Influence of morphology and crystalline structure of TiO₂ nanotubes on their electrochemical properties and apatite-forming ability

To study the synergetic influence of TiO₂ nanotubes (NTs) morphology and crystalline structure on their electrochemical performances and apatite-forming ability, various sizes of nanotubes were synthesized via anodic oxidation of Ti and then annealed at different temperatures. XRD analysis and SEM observations confirmed that as-anodized amorphous nanotubes crystallize into anatase phase when annealed at 450 °C and into a mixture of anatase and rutile when annealed at 550 °C, without significant morphological modifications. Corrosion resistance was assessed by Open Circuit Potential measurements (OCP) and by potentiodynamic polarization curves while apatite-forming ability was evaluated by measuring the amount of Hydroxyapatite (HAp) precipitated on samples surfaces when soaked in Simulated Body Fluid (SBF) solution. Experiments confirmed that anodized titanium possesses much better corrosion resistance and bioactivity than flat Ti substrate and that annealed nanotubes are more suitable for biomedical applications than amorphous ones. Additionally, this study highlights paradoxical features such as plain anatase structure showed high bioactivity, but a mixed structure was preferable because of its synergistically better chemical stability and mechanical properties. Longer nanotubes had high corrosion resistance, but their apatite-forming ability after 14 days was poor; shorter nanotubes were less corrosion resistant, but induced thicker layer of HAp when immersed in SBF. Finally, the best compromise for implants surfaces was discussed regarding thermal, mechanical, electrochemical, chemical and bioactive properties.