Controlled release of strontium ions from a bioactive Ti metal with a Ca-enriched surface layer

A nanostructured sodium hydrogen titanate layer ∼1 μm in thickness was initially produced on the surface of titanium metal (Ti) by soaking in NaOH solution. When the metal was subsequently soaked in a mixed solution of CaCl2 and SrCl2, its Na ions were replaced with Ca and Sr ions in an Sr/Ca ratio in the range 0.18-1.62. The metal was then heat-treated at 600 °C to form strontium-containing calcium titanate (SrCT) and rutile on its surface. The treated metal did not form apatite in a simulated body fluid (SBF) even after 7 days. When the metal formed with SrCT was subsequently soaked in water at 80 °C, the treated metal formed bone-like apatite on its surface within 1 day in SBF since the Ca ions were partially replaced with H3O+ ions. However, it released only 0.06 ppm of Sr ions even after 7 days in phosphate-buffered saline. When the metal was soaked after the heat treatment in 1 M SrCl2 solution instead of water, the treated metal released 0.92 ppm of Sr ions within 7 days while maintaining its apatite-forming ability. The Ti formed with this kind of bioactive SrCT layer on its surface is expected to be highly useful for orthopedic and dental implants, since it should be able to promote bone growth by releasing Sr ions and tightly bond to the bone through the apatite formed on its surface.