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A highly bioactive bone-bonding Ti metal was obtained when Ti metal was simply heattreated after a common acid treatment. This bone-bonding property was ascribed to the formation of apatite on the Ti metal in a body environment. The formation of apatite on the Ti metal was induced neither by its surface roughness nor by the rutile phase precipitated on its surface, but by its positively charged surface. The surface of the Ti metal was positively charged because acid groups were adsorbed on titanium hydride formed on the Ti metal by the acid treatment, and remained even after the titanium hydride was transformed into titanium oxide by the subsequent heat treatment. These results provide a new principle based on a positively charged surface for obtaining bioactive materials.

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In order to elucidate the main factor governing the capacity for apatite formation of titanium (Ti), Ti was exposed to HCl or NaOH solutions with different pH values ranging from approximately 0 to 14 and then heat-treated at 6008C. Apatite formed on the metal surface in a simulated body fluid, when Ti was exposed to solutions with a pH less than 1.1 or higher than 13.6, while no apatite formed upon exposure to solutions with an intermediate pH value. The apatite formation on Ti exposed to strongly acidic or alkaline solutions is attributed to the magnitude of the positive or negative surface charge, respectively, while the absence of apatite formation at an intermediate pH is attributed to its neutral surface charge. The positive or negative surface charge was produced by the effect of either the acidic or alkaline ions on Ti, respectively. It is predicted from the present results that the bone bonding of Ti depends upon the pH of the solution to which it is exposed, i.e. Ti forms a bone-like apatite on its surface in the living body and bonds to living bone through the apatite layer upon heat treatment after exposure to a strongly acidic or alkaline solution.

Ti-15Zr-4Nb-4Ta alloy free from cytotoxic elements shows high mechanical strength and high corrosion resistance. However, simple NaOH and heat treatments cannot induce its ability to form apatite in the body environment. In the present study, this alloy was found to exhibit high apatite-forming ability when it was treated with NaOH and CaCl(2) solutions, and then subjected to heat and hot water treatments to form calcium titanate, rutile, and anatase on its surface. Its high apatite-forming ability was maintained even in 95% relative humidity at 80 degrees C after 1 week. The surface layer of the treated alloy had scratch resistance high enough for handling hard surgical devices. Thus, the treated alloy is believed to be useful for orthopedic and dental implants.

The surface structure developed on Ti metal after an NaOH and heat treatment and subsequent soaking in a simulated body fluid (SBF) was investigated using cross-sectional analysis involving SEM observations and EDX analysis, as well as an outer surface analysis involving SEM observations, thin-film X-ray diffraction, and Raman spectroscopy. A 1 μm-thick layer, which consisted of lathlike sodium hydrogen titanate (NaxH2-xTi3O7) elongated perpendicular to the surface, formed on the surface of the Ti metal after the initial NaOH treatment. This layer gradually changed into Ti metal at the boundary. The surface layer was densified by the subsequent heat treatment, accompanied by a transformation of the sodium hydrogen titanate into sodium titanate (Na2Ti6O13), rutile, and anatase. The scratch resistance of the surface layer significantly increased after the heat treatment. When this Ti metal with a modified surface was soaked in SBF, apatite began to precipitate in the interior of the surface layer, filled the interspaces of the lathlike phases to integrate with the latter giving a dense composite structure, and grew over the surface layer.

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