Conduction properties of non-stoichiometric hydroxyapatite whiskers for biomedical use

Abstract

Hydroxyapatite (HA) whiskers were synthesized by a hydrothermal method and ion conductive property was investigated in order to develop HA whisker electrets for biomedical utilization. Based on infrared (IR) spectroscopy and inductively coupled plasma optical emission spectrometry (ICP-OES), the obtained whiskers were assumed to be Ca deficient HA with a composition of Ca_9.18[HPO₄]_0.82[PO₄]_5.18[OH]_1.18·n H₂O. The results of complex impedance measurements proved that the conduction properties of whiskers and powders showed almost the same temperature dependence as those of sintered HA above 600°C. The mechanism is understood by the migration of protons originating in OH^- ions. In the temperature range below 600°C, however, the non-linear Arrhenius relationship of conductivity (σ vs. 1/T) was only observed in the whiskers and powders. In particular, a drastic change in conductivity with temperature appeared in the whiskers within 250-600°C; the conductivity increased to 5.0 × 10^-9 S/cm at 500°C, then decreased to 1.7 × 10^-9 S/cm with the temperature elevated to 600°C. Since HPO₄^2- ions are reportedly supposed to convert into P₂O₇^4- in almost the same temperature range where the drastic change in conductivity appeared, the increase in whisker conductivity up to 500°C is presumably the result of an increase in the number of mobile protons generated from HPO₄^2-. The subsequent decrease in conductivity seemed to be caused by a decrease in the number of protons achieved by water elimination, causing the conversion of HPO₄^2- to P₂O₇^4-.