Surface modification of TiO2 nanotubes via pre-loaded hydroxyapatite towards enhanced bioactivity

Anodic TiO2 nanotubes (NTs) are widely established in biomedical applications, as the sub-100 nm morphology significantly impacts their biological activity. In this study, we examine the use and surface functionalization of TiO2 nanotube layers on titanium substrates to facilitate the formation of hydroxyapatite, a crucial ability for implant applications. TiO2 NT layers are grown by electrochemical anodization and the focus is on as-formed and anatase NTs with 100- and 15-nm-diameters, the latter amorphous and available in double-wall (DT) or singlewall (ST) structures. Surface modification of the TiO2 NTs is achieved through an alternate immersion method (AIM) or a simple CaCl2 immersion. The former deposits hydroxyapatite (HA) coatings on/in the NT layers, while the latter forms a thin Ca-surface-modified layer on the TiO2 surface. Both methods effectively induce the formation of an HA layer on 100-nm-diameter NTs after five days of immersion in simulated body fluid (SBF). The chemical composition of NTs is a deciding factor, as the 100-nm-diameter NTs that already contain phosphates (from the anodizing electrolyte) also lead to HA formation by modification via Ca-functionalization (CaCl2 immersion). Whereas, for smaller diameter NTs, the surface nanotopography of the DT and ST NTs is key for the HA nucleation and formation via the AIM approach, but not via immersion in a calcium-containing solution. This promising approach accelerates the growth of HA on TiO2 nanomaterials by initiating apatite nucleation on TiO2 NTs and, thus, has significant implications for increasing their bioactivity in biomedical applications.