Because electricity is the energy vector for the neural system, there’s a current generator within the cochlea of the inner ear of mammals that works like a battery. Though the current generated there is very small, a team of researchers from MIT, the Massachusetts Eye and Ear Infirmary and the Harvard-MIT Division of Health Sciences and Technology have developed and successfully tested a chip that harvests that electricity to power itself.
The chips were implanted into the ears of guinea pigs and wired via electrodes to the natural battery. Thanks to some smart circuitry, the device was able to do some basic chemical sensing while transmitting the data via radio to an external receiver, all without using traditional man made batteries. Moreover, the animals had their hearing preserved, a considerable initial worry for the team. The researchers believe that a variety of implanted medical devices could take advantage of this technology to provide long term viability without battery changes.
Some details from MIT:
The ear converts a mechanical force — the vibration of the eardrum — into an electrochemical signal that can be processed by the brain; the biological battery is the source of that signal’s current. Located in the part of the ear called the cochlea, the battery chamber is divided by a membrane, some of whose cells are specialized to pump ions. An imbalance of potassium and sodium ions on opposite sides of the membrane, together with the particular arrangement of the pumps, creates an electrical voltage.
Although the voltage is the highest in the body (outside of individual cells, at least), it’s still very low. Moreover, in order not to disrupt hearing, a device powered by the biological battery can harvest only a small fraction of its power. Low-power chips, however, are precisely the area of expertise of Anantha Chandrakasan’s group at MTL.
The MTL researchers — Chandrakasan, who heads MIT’s Department of Electrical Engineering and Computer Science; his former graduate student Patrick Mercier, who’s now an assistant professor at the University of California at San Diego; and Saurav Bandyopadhyay, a graduate student in Chandrakasan’s group — equipped their 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 cellphones, it still couldn’t run directly on the biological battery. So the MTL chip also includes power-conversion circuitry — like that in the boxy converters at the ends of many electronic devices’ power cables — that gradually builds up charge in a capacitor. The voltage of the biological battery fluctuates, but it would take the control circuit somewhere between 40 seconds and four minutes to amass enough charge to power the radio. The frequency of the signal was thus itself an indication of the electrochemical properties of the inner ear.
Press release: Medical devices powered by the ear itself