Cochlear implants - devices that help people who would otherwise be deaf have some limited hearing - currently require hardware mounted on the outside of the skull to accommodate a recharger and microphone. Now, researchers in the US have developed a new low-powered chip that offers the prospect of eliminating these bulky, visible externals.

The new chip is the work of engineers in the Microsystems Technology Laboratory at Massachusetts Institute of Technology (MIT) together with team members from Harvard Medical School and the Massachusetts Eye and Ear Infirmary.

They are presenting a paper about their work at the 2014 IEEE international Solid-State Circuits Conference being held in San Francisco, CA, this week.

Cochlear implants are used by hundreds of thousands of people worldwide whose hearing is impaired because sensory hair cells in their cochleas, within the inner ear, do not pass on sound vibrations to the brain.

In the US, around 70,000 people have them, many of them children. The device works by electrically stimulating the auditory nerve to receive sound signals that pass from an external microphone into the ear.

Current designs mean that users have to wear a 1-inch diameter disk-shaped transmitter on the skull, attached by a wire to a microphone and power source inside what looks like a large hearing aid around the ear.

But the new low-powered signal-processing chip could lead to a new implant design that eliminates the need for any external hardware, say the researchers. The implant could be wirelessly charged - it could run for about 8 hours between charges - and instead of an external microphone, it could pick up sound using the natural microphone chamber of the inner ear, which is often intact in implant users.

One of the researchers, Anantha Chandrakasan, a professor of electrical engineering at MIT, says:

"The idea with this design is that you could use a phone, with an adaptor, to charge the cochlear implant, so you don't have to be plugged in. Or you could imagine a smart pillow, so you charge overnight, and the next day, it just functions."

Lawrence Lustig, director of the Cochlear Implant Center at the University of California at San Francisco (UCSF), who describes the device as "very cool," says people often experience more stigma with hearing loss than vision loss, so "people would be very keen on losing the externals for that reason alone."

And, he says, there would also be practical benefits, such as "not having to take it off when you're near water or worrying about components getting lost or broken or stolen."

Design is based on middle-ear implant mechanism

The researchers based their new design on the mechanism of a middle-ear implant. The idea is to pick up the sound vibrations in the delicate bones of the middle ear and instead of conveying them to the cochlea, send them to a microchip implanted in the ear that converts them to electrical signals passed to an electrode in the cochlea.

Lowering the power requirements of the chip was the key to eliminating the need for the external skull-mounted hardware, say the researchers.

The device has been tested on patients already with cochlear implants to check it does not affect ability to hear. And the researchers showed the chip can pick up and process speech played into the middle ear.

Lustig says such a device would require more complex surgery to implant than existing designs. A current operation takes about an hour - the new design would probably need about 3 to 4 hours of surgery but would still be a relatively straightforward procedure.

"I don't anticipate putting a lot of extra risk into the procedure," he adds.

Medical News Today recently reported a study that showed short stays in darkness can boost hearing. Another US team working with mice found that preventing sight for as little as a week was enough to help the brain process sound more effectively.