Chronic electrical stimulation of the auditory nerve using high surface area (HiQ) platinum electrodes

High surface area cochlear implant electrodes with much smaller geometric surface areas than current designs might be used in the future to increase the number of stimulating electrodes along the carrier. Potential problems with an increase in charge density for a common stimulus resulting from decreasing the geometric surface area would be reduced by the enlarged real surface area of such electrodes. Electrochemically modified (HiQ) platinum (Pt) electrodes, with a real surface area ∼75 times greater than the current standard Pt electrodes of the same geometric size, had shown in vitro a low polarization ( Z pol) and electrode impedance ( Z e), as well as a low residual direct current (DC). In this study we examined the chronic performance of HiQ electrodes in cats, which were bilaterally implanted with a two-channel HiQ or standard Pt scala tympani electrode array and unilaterally stimulated for periods of up to 2390 h. Stimuli consisted of 50 μs/phase charge-balanced biphasic current pulses presented at 2000 pulses/s/channel with a 50% duty cycle. Electrode impedance ( Z e), access resistance ( R a) and polarization impedance ( Z pol) were calculated from current and voltage measurements obtained periodically throughout the implantation period. Immediately following implantation HiQ electrodes showed a significantly smaller Z pol, resulting in a reduced Z e ( P0.05) for either animals implanted with HiQ electrodes ( n=6) or standard electrodes ( n=2). Nor were there any significant differences between the spiral ganglion cell density of the basal turn implanted with HiQ or standard electrodes fo