Biomechanical aspects in implantable microphones and hearing aids and development of a concept with a hydroacoustical transmission

The middle ear functions as a sensitive pressure receptor. This implies tha t it not only transmits sound pressure waves with molecule-sized vibrationa l amplitudes, but that it also reacts to the million times larger variation s of ambient air pressure. This pressure induces movements of the lympanic membrane and of the attached ossicular chain of up to 1,000 mum. Any artifi cial device that contacts these sound-transporting elements, be it a receiv er 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 consistin g 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 n ot restrain the free movement of the underlying ossicle during ambient pres sure variations. Temporal bone experiments showed that the device transmits vibration energy in good acoustical quality. The device can also be used i n a reverse mode, working as a microphone, for example for a totally implan table hearing aid or a cochlear implant. This microphone concept has the ad vantage that it incorporates the biologic sound receiving components (the t ympanic membrane and the ossicular chain micromechanics) into a technical d evice.