Future Directions in Cochlear Implants

Cochlear implants have restored hearing for more than 350,000 people, and that number continues to grow, as clinicians and engineers around the world are working toward improving the technology further.

In light of a recent collaboration between Konstantina Stankovic, MD, PhD, FACS, an otologic surgeon and researcher at Mass. Eye and Ear/Harvard Medical School, and Anantha Chandrakasan, PhD, Head of Electrical Engineering and Computer Science at MIT, the implant’s bulky exterior unit, which raises concerns in some individuals with social stigma and has limited use in the shower and during water sports, may soon no longer be necessary.

With a team of researchers, including then-graduate students Marcus Yip and Rui Jin, as well as Heidi Nakajima, MD, PhD, they have developed a prototype system-on-chip that makes possible a fully implantable cochlear implant. The system-on-chip electrically stimulates the auditory nerve in a similar fashion to the conventional cochlear implant, with one important difference.

Where conventional cochlear implants are made up of an external unit with a microphone and sound processor to transmit information to implanted electrodes, the system that Drs. Stankovic and Chandrakasan developed relies on a piezoelectric sensor — implanted beyond the ear drum in the middle ear — to transmit sound information by picking up on vibrations from the hearing bones.

“In a way, it is the body’s natural microphone,” Dr. Stankovic said. “When sound waves hit the eardrum, it sets into motion the smallest bones in the body, and their vibration leads to the vibration of fluids in the inner ear, which stimulates sensory cells and the auditory nerve, thus completing conversion from mechanical vibrations into electrical impulses.”

After pairing the system-on-chip with the piezoelectric sensor, which was already developed for the middle ear implant and readily available “off the shelf,” Drs. Stankovic and colleagues tested the device on a human cadaver. Results have been very encouraging.   

“The micromechanics of how these little bones vibrate is very similar in cadavers and in live humans, so we were able to study the output of our system,” Dr. Stankovic said. “It seems very comparable to the existing cochlear implant.”

With Dr. Chandrakasan, a renowned expert in ultra low-power electronics, the team is working to optimize the power supply of their device. The chip is specially designed to be charged wirelessly through a smart phone while the user is making a phone call. They have shown that it takes just a few moments to charge and that the charge lasts for 8 hours.

“It is a requirement that anything fully implanted in the body must be low power,” Dr. Stankovic said. “We’re looking into every opportunity to make the device more energy efficient.”

Dr. Stankovic speculates that they may eventually be able to harness energy from the user’s own body to power the implant. In 2012, the same team working on the system-on-chip found that they could extract energy from the inner ear to function like a “biological battery.” But the energy extracted from the inner ear turned out to be insufficient to power a cochlear implant.

“Others are exploring this further, because the body is a huge reservoir of energy,” Dr. Stankovic said. “At some point, that may become practical.”

Now that the team has demonstrated feasibility of the system-on-chip paired with a sensor and wireless power supply, they are working to fully package the device and prepare for a clinical trial.

“We have a little more work to do to have a fully assembled, packaged device that can be surgically implanted,” Dr. Stankovic said. “The next step would be a clinical trial, and we need to show that it’s at least as successful as the existing devices with the additional advantage of not having external components.”

Dr. Stankovic is eager to study through clinical trial a potential functional benefit that they’ve not yet had the opportunity to test. The fully implanted cochlear implant may offer improved sound localization.

“It turns out that the outer ear and the ear canal are important in filtering sounds,” she said. “So if you have a microphone that’s sitting next to the ear, you lose all of those directional cues. But with our system, where the sensor is on the other side of the eardrum, you still maintain all of those directional cues.”

Dr. Stankovic, who tackles otologic conditions through a variety of approaches in her research, is excited about the potential of this future direction for the cochlear implant, a therapeutic approach she’s been working on for many years. Dr. Stankovic began working at Mass. Eye and Ear in the early 1990s as an undergraduate student at MIT, working on her physics thesis in the cochlear implant research laboratory. The fully implantable cochlear implant project is the latest step in a mission that hits close for Dr. Stankovic.

“It’s very near and dear to my heart,” she said, of the project. “Cochlear implants are the reason I’m in the auditory field today.”