Platinum electrodes were examined for evidence of corrosion using a scanning electron microscope (SEM). In vivo electrodes, stimulated using charge-balanced biphasic pulses for periods of up to 2000 h at charge densities of 0.18–0.32 μC mm−2 geom. per phase, were compared with in vitro electrodes stimulated in inorganic saline using similar stimulus parameters, and with in vivo control electrodes. The in vitro stimulated electrodes showed evidence of platinum corrosion at high charge density and aggregate charge injection. Significantly, the in vivo stimulated electrodes showed no evidence of stimulus induced corrosion. Indeed, their surfaces were similar to the in vivo control electrodes. In vitro electrochemical studies have demonstrated that proteins play a significant role in the inhibition of platinum dissolution: the present study has demonstrated an inhibitory effect in vivo. This may be due to the presence of proteins.

We have performed a number of temporal bone and animal studies in order to evaluate the histopathological effects of intracochlear electrode implantation and chronic electrical stimulation. Our results indicate that (a) the insertion of a free-fit scala tympani array results in minimal damage to the membranous labyrinth; (b) the materials used in the electrode array evoke mild tissue reactions when implanted subcutaneously, in muscle, or within the scala tympani; (c) intracochlear electrical stimulation for periods of 500 to 2000 hours, using carefully controlled biphasic pulses, does not adversely affect the population or neural activity of the primary auditory neurones; (d) labyrinthine infection severely reduces the number of viable spiral ganglion cells; (e) an adequate fibrous tissue seal of the round window can prevent the spread of infection from the bulla to the implanted cochlea in cats, following inoculation of the bulla cavity with bacteria; (f) bone growth is not associated with electrical stimulation per se; (g) the electrode arrays show minimal platinum dissolution and no apparent degradation of the Silastic carrier following periods of long-term intracochlear electrical stimulation.

A description is presented of two separate methods of calculating the far-field and near-field pressure distributions associated with an acoustic radiator. It is assumed that the normal velocity distribution is known over the surface of the radiator. One method describes the solution to the wave equation in terms of a series of prolate spheroidal wave-functions. Application of the method is restricted to the lower end of the frequency scale; the body geometry is not constrained, but the method is best suited to relatively long, thin bodies. The other method is based on a high frequency asymptotic solution to the wave equation, obtained by using Fourier transform techniques. Accurate solutions are obtained only at the higher end of the frequency scale; the only constraint on the geometry of the radiator is that the surface should be everywhere convex. Numerical examples are presented for both methods; it is shown that for some bodies the frequency ranges for which the methods are valid overlap, but that for other bodies there is a gap in the frequency range.