Title:  Micromagnetic Intracochlear Stimulation: Dual-Axial Microcoil Modeling, Fabrication, and Validation

Committee:

Dr. Pamela Bhatti, ECE, Chair, Advisor

Dr. Waymond Scott, ECE

Dr. Omer Inan, ECE

Dr. Albert Frazier, ECE

Dr. Flavio Fenton, Physics

Abstract: Over 700,000 cochlear implant users worldwide utilize commercialized cochlear implants that operate using direct current injection from electrodes. The cochlea is filled with a conductive fluid and electrical currents extend to nearby areas, causing a spread of excitation which impacts user hearing quality. Magnetic stimulation serves as a solution since magnetic stimulation sites are more localized when compared to electrical stimulation and magnetic fields are impervious to the material properties of the biological environment. The objective of this research project is to develop, characterize, and demonstrate a functional magnetic alternative to conventional cochlear implant electrodes. Microcoils designed for a cochlear implant array were developed to stimulate neural elements with an improved spatial resolution while operating safely within the constraints of the cochlea. Previous reports show that micromagnetic stimulators demonstrate an improved spatial resolution of stimulation sites by comparing the activating functions of electrical and micromagnetic stimulators. In this work, microcoils were designed using finite-element modeling, and the activating functions were calculated to evaluate the spatial resolution of the microcoil stimulation channels. During fabrication, additive manufacturing techniques were utilized to integrate the microcoils onto existing cochlear array substrates that minimize insertion trauma. The physical, electrical, and electromagnetic properties of the microcoil components were characterized and validated experimentally, and additional tests were conducted to observe the microcoil heating profile, power characteristics, and stability. The microcoils are comprised of a four-turn, 600-μm diameter planar component and a four-turn, 1-mm diameter solenoid component. The activating function spans 1 mm which is 73% narrower than the activating function of conventional electrodes. The microcoils are capable of transmitting 5 nW to target neurons and can be safely operated with a 60 mA maximum amplitude, 5 kHz sinusoidal input pulsed at 1 kHz with a 50% duty cycle for 2.8 hours.