Influence of materials and geometry on fields produced by cochlear electrode arrays

Most cochlear prostheses use multichannel electrode arrays implanted in the scala tympani to generate multiple parallel channels of activity in the tonotopically mapped auditory nerve. Various materials and geometrical arrangements of contacts and locations within the scala tympani have been used, resulting in substantial differences in the required stimulus strengths and resulting spatial distributions of neural activity. Activated iridium (AIR) permits systematic adjustment of its metal-electrolyte interface properties by changing the degree of electrochemical activation. Planar equivalent models of bipolar cochlear electrode geometries using iridium have been built, and their impedance magnitude and phase versus frequency for various degrees of activation have been characterised. The electrical fields produced at various distances from bipolar contacts in saline are measured. In comparison to platinum and unactivated iridium, AIR provides a much lower metal-electrolyte interfacial impedance. When two AIR contacts are placed with their edges close together, the resulting field distributions are determined largely by the high (but safe) current densities at their apposed edges rather than the overall extent of the contacts. These edge effects permit a novel design for a bipolar array that produces N channels of radially distributed, non-overlapping fields and that requires only N + 1 contacts.