Apatite-forming ability on titanium surface modified by hydrothermal treatment and ultraviolet irradiation
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Titania coatings with various morphologies were formed on titanium surfaces by hydrothermal treatment using a dilute alkaline solution and evaluated in their hydroxyapatite (HA)-forming abilities in simulated body fluid (1.5SBF) under ultraviolet (UV) irradiation. The HA formation on the titania coating in 1.5SBF was enhanced by UV irradiation. The amount of phosphate groups adsorbed on the titania, after soaking in 1.5SBF for 24 h under UV irradiation, was estimated to be larger than that of calcium ions, whereas that of calcium ions on the titania, after soaking without UV irradiation, was larger than that of phosphate groups. It was suggested that the titania generated much basic Ti–OH groups at its surface by UV irradiation and subsequently adsorbed phosphate groups, such as H2PO4−, resulting in the formation of a new surface rich in the amount of the groups, which eventually enhanced the HA formation in 1.5SBF.
I. INTRODUCTION
Titanium and its alloys have been applied as implant materials for use in bone and teeth reconstructions under high loading conditions because of their high fracture toughness and corrosion durability. Various surface modifications of the metal materials have been investigated to provide a bone-bonding ability, that is, bioactivity for them. It has been reported to coat the metal materials with bioactive materials, such as hydroxyapatite (HA) and bioactive glasses, and to provide an HAforming ability in body for their surfaces by chemical modifications.1–8 The metal materials bond with bone via the bioactive materials or bonelike HA layer precipitated on their surfaces in body and show excellent fixation with bone. For example, chemical modifications using NaOH solution (1–10 mol/L) or H2O2 solution (10 mass%) were developed to provide the HAforming ability in body for a titanium surface.2–6 Strategies of these modifications were to form a sodium titanate or titania hydrogel layer on a titanium surface, and then to induce HA nucleation by Ti–OH groups formed in the hydrogel layer. The basic Ti–OH group (denoted by Ti–OH), which has a terminal OH group, was reported to induce HA nucleation in simulated body fluid (SBF).5 Titania is well known to have the ability of HA nuclea-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0385 J. Mater. Res., Vol. 23, No. 12, Dec 2008
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tion in SBF and body, and the anatase phase was reported to be especially effective.9 Titanium is able to have excellent bioactivity with little damage to its mechanical properties by these chemical modifications. Nanocrystalline titania particles have been prepared for various applications, such as a solar cell and catalysis by a hydrothermal treatment.10,11 Anatase is well known to show the highest photoactivity in comparison with other polymorphs, rutile, and brookite. It is also widely regarded that an anatase powder, with a high degree of crystallinity, is desirable to enhance the photocatalystic activity.1
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