Amelie A. Guex,Rohit U. Verma, Ariel E. Hight, Elliott Kozin, Keith Darrow, M. Christian Brown,Philippe Renaud, Daniel J. Lee, Stephanie P. Lacour
Title: Electrical stimulation of the cochlear nucleus with a thin flexible polymer microelectrode array: Designing the next generation auditory brainstem implant
Background: The auditory brainstem implant (ABI) provides meaningful hearing sensations to patients who are not candidates for cochlear implants (CI). Clinical outcomes among ABI users, however, vary among similar cohorts. This variable performance may be due to the rigid electrode design of current ABIs that only access a limited range of the tonotopical arrangement of the dorsal cochlear nucleus (DCN) and may induce extra-auditory sensations due to the spread of electrical current to neighboring non-auditory structures. In this study, we examine thin (<100µm width) and small (<200µm diameter) electrodes integrated on flexible polyimide (PI) substrates. Our novel ABI design may offer a better physical interface with the curvilinear surface of the cochlear nucleus. Further, our design may allow for a reduction of the spread of electrical currents, access a greater tonotopic range of the CN, and limit the recruitment of neighboring non-auditory neurons.
Methods 5-channel microelectrode arrays (MEAs) are fabricated on polyimide substrate using standard microfabrication processes. The electrode sites are coated with PEDOT, a conducting polymer, electropolymerized at the sites to decrease electrode impedance and increase charge injection capacity. In vivo experiments are performed on anesthetized Sprague Dawley rats (350-500g). The MEA is placed on the exposed surface of the DCN and a bipolar stimulation is induced, consisting of symmetric biphasic waveforms with 0.2ms phase duration, at a frequency of 23Hz. Responses of the auditory system are assessed by auditory brainstem responses (ABR) and recordings from the central nucleus of the inferior colliculus (CNIC) using a 16-channel electrode array (Neuronexus Technologies, Inc.)
Results ABRs were successfully generated using our newly designed implant and activation of the central auditory pathways was identified at the level of the inferior colliculus using both monopolar and bipolar conditions. Stimulation at different electrode locations on the DCN surface of the same animal exhibited a relatively broad activation of the neuronal population.
Conclusions Our results demonstrate that ABI electrodes can be manufactured with well-established MEMS technology. Electrode geometry and density may be optimized to produce implants with higher selectivity and better conform to DCN compared to current designs. Future work includes the optimization of implant geometry and mechanical compliance. We believe this technology may be readily scaled up for use in larger animal models and ultimately employed for clinical use in the next generation of auditory brainstem implants.
This work is funded by Fondation Bertarelli