Feasibility analysis of an implantable middle ear cavity pressure microphone

Current cochlear implant systems do not yet provide recipients invisible, carefree, and 24/7 hearing. The main technical challenge for a totally implantable cochlear implant (TICI) system is the development of a suitable implantable microphone (IM). One promising approach involves an IM that detects sound pressure within the middle ear cavity (MEC), eliminating the need for complex mechanical coupling with the ossicular chain. The present study investigates whether and under what conditions an MEC microphone (MECM) can meet the performance criteria required for TICI systems.

The analysis considers an MECM design concept that uses a micro-electromechanical systems (MEMS) microphone enclosed in a hermetic and biocompatible housing. A lumped element model (LEM) of the MECM, combined with sound pressure measurements in an intact and surgically modified MEC of human cadaver temporal bones, was employed to address the research question.

The experimental study showed that sealing the MEC at the aditus ad antrum increases sound pressure by up to 15 dB at frequencies below 3 kHz compared to an MEC configuration with an intact mastoid. However, a decrease of approximately 10 dB in the middle ear transfer function (METF) was observed due to stiffening of the middle ear caused by a smaller MEC size. The LEM indicated that an MECM with a 5 mm diameter and optimized enclosure geometry meets the performance criteria for a TICI, but only when the MEC is acoustically isolated from the mastoid cavity. Our analysis shows that the MECM can be seen as a promising IM concept for TICIs.