Titania Bioactive Coatings Prepared On Titanium Substrate Using Sol-gel And Anodic Oxidation

Titanium has been used as implant materials for a long time in dental and orthopaedic applications due to its excellent mechanical properties. Titanium is apt to be oxidized and covered by a titania layer. Biochemical reactions occur between titania and host tissue when the titanium implant in contact with body fluids or tissues. In addtion, titania possesses good biocompatibility and corrosion resistance, and can be potentially used in the area of metal implant's surface modification. In this work, sol-gel and anodic oxidation were used to deposit titania bioactive coatings on titanium substrate.The porous titanium substrate was prepared by powder metallurgy process, and then TiO₂ coating was produced by sol-gel method. The coating samples were annealed at different temperatures. It was found that the TiO₂ coating covered all the outer and the inner surface of pores in the porous Ti substrate, which did not destroy the porous structure of the substrate. The as-deposited TiO₂ layer was amorphous. Some amorphous transformed into anatase titania for the coating annealed at 500℃, anatase began to ransform into rutile when the coating annealed at 600℃, and all anatase transformed into rutile when the annealed temperature up to 800℃. The corrosion resistance of the TiO₂ coating annealed at 800℃was the best. While the corrosion resistance of the coatings after annealed at 500℃and 600℃was equal to that of the Ti substrate.Hollow TiO₂ nanotubes, which were in a vertically aligned and parallel configuration, were successfully prepared by a simple anodic oxidation method when the oxidation voltage between 10～20V. There were no nanotube formation on the Ti plate when the oxidation voltage was too low or too high. At the constant voltage 20V, the average diameter of the nanotubes was unchanged and the length of the nanotubes was lengthened slightly with the oxidation time increased. The annealed temperature had a signifiant effect on the crystallization of the TiO₂ nanotubes. The as-deposited TiO₂ nanotubes were amorphous,which was similar to that of the coating prepared by sol-gel. Anatase occurred for the TiO₂ nanotubes annealed at 300℃. Anatase transformed into rutile at 500～700℃and all anatase transformed into rutile at 700℃. The configuration of the TiO₂ nanotubes existed stably when the annealed temperature was or lower than 500℃, while that was destroyed when the temperature was or above 600℃.The thickness of TiO₂ nanotube layer produced by anodic oxidation process was much thinner than that of the layer prepared by sol-gel method. And the corrosion resistance and the bioactivity of the former was not as good as that of the latter. The bioactivity assessments in vitro were performed by soaking the specimens into the simulated body fluid (SBF) for various periods. It was found that the TiO₂ coating surface was covered thoroughly with spherules apatite layer after soaking in SBF for 10 days for the sample prepared by sol-gel method and annealed at 600℃. But no obvious apatite was observed on the TiO₂ nanotubes made by anodic oxidation process even up to 30 days immersion in the SBF. The bioactivity of TiO₂ nanotubes was enhanced by the pre-calcification procedure. There was no apatite formed on the surfaces of the TiO₂ nanotubes for the sample without pre-calcification, which were soaked in SCP (supersaturated calcium phosphate solution) for 14 days. While dense and flake-like apatite formed on the surface for the pre-calcificatied TiO₂ nanotube sample, which were soaked in SCP only for 7 days. Quantitative analysis yielded a Ca to P ratio of around 1.34 that is less than the value of HA (1.67) and equal to that of the apatite induced by TiO₂ film prepared by sol-gel method.