A team of surgeons, neuroscientists, and electrical engineers has developed a cochlea chip that extracts electrical signals from the inner ear to power itself.

The chip is the latest in a series of inventions aimed at creating entirely self-sufficient, self-powering implants that will remove the need for external power and enable permanent surgical implantation in some cases. This year alone, Stanford University announced the creation of its radio wave-powered heart implant and infrared light-powered retinal implants.

Cochlear implants have been around for decades, with the first electrical stimulation of auditory nerves taking place in the 50s. Though the implant has been a great success, with hundreds of thousands of people with severe hearing difficulties receiving the implant each year, they still run on batteries so are fairly cumbersome. MIT hopes to change all that by taking advantage of a natural battery that lies dormant within the ear.

"In the past, people have thought that the space where the high potential [in the ear] is located is inaccessible for implantable devices, because potentially it's very dangerous if you encroach on it," said Konstantina Stankovic, an otologic surgeon at the Massachusetts Eye and Ear Infirmary. "We have known for 60 years that this battery exists and that it's really important for normal hearing, but nobody has attempted to use this battery to power useful electronics."

Inside the ear chamber, negatively charged potassium ions and positively charged sodium ions are active either side of a membrane—this combination of ions helps convert the mechanical force of eardrum vibrations into electrochemical signals transmitted to the brain. It's this potential for electric charge Stankovic and her team are taking advantage of with their tiny electronic chips, fitted with several low resistance electrodes and a low-power radio transmitter. Although the inner ear is capable of producing a significant electrical current, it's still very low and the team could only extract a little so as to keep the charge low and not interfere with the hearing. Power-conversion circuitry was therefore also included to help power the low-energy chip.

The implant was tested on guinea pigs (their inner ears operate very much like humans'), with the electrodes attached to both sides of the cell membrane. Though the biological battery within the ear had the power to charge the chip, it needed a kick-start in the form of a radio wave burst. It took the capacitor between 40 seconds and four minutes to fill with enough energy to power the radio, after which time it ran without incident for up to five hours without affecting the guinea pigs' hearing, all the while transmitting chemical data to an external receiver.

The team hopes to make the devices even smaller and less invasive, since running them for long periods of time could mean damaging the inner tissue. But the results are promising and suggest that future cochlear implants could one day transmit vital biological and chemical statistics to monitor therapy progress, or even transmit the therapy itself.

Stankovic says the findings suggest there is a whole world of possibilities when it comes to using our own cells to self power devices. "A very futuristic view is that maybe we will be able to extract energy from individual cells using similar designs," she says.

The study was published in the journal Nature Biotechnology.