Bioactivity and generation of anodized nanotubular TiO2 layer of Ti–6Al–4V alloy in glycerol solution

• We examined the growth behavior of anodic TiO2 nanotubular layer.
• The nanotubular TiO2 layer showed a dense structure on the Ti–6Al–4V alloy implant.
• Diameter of the TiO2 nanotubes increase with applied potential and water content.
• Bioactivity of Ti–6Al–4V alloy implant was improved by precalcification treatment.

This study examined the growth behavior of anodic TiO2 nanotubular layer in glycerol solutions and the bioactivity of a TiO2 nanotubular layer that was modified by precalcification treatment. A glycerol solution containing 1 wt.% NH4F and 20 wt.% deionized water was used as the electrolyte. Anodization was carried out at 20 V at a current density of 20 mA/cm2 for 1 h. The anodized specimens were precalcified by soaking in a Na2HPO4 solution at 80 °C for 30 min followed by soaking in a saturated Ca(OH)2 solution at 100 °C for 30 min and heat-treatment at 500 °C for 2 h. To evaluate the bioactivity of the precalcified TiO2 nanotube layer, hydroxyapatite precipitation was examined in simulated body fluid at pH 7.4 and 36.5 °C for 10 days. During the electrochemical oxidation process, a thin strip-like net structure was formed at the initial stage, which then developed into a well-arranged and self-organized nanotubular TiO2 layer. The nanotubular TiO2 layer had a dense structure due to the interpositioning of small diameter nanotubes between the larger diameter nanotubes, and the diameter of the nanotubes increased in the direction towards the substrate. The diameter of the TiO2 nanotubes increased with increasing applied potential and water content in the glycerol electrolyte solution. The diameters of the nanotubes grown on the samples treated in a glycerol solution containing 20 wt.% H2O and 1 wt.% NH4F at 20 V were 122.9 ± 15.2 nm and 88.5 ± 9.5 nm, and the length of nanotubes was 567.0 ± 27.2 nm. The bioactivity of the Ti–6Al–4V alloy implant was improved with the formation of a nanotubular TiO2 layer and precalcification treatment in a 0.5 M Na2HPO4 and saturated Ca(OH)2 solution.