of the DCPD.The bionic DCPD layer improved the biocompatibility of the siloxane layer.In the electrochemical test,the corrosion resistance of the composite coating increased obviously.The corrosion potential and corrosion current density of the DCPD/PMTMS/DCPD-CS composite coating were-1.38 V and 0.03μA cm-2,respectively.The corrosion potential of the composite coating was positive shifted by 0.13 V than that of Mg alloy,and the corresponding corrosion current density was reduced by 3 orders of magnitude.The hydrogen evolution capacity of the composite coating was about 1/9 of that of Mg alloy in the long-term immersion experiment,and the composite coating showed good biomineralization ability.In vitro cell experiments evidenced that the survival rate and adhesion status of the cells on the surface of the samples modified by the composite coating were improved,the biocompatibility of Mg alloy was enhanced.（2）In order to further improve the biocompatibility of the coating,natural polymer Col-CS was used to seal the DCPD coating.The DCPD/Col-CS composite coating was prepared by chemical conversion method and immersion method.The Col-CS blend improved the fracture phenomenon of Col film forming alone and completely covered the pores of DCPD coating.The composite coating thickness was about 8.47μm.Compared with the DCPD coating,the thickness of the composite coating was not increased much,but the scaly shape was retained,leaving the binding site for cell adhesion.In the electrochemical tests,the corrosion current density of DCPD/Col-CS composite coating was reduced by 2 orders of magnitude compared with Mg alloy.The hydrogen evolution amount of the immersion experiment was about 1/7 of that of Mg alloy.The good biocompatibility of Col provided sites for the deposition of apatite,so that the composite coating was covered by the biological mineralized layer.In vitro cell experiments showed that the composite coating had good biocompatibility,and cell survival rate and adhesion state were greatly improved. |
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