Crystal Growth Modification by Surface Charges on Ceramic Electret in Simulated Body Fluid

The interactions at the interface of the polarized hydroxyapatit e (HAp) surface and the simulated body fluid were investigated in the early stage. The i nteraction mechanism was discussed in view of the electrostatic effects of HAp electrets. The HAp ceramics sinterd at 1250 °C for 2 h in saturated water vapor was polarized in air at a temperature ra nging from 300 to 500 °C in a dc electric field. The thermally stimulated depolarization current spectrum of the polarized HAp ceramics increased at ca. 180°C and reached maximum at 420 °C, and then decreased. On the electrically polarized HAp ceramic substrates after the SBF immersion, spher oidal HAp crystals were deposited on each of the ceramic substrate grain with a uniformly orientati o . The SEM observation revealed that the induced surface charges of the HAp electrets affected the overgrowth phenom eno . Introduction An immersion test using simulated body fluid (SBF) has been widely accepted as an osteobonding estimation of biomaterial surfaces [1]. The effectivity of the SBF test is based on the similarity of the thin amorphous calcium phosphate layer at the living bone / bioactive mat erial and the bone-like apatite sub-microcrystals overgrown on the material in SBF. We hav been demonstrated by SBF tests that the negative surface charges induced by an electrical polari zation ncreased the overgrowth conductivity on the hydroxyapatite (Ca 10(PO4)6(OH)2; HAp) ceramic surface but that the positive charge diminished the overgrowth ability [2]. The in vivo osteobonding ability of the polarized HAp obtained from canine bones accurately corresponded the results of the SB F test [3, 4]. However, it has not been clarified yet the mechanism of the overgrowth modification by the polarization charges. In the present study, the interactions at the interface of the polarized HAp surfa ce and the solution were investigated in the early stage. The interaction mechanism was discussed in vie w of th electrostatic effects of HAp electrets. Materials and Methods Powder of HAp was prepared by a precipitation synthesis from calci um hydroxide aqueous suspension and phosphoric acid at room temperature (RT) [5]. The obtained slurry wa s filtrated and then, dried at 60 °C for 24 h. The powder calcined at 850 °C was uniaxially pressed at 220 MPa. The pellets were sinterd at 1250 °C for 2 h in saturated water vapor [6]. The obtained dense HAp cerami disks with the size of 10 mm diameter and 0.7 mm thickness were ident ified by powder X-ray diffractometry (XRD) and FT-infrared spectroscopy (FT-IR). The HAp ceramic disk cramped with a pair of Pt electrodes was pol rized in air at a temperature ranging from 300 to 500 °C for 1h in a dc electric field of 1 or 10 kVcm . The thermally stimulated depolarization current (TSDC) spectra were examined with a handma de easurement cell shielded against stray field and a pico ammeter (Hewlett-Packard 4140B) from RT to 850°C at a heating rate of 5°C/min. The stored electrical charge ( Q) was calculated by numerical integration from each of the measured current spectrum [7]. Key Engineering Materials Online: 2003-05-15 ISSN: 1662-9795, Vols. 240-242, pp 445-448 doi:10.4028/www.scientific. net/KEM.240-242.445 © 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-13/03/20,21:20:44) 2 Title of Publication (to be inserted by the publisher) The SBF with ion concentrations almost equal to those of human blood plasma was prepared according to the method described by Kokubo [1]. The HAp samples were im mersed in 10 cm of the SBF at 37.0°C up to 9 days. The immersed samples were washed with di stilled-deionized water, and then dried at RT for SEM microstructure observation. Results The obtained HAp ceramics had an average relative density of 98%. The mean gain size of the HAp ceramics was 2 μm, based on SEM observation. The XRD pattern (Fig.1) of the HAp specimen was well corresponded to the published data of HAp (JCPDS No. 9-432) verified t hat the ceramics consisted of pure HAp phase. The peaks at 3570 and 630 cm —1 in the IR spectrum (Fig.2) were assigned to the stretching and bending absorptions of hydroxide ions, respect iv ly. The broad absorption bands at 3400 and 1650 cm —1 were attributed to adsorbed water. No significant difference in XRD patterns and FT-IR spectra was found before and after polarization under any condit ion. The TDSC spectrum (Fig.3) of the HAp ceramics polarized in a dc field of 1.0 kVcm —1 at 400°C for 1h. The curve increased at ca. 180°C and reached maximum at 420 °C, and then decreased. The calculated charge storage ( Q) was 15.1 μCcm [8]. Figure 4 shows HAp overgrowth on the negative charge induced surface (N-surface) of HAp ceramic substrate after 2 days’ immersion of SBF. Although no overgrown particle was found on the some part of the ceramic substrate, HAp crystals with the size of ca. 50 nm were overgrown on almost all the HAp ceramic surface. Almost of the particles were deposited on each of the ceramic substrate grain. The grain boundaries of the substrate clearly appeared as linear lacks of deposition. The majority of the overgrown crystals were spheroidal and uniformly orientated, whereas some particles we r aggregated at random on the N-surfaces. 4000 300