The energy produced in the inner ear was harnessed to power an implantable electronic device in new research from Massachusetts Eye and Ear Infirmary and Massachusetts Institute of Technology (MIT). While the technology is still in an early stage of development, the hope is that the cochlea's “natural battery” could someday be used to power miniature implantable diagnostic sensors or perhaps tiny drug delivery pumps.
“We have known for decades that the endocochlear potential is the largest source of energy outside of individual cells,” said Konstantina Stankovic, MD, PhD, assistant professor of otology and laryngology at Harvard Medical School. Although it was thought that hearing could be harmed if the inner ear was encroached upon, this concern wasn't borne out by the new research.
“There is enough power in the inner ear to run electronics without damaging hearing,” said Dr. Stankovic, who is an otologic surgeon at Massachusetts Eye and Ear Infirmary. This approach to harnessing the ear's “biologic battery” was published in Nature Biotechnology (2012;30:1240-1243).
So far the work has only been done in guinea pigs, but the researchers expect that, with further refinement, the technology could be used to power a sensor to monitor biologic activity in the ears of people with hearing issues. The approach could also be used to measure the response to a treatment or deliver a drug therapy.
“It's a very interesting idea,” said Lawrence Lustig, MD, professor of otolaryngology–head and neck surgery at the University of California, San Francisco. “The research is proof of principle that you can harness the inner ear's energy in a useful way.”
‘HUGE ENERGY FACTORY’
The human body is “a huge energy factory” that could be put to use for medical purposes, said Patrick Mercier, PhD, who designed the electronics for the project as part of his doctoral thesis at MIT. Fat cells, for instance, are loaded with energy.
“If you could somehow convert one gram of fat into pure energy, it would be enough to download 10 blu-ray movies on your smartphone,” Dr. Mercier said.
According to background information in the Nature Biotechnology article, endocochlear potential “is the largest positive direct current electrochemical potential in mammals.” The electrical voltage is generated by a naturally occurring imbalance of sodium and potassium ions on either side of the membrane dividing the cochlea.
The challenge was to design a device that would be powerful enough to harness that energy source but not so powerful that it would disrupt the process involved in hearing, said Dr. Mercier, who is now an assistant professor of electrical and computer engineering at University of California, San Diego.
“This was a significant challenge because in order not to disrupt hearing the device would have to consume less power than almost any other device available today.”
The researchers tested their concept in guinea pigs. Electrodes attached to an ultralow-power energy harvester chip were implanted in the cochlea. For these feasibility studies, the chip was positioned outside the animal instead of being implanted in the middle ear. The gathered energy was used to power a wireless transmitter and pick up readings from the inner ear. The short-term effect of the approach on the animals’ hearing was minimal, the researchers reported.
The device would need to be refined before it could move on to be tested in humans, Dr. Mercier said. While some reports in the media suggested the new technology could be used to power cochlear implants or battery-free hearing aids, Dr. Mercier said the amount of energy he and his team were able to harvest from the ear would not be enough to accomplish that. (In comparison, there's two million times more energy stored in a typical watch battery, he noted.) Still, if further developed, the device could power small sensors used to monitor ear health or look for signs of infection after surgery or other therapy, he said.
The research opens up the possibility of developing sensors that could be implanted in the cochlea to monitor chemical and molecular conditions, Dr. Stankovic agreed. There is much to be learned about the inner ear, she added.
“We could gain fundamental new knowledge about how the ear works and diagnostically find out what is happening with a given person,” Dr. Stankovic said, noting that such implantable devices could possibly flag problems before they became serious and even be used to deliver new therapies, such as nerve regenerative agents.
BROADER USE POSSIBLE
The inner ear's energy source is a desirable target for broader medical applications as well, Dr. Stankovic said.
“Just like in real estate, it's all about location, location, location.” Because the ear is close to key structures within the head and neck, its energy source could be tapped for diagnostic and therapeutic use in those places, she said.
Implantable medical devices have vast potential, but current technology is limited in its usefulness because of the need for a long-lasting power supply.
“Anatomy often limits the size of implantable batteries, requiring surgical reimplantation or cumbersome external wireless power sources for long-term operation,” the researchers reported in the Nature Biotechnology article. “Biologically based energy harvesting is a potential solution to power implantable devices.”
There's more work to be done to develop a fully implantable device for use in humans, Dr. Stankovic said.
“Although we have identified and overcome key challenges in developing the components of the system necessary to harvest net positive energy from the inner ear for hours at a time, important future work is required in electrode miniaturization and surgical approaches in order to advance from demonstrating feasibility to integrating all components into a safe, fully implantable system for long-term operation,” she and her colleagues concluded in their report.
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