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Using the Inner Ear's Biological Battery

Scientists power a radio transmitter with the electrochemical potential of the inner ear

2 min read
Using the Inner Ear's Biological Battery

Scientists have harvested energy from a guinea pig's inner ear and used it to power a small wireless transmitter. With further design work, researchers could harvest this biological battery to power implanted devices near the human ear, such as molecular sensors and drug delivery vehicles for hearing loss and other disorders, according to a study to be published today in Nature Biotechnology.

It has been known for decades that the inner ear contains this biological battery, but until now, no one has harvested it. The authors of the paper, led by Anantha Chandrakasan at Massachusetts Institute of Technology and Konstantina Stankovic at Massachusetts Eye and Ear Infirmary, succeeded without damaging the guinea pigs' hearing. 

The inner ear's biological battery is located in a spiral-shaped auditory region called the cochlea. The electric potential in this region arises from the electrical difference between two different chambers in the cochlea, which contain charged particles such as potassium and chloride ions. A nearby specialized structure known as the stria vascularis transports the ions through its unique arrangement of electrogenic ion pumps, generating an electrochemical potential known as the endocochlear potential

At 70-100 mV, the electrochemical potential of the inner ear is the highest in the mammalian body. But it's still a very small amount of energy, and only a fraction of it can be extracted without disrupting hearing. To address this challenge, the researchers chose to power a specially designed chip equipped with an ultralow-power radio transmitter. 

In the experiments, the researchers implanted electrodes in the cochlea of anesthetized guinea pigs. The electrodes were connected to the chip, which was located outside the animals' ears. (It is small enough to fit in a human ear.) The chip included power-conversion circuitry that gradually builds up charge in a capacitor. To kick-start the control circuit, the researchers applied a one-time burst of radio waves. The device wirelessly transmitted measurements of the endocochlear potential to an external receiver. About 1 nW of power was extracted for up to 5 hours—long enough to enable the 2.4 GHz radio to transmit measurements every 40-360 seconds.

Harvesting energy from the human ear to power small electronic devices could be a huge breakthrough for people grappling with hearing loss and other disorders. Implantable electronics usually require large energy reservoirs to operate reliably over long periods of time. But human anatomy limits the size of implantable batteries, and often requires surgical re-implantation or cumbersome external wireless power sources. Harvesting enough energy from the body's own energy sources is a way to extend implant life, and maybe even allow it to operate autonomously, the authors report.

Images: Patrick P. Mercier


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Restoring Hearing With Beams of Light

Gene therapy and optoelectronics could radically upgrade hearing for millions of people

13 min read
A computer graphic shows a gray structure that’s curled like a snail’s shell. A big purple line runs through it. Many clusters of smaller red lines are scattered throughout the curled structure.

Human hearing depends on the cochlea, a snail-shaped structure in the inner ear. A new kind of cochlear implant for people with disabling hearing loss would use beams of light to stimulate the cochlear nerve.

Lakshay Khurana and Daniel Keppeler

There’s a popular misconception that cochlear implants restore natural hearing. In fact, these marvels of engineering give people a new kind of “electric hearing” that they must learn how to use.

Natural hearing results from vibrations hitting tiny structures called hair cells within the cochlea in the inner ear. A cochlear implant bypasses the damaged or dysfunctional parts of the ear and uses electrodes to directly stimulate the cochlear nerve, which sends signals to the brain. When my hearing-impaired patients have their cochlear implants turned on for the first time, they often report that voices sound flat and robotic and that background noises blur together and drown out voices. Although users can have many sessions with technicians to “tune” and adjust their implants’ settings to make sounds more pleasant and helpful, there’s a limit to what can be achieved with today’s technology.

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