A new kind of tiny microphone is shown here attached at right to a cadaver’s umbo, where the eardrum (under left part of device) meets the hearing bones. 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, and from there to the brain. The device measures about one-tenth inch by one-quarter inch. Credit: Case Western Reserve University, University of Utah.
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.
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.
proof-of-concept device has been successfully tested in the ear canals
of four cadavers, the researchers report in a study just published
online in the Institute of Electrical and Electronics Engineers journal
Transactions on Biomedical Engineering.
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.
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.
current prototype of the packaged, middle-ear microphone measures
2.5-by-6.2 mm (roughly one-tenth by one-quarter inch) and weighs 25 mg,
or less than a thousandth of an ounce. Young wants to reduce the package
to 2-by-2 mm.
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
The study was funded by the National Institutes of Health (NIH-DC-006850).
Problems with external parts on cochlear implants
National Institutes of Health 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 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.
a disadvantage having all these things attached to the outside” of the
head, Young says. “Imagine a child wearing a microphone behind the ear.
It causes problems for a lot of activities. Swimming is the main issue.
And it’s not convenient to wear these things if they have to wear a
adds that “for adults, it’s social perception. Wearing this thing
indicates you are somewhat handicapped and that actually prevents quite a
percentage of candidates from getting the implant. They worry about the
As for reliability, “if you have wires connected from the microphone to the coil, those wires can break,” he says.
How sound moves
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.
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.
a cochlear implant, the microphone, signal processor and transmitter
coil worn outside the head send signals to the internal
receiver-stimulator, which is implanted in bone under the skin and sends
the signals to the electrodes implanted in the cochlea to stimulate
auditory nerves. The ear canal, eardrum and hearing bones are bypassed.
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.
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.
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.
Testing the microphone in cadavers
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.
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.
is the same as a conventional cochlear implant, except we use an
implantable microphone that uses the vibration of the bone,” Young says.
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.
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.
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.
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,” says Young.