Enhancement of the Capability of Hydroxyapatite Formation on Zr with Anodic ZrO₂ Nanotubular Arrays Via an Effective Dipping Pretreatment

Hydroxyapatite (HA) depositions on metallic biomedical implants are widely applied to generate bioactive surfaces in simulated biological environments. Highly ordered anodic ZrO₂ nanotubes have attracted increasing interest for biomedical applications. However, previous reports showed that at least 14-28 days were required to obtain HA coating on ZrO₂ nanotubular arrays under biomimetic condition, thus capability to grow HA coating on ZrO ₂nanotubular at room temperature needs to be enhanced. In the present work, we demonstrate that ZrO₂ nanotubular arrays are suitable for an effective dipping treatment to induce more rapid HA coating. A series of ZrO₂ nanotubular arrays having different dimensions were fabricated in fluoride containing electrolyte. Then, we used a dipping treatment for biomimetic formation of an adhesive HA coating on the nanotubular arrays. The coatings formed rapidly using this procedure under biomimetic conditions and did not require a high-temperature annealing process. The as-formed ZrO₂ nanotubular arrays were treated using several dip-and-dry steps, through which the nanotubular arrays were filled and covered with calcium phosphate (CaP) nucleation sites. The specimens readily grew HA once immersed in the simulated biological fluid after 2 days immersion. The carbonated HA coating had several micron thickness after 8 days of immersion while only a thin layer of CaP were observed on annealed ZrO₂ nanotubes immersed in the same solution for the same duration. Tensile testing showed that bonding strength between HA coating and substrate was 21.6 ± 1.6 MPa. This treatment dramatically improves efficiency for promoting HA formation on anodic ZrO₂ nanotubes at room temperature.