University of Utah

Engineers from the universities of Utah and Ohio have developed a tiny prototype microphone that can be implanted in the middle ear of deaf patients as an alternative to wearing external electronics.

Cochlear implants have restored basic hearing for around 220,000 deaf people, but the related electronics, including the microphone, a speech processor and a radio transmitter coil, must be worn outside the head. This causes reliability issues, creates social stigma and prevents patients from swimming.


Sound usually moves into the ear canal and makes the eardrum vibrate. 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 on the cochlea's inner membrane move, which triggers the release of a neurotransmitter that carries the sound to the brain.

In cochlear implant candidates, the hair cells don't work -- this could be down to a birth defect, exposure to loud sounds or from certain viruses. In conventional cochlear implants, 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 altogether.

The system, developed by Darrin Young, associate professor of electrical and computer engineering at the University of Utah and his team, implants all of the aforementioned external components. Sound moves through the ear canal to the eardrum, which vibrates as normal, but the vibrations are picked up by an accelerometer. This accelerometer is attached to a chip, which converts the sound vibrations and converts them into electrical signals sent to the electrodes in the cochlea.

The device -- which is about the size of an eraser on a pencil -- has been successfully tested in the ear canals of four cadavers, although tests on living people are about three years away, according to lead researcher Young.

The prototype will have to be reduced in size and tuned to better detect quieter, low-pitched sounds before this can happen. Patients wearing the system would still need to wear a charger behind the ear while sleeping to recharge the implanted battery.

They tested the system by inserting tubing with a small speaker into the ear canal of the cadavers to generate tones and various frequencies and volumes. As the sounds were picked up by the implanted microphone, the researchers used lasers to measure the vibrations of the tiny ear bones. The output of the microphone went to speakers; in a real person it would send sound to the implanted speech processor.

In order to put the microphone to the test, Young used it -- somewhat bizarrely -- to record the start of Beethoven's Ninth Symphony while implanted in a cadaver ear. It was recognisable, if a little fuzzy. "The muffling can be filtered out," said Young.

The team found that the prototype microphone could pick up medium pitches at conversational volumes, but had trouble detecting quieter, lower-frequency sounds.

The study -- published in the Institute of Electrical and Electronics Engineers journal Transactions on Biomedical Engineering -- also showed that sound was transmitted most efficiently to the microphone if surgeons first removed one of the three small middle-ear bones -- the incus, aka the anvil. 

The prototype currently measures 2.5 by 6.2mm and weighs just 25 milligrams, but the team wants to reduce it to just two-by-twomillimetres. The team plans to create further prototypes to test on living patients.

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