Mechanism of Apatite Formation on Bioactive Tantalum Metal in Simulated Body Fluid

Mechanism of the apatite formation on bioactive tantalum metal prepared by NaOH treatment was investigated by soaking the metal in a simulated body fluid and surveying the surface structual change using transmission electron microscope attached with energy dispersive X-ray spectrometry. The NaOH-treated tantalum metal was covered with nano-particles of crystalline sodium tantalate. Immediately after immersion in simulated body fluid, the sodium tantalate on the metal released sodium ions to form Ta-OH groups, which combined with calcium ions in the fluid to form an amorphous calcium tantalate. The calcium tantalate, after fairly a long immersing period, combined with the phosphate ions as well as the calcium ions in the fluid to form an amorphous calcium phosphate with a low Ca/P ratio. The amorphous calcium phosphate during immersion increases its Ca/P ratio subsequently to convert into crystalline apatite. These results indicate that the Ta-OHgroups on the metal induce the apatite nucleation not directly, but via formation of an amorphous calcium tantalate and calcium phosphate. Introduction Tantalum metal, owing to its excellent malleability and biocompatibility, is a material of much interest in biomedical implant applications [1-4]. Being induced with bone-bonding ability, tantalum metal is expected to find a number of applications as load-bearing bone substitutes. It has been shown that tantalum metal can form a biologically active bonelike apatite layer on its surface in the body and tightly bond to living bone through the apatite layer, when it was previously treated with NaOH aqueous solution. The induction of bonelike apatite formation on the metal was attributed to a sodium tantalate formed on the metal by the NaOH treatment, of which the mechanism is not clear. It has been proposed that when exposed to body environment, the sodium tantalate exchanges its sodium ions with hydronium ions in the fluid to form Ta-OH groups, which is able to induce apatite formation [5]. In the present study, the process of apatite formation on the NaOH-treated tantalum metal in simulated body fluid (SBF) was investigated by transmission electron microscope (TEM) accompanied with energy-dispersive X-ray spectrometry (EDX). Materials and Methods Foil of tantalum metal 0.075 mm thick was cut into a rectangular form 5 mm x 5 mm in size, and digged with a hole to reveal sharp edge side. The metal was soaked in 0.5M-NaOH aqueous solution at 60°C for 24 h, and then dried at 40°C for 24 h. The treated metal was soaked in 30 mL of simulated body fluid (SBF) with pH (7.40) and ion concentrations (K 5.0, Na 142, Mg 1.5, Ca 2.5, Cl 148.0, HCO3 4.2, HPO4 1.0, SO4 0.5 mM) nearly equal to those of human blood plasma at 36.5°C. After some pre-determined period, the specimens were removed from the fluid, Key Engineering Materials Online: 2003-05-15 ISSN: 1662-9795, Vols. 240-242, pp 11-14 doi:10.4028/www.scientific. net/KEM.240-242.11 © 2003 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-13/03/20,21:20:23) Fig. 1. TEM photograph and EDX spectrum of NaOH-treated tantalum metal before soaking in SBF (*: center of electron diffraction and EDX analysis). Fig. 2. TEM photograph and EDX spectrum of NaOH-treated tantalum metal after soaking in SBF for 0.5h (*: center of electron diffraction and EDX analysis). washed with distilled water and dried at room temperature. The metal specimens before and after soaking in SBF were analyzed using transmission electron microscope (TEM: JEM-2000FXIII, Jeol Co., Tokyo, Japan) and energy dispersive X-ray spectrometer (EDX: Voyager III, Noran Instruments, Inc., Middleton, USA). Results and Discussion TEM image indicated that the tantalum metal after the NaOH treatment was covered with a layer about 150 nm thick, in which a large number of particles 4-5 nm in size were observed and EDX detected Na, Ta and O, as shown in Fig. 1. Electron diffraction pattern around these particles was ascribed to crystalline sodium tantalate (Na2Ta4O11). After soaking in SBF for 0.5 h, the TEM image and electron diffraction pattern were apparently unchanged from those before soaking, as shown in Fig. 2. However, the EDX newly detected Ca in addition to Na, Ta and O, and the intensity of the Na peak was decreased. 100 nm 100 n 100 100 * Sodium tantalate (Na2Ta4O11) 1 1 3 0 0 6 3 0 0 3 0 12 0 h 0 h