Formation of Hydroxyapatite Coating on Anodic Titanium Dioxide Nanotubes via an Efficient Dipping Treatment
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TRODUCTION
TITANIUM and its alloys are widely applied in orthopaedic and orthodontic implants owing to their excellent mechanical properties, adequate corrosion resistance, and good biocompatibility.[1–4] Moreover, titanium and its alloys are bioinert materials, which can be covered by the host organism without being integrated with bone.[5,6] Thus, surface modification on titanium and its alloys was required and has been reported during the last several decades.[7–9] For example, chemical etching in either acidic or alkaline solutions or anodization above the breakdown potential to obtain a porous structure was reported by numerous groups.[10–15] Besides the etching and oxidation modification methods, different calcium phosphates (CaP), such as hydroxyapatite (HA, Ca10(PO4)6(OH)2), have been used often as a coating on titanium to improve its biocompatibility.[7,16–18] The plasma-sprayed HA-coated titanium alloys have been clinically applied.[19–21] However, reports describe the difficulty of controlling the quality, composition, and crystallinity of plasmasprayed HA coatings.[22–24] An alternative dipping method was recently reported to form calcium phosphate coating by dipping and withdrawing the metallic implant in Ca(OH)2 and (NH4)2HPO4 solutions for LU-NING WANG, PhD Candidate and JING-LI LUO, Professor, are with the Department Chemical and Materials Engineering, University of Alberta, Edmonton, AB, T6G 2V4 Canada. Contact e-mail: [email protected] Manuscript submitted March 15, 2010. Article published online December 15, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
various times, resulting in HA-covered surfaces. This method was applied to efficiently deposit synthetic HA after short-term immersion and can be applied on various porous metallic substrates such as aluminum and magnesium.[25] Anodic oxidation is increasingly applied for surface treatment on titanium and its alloys to obtain nanotube structures at the surface. During the last decade, there have been numerous studies on the formation of TiO2 nanotubes by anodization.[26–30] It was reported that the tubes with lengths ranging from 15 to 500 nm with about several micron thickness could be grown under anodic potentials ranging from 1 to 20 V in F– containing acidic electrolyte,[26,27] resulting in TiO2 nanotubes possessing a hollow structure for filling with bioactivating species and providing an interface suitable for anchoring connective tissue. It was reported that the TiO2 nanotubes have many potential biomedical applications, for example, use as a bond scale and supporting platform for bone and stem cells, local delivery of antibiotics off-implant at the site of implantation, and the control of hemorrhaging by forming significantly stronger clots with reduced clotting times.[31–33] Recently, HA coatings on TiO2 nanotubes were developed for biomedical applications. It has been reported that HA coating was formed on the annealed TiO2 nanotubes by immersion in 1.5 SBF (simulated biological fluid (SBF) with 1.5 times of Ca and P concentration than stan
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