| The density of magnesium（Mg）and its alloys is very close to bone,and their mechanical properties are very matched with bone.Moreover,they have good biocompatibility and biodegradability,showing a wide application prospect as bone repairing implant materials.However,the over-rapid degradation rate of magnesium alloy in human body will result in serious hydrogen production,loss of mechanical properties and affect bone repair,which limits its further application in orthopaedical implantation.Surface coating technology can effectively delay the degradation of Mg alloy and endow it with certain biological functions,but the strong interface bonding strength is the key to the long-term service of in vivo.Therefore,how to enhance the interface strength,control the degradation rate,improve the biocompatibility,ensure the long-term reliability and mechanical integrity of implants is very important.In this study,inspired by biomimetic bone composition,a silk fibroin（i.e.,SF）composite coating was prepared on the surface of Mg-Zn-Ca alloy by plasma activation process,considering the excellent osteogenic activity properties of silk fibroin and hydroxyapatite（i.e.,HA）.By systematically studying the influence of plasma activation process parameters on the surface and interface,a stable silk fibroin coating was constructed.Considering uniformity and compactness of silk fibroin composite coating,silk fibroin/hydroxyapatite（i.e.,SF/HA）composite coating was prepared by controllable ultrasonic atomization spraying technology to further improve the anti-corrosion ability and biological activity.The properties of the composite coating were systematically studied by long-term immersion test,electrochemical test,in vitro cell experiment and in vivo animal experiment,moreover the corrosion behavior in the degradation process was analyzed.At the same time,the reasons for accelerating the self-healing rate of the coating after oxygen plasma secondary treatment were analyzed by scratch method.Finally,based on in vivo experimental results,biomechanical behavior of coated-magnesium alloy nail-human tibia model were evaluated by finite element simulation.By using oxygen,nitrogen and argon as plasma activation gases,the effects of gas composition and activation time on the surface wettability,roughness,microstructure and chemical state of Mg alloy were studied,and the parameters were selected.The surface effect of Mg alloy substrate was completed by etching,oxidation and chemical modification.The results showed that the surface roughness of Mg alloy was improved,and hydrophilic functional groups were increased under the oxygen/nitrogen hybrid plasma（i.e.,O2/N2 plasma）activation for 120 s.Thus,the bonding interface was enhanced between silk fibroin coating and Mg alloy substrate by strong mechanical interlocking and partial chemical bonds,and the bonding strength reached 10.98 MPa,which was 13.5 times that of un-activated process.After determining the O2/N2 plasma as the surface activation process of magnesium alloy,the silk fibroin/hydroxyapatite composite coating was prepared on the surface of the activated substrate by the self-built ultrasonic atomization spraying device with the gas velocity of 2.5 L/min and the liquid velocity of 27 m L/h.Both gas velocity and liquid velocity affected the uniformity of silk fibroin coating,and with the increase of spraying times,the compactness of the coating was improved,and its anti-corrosion ability was improved as well.In addition,there was chemical interaction between silk fibroin and hydroxyapatite in the composite coating,and hydroxyapatite made silk fibroin transform into more stableβ-sheet structure.When the mass ratio of silk fibroin to hydroxyapatite reached 10:1,the mechanical property of the coating was highly raised.Due to the barrier of the coating,the corrosion of Mg alloy was delayed,and the p H and hydrogen release of the coated samples in Hank’s solution were inhibited.The Hank’s medium diffused to the interface through coating defects,and electrochemical corrosion happened with Mg substrate.With the extension of immersing time,the surface defects of the coating gradually expanded and the protective effect on the substrate decreased.The corrosion products generated during the corrosion process can be gradually metabolized by the human body,and the calcium phosphate-based components can help the formation of new bone.At the same time,the silk fibroin/hydroxyapatite composite coating not only enhanced the biocompatibility of Mg alloy,but also reduced the in vivo degradation rate of the coating structure to0.0563 mg/day·cm2,which meets the requirements of the in vivo degradation rate in the clinic application of implant material.In addition,it was found that the secondary treatment of O2 plasma can improve the surface hydrophilicity of the silk coating,accelerate the deposition reaction of ions in the immersion solution,and improve the self-healing efficiency of the coating.With the continuous corrosion of Mg alloy implants,their mechanical properties gradually decreased.Considering the loss of mechanical properties caused by corrosion,taking from animal experiments as the initial material parameters in finite element simulation,a magnesium nail-tibia composite model was established.The simulation results showed that the stress of the tibia structure is mainly concentrated near the anterior column of the distal tibia,while the stress of the magnesium nail was mainly concentrated at the contact position between the nail and the bone.Meanwhile,the biomechanical properties of the coated magnesium bone nail were evaluated by the stress distribution on the coating surface. |
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