Low-temperature Deposition Of Crystalline Titania Films On Titanium Substrates And The Bioactivity In Vitro

Commercially available pure titanium (designated as CPTi) and its alloys attract the most attention in surgical and dental applications due to their relatively low density, good wear and corrosion resistance, excellent biocompatibility, and long fatigue life. Many efforts have been made to provide them with ability (bioactivity) to bond directly and reliably to living bone in a shorter period after they were implanted in vivo. An alternative is to coat the surface with a titania layer, which is thought to be bioactive due to both the chemisorbed hydroxyl (OH) and the negatively charged surface. A titania layer involving anatase derived by heating an amorphous gel layer at appropriate temperatures favors apatite (Ca5(PO4)3·OH) deposition in the simulated body fluid (SBF), a metastable calcium phosphate solution generally used to evaluate the in vivo bioactivity. Unfortunately, heat treatment decreases the amount of Ti-OH functional groups in the layer and hence the bioactivity of such treated CPTi. Therefore, it is reasonable to anticipate that crystalline titania films obtained directly from aqueous solution under mild conditions, without subsequent heat treatment, should further improve the bioactivity. This work reports a simple, economical and effective technique to provide CPTi with excellent bioactivity, under a low processing temperature of 80 ℃.Crystalline titania films consisted of anatase and rutile, with different weight ratio, were deposited directly by soaking pickled CPTi in the 30 wt. % H2O2 solution containing 3.0 mM TaCl5 at 80 ℃ for 3 d, or soaking for up to 12 h followed by ageing at 80 ℃ for 3 d in distilled water and 0.25 M HC1 solution. The crystalline titania films were suggested to come into being predominantly through the dissolution of the previously formed amorphous titania gel and the subsequent precipitation of the crystalline one back to the substrates. Another possible mechanism is the in situ crystallization. TaCl5 as well as high H+ concentrations in the solution inhibited the full development of the TiO6 net in the amorphous titania and hence favored the formation of rutile.The low-temperature derived crystalline titania film, regardless of the relative weight percentage of anatase and rutile, induced apatite deposition in SBF with an induction time of less than 1 d. The excellent in vitro bioactivity is attributed to both the crystalline substrates and the large amounts of Ti-OH functional groups since no subsequent heat treatment, whichis generally used to obtain anatase in the titania gels, has been employed. The titania film consisting mainly of rutile was found, for the first time, to induce also effectively apatite deposition in SBF, provided that it contained enough Ti-OH groups. The rutile phase was suggested to exhibit a comparable apatite epitaxial growing ability as anatase.Effects of both hydrogen peroxide concentrations and temperatures were studied in detail. Results reveal that a concentration of hydrogen peroxide as low as 6 wt. % was enough to induce the low-temperature deposition of a mixture of anatase and rutile with excellent in vitro bioactivity on titanium surface. The increased concentration of hydrogen peroxide for up to 30 wt.% had no significant effects. The ability of titanium to induce apatite deposition in SBF was readily affected by the processing temperatures.Effects of surface morphologies and film thickness of titania on the in vitro bioactivity were also focused on. Anatase films with porous and non-porous morphologies were prepared through different approaches using hydrogen peroxide solutions. A porous surface was found to favor apatite deposition in SBF, due probably to the increased supersaturation of apatite within the pores. Increased film thickness with increased crack gaps also improved the in vitro bioactivity. However, these effects were not so evident compared to the effect of the abundance of Ti-OH functional groups incorporated in the anatase films.