"Implantable Microphone", figure from ERCD licensed under CC BY-SA 4.0Implantable Microphone for Cochlea Implants
Introduction - Cochlear Implants
A Cochlea Implant is a medical device that directly provides a „hearing signal“ to the hearing nerve in the inner ear in cases where the sensory structures of the inner ear are out of order. The Cochlear Implant consists of an inner, implanted part, the electrode which stimulates the hearing nerve in the cochlear. The outer part comprises microphone, power supply and speech processor. It receives the sound signal, transforms and processes it and sends it to the implanted electrode.
The main group of patients for cochlear implants are children (age 1-2 years) where the outer parts of the implant are very strained, thus increasing the risk of device failure. Since device failure leads to sudden deafness again, the development aim is the completely implantable device. Beside this and aesthetic reasons protection of the device from other environment variables like sweat, dust, pressure and temperature variations are further motivations. Recent progress in micro-electronics already enables most components to be implanted. The implantable microphone, however, is still an engineering challenge. Currently available implantable microphones and prototypes, like devices in the wall of the ear canal, in the skin around the ear or in the inner ear, have several drawbacks which limit their usage.
Microphone technique
The basic idea is to incorporate the normal middle ear into the implantable microphone and to use the middle ear sound transfer characteristics and static pressure protection. The tympanic membrane serves as a natural microphone membrane that transfers the sound signal into structural vibrations of the ossicular chain. These vibrations are picked up by the sensor at the long process of the incus. The sensor is a small titanium tin which is inserted into the incudostapedial joint gap (see fig. 1). A membrane with a strip of single crystal piezo-ceramic sticked on it forms the lid of the tin. The structural vibrations are transferred to the plate and transformed into electrical signals by the piezo. These signals can be further processed by cochlear implants or implantable hearing aids. The new microphone design makes use of the micro mechanics of the middle ear and thus perfectly combines biology and engineering. A variety of functionality of the normal ear can be incorporated into the new microphone. This comprises for instance the resonance of the ear canal and the directivity pattern of the outer ear.
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| Fig. 1: Sensor schematic | Fig. 2: Sensor finite element model |
Results
The microphone has a flat frequency response function from 100 Hz up to 5 kHz in temporal bone measurement data (see fig. 3). The surgical procedure of insertion has been shown feasible and quite reversible. The sensors sensitivity is resilient against small dislocations, usual air pressure variations and other changing environmental conditions. There is a good correlation between experimental and theoretical results. Theoretical results have been obtained with numerical models (finite element models and electro-acoustic network models, see fig. 2). The models are used to predict the results of modifications to the microphone to further improve its sensitivity.
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| Fig. 3: sensitivity measurements and simulation |
Funding
- Ministry of Education and Research, grant no. 01 EZ 0001. Period 1.4.2000 - 28.2.2003.
Technique and application of Chochlear implants are described in detail on the homepages of the manufacturers
Contact
Tel.: 0351 458-5165
Hannes Seidler
Tel.: +49 0351 458-2247
Hannes.Seidler
Matthias Bornitz
Tel.: 0351 458-12025
matthias.bornitz