Fabrication of Al2O3 by anodic oxidation and hydrothermal synthesis of strong-bonding hydroxyapatite coatings on its surface

Due to excellent mechanical properties and biocompatibility, Al2O3 is considered as a promising biomaterial for biomedical applications. However, the inherent bio-inertness of Al2O3 have limited its wider application. Although applying bioactive hydroxyapatite (HA) coatings on Al2O3 is an effective method to improve surface bioactivity, the bonding strength between the HA coatings and the flat Al2O3 substrate is low. In this study, Al2O3 with micro-porous surface structure and hydrophilic surface was fabricated by anodic oxidation to improve the bonding strength between HA coatings and Al2O3. Then, strong-bonding HA coatings were prepared on the porous Al2O3 by hydrothermal synthesis. The results reveal the uniformity of pore distribution and the wettability of Al2O3 surface are significantly improved with the increase of current density and oxidation time. When the porous Al2O3 is prepared by anodic oxidation at 7 mA/cm(2) for 4 h, the diameter of pores is 4.3-7.1 mu m and the porous Al2O3 has a contact angle of 42.2 +/- 2.6 degrees. Nano-rod-like HA coatings completely and uniformly cover the surface of Al2O3 by hydrothermal synthesis at 180 degrees C for 24h. The thickness of HA coatings is almost 30 +/- 1.01 mu m without delamination and/or cracking at the interface between HA coatings and Al2O3. The highest bonding strength of HA coatings on porous Al2O3 is 18.3 +/- 2.4 MPa, which is higher than that of HA coatings formed on flat Al2O3 by other methods. In addition, simulated body fluid (SBF) immersion tests indicate that HA-coated Al2O3 possesses excellent biological activity in vitro. Hence, Al2O3 with bioactive HA coatings could be a candidate biomaterial for biomedical applications.