Co-Optimization of in Vitro Biofunctionality and Electrochemical Passivity in Self-Doped TiO ₂ Surfaces

Titanium is a well-established biomaterial, with its passive oxide film playing a key role in regulating interfacial chemistry and biofunctionality. However, the relationship between the biofunctionality of the passive TiO₂ film and its semiconducting properties remains underexplored. To address this gap, self-doped TiO₂ was fabricated on titanium via hydrothermal oxidation in hydrogen peroxide. This additive-free approach enabled the intrinsic effects of semiconducting behavior to be studied independently. Thin-film X-ray diffraction and Raman microspectroscopy identified partial reduction of Ti⁴⁺ to Ti³⁺, consistent with the formation of oxygen-deficient states characteristic of <i>n</i>-type semiconductors. Treatment conditions were varied to control the defect level. Moderate treatment produced a pronounced <i>n</i>-type character while preserving electrochemical passivity, whereas prolonged treatment yielded thicker, cracked films with reduced semiconducting response. The modified surfaces exhibited antibacterial activities against <i>Porphyromonas gingivalis</i> and <i>Aggregatibacter actinomycetemcomitans</i> and promoted in vitro osteogenic functions, including upregulation of the redox-responsive genes <i>Hic-5</i> and <i>Sod2</i>. These results demonstrate that defect-mediated <i>n</i>-type semiconducting properties are closely linked to biofunctionality, and that tuning self-doping to moderate levels is the key for co-optimizing the biological performance and corrosion resistance of titanium.