Mechanism of Apatite Formation on Anodically Oxidized Titanium Metal in Simulated Body Fluid

Mechanism of apatite formation on anodically oxidized titanium me tal in a simulated body fluid was investigated by XPS and TEM observation. The anodically oxidized metal was found to have rutile and anatase titania with a large number of Ti-O H groups on its surface. On immersion in SBF, the metal formed a bonelike apatite on its surface through formations of an amorphous calcium titanate and an amorphous calcium phosphate. The format ion of the calcium titanate was induced by the Ti-OH groups, which reveals negative charge t o int ract selectively with positively charged calcium ions in the fluid. The calcium titanate is postulated to reveal positive charge, thereby interacting with the negatively charged phosphate ions in the fluid to form the calcium phosphate, which eventually crystallized into bonelike apatite. Introduction An essential requirement for an artificial material to bond to living bone is the formation of a biologically active bonelike apatite layer on its surface in a body environment [1]. Titania hydrogel has been documented to induce formation of bonelike apatite formation on its surface in body environment [2]. Based upon these fundamentals, titanium metal has been subj cted to anodic oxidation (AO) to form a bioactive titania on its surface. When the AO was applied to the metal at spark discharge field in some specific aqueous electrolytes such a H2SO4, the treated metal formed a bonelike apatite on its surface in SBF and in vivo, through which integr ates with living bone [1, 3]. While this indicates that the AO is an effective method to pre pare bioactive surface in situ on titanium metal, the characteristics of the AO surface gove rning the formation of apatite is not yet clear. In this study, we investigated the mechanism of apatite format ion on an AO titanium metal in SBF. The surface composition and structure of the AO metal were surveyed as a function of soaking time in the simulated body fluid (SBF) using X-ray photoelectron s pectroscopy (XPS) and transmission electron microscopy (TEM) attached with energy di spersive X-ray spectrometry (EDX). The process of apatite formation on the AO metal was dis cussed in terms of electrostatic interaction between the surface of the metal and ions in the SBF. Key Engineering Materials Online: 2003-12-15 ISSN: 1662-9795, Vols. 254-256, pp 741-744 doi:10.4028/www.scientific. net/KEM.254-256.741 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific. net. (Semanticscholar.org-13/03/20,19:47:19) Materials and Methods Plates (10×10×1 mm) or mesh-grids (3 mmø) of pure titanium metal (Ti > 99.8%, Nilac o Co., Tokyo, Japan) were subjected to AO by being soaked in 1M-H 2SO4 solution and applied with electrical field of 150 V for 1 min at room temperature. The AO s pecimens of plates and grids were then gently washed with distilled water, and dried at 40°C for 24 h. They were soaked at 36.5°C in 30 ml of SBF [4] with a pH (7.40) and ionic concentrations (Na + 142, K 5.0, Mg 1.5, Ca 2.5, Cl 148.0, HCO3 4.2, HPO4 21.0, SO4 20.5 mM) nearly equal to those in human blood plasma. After soaking for various periods, the plate and grid specimens wer e subjected to XPS (Model 5500, ULVAC-PHI Co., Ltd., Chigasaki, Japan) and TEM (JEM-2000FXIII, JEOL , Co., Tokyo, Japan) attached with EDX (Voyager III, NORAN Instruments, In c., Middleton, WI), respectively. The analyses were performed adopting reference materials of unt reated titanium metal, titania gel prepared by sol-gel process, tricalcium phosphate (Ca 3(PO4)2; Taihei Chem., Osaka, Japan) and hydroxyapatite (Ca 10(PO4)6(OH)2; Taihei Chem.). Results and Discussion Figure 1 shows the O 1s Ti2p, Ca2p and P2p XPS spectra of the surfaces of AO titanium metals soaked in SBF for various periods. The AO surface before soaking in SBF (0 h) revealed O1s and Ti2p peaks ascribed to Ti-O and Ti-OH bonds. After soaking in SBF for 0.5 h, the surface reveal ed C 2p peak in addition to the O1s and Ti2p peaks. While the Ti-OH peak appeared to slightly increase in O 1s spectrum, binding energy positions were apparently unchanged. After soaking for 24 h, the surface revealed P2p peak in addition to the O 1s Ti2p and Ca2p peaks. The binding energy position of O 1s spectrum shifted to those ascribed to P-O and P-OH bonds. Thereafter by increasi ng so k ng time to 72 h, both the Ca 2p and P2p peaks increased appreciably their intensities. Figure 2 shows the TEM-EDX profiles of the surfaces of AO t itanium metals soaked in SBF for various periods. The profile together with electron diffraction patte rn indicates that the AO surface of the metal (0 h) is composed of porous titania containing spherical na no-crystals of anatase and rutile (TiO2). After soaking in SBF for 0.5 h, the surface titania was appare ntly unchanged in morphology and diffraction pattern, but the EDX detected Ca in addition to O and Ti. After 24 h, new particle-like nano-deposits were observed to precipitate on the surface titania, where EDX detected Ca and P with Ca/P atomic ratio of 1.39, but electron diffr action found no resolvable pattern. After 72 h, new aggregates of needle-like deposits were obser ved to precipitate on the surface, where EDX detected Na and Mg in addition to Ti, O, Ca and P, and the Ca/P ratio of 1.58. This aggregates revealed electron diffraction pattern ascribed to apatit e. The surface of AO titanium metal was shown above being composed of nano-crystalline anatase and rutile, which on immersion in SBF forms a thin amorphous calcium titanate within 0.5 h, thereon an amorphous calcium phosphate with a low Ca/P ratio within 24 h, and eventually the crystalline apatite with bonelike composition and structure withi n 72 h. A distinctive feature of the titania on the AO titanium metal is abundance in Ti-OH groups able to induce the formation of apatite [1, 2]. Therefore, the AO titanium metal appears to induce the apatite on its surface in the SBF via the following process. Immediately after soaking in SBF, the Ti-OH groups of the surfa ce natase and rutile on the metal are dissociated into negatively charged units of Ti-O owing to its isoelectric point considerably lower than the pH of SBF, i.e., 7.40. These Ti-O units combine with the positively charged calcium ions in the fluid to form an amorphous calcium titanat e, which is postulated to reveal positive charge. This calcium titanate thereby combines with the negatively charged phosphate ions in the fluid to form an amorphous calcium phosphate. The amorphous calci m 742 Bioceramics 16