Our ears contain an elaborate system of chambers that convert mechanical energy into an electrochemical signal, much like a battery. Without those signals, we'd be unable to balance or hear. Now a new wireless microdevice can actually run on that scant energy. The technique could provide an inexhaustible power source for localized therapies and even hearing aids.
"This is uncharted territory in terms of super low power chip design," said Anantha Chandrakasan, the head of electrical engineering and computer science at MIT. Chandrakasan created the ear-powered device with an interdisciplinary team that included Dr. Konstantina Stankovic, an ear and skull surgeon at Massachusetts General Hospital who teaches at Harvard Medical School. Their paper will be published in a forthcoming issue of Nature Biotechnology.
Medical experts have known about the electrochemical gradient generated inside our ears -- known as the endocochlear potential -- for decades. But carefully tapping into a small amount of that already minuscule current without damaging hearing was thought to be impossible. Stankovic wondered if there was a way to overcome that so she connected with Chandrakasan, who is known for developing devices powered by body heat.
Chandrakasan was used to dealing in microwatts, though. “Now we’re talking about nanowatts,” he said. “It required very new kinds of low-power circuit techniques.”
The team, which included researchers from MIT’s Microsystems Technology Laboratories, began by designing a tiny chip containing a 2.4-gigahertz radio transmitter. They used the simplest possible architecture with very few gates. This allowed for duty cycling, meaning the power would be shut off when nothing was transmitted to avoid leaking away the charge, Chandrakasan said.
Converting 100 millivolts into a higher voltage wasn’t the problem, Chandrakasan added. Powering the electronic converter was. To get around that, they incorporated a “jumpstart” with radio waves that would help the system begin running on its own. Then the chip took three months to fabricate.
The system was tested by attaching electrodes to a guinea pig’s inner ear and then connecting those electrodes to the chip. Although electrodes were used for the tests, there’s no reason the chip couldn’t be implanted directly inside the air-filled middle ear, Chandrakasan said. At 9-by-11 millimeters, the system is already small enough.
After a quick jumpstart, the chip setup began to extract energy from the guinea pig’s endocochlear potential and transmit information wirelessly to a nearby radio.
“We were able to do this for five hours in these pilot experiments without damaging hearing,” Stankovic said. They used electrical responses similar to EEG measurements to verify that the guinea pig’s hearing was unchanged.
The team envisions building implantable sensors that could monitor and report on health conditions in real time.
“The idea is fantastic,” said Dr. Lawrence Lustig, director of Cochlear Implant Center at the University of California San Francisco. Although the energy amount is low, he envisions greater efficiency with smaller and smaller machines. Thirty years from now we could use the self-powered system to regenerate inner ear processes, Lustig said.
Dr. John Niparko is an ear surgeon who currently heads hearing care programs at Johns Hopkins. He specializes in the use of devices to restore hearing, particularly for those with congenital deafness. Niparko called the Nature Biotechnology paper an exciting proposal.
“What the group has shown here is that you can tap off, basically capture that supply of energy without destroying the generation of electrical current,” he said.
We can already epileptic seizures and heart arrhythmias. The ear-powered device could enable new diagnostics for disorders, Niparko added. “That’s a whole new set of textbooks right there.”
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