MEMS acoustic sensors for totally implantable hearing aid systems

There is a need for high quality implant microphone for existing semi-implantable hearing aid systems, including cochlear systems and middle ear semi-implant systems. A totally implant hearing aids system integrating implant microphone, interface electronics, speech processor, stimulator, and cochlear electrodes, can overcome the discomfort, inconvenient, and social stigma associate with the conventional and semi-implantable hearing aids. This paper presents the results of four years in vitro study to use micro-chip acoustic sensors, serving as implant microphones, to directly sense the umbo acoustic vibration and convert it into high quality sound signal at the output. Umbo vibration characteristics were extracted from literatures and laboratory data. A PZT vibration source was built to simulate the umbo receiving 40 to 100 dB SPL sound input. A laboratory model using commercial electrets- microphone as the sensor was studied in laboratory and on several temporal bones. Near flat frequency response from 250 to 8000Hz was measured with minimum detectable signal about 65 dB SPL at 1 kHz, limited by the sensitivity of the microphone and amplifier noise. From these results, an MEMS acoustic displacement sensor chip was designed and fabricated to increase the sensitivity and to provide an easy mounting method on umbo. The prototype device can detect 40 dB SPL sound in 1-2 kHz region, assuming 100Hz channel bandwidth. Test results on temporal bones are being cumulated and was presented.