Deep in the inner ear of mammals is a natural battery—a chamber filled with ions that produces an electrical potential to drive neural signals. MIT researchers, together with colleagues at the Massachusetts Eye and Ear Infirmary (MEEI) and the Harvard-MIT Division of Health Sciences and Technology (HST), have demonstrated that this battery could power implantable electronic devices without impairing hearing.
The devices could monitor biological activity in the ears of people with hearing or balance impairments or gauge responses to therapies. Eventually, they might even deliver therapies themselves.
MEEI otologic surgeon Konstantina Stankovic and HST graduate student Andrew Lysaght implanted electrodes in the biological batteries in guinea pigs’ ears. Attached to the electrodes were low-power electronic devices developed by MIT’s Microsystems Technology Laboratories (MTL). After the implantation, the guinea pigs responded normally to hearing tests, and the devices were able to wirelessly transmit data about the chemical conditions of the ear to an external receiver.
A device powered by the biological battery can harvest only a small fraction of its power without disrupting hearing. Low-power chips, however, are precisely the area in which Anantha Chandrakasan’s group at MTL has expertise.
The MTL researchers equipped each guinea pig’s chip with an ultralow-power radio transmitter: after all, an implantable medical monitor wouldn’t be much use if there were no way to retrieve its measurements.
But while the radio is much more efficient than those found in cell phones, it still couldn’t run directly on the biological battery. So the MTL chip also includes power-conversion circuitry that gradually builds up charge in a capacitor.
To reduce its power consumption, the control circuit had to be drastically simplified, but like the radio, it still required a higher voltage than the biological battery could provide. Once the control circuit was up and running, it could drive itself; the problem was getting there.
The MTL researchers solve that problem with a one-time burst of radio waves. “In the very beginning, we need to kick-start it,” Chandrakasan says. “Once we do that, we can be self-sustaining. The control runs off the output.”