Biocompatibility of Nanoscale Hydroxyapatite Coating on TiO₂ Nanotubes

In this study, a highly-ordered TiO₂ nanotube array was successfully fabricated on the surface of a pure titanium foil using the anodization method, and a hydroxyapatite (HA) layer was electrochemically deposited on the vertically aligned titania (TiO₂) nanotube array. The TiO₂ nanotubes exhibited an inner diameter ranging from 44.5 to 136.8 nm, a wall thickness of 9.8 to 20 nm and a length of 1.25 to 3.94 µm, depending on the applied potential, and the anodization time and temperature. The TiO₂ nanotubes provided a high number of nucleation sites for the HA precipitation during electrochemical deposition, resulting in the formation of a nanoscale HA layer with a particle size of about 50 nm. The bond strength between the HA coating and the nanotubular layer with an inner diameter of 136.8 nm was over 28.7 MPa, and the interlocking between the nanoscale HA and the TiO₂ nanotubes may have been responsible for the high bond strength. The biocompatibility assessment was conducted on Ti foil with a composite coat of nanoscale HA and the TiO₂ nanotube array by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) array with mesenchymal stem cells (MSCs). The mesenchymal stem cells adhered to and spread onto the nanoscale HA layer with plenty of extending filopodia, indicating excellent biocompatibility of the HA coat, the composite coat of nanoscale HA and the TiO₂ nanotube array. The findings suggest that the nanoscale HA coating on the TiO₂ nanotube array might be a promising way to improve the bond strength and the compatibility of the HA layer.