Middle Ear MEMS Microphone Could Restore Hearing

Researchers at the University of Utah have developed a MEMS microphone that can be implanted in the middle ear to restore hearing and requires no clunky external electronics. The device improves upon conventional cochlear implants, which have restored basic hearing to some 220,000 deaf people but require a microphone and related electronics to be worn outside the head, creating reliability issues and social stigma.

The MEMS accelerometer-based microphone has been tested in the ear canals of four cadavers, the researchers reported in IEEE Transactions on Biomedical Engineering. Led by electrical engineer Darrin Young, the researchers tested the quality of the sound generated by the microphone by inserting tubing with a small speaker into cadaver ear canals containing the implants, and played Beethoven’s Symphony No. 9. Young made a recording of the sound picked up by the microphone and compared it with a regular recording. You’ll hear (yes, through a dead person’s ear) that the music sounds a bit muffled, but probably resonates better than it ever did to Beethoven, who was deaf when he composed his ninth symphony.

The sensor works by converting bone vibration in the ear to an electrical signal that represents the acoustic information. The prototype can detect a sound pressure level (SPL) of 60 dB at 500 Hz, 35 dB at 2 kHz and 57 dB at 8 kHz. An improved sound detection limit of 34 dB SPL at 150 Hz and 24 dB SPL at 500 Hz might be achieved by employing start-of-the-art MEMS fabrication technology, the researchers said.

Conventional microphones include a membrane or diaphragm that moves and generates an electrical signal change in response to sound. But these require a hole through which sound must enter—a hole that would get clogged by human tissue if implanted. Young's prototype avoids that problem by placing the accelerometer in a sealed package with a low-power silicon chip to convert sound vibrations to outgoing electrical signals.

The current prototype 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.

Sound normally moves into the ear canal and makes the eardrum vibrate. At a structure known as the umbo, the eardrum connects to a chain of three tiny bones—the hammer, anvil and stirrup—that vibrate. The stirrup touches the cochlea, the inner ear’s fluid-filled chamber, and hair cells on the cochlea’s inner membrane move, triggering the release of a neurotransmitter chemical that carries the sound signals to the brain.

In profoundly deaf people who are candidates for cochlear implants, the hair cells don’t work. So in a traditional cochlear implant, the microphone, signal processor and transmitter coil, which are worn outside the head, send signals to a receiver-stimulator under the skin, which then sends the signals to electrodes implanted in the cochlea and stimulates auditory nerves. The ear canal, eardrum and hearing bones are bypassed.

The system developed by Young moves all the external components inside the body. Sound moves through the ear canal to the eardrum, which vibrates as it does normally. An accelerometer is attached to the umbo detect the vibration. The accelerometer is also attached to a chip, and together they serve as a microphone that picks up the sound vibrations and converts them into electrical signals sent to the electrodes in the cochlea.

The package is glued to the umbo so the accelerometer vibrates in response to eardrum vibrations. The moving mass generates an electrical signal that is amplified by the chip, which then connects to the a speech processor and stimulator wired to the electrodes in the cochlea. “Everything is the same as a conventional cochlear implant, except we use an implantable microphone that uses the vibration of the bone,” Young says.

The device would require users to wear a charger behind the ear while sleeping at night to recharge an implanted battery. Young says he expects the battery would last one to several days between charging. Young also says he must reduce the size of the device and improve its ability to detect quieter, low-pitched sounds, and that live human testing is about three years away.

The study showed that sound is transmitted most efficiently to the microphone if a small middle-ear bone called the anvil is first removed. U.S. Food and Drug Administration approval for an implant requiring such surgery would be needed.

Photo: Case Western Reserve University, University of Utah.

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