'Middle-ear' microphone may make cochlear implants more convenient

Washington, Apr 30 (ANI): Cochlear implants have restored basic hearing to around 220,000 deaf people, still a microphone and related electronics must be worn outside the head, raising reliability issues, preventing patients from swimming and creating social stigma.

Now, a University of Utah engineer and colleagues in Ohio have developed a tiny prototype microphone that can be implanted in the middle ear to avoid such problems.

The proof-of-concept device has been successfully tested in the ear canals of four cadavers, the researchers reported.

The prototype - about the size of an eraser on a pencil - must be reduced in size and improved in its ability to detect quieter, low-pitched sounds, so tests in people are about three years away, said the study's senior author, Darrin J. Young, an associate professor of electrical and computer engineering at the University of Utah and USTAR, the Utah Science Technology and Research initiative.

The study showed incoming sound is transmitted most efficiently to the microphone if surgeons first remove the incus or anvil - one of three, small, middle-ear bones. U.S. Food and Drug Administration approval would be needed for an implant requiring such surgery.

The current prototype of the packaged, middle-ear microphone measures 2.5-by-6.2 millimeters (roughly one-tenth by one-quarter inch) and weighs 25 milligrams, or less than a thousandth of an ounce. Young wants to reduce the package to 2-by-2 millimeters.

Young, who moved the Utah in 2009, conducted the study with Mark Zurcher and Wen Ko, who are his former electrical engineering colleagues at Case Western Reserve University in Cleveland, and with ear-nose-throat physicians Maroun Semaan and Cliff Megerian of University Hospitals Case Medical Center.

The device still would require patients to wear a charger behind the ear while sleeping at night to recharge an implanted battery. Young said he expects the battery would last one to several days between charging.

Young said the microphone also might be part of an implanted hearing aid that could replace conventional hearing aids for a certain class of patients who have degraded hearing bones unable to adequately convey sounds from conventional hearing aids.

Conventional microphones include a membrane or diaphragm that moves and generates an electrical signal change in response to sound. But they require a hole through which sound must enter - a hole that would get clogged by growing tissue if implanted.

So Young's middle-ear microphone instead uses an accelerometer - a 2.5-microgram mass attached to a spring - that would be placed in a sealed package with a low-power silicon chip to convert sound vibrations to outgoing electrical signals.

To test the new microphone, the researchers used the temporal bones - bones at the side of the skull - and related ear canal, eardrum and hearing bones from four cadaver donors.

The researchers inserted tubing with a small loudspeaker into the ear canal and generated tones of various frequencies and loudness. As the sounds were picked up by the implanted microphone, the researchers used a laser device to measure the vibrations of the tiny ear bones.

They found the umbo - where the eardrum connects to the hammer or malleus - produced the greatest sound vibration, particularly if the incus or anvil bone first was removed surgically.

The experiments showed that when the prototype microphone unit was attached to the umbo, it could pick up medium pitches at conversational volumes, but had trouble detecting quieter, low-frequency sounds.

Young plans to improve the microphone to pick up quieter, deeper, very low pitches.

In the tests, the output of the microphone went to speakers; in a real person, it would send sound to the implanted speech processor.

To demonstrate the microphone, Young also used it to record the start of Beethoven's Ninth Symphony while implanted in a cadaver ear. It is easily recognizable, even if somewhat fuzzy and muffled.

"The muffling can be filtered out," Young added.

The study has been published online in the Institute of Electrical and Electronics Engineers journal Transactions on Biomedical Engineering. (ANI)