Our ears work by converting the vibrations of the eardrum into electrochemical signals that can be interpreted by the brain. The current for those signals is supplied by an ion-filled chamber deep within the inner ear – it’s essentially a natural battery. Scientists are now looking at using that battery to power devices that could be implanted in the ear, without affecting the recipient’s hearing.
The “battery chamber” is located in the cochlea. It is internally divided by a membrane, some of the cells of which are designed to pump ions. The arrangement of those specialized cells, combined with an imbalance of potassium and sodium ions on opposite sides of the membrane, are what creates the electrical voltage.
A team of scientists from MIT, the Massachusetts Eye and Ear Infirmary, and the Harvard-MIT Division of Health Sciences and Technology have recently succeeded in running an ultra-low-power radio-transmitting chip using power from these battery chambers – in guinea pigs’ ears.
The chip itself is located outside the animals’ ears (although it’s small enough to fit inside the middle ear), with electrodes running from it to both sides of the membrane in their battery chamber. The voltage generated by the chamber isn’t enough to run the chip continuously, however. Therefore, the chip incorporates power conversion circuitry, that allows a charge to build up in a capacitor.
Once that capacitor is sufficiently charged, the chip transmits a signal to an external receiver – it can take anywhere from 40 seconds to four minutes to build up enough power to do so. In this way, the timing of the chip’s transmissions can be used as a measure of the electrochemical properties of each guinea pig’s ears.
The chip’s control circuit likewise requires more voltage than the chamber can provide – at least, to initially get running. An external burst of radio waves is used to supply that initial boost. Once it’s going, the circuit is able to sustain itself using only ear-power.
Even if the battery chamber did provide a stronger current, the chip could still only use a fraction of it – if it drew too much power from the chamber, it would affect the animals’ hearing. The guinea pigs used in the experiments showed no loss of hearing, however, when tested.
Although practical chips in human ears are likely still some years away, the scientists envision the technology some day being used for things like biomedical sensors for people with hearing or balance problems, drug-delivery devices, cochlear implants, and even hearing aids.