A biological battery

VIJAYSREE VENKATRAMAN

                Plugging into sources of energy within our body — such as heat, internal motion or metabolites — to power implanted medical devices has long been the goal of biomedical engineers. Now researchers based in Cambridge, Massachusetts have demonstrated that a sensing device embedded in the ear can be powered by the ear’s own electrochemical battery.

Our auditory mechanism picks up external sounds and sends information to the brain in the form of neural signals. When the sound wave hits the ear, the eardrum vibrates in response. This mechanical energy must to be converted into an appropriate electrochemical impulse.

Deep inside the ear, the cochlea perceives the frequency of the vibration. It maintains a gradient of potassium and sodium ions across a delicate membrane via a system of pumps and channels. This natural battery, which makes neurotransmission of sound possible, generates a net positive voltage.

Researchers have known about the existence of this endocochlear potential (EP) for decades, but had not devised ways of using this voltage without interfering with the mammal’s hearing, says Konstantina Stankovic, otologic surgeon at Massachusetts Eye and Ear Infirmary, medical lead of the collaborative team. “What we have is both a conceptual and technological breakthrough. New electrodes and new electronics had to be developed to make safe harvesting possible,” she says.

Prof. Anantha Chandrakasan’s group at Massachusetts Institute of Technology designed the chip to extract current from the ear, keeping in mind the many physiological constraints. In the prototype, the harnessed power drives a wireless sensor that can monitor the value of the EP. A radio transmitter relays data to the clinician who uses the numbers to gauge the ear’s condition.

Though our ear functions on EP ranging from 70-100 millivolts, this voltage is not enough for electronic implants. “Since the power from the source is so small, we accumulate energy on a capacitor. Once the capacitor fills up, it can drive a higher power electronic circuit,” says Chandrakasan. “We power a 2.4 Gigahertz radio in this case.”

But transistor-based electronics need hundreds of millivolts to start. A wireless receiver on the integrated circuit gets a short burst of radio waves to kick-start the system.

The setup, implanted in the ear of a guinea pig, could transmit data for five hours without compromising normal hearing. Design optimization and more testing lie ahead.

“Thus far, we have demonstrated feasibility of sensing the EP, powered by the EP,” says Stankovic. “But we are eager to couple this energy-harvesting chip to a variety of molecular and chemical sensors to sense the inner ear and its environment and identify the most promising biomarkers relevant for the ultimate human application.”

The device cannot power multichannel cochlear implants or hearing aids as yet. But Charley C. Della Santina, professor of Otolaryngology and biomedical engineering at Johns Hopkins University, who is unconnected to the research team, points out that there is a real need for a system that can monitor the EP in animal models of Meniere’s disease — an inner ear disorder that affects balance and hearing. And, this device, he says, may just fit the bill.

Plus, the data collected in vivo could transform our understanding of how the mammalian ear works, says Stankovic. The paper that describes the findings appears in the latest issue of Nature Biotechnology.