Osseous Cell Response to Electrostatic Stimulations of Poled Hydroxyapatite Ceramics in Canine Diaphyses

Cell responses of electrically charged surfaces of poled hydrox yapatite (HA) ceramics were investigated by implantation in wide defects of canine femora , c mpared with the uncharged surfaces. The HA ceramic specimens were poled in a dc field i n air at 300-400°C. Base on the thermo stimulated current measurements, the stored charge and the half–value period of HA ceramics polarized at 300°C and 1.0 kVcm —1 were 4.2 μCcm and 7.3×10 s (at 37.0°C), respectively. Although the non-polarized HA ceramic surfaces were still isolat ed from osteoid tissues by dominant fibrin multi-layers 7 days after the implantation, newly formed bone lay rs of 0.01-0.02 mm in thickness contacted the negatively charged surfaces without any incl usion. On the contrary, The osteoid tissues surrounded by osteoblastic cells occupied the gap betwee n the positively charged surface and the cortical bone. We have demonstrated that the polarize d HA ceramic had a large and long durable charge suitable for biomedical applications and that the HA surf ace charges induced by electrical polarization stimulated osseous cells and promoted bone reconstruction. Introduction Surface charges of biomaterials are recognized as one of the im portant factors to determine cell responses [1, 2]. The cells receiving the stimulation exhibited vari ous modulated reactions, such as migration, alteration of differentiation, cell phase, and extracellular matr ix secretion. We have more recently disclosed that exclusively large surface charges we re inducible on hydroxyapatite (HA) ceramics by proton transport polarization [3, 4] and demonstrated that the negatively charged surface of the HA ceramics enhanced their osteobonding ability in canine femor a [5, 6]. In the present study, the cell responses of the negatively and the positively charged surf aces of the electrically poled HA ceramics were investigated by implantation in wide defects of c anine femora, compared with the uncharged surfaces. Materials and Methods HA powder was precipitated from calcium hydroxide aqueous suspension and phos phoric acid solution. The HA powder, calcined at 850°C, was pressed in a mold at 200 MPa . Dense HAp ceramics prepared by sintering at 1250°C for 2 h in saturated water vapo were employed as the specimens. The HA ceramic specimens with a size of 5.0 ×8.0×1.0 mm were electrically poled in a dc field of 1.0 kVcm with a pair of Pt electrodes in air at 300-400°C for 1 h (Fig. 1a). The confirmation of the poling charges of the samples chosen at random was examined by the thermally stimulated current (TSC) method using a handmade measurement cell [4]. Key Engineering Materials Online: 2003-12-15 ISSN: 1662-9795, Vols. 254-256, pp 849-852 doi:10.4028/www.scientific.net/KEM.254-256.849 © 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-13/03/20,19:47:24) Title of Publication (to be inserted by the publisher) The samples thus polarized at 300°C were implanted in the femoral and tibi l iaphyses after being sterilized by the ethylene-oxide gas method. The experiments wer e carefully completed by veterinarians in accordance with the Guideline for Animal Experime ntation (Tokyo Medical & Dental University) as well as the Guide for the Care and Use of Laboratory Animals (National Institutes of Health Pub. No. 85-23, Rev. 1985). Six male beagle dogs weighing 12-13 kg were inhalationally anesthetized with isoflurane using tracheal tubes. W e exposed the femola by lateral luxation and bored rectangular through-holes on the lateral faces of the bones with a 0.7-mm dental fisher bur (Fig. 1b). The gaps between the observational faces of the samples and the cut cortical bone faces were fixed at 0.2 – 0.5 mm. The samples were rigidly held by the friction between the lateral faces of the samples and the bone faces [5]. The bones containing the sampl s harvested at 7, 14 and 28 days after the implantation were histologically observed. Results The magnitude and time durability of the polarization charge were e stimated from the TSC spectra measured at 1 day after the polarization [4]. The peak temperature (Tpeak) and maximum current density (Jmax) increased with increasing polarization temperature, as shown in Tabl e 1. The increase in the polarization temperature raised the calculated stored charge ( Q). The peak temperature ( Tpeak) giving the maximum current widely varied from 295 to 420°C, depending on the pol arization temperature. This result suggests that the polarization of HA w as not due to a phase transition but by mass transport. The relaxation time ( τ) is described by the Arrhenius law: τ (T) = τ0 exp(H / kT ), (1) where H is the activation energy, τ0 is a pre-exponential factor and k is Boltzmann’s constant. The activation energies of 0.84-0.89 eV for the HA polarized in a field of 1.0 kVcm were almost independent of the polarization temperature. The obtained τ0 increased with increasing polarization temperature. Polarization charge ( Q) is given by a function of time: Q(t) = Q0 exp(— t / τ ), (2) Where Q0 is the initial polarization charge. The calculated half–value per iod (t50) of HA polarized at 300°C and 1.0 kVcm was 7.3×10 s (ca. 230 years) at the biological temperature of 37.0°C. Therefore, the polarized HA ceramic had a long durable charge for biomedical applica tions [4]. Fig. 1 Schematic illustrations of setups of electrical poling procedures (a: left) and implantation geometries (b: right). 850 Bioceramics 16