For the first time, an electrical device has been powered by the ear alone.
The team behind the technology used a natural electrochemical gradient in cells within the inner ear of a guinea pig to power a wireless transmitter for up to five hours.
The technique could one day provide an autonomous power source for brain and cochlear implants, says Tina Stankovic, an auditory neuroscientist at Harvard University Medical School in Boston, Massachusetts.
Nerve cells use the movement of positively charged sodium and potassium ions across a membrane to create an electrochemical gradient that drives neural signals. Some cells in the cochlear have the same kind of gradient, which is used to convert the mechanical force of the vibrating eardrum into electrical signals that the brain can understand.
A major challenge in tapping such electrical potential is that the voltage created is tiny – a fraction of that generated by a standard AA battery.
"We have known about DC potential in the human ear for 60 years but no one has attempted to harness it," Stankovic says.
Now, Stankovic and her colleagues have developed an electronic chip containing several tiny, low resistance electrodes that can harness a small amount of this electrical activity without damaging hearing.
The implant was inserted into a guinea pig's inner ear and the electrodes attached to both sides of cochlear cell membranes. Attached to the chip was a low power radio transmitter.
The device needed kick-starting with a short burst of radio waves, but was then able to use the electrical gradient running across the membrane to sustain the transmitter for up to five hours. Tests showed that the guinea pig's hearing was not affected.
The device works well for short durations but long-term use of the electrodes risks damaging the sensitive tissue inside the ear. The next step will be to make the electrodes even smaller, reducing their invasiveness.
Stankovic says that this is proof of concept that biological sources of energy exist that have not yet been fully considered. "A very futuristic view is that maybe we will be able to extract energy from individual cells using similar designs," she says.