Biologically based energy harvesting offers a viable solution to the long-term powering of implantable electronics for medical use, which currently require large external energy sources. Methods such as heat capture using thermoelectric devices and muscle-movement capture using piezoelectric devices or induction generators offer some hope but are typically not suitable for daily use as they require bulky, externally worn apparatuses.

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Researchers at Massachusetts Institute of Technology (Cambridge, MA) have, however, recently identified another untapped source of stable energy (Nat. Biotechnol. published online 8 November 2012; doi:10.1038/nbt.2394). Inside every mammalian ear resides a biological battery. Endocochlear potential (EP) is an electrochemical gradient found in the inner ear; at 70–100 mV, it is the largest positive direct current electrochemical potential in mammals. Arising from the difference in ionic concentration between endolymph (an extracellular fluid in the inner ear) and perilymph (an extracellular fluid that bathes the surrounding spaces), EP drives cochlear mechanotransduction of sound pressure vibrations to neurotransmitter release and excitation of the auditory nerve. Further, its intrinsic stability over the lifetime of a mammal makes it an ideal candidate for powering long-term, energy-autonomous implantable devices.

In 'proof of concept' experiments, Konstantina Stankovic and her colleagues implanted tiny electrodes into the biological battery of a guinea pig's ear. Attached to the microelectrodes was an electronic chip equipped with an ultralow-power radio transmitter and power conversion device. Powering up the chip required an initial 'kick-start' from an external source, but after that, the biological battery took control, extracting a minimum of 1.12 nW from the EP for up to 5 hours, enabling the 2.4-GHz radio to transmit every 40–360 s. Insertion of the electrodes did not adversely affect the guinea pig's hearing or cause excessive trauma to the ear tissue.

This study is the first of this kind, and although the results indicate great promise, the amount of energy extracted from the EP is still too low to provide any applicable use. The authors believe that major improvements in their electrode design are necessary, reasoning that smaller diameter electrodes would be less invasive and thereby reduce the amount of ion leakage across the endolymph-perilymph barrier.

In the future, energy harvesting from the cochlea may be applicable to humans. Self-sustaining, implantable electronics could provide myriad medical benefits, including monitoring deafness and hearing loss or tracking the health of organs in and around the inner ear (e.g., the carotid artery, facial nerve or temporal lobe).