Biomaterials 25 (2004) 2533–2538 Hydroxyapatite coating on titanium substrate with titania buffer layer processed by sol–gel method Hae-Won Kim, Young-Hag Koh, Long-Hao Li, Sook Lee, Hyoun-Ee Kim* School of Materials Science and Engineering, Seoul National University, Seoul 151-742, South Korea Received 31 March 2003; accepted 4 September 2003 Abstract Hydroxyapatite (HA) was coated onto a titanium (Ti) substrate with the insertion of a titania (TiO 2 ) buffer layer by the sol–gel method. The HA layer was employed to enhance the bioactivity and osteoconductivity of the Ti substrate, and the TiO 2 buffer layer was inserted to improve the bonding strength between the HA layer and Ti substrate, as well as to prevent the corrosion of the Ti substrate. The HA layer coated over the TiO 2 showed a typical apatite phase at 400  C and the phase intensity increased above 450  C. The sol–gel derived HA and TiO 2 films, with thicknesses of approximately 800 and 200 nm, respectively, adhered tightly to each other and to the Ti substrate. The bonding strength of the HA/TiO 2 double layer coating on Ti was markedly improved when compared to that of the HA single coating on Ti. The highest strength of the double layer coating was 55 MPa after heat treatment at 500  C. The improvement in bonding strength with the insertion of TiO 2 was attributed to the resulting enhanced chemical affinity of TiO 2 toward the HA layer, as well as toward the Ti substrate. Human osteoblast-like cells, cultured on the HA/TiO 2 coating surface, proliferated in a similar manner to those on the TiO 2 single coating and on the pure Ti surfaces. However, the alkaline phosphatase activity of the cells on the HA/TiO 2 double layer was expressed to a higher degree than that on the TiO 2 single coating and pure Ti surfaces. The corrosion resistance of Ti was improved by the presence of the TiO 2 coating, as confirmed by a potentiodynamic polarization test. r 2003 Elsevier Ltd. All rights reserved. Keywords: Hydroxyapatite coating; Titania buffer layer; Titanium substrate; Sol–gel method; Bonding strength; Cell response; Corrosion resistance 1. Introduction Titanium (Ti) and its alloys have long been used as implant materials in dental and orthopedic applications [1]. To improve the implant-tissue osseointegration, much effort has gone into the modification of the Ti surface [2– 4]. Among the various attempts which have been made to improve the osseointegration, hydroxyapatite (HA, Ca 10 (PO 4 ) 6 (OH) 2 ) coatings on Ti implants have shown good fixation to the host bone and increased bone ingrowth to the implant [5]. The improved biocompat- ibility provided by the HA coatings is due to the chemical and biological similarity of HA to hard tissues, and its consequent direct bonding to host bones [6]. Parallel with this development, titania (TiO 2 ) coatings on Ti have been used to improve the corrosion resistance of Ti, which otherwise restricted its usage in load-bearing implants over a prolonged period of time [7,8]. In practice, the very thin (at most several tens of nanometers) oxide film on the Ti surface, which is formed in an aqueous environment, plays a decisive role in determining the biocompatibility and corrosion behavior of the Ti implant [9]. Since the corrosion resistance is known to increase with the thickness of the oxide layer [10,11], many attempts have been made to form a thick TiO 2 layer on the Ti substrate using various methods, such as anodization, thermal oxidation, and the sol–gel process [12–16]. Therefore, this study was performed to fabricate an HA/TiO 2 double layer coating on the Ti substrate, in order to optimize the biocompatibility of the Ti implant. The HA layer is expected to enhance the bioactivity and osteoconductivity during the initial stage following implantation, by acting as an outer coating layer; the inner TiO 2 layer is designed to prevent the Ti substrate from becoming corroded, even after the HA layer has been completely dissolved due to biological processes. More importantly, the TiO 2 layer, placed between the ARTICLE IN PRESS *Corresponding author. Tel.: +82-02-880-7161; fax: +82-02-884- 1413. E-mail address: kimhe@snu.ac.kr (H.-E. Kim). 0142-9612/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.biomaterials.2003.09.041