Research On Preparation And Healing Mechanism Of Self-Healing Coatings On Biomedical Magnesium Alloys

Mg metals can be completely degraded and absorbed in the human body,and exhibit good biological and mechanical compatibility,which are an ideal material for new biodegradable orthopedic implant devices.However,the excessive degradation rate is the main problem limiting the clinical application of biomedical Mg-based bone repair implants.Surface coatings are a significantly effective approach to improving the corrosion resistance of biomedical Mg,but the surface coatings are easily damaged,resulting in local corrosion of Mg.The damaged coatings play a negative role in corrosion protection,causing the premature failure of Mg-based implants.Therefore,bio-safe self-healing coatings have been developed as a hot topic in the field of biomedical Mg research in recent years.In this work,a series of new biomedical self-healing coatings with good biocompatibility have been developed,and the self-healing mechanisms of these coatings have been explored.The research results not only enrich the theory of self-healing coatings on Mg alloys,but also provide theoretical guidance for promoting the research and development of clinical applications of biomedical Mg-based implant devices.In this work,self-healing coatings loaded with bio-safe corrosion inhibitors were prepared on biomedical Mg alloys,and the microstructures and properties were systematically investigated.The anti-corrosion properties and self-healing behaviors of the coatings in Hanks’balanced salt solution（HBSS）and normal saline solution（NS）were evaluated by in-vitro degradation tests and corrosion electrochemical methods,and the self-healing mechanism was analyzed by the phase evolution and material migration.The main research contents and conclusions are as follows:（1）This work is the first to discover the self-healing behavior and mechanism of dicalcium phosphate dihydrate（DCPD）coatings.DCPD coating was prepared on pure Mg by chemical conversion treatment.The research showed that DCPD acted as not only a physical barrier layer but also a corrosion inhibitor source,regarded as a biomedical surface coating with both good anti-corrosion ability and self-healing function.The self-healing mechanism was that the local alkaline environment in coating scratches stimulated p H-sensitive DCPD to release Ca²⁺and PO₄^3-ions,resulting in the formation of corrosion-resistant products to inhibit local corrosion in the coating scratches.Besides,the HBSS could provide extra corrosion inhibitors to synergistically promote self-healing ability.（2）Based on the concept of composite enhancement,phytic acid（H_xPhy）groups were introduced into the DCPD coating,and the DCPD/H_xPhy hybrid coating was prepared on biomedical Mg.When the DCPD/H_xPhy hybrid coating was damaged,the exposed metallic Mg was corroded along with local enrichment of OH^-and Mg²⁺ions.OH^-ions stimulated DCPD to dissolve and release Ca²⁺and PO₄^3-ions to form corrosion-resistant apatite compounds.Besides,the H_xPhy group was sensitive to Mg²⁺ion enhancement,and tended to chelate with Mg²⁺ions to form Mg-H_xPhy compounds for localized corrosion mitigation.DCPD/H_xPhy9hybrid coating exhibited an efficient dual self-healing behavior with Mg corrosion by-products(Mg²⁺and OH^-)as dual responsive stimuli.（3）In order to avoid premature dissolution of conversion coatings and unnecessary loss of corrosion inhibitors,a novel coating（PEO-LDH-P）was constructed on Mg alloy（Mg-2Zn,Z2）,including a plasma-electrolytic-oxidation（PEO）coating as the physical barrier layer,layered double hydroxide（LDH）as the corrosion inhibitor carrier,and phosphate ions as the corrosion inhibitor.Phosphate corrosion inhibitors were introduced not only into the interlayer but also on the surface of LDH.PEO-LDH-P coating exhibited good self-healing behavior,and the released PO₄^3-ions cooperated with Ca²⁺and PO₄^3-ions in HBSS to form anti-corrosion Ca-P compounds for local corrosion mitigation.PEO-LDH-P coating displayed a dual-response self-healing mechanism.On the one hand,the PO₄^3-ions intercalated in LDH were released by interlayer anion exchange.On the other hand,the PO₄^3-ions existed on the surface of LDH in the form of Mg HPO₄ compounds.Therein,Mg HPO₄ exhibited p H-dependent dissolution performance and tended to release PO₄^3-ions in the alkaline environment.（4）Finally,the cytocompatibility of Mg-based alloys and coating samples was evaluated by the cell co-culture method.These self-healing coatings exhibited good hydrophilicity,and could improve the cytocompatibility of Mg-based materials.The enhanced cytocompatibility was due to the bio-safe coating composition,good anti-corrosion performance,and hydrophilic surface,providing a good environment for cell adhesion,growth,and proliferation.