Formation Mechanism And Bioactivity Of Hierarchical TiO₂ Coatings By Micro-arc Oxidation

Titanium(Ti)has been widely adopted for dental and orthopedic implants.However,the biological inertia of Ti has discouraged its application.Thus surface modification is required.The main methods to improve bioactivity are building surface structures(particularly hierarchical structures that consist of structures in different scales).improving hydrophilicity.and incorporating bioactive elements.Micro-arc oxidation(MAO)is an electrochemical technique that has been broadly used in the surface modification of Ti.Mg,AI.etc.,on the surface of which ceramic coatings are in situ formed during the MAO treatment.The roughness and porosity of the MAO coatings are usually high.which is beneficial for biocompatibility.Still,because of the lack of hierarchical structures,the biocompatibility of traditional MAO coatings has been relatively limited.A signimcant improvement of the biocompatibility of MAO coatings would be achieved through building hierarchical structures and incorporating bioactive elements."Cortex-like" MAO coatings with nano/micro hierarchical structures are prepared on pure Ti surface in the electrolyte of tetraborates.These coatings combine microslots and nanopores,presenting super-hydrophilicity.A forming mechanism was revealed by a systematic study on the properties and formation of the coatings.Compared with the formation of discrete micropores on traditional MAO coatings,the structure of the coating formed in the tetraborate electrolyte evolves as a“nucleation-lateral growth,process of micropores to form micro?groove.Due to the dissolution of TiO2 in the tetraborate electrolyte,the oxides hardly deposit around the micron-scale discharge channels and the micropores "nucleate".Adjacent nanopores are retained and become weak sites for the successive discharging due to the lack of sediment.Thus,the micropores laterally grow and transform into micro-grooves.The hierarchical structure is formed by the micro-grooves together with the nanopores.The hierarchical structure and super-hydrophilicity of the coatings stimulate the adhesion,spreading,and osteogenic differentiation of mesenchymal stem cells and enhance in vivo osteointegration.Calcium(Ca)/Strontium(Sr)incorporated coatings with hierarchical structures were prepared by adding CaO and SrO into the tetraborate electrolyte.The Ca and Sr exist in forms of carbonates and oxides.The addition of oxides and the increase of the electrolyte conductivity enhance the formation of the oxides during the MAO process and increase the sediments around the discharge channels,forming micropores.A hierarchical structure is formed by the combination of the micropores and the nanopores that are not covered by the sediments.The insoluble oxides in the electrolyte deposit on the surface after the disappearance of the discharge plasma,forming amorphous layers under synergy of the heat of discharges and the cooling of the electrolyte.These coatings present high porosity,and super-hydrophilicity and contain bioactive elements.The adhesion and spread of cells are mainly determined by surface structures,the released Ca and Sr in a certain range of concentration promote the osteogenic differentiation of mesenchymal stem cells,and the released Sr in a certain range of concentration facilitates the angiogenesis of endothelial cells.A two-step treatment is proposed to prepare submillimeter-scale macropores and superimpose "cortex-like" coatings on the macropores,forming a submillimeter/micro/nano triple structure.The macropores are formed under the plasma-enhanced corrosion in the NaNO3 and NaOH hybrid electrolyte,regularly shaped and evenly distributed.After the ultrasonic acid washing,the surface composition is restored as Ti,thus the followed treatment is conducted with little influence.In brief,it is an effective method to improve the bioactivity of the Ti to prepare MAO coatings on the surface using tetraborate-based electrolytes.The formation mechanism revealed in this study provide a helpful reference for the building of hierarchical structures,the incorporation of bioactive elements,and the improvement of biocompatibility of titanium.