Preparation And Bioactivity Of Titania Coatings On Medical NiTi Alloy

The micro-arc oxidation （MAO） method has been employed for the surfacemodification of NiTi alloy to improve its corrosion resistance, reduce the releasingamount of the Ni ions, and improve the biological activity of the NiTi alloy. Thesurface morphology, phase composition, element component and valence of thecoatings have been characterized by SEM, XRD, EDS, and XPS. The corrosionresistance and the Ni ion releasing rate of the samples have been evaluated byelectrochemical method and Hank’s solution immersion test. SBF soakingexperiment has been employed to study the apatite-induce ability, and evaluate thebioactivity of the micro-arc oxidation coating. The osteoblast cell culture studyhas been employed to study the biocompatibility of the coating. The SEM andfluorescence microscopy have been employed to observe the growth andreproductive behavior of the cells on the surface of the MAO coating. The cellbioactivity on coating surface has been also evaluated by monitoring alkalinephosphatase activity.After NiTi alloy had been MAO treated in the concentrated sulfuric acidelectrolyte, a porous TiO₂ceramic coating with the thickness of0.4-0.5μm couldbe prepared on its surface. The coating was tightly adherented to the NiTi alloy,and could effectively inhibit the Ni releasing. When the concentrated phosphoricacid was employed as the MAO electrolyte to treat the NiTi alloy, a P-containingTiO₂coating could be formed on the surface of NiTi alloy. TiO₂in the film wasmainly in the amorphous state, and the amorphous TiO₂could be transformed toanatase after the hydrothermal treatment. The thickness of the MAO coating firstincreased and then decreased with the increase of the MAO processing time, andthe roughness increased with the growth of the processing time. The increase ofcurrent density could make the coating thickness thicker. Both the corrosionresistance and Ni ions inhibition ability of the MAO coating formed inconcentrated phosphoric acid electrolyte first increased and then decreased withthe increase of the MAO processing time or the increase of current density.When the NiTi alloy was MAO treated in the ethylene glycol electrolytewhich contained TiO₂Sol （TiO₂-Sol）, a P-containing TiO₂coating could form onthe surface of the alloy, and the thickness of the coating was significantly thickerthan the coating formed in the concentrated acid system. The substance in thecoating was mainly in the amorphous state, and the amorphous TiO₂in the coatingcould be transformed to anatase after the hydrothermal treatment. The thicknessand roughness of the MAO coating increased with the increase of MAO processing time or the increase of current density. The corrosion resistance and Ni ionsinhibition ability of the MAO coating first increased and then decreased with theincrease of the MAO processing time or the increase of current density. When thecalcium glycerophosphate was added to the Ti-Sol electrolyte, Ca and P elementscould be found in the prepared TiO₂coating. The XRD results showed thatCa₂P₄O₁₂4H₂O had been formed in the coating. The addition of the Ca elementsin the MAO electrolyte increased the thickness and roughness of the MAO coating,reduced the corrosion resistance and the Ni ions inhibition ability of the MAOcoating, and also reduced the binding-strength between the coating and substrate.All the MAO coatings prepared in different electrolytes could induce theformation of hydroxyapatite when soaked in SBF solution, which indicated theMAO treatment could improve the bioactivity of the NiTi alloy. XRD, EDS, XPS,and FT-IR results showed that the induced apatite was a kind of carbonatecontaining bone-like apatite. Ca and P elements combination could improve theapatite-induced ability of the MAO coating. The order of the apatite-inducedability of the coatings form in different electrolytes was: TiO₂-Sol-Ca> TiO₂-Sol>concentrated phosphoric acid> concentrated sulfuric acid.The MAO coatings had the appropriate condition for the cell proliferation.The results of in-vitro cell culture showed that the proliferation of osteoblast cellhas rosen with the increase of the incubation time, and the proliferation ofosteoblasts was significantly higher than the untreated NiTi alloy substrate. SEMshowed that the cells spread well on the surface of the coating and tightlyadherented to the surface and it could also be found that the cells were intact andfull. Fluorescence microscopy revealed that the cells lived well in the surface ofthe coating and growed strongly. That indicated the micro-arc oxidation layer hasgood cell compatibility. Osteoblasts multiplication behavior and alkalinephosphatase results showed that osteoblasts had good bioactivity and proliferationablity on the MAO coatings prepared in concentrated sulfuric acid, concentratedphosphoric acid, and the TiO₂-Sol electrolyte. It could be included that NiTi alloysafter MAO treatment in these three electrolyte can improve its activity andbiocompatibility. But due to the high amount of Ni ions released from the MAOcoating prepared in TiO₂-Sol-Ca electrolyte, the osteoblast adsorption andreproduction was inhibited, and the biocompatibility of the coating declined.