The essential prerequisite for an artificial material to show bone-bonding ability, i.e., bioactivity, is the formation of a bone-like apatite layer on its surface in a body environment. This can be reproduced in a simulated body fluid (SBF) with inorganic ion concentrations nearly equal to those of human blood plasma. It is known that some surface functional groups are effective for inducing heterogeneous apatite nucleation in SBF. Niobium is used clinically as a component in titanium alloys to improve their workability. In the present study, in order to investigate whether or not Nb-OH groups are effective for apatite nucleation, sol-gel-derived niobium oxide gels subjected to various heat treatments, and niobium metals subjected to various NaOH treatments were examined for their apatite-forming ability in SBF. The sol-gel-derived niobium oxide gels, both as-prepared, and heat-treated at 600°C, assumed an amorphous structure, and those heat-treated at 800 and 1000°C assumed orthorhombic and monoclinic structure, respectively. Apatite formation was observed on the niobium oxide gels both as-prepared, and heat-treated at temperatures below 800°C in SBF within one week, but not on the gel heat-treated at 1000°C. Niobium metals treated with the NaOH solutions did not form apatite on their surfaces, even after soaking in SBF for two weeks. It was confirmed that niobium metal formed a sodium niobate on its surface after the NaOH treatment, and that the sodium niobate formed a niobium oxide hydrogel by exchange of a Na+ ion with the H3O+ ion in SBF. Niobium oxide gels prepared by the sol-gel method have the ability to form an apatite layer on their surfaces in a body environment, when the gel has an amorphous phase structure, or an orthorhombic phase with abundant Nb-OH groups. Apatite deposition is not induced by Nb-OH groups formed on niobium metal after NaOH treatment and subsequent soaking in SBF. A niobium-based bioactive material can be developed through the design of a surface structure that effectively induces apatite nucleation.
