  Electrical and electrochemical processes in a bioactive soda-lime
phosphosilicate glasses and in a bioabsorbable soda-lime phosphate glass during
thermal poling were studied by means of thermally stimulated depolarization
current measurements, ac impedance spectroscopy, and SEM/EDX analyses. The
thermal poling was done by sputtering thin Pt electrode films onto the faces of
the glass samples and by applying voltages up to 1 kV to the electrodes at
temperatures up to 513 K. The poling leads to the formation of interfacial
layers under the electrodes which are responsible for two depolarization
current peaks and for one additional semicircle in a Nyquist plot of the ac
impedance. The SEM/EDX profiles suggest that redox and transport processes of
Na+ ions are responsible for the formation of the interfacial layers and that
Ca2+ ions are immobile under the poling conditions. The sodium depletion layer
under the anode leads to a negative surface charge of the glass samples which
may enhance their bioactivity.
