Design of a Surgically-Viable Umbo Microphone For Implantable Assistive Hearing Devices

Abstract

Assistive hearing devices have enabled the restoration of one the most important senses. These technologies have changed the lives of millions, but there are also hindrances which arise. Today’s hearing devices are bulky, can only be worn during the day, and do not function well in noisy environments. These issues can be addressed by implantable devices. As these devices are entirely encapsulated in the body, these implants can take advantage of the natural acoustic filtering of the ear, are not a detriment to a person’s ability to be physically active and can be worn at night. However, microphones within these devices are a limiting factor.

This thesis develops key components for a fully-implantable assistive hearing device. Specifically, a microphone that transduces umbo motion together with a signal conditioning amplifier is presented. The output of the system ultimately drives a cochlear implant. The microphone takes the form of a 3-mm-diameter drum pressed by the umbo in which the drum head is piezoelectric PVDF. The amplifier is a lowinput-impedance charge amplifier.

Continuum electromechanical modeling of the microphone, electrical modeling of the charge amplifier, system design based on the models, and microphone fabrication are presented. A system demonstration of both bench-top and cadaveric temporalbone experiments yield results which match well to expectations from theory. The system achieves a bandwidth of 200 Hz to 7 kHz, a sensitivity of 220 dB ref. 1mV/m of voltage output per meter of umbo displacement and 62 dB ref. 1mV/Pa of voltage output per Pascal of ear canal pressure at 1 kHz. The signal-to-noise ratio is 30 dB.

The importance of an umbo-induced static offset on the microphone, mechanical coupling between the umbo and microphone, and shielding are demonstrated. The microphone behaves according to plate bending mechanics, yet plate stretching produces the charge which is amplified.