Effect of pulse current on structure and adhesion of apatite electrochemically deposited onto titanium substrates
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Masakazu Kawashitaa) Center for Research Strategy and Support, Tohoku University, Aoba-ku, Sendai 980-8579, Japan
Gikan H. Takaoka Photonics and Electronics Science and Engineering Center, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan
Toshiki Miyazaki Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, Wakamatsu-ku, Kitakyushu 808-0196, Japan (Received 20 April 2008; accepted 15 August 2008)
Apatite films were deposited onto titanium (Ti) metal substrates by an electrodeposition method under a pulse current. Metastable calcium phosphate solution was used as the electrolyte. The ion concentration of the solution was 1.5 times that of human body fluid, but the solution did not contain magnesium ions at 36.5 °C. We used an average current density of 0.01 A/cm2 and current-on time (TON) equal to current-off time (TOFF) of 10 ms, 100 ms, 1 s, and 15 s. The adhesive strength between apatite and Ti substrates were relatively high at TON ⳱ TOFF ⳱ 10 ms. It is considered that small calcium phosphate (C–P) crystals with low crystallinity were deposited on the Ti surface without reacting with other C–P crystals, H2O, and HCO3− in the surrounding environment. This resulted in relaxation of the lattice mismatch and enhancement of the adhesive strength between the apatite crystals and Ti substrates.
I. INTRODUCTION
Titanium (Ti) metal and its alloys are widely used for orthopedic and dental applications because they exhibit high mechanical properties and biocompatibility.1–3 Ti metal has a passive layer of TiO2 on its surface, which is responsible for its chemical stability and therefore its biocompatible characteristics. This oxide layer naturally has a thickness of a few nanometers, but the thickness can be increased to a few micrometers by chemical and thermal treatments.4 Although Ti metal is biocompatible, it is not bioactive and hence it cannot directly bond to living bones. To improve its bone-bonding property, apatite coating is applied on Ti implants.5 Several coating methods, such as plasma spraying,6 biomimetic precipitation,7,8 and electrodeposition,9–11 have been successfully used to deposit an apatite layer on Ti or titanium alloy implants. Each of these methods has some advantages as well as drawbacks. Among these coating tech-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0386 3176
http://journals.cambridge.org
J. Mater. Res., Vol. 23, No. 12, Dec 2008 Downloaded: 11 Apr 2015
niques, only the plasma spraying of apatite is clinically used. However, the high temperature of the plasma flame may adversely affect the implant structure and the homogeneity of the coating, resulting in delamination of the coating.12–14 The electrodeposition method is an alternative process that uses aqueous solutions at low temperatures; this method hardly affects the implant structure and can be applied to complex shapes.15 In addition, the apatite layer can be rapidly formed on Ti substrates by el
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