Co-Optimization of in Vitro Biofunctionality and Electrochemical Passivity in Self-Doped TiO₂ 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 n-type semiconductors. Treatment conditions were varied to control the defect level. Moderate treatment produced a pronounced n-type character while preserving electrochemical passivity, whereas prolonged treatment yielded thicker, cracked films with reduced semiconducting response. The modified surfaces exhibited antibacterial activities against Porphyromonas gingivalis and Aggregatibacter actinomycetemcomitans and promoted in vitro osteogenic functions, including upregulation of the redox-responsive genes Hic-5 and Sod2. These results demonstrate that defect-mediated n-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.