Biomechanical Aspects in Implantable Microphones and Hearing Aids and Development of a Concept with a Hydroacoustical Transmission

Abstract

The middle ear functions as a sensitive pressure receptor. This implies that it not only transmits sound pressure waves with molecule-sized vibrational amplitudes, but that it also reacts to the million times larger variations of ambient air pressure. This pressure induces movements of the tympanic membrane and of the attached ossicular chain of up to 1,000 microm. Any artificial device that contacts these sound-transporting elements, be it a receiver for an implantable microphone or a transducer for an implantable hearing aid, has to respect these two different modes of biomechanical behavior. A hydroacoustical transmission system has therefore been developed consisting of a water-filled flexible tube, which contacts the ossicular chain with a balloon tip, and which is connected to a piezo-electric transducer at its other end. This soft contact prevents a localized pressure load and does not restrain the free movement of the underlying ossicle during ambient pressure variations. Temporal bone experiments showed that the device transmits vibration energy in good acoustical quality. The device can also be used in a reverse mode, working as a microphone, for example for a totally implantable hearing aid or a cochlear implant. This microphone concept has the advantage that it incorporates the biologic sound receiving components (the tympanic membrane and the ossicular chain micromechanics) into a technical device.