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Tiny mic could improve cochlear implants

Electrical engineers unveil a prototype the size of a pencil eraser that would reside in the middle ear instead of worn externally behind the ear.

The microphone includes an accelerometer and silicon chip that detect vibrations and convert them to electronic signals sent to electrodes in the inner ear's cochlea. Case Western Reserve University and University of Utah

Cochlear implants, which help 220,000 deaf people around the world hear, come with a few unfortunate side effects.

Because the implants also consist of external parts (the mic, a speech processor, and a radio transmitter coil) worn rather conspicuously behind the ear, users are often unable to swim or wear helmets comfortably, must fully rely on a microphone exposed to the elements, and have to deal with appearing at least somewhat handicapped.

So an electrical engineer at the University of Utah has developed a prototype that moves all these external parts into the middle ear, allowing cochlear implants to fully live up to their name.

In profoundly deaf people who are candidates for these implants, there is a failure of the hair cells whose movements are supposed to trigger the release of a chemical that then carries signals to the brain. This can be due to a viral infection, birth defect, exposure to loud sounds, or the side effects of drugs.

In a cochlear implant, sound signals are sent from the external parts directly to implanted electrodes in the cochlea that stimulate auditory nerves, bypassing the ear canal and eardrum altogether. In the fully implanted Utah prototype, however, sound vibrates normally through the ear canal to the eardrum, and an accelerometer attached to a chip in the middle ear detects the vibration, acting as a mic that converts those vibrations into electrical signals then sent to the electrodes in the cochlea.

The device still requires the nightly use of one external part behind the ear: a charger so that the implanted battery will last anywhere from one to several more days.

"Everything is the same as a conventional cochlear implant, except we use an implantable microphone that uses the vibration of the bone," Darrin Young, lead author of the study that appears in the journal Transactions on Biomedical Engineering, said in a school news release.

The development is a bit of a tease for the thousands of people who might stand to benefit from the device being fully implanted, because Young and his colleagues have only tested it on four, ahem, cadavers.

Rest assured, the implanted mic picked up the sounds the team generated via a small loudspeaker, and the levels were confirmed via a laser device that measured the vibrations of the cadavers' tiny ear bones. But while the implant handled medium pitches at conversational volumes well, it had trouble detecting quiet, low-frequency sounds.

Young hopes to improve on this shortcoming and bring the size of the mic down from 2.5 by 6.2 millimeters to 2 by 2 millimeters, so clinical trials are likely still a good three years away.

To get an idea of how the world sounds through the prototype device, compare this regular recording of Beethoven's Symphony No. 5 to this recording as picked up through the mic in the middle ear of a cadaver. "The muffling can be filtered out," Young says.

Beethoven, whose own hearing is said to have deteriorated to the point of not being able to hear the applause at the premiere of his Ninth Symphony, would surely approve.