Induced deposition of bone-like hydroxyapatite on thermally oxidized titanium substrates using a spatial gap in a solution that mimics a body fluid

We report on the effect of using a spatial gap on heterogeneous nucleation on the surface of thermally oxidized titanium substrates. Induction of heterogeneous nucleation of bone-like hydroxyapatite (BHAp) was evaluated in the spatial gaps between substrates that were thermally oxidized at temperatures of 100-800°C on exposure to a simulated body fluid (SBF). After soaking in a SBF for 7 d, BHAp spontaneously deposited inside the gap on the surface of samples that were thermally oxidized at temperatures above 400°C, but not on samples that were thermally oxidized at temperatures of 300°C or less. Among the substrates studied, BHAp particles were most readily deposited inside the gap on the surface of the samples that were thermally oxidized at 400°C after soaking in an SBF. A smaller gap led to a higher number of BHAp particles being deposited on the surface of the samples that were thermally oxidized at 400 or 500°C. Our results suggest that the formation of BHAp in a SBF is dependent on the temperature during thermal oxidization, and also on the spatial gap between the samples. The ease of formation of BHAp on thermally oxidized titanium increases with increasing thickness of the rutile phase and the number of Ti-OH groups, which are produced during the thermal oxidization process. In addition to the surface structure of the substrates, the spatial gap is regarded as an important parameter for enhancing the deposition of BHAp. Since the formation of BHAp allows osteoconduction to occur after implantation in a bony defect, it is possible to design titanium-based implants with a high biological affinity to bone by processing using an appropriate spatial design of the substrate.