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In the present study, pure titanium (Ti) plates were firstly treated to form various types of oxide layers on the surface and then were immersed into simulated body fluid (SBF) to evaluate the apatite-forming ability. The surface morphology and roughness of the different oxide layers were measured by atomic force microscopy (AFM), and the surface energies were determined based on the Owens–Wendt (OW) methods. It was found that Ti samples after alkali heat (AH) treatment achieved the best apatite formation after soaking in SBF for three weeks, compared with those without treatment, thermal or H2O2 oxidation. Furthermore, contact angle measurement revealed that the oxide layer on the alkali heat treated Ti samples possessed the highest surface energy. The results indicate that the apatite-inducing ability of a titanium oxide layer links to its surface energy. Apatite nucleation is easier on a surface with a higher surface energy.

I. INTRODUCTION

It is important for new generation cementless Ti implants to bond directly to bone (osseointegration) after implantation into a living body. To achieve osseointegration, an essential requirement is the formation of a bonelike apatite layer between the implant and bone.1 Titanium oxide on a Ti implant surface has demonstrated its critical effect on the apatite layer deposition.2 Over the past decades, various surface modification techniques, such as sol-gel,3–5 microarc oxidation treatment (MAO),6 and alkali heat (AH) treatment,7–15 have been used to create titanium oxide layers with different morphologies and structures. The in vitro bioactivities of those oxide layers have been evaluated and reported extensively. The formation of apatite on various titanium oxide surfaces in simulated body fluid (SBF) is believed to be caused by the interaction of metal surface and ions in the SBF. The mechanism of apatite formation on AH treated titanium was once believed to be different from that on titanium after thermal oxidation or acid treatment.16 Na+ dissolved from sodium titanate layer accelerated the apatite deposition on the AH treated titanium in SBF.8 However, the process of Na+ dissolving from sodium titanate recently has demonstrated little influence on apatite formation on AH treated titanium surface7,9; instead, the abundant Ti–OH groups on various titanium oxide surfaces are beneficial to apatite deposition. a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0195 1682 J. Mater. Res., Vol. 23, No. 6, Jun 2008 http://journals.cambridge.org Downloaded: 25 Mar 2015

There are two types of Ti–OH groups present on the TiO2 surface, namely acidic Ti–OH group and basic Ti– OH group. At an electrolyte with pH higher than isoelectric point (IEP) of titanium oxide, the acidic Ti–OH groups dominate the surface and formation of [Ti–O]− causing a positive surface charge. IEP is the solution pH at which TiO2 surface has zero net charge. The IEP of titanium oxide is 5 to 6.17 Therefore, the titanium oxide surface is slightly negatively charged