Development of a Micro-coil Based Cochlear Implant

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We have been evaluating magnetic stimulation from tiny, implantable coils (referred to as microcoils) for use in a next-generation cochlear implant (CI). Existing CIs enable speech discrimination, but their effectiveness decreases when background noise levels are high, and most users cannot appreciate music. While a number of factors are thought to contribute to these limitations, it is generally agreed that complex auditory signals, such as those arising from speech in the presence of background noise, or music, require more independent spectral channels than are created by existing, electrode-based CIs. Increasing the number of channels has proven challenging however, as the highly conductive solution surrounding implants (perilymph) expands the spread of activation from each electrode so that fields from neighboring electrodes overlap and channels are no longer independent. The spread of fields is worsened because the targets of stimulation (spiral ganglion neurons) are within one of the bony cavities of the cochlea and thus higher stimulus levels are required for activation which lead to increased current spread. Microcoils may be an attractive alternative to electrodes because the physics governing the spread of induced fields (Maxwell’s equations) suggests narrower confinement of activation. Further, the high permeability of biological tissues to magnetic fields allows stimulation to pass readily through the bony wall, without the need for increased stimulation levels (and the resulting spread of activation). Consistent with this, stimulation from micro-coils implanted in the cochleae of both mice (Lee et al., 2022) and guinea pig (present proposal) results in narrow channels of activation in the inferior colliculus, i.e., better approximating the normal physiological signal, and smaller than those from electrodes. The ability to create narrow spectral channels suggests a larger number of independent channels are possible with microcoils and thus the potential exists for improved rehabilitation of hearing. Our goal here is to further evaluate the potential of microcoils for use in CIs. The Aims focus on (1) electrophysiological evaluation of implanted microcoils, (2) evaluation of the interactions between neighboring channels on the multi-coil array, (3) chronic testing of coil-based implants, and (4) development of a computer model to help understand the mechanism(s) of activation. All physiological testing will take place in guinea pigs, a well- established animal model for evaluation of CI performance; our team has previous experience with this animal and new preliminary results validate the overall viability of our device and the approach. Our multi-disciplinary team has strong expertise in microcoil design and development, magnetic stimulation, computer modeling, cochlear implants and auditory physiology. Almost all of the team is located at Mass. General Hospital or next door at Mass. Eye and Ear; the results presented here are the result of a 2+ year collaboration between team members.