Middle Ear Ossicular Chain Vibration Detection by Means of an Optimized MEMS Piezoelectric Accelerometer

Abstract

Totally implantable hearing devices have been proposed as a solution to mitigate limitations associated with external elements in traditional hearing devices. However, requirements for an implantable sensor are difficult to be achieved concurrently, and therefore, no sensor developed fulfills them entirely. In this paper, we investigate how various geometries of MEMS piezoelectric accelerometers, optimized via different optimization techniques, behave in terms of the equivalent input noise (EIN) in sound pressure level (SPL) when coupled to the middle ear ossicular chain. An alternative optimization goal based on the minimization of the EIN is proposed in contrast to the traditional charge sensitivity maximization, and this alteration rendered near 20-dB improvement in EIN in low frequencies. Prototypes are fabricated through a multi-user batch process featuring a 500-nm aluminum nitride piezoelectric layer and experimentally characterized for the validation of the finite-element analysis and analytic modeling of spectral noise. Despite limitations imposed by the current batch fabrication process, we are able to design a highly competitive implantable sensor in the form of a $2\times 2\,\,\textrm {mm}^{2}$ footprint cantilever accelerometer, which should be able to detect 60-dB SPL between 250 Hz and 8 kHz. These promising results establish piezoelectric sensors as a viable alternative for totally implantable hearing devices and put an implantable sensor, which fulfills all requirements concurrently within our grasp.