Medical devices powered by the ear itself

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.

A team of researchers from MIT, the Massachusetts Eye and Ear Infirmary (MEEI) and the Harvard-MIT Division of Health Sciences and Technology (HST) demonstrate for the first time 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 responses to therapies. Eventually, they might even deliver therapies themselves.

In experiments, Konstantina Stankovic, an otologic surgeon at MEEI, 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.

''In the past, people have thought that the space where the high potential is located is inaccessible for implantable devices, because potentially it's very dangerous if you encroach on it,'' Stankovic says. ''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.''

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 specialised 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.