A tiny prototype microphone and related electronics that can be implanted in the middle ear has been developed by University of Utah and Case Western Reserve University engineers.

Cochlear implants have restored basic hearing to some 220,000 deaf people, yet a microphone and related electronics must be worn outside the head, raising reliability issues, preventing patients from swimming, and creating social stigma.

The proof-of-concept device, funded by the National Institutes of Health (NIH), has been successfully tested in the ear canals of four cadavers.

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, says 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 and weighs 25 milligrams, Young wants to reduce the package to 2-by-2 millimeters.

The NIH says almost 220,000 people worldwide with profound deafness or severe hearing impairment have received cochlear implants, about one-third of them in the United States, where two-fifths of the recipients are children.

Conventional cochlear implants

In conventional cochlear implant, there are three main parts that are worn externally on the head behind the ear: a microphone to pick up sound, a speech processor,and a radio transmitter coil. Implanted under the skin behind the ear are a receiver and stimulator to convert the sound signals into electric impulses, which then go through a cable to between four and 16 electrodes that wind through the cochlea of the inner ear and stimulate auditory nerves so the patient can hear. The ear canal, eardrum and hearing bones are bypassed.

Sound normally moves into the ear canal and makes the eardrum vibrate. At what is known as the umbo, the eardrum connects to a chain of three tiny bones: the malleus, incus and stapes, also known as the hammer, anvil and stirrup. The bones vibrate. The stapes or stirrup touches the cochlea, the inner ear’s fluid-filled chamber. Hair cells (not really hair) 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 for a variety of reasons, including birth defects, side effects of drugs, exposure to excessively loud sounds or infection by certain viruses.

How it works

The system developed by Young implants all the external components. Sound moves through the ear canal to the eardrum, which vibrates as it does normally. But at the umbo, a sensor known as an accelerometer is attached to detect the vibration. The sensor also is 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 device still would require patients 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.

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.

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 conventional parts of a cochlear implant: a speech processor and stimulator wired to the electrodes in the cochlea.

Young says 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.

Performance

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.

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 (which can be improved).

To hear a regular recording of the start of Beethoven’s Ninth Symphony, click here.

To hear it recorded through the new microphone in the middle ear of a cadaver, click here.

Ref.: Darrin J. Young, MEMS Capacitive Accelerometer-Based Middle Ear Microphone, Transactions on Biomedical Engineering, 2012, DOI: 10.1109/TBME.2012.2195782