Cochlear implants, small electronic devices that provide a sense of sound to those who are deaf or hard of hearing, have improved hearing for over a million people worldwide, according to the National Institutes of Health. However, current cochlear implants are only partially implanted, relying on external hardware that sits on the side of the head. This external component restricts users, preventing them from swimming, exercising, or sleeping with the device, leading some to forgo the implant altogether.

A multidisciplinary team of researchers from MIT, Massachusetts Eye and Ear, Harvard Medical School, and Columbia University has made significant progress toward creating a fully internal cochlear implant. They have developed an implantable microphone that performs as well as commercial external hearing aid microphones, addressing one of the largest hurdles in achieving a fully internalized cochlear implant.

This innovative microphone is a sensor made from biocompatible piezoelectric material that measures minuscule movements on the underside of the eardrum. Piezoelectric materials generate an electric charge when compressed or stretched. To maximize performance, the team also developed a low-noise amplifier that enhances the signal while minimizing electronic noise.

While many challenges remain before this microphone can be used in cochlear implants, the collaborative team is optimistic about further refining and testing their prototype. This work builds on research that began at MIT and Mass Eye and Ear over a decade ago.

“It starts with the ear doctors who are with this every day, trying to improve people’s hearing, recognizing a need, and bringing that need to us. If it weren’t for this team collaboration, we wouldn’t be where we are today,” said Jeffrey Lang, the Vitesse Professor of Electrical Engineering at MIT and co-senior author of a paper on the microphone.

Lang’s coauthors include Emma Wawrzynek, an electrical engineering and computer science (EECS) graduate student; Aaron Yeiser SM ’21; mechanical engineering graduate student John Zhang; Lukas Graf and Christopher McHugh of Mass Eye and Ear; Ioannis Kymissis, the Kenneth Brayer Professor of Electrical Engineering at Columbia; Elizabeth S. Olson, a professor of biomedical engineering and auditory biophysics at Columbia; and co-senior author Hideko Heidi Nakajima, an associate professor of otolaryngology-head and neck surgery at Harvard Medical School and Mass Eye and Ear. The research is published in the Journal of Micromechanics and Microengineering.

Current cochlear implant microphones are usually placed on the side of the head, which prevents users from taking advantage of the noise filtering and sound localization provided by the outer ear. Fully implantable microphones offer many advantages, but most devices in development, which sense sound under the skin or through middle ear bone motion, struggle to capture soft sounds and a wide range of frequencies.

The team targeted a part of the middle ear called the umbo for their new microphone. The umbo vibrates unidirectionally (inward and outward), making it easier to sense these simple movements. Although the umbo moves only by a few nanometers, developing a device to measure such small vibrations presents challenges. Any implantable sensor must be biocompatible and withstand the body’s humid, dynamic environment without causing harm.

“Our goal is that a surgeon implants this device at the same time as the cochlear implant and internalized processor, which means optimizing the surgery while working around the internal structures of the ear without disrupting any of the processes that go on in there,” Wawrzynek said.

The team created the UmboMic, a 3-millimeter by 3-millimeter motion sensor made of two layers of biocompatible piezoelectric material called polyvinylidene difluoride (PVDF). These layers are sandwiched on either side of a flexible printed circuit board (PCB), forming a microphone about the size of a grain of rice and 200 micrometers thick (an average human hair is about 100 micrometers thick). The narrow tip of the UmboMic is placed against the umbo. When the umbo vibrates, it pushes against the piezoelectric material, causing the PVDF layers to bend and generate electric charges, which are measured by electrodes in the PCB layer.

Using a “PVDF sandwich” design reduces noise. When the sensor bends, one PVDF layer produces a positive charge and the other a negative charge. Electrical interference affects both equally, so the difference between the charges cancels out the noise.

Developing the sensor was only half the battle — umbo vibrations are so tiny that the team needed to amplify the signal without introducing too much noise. Unable to find a suitable low-noise amplifier that used very little power, they built their own. With both prototypes in place, the researchers tested the UmboMic in human ear bones from cadavers and found it had robust performance within the intensity and frequency range of human speech. The microphone and amplifier together have a low noise floor, distinguishing very quiet sounds from the overall noise level.

“One thing we saw that was really interesting is that the frequency response of the sensor is influenced by the anatomy of the ear we are experimenting on, because the umbo moves slightly differently in different people’s ears,” Wawrzynek said.

The researchers are preparing to launch live animal studies to further explore this finding and determine how the UmboMic responds to being implanted. They are also studying ways to encapsulate the sensor to ensure it can remain in the body safely for up to 10 years while still being flexible enough to capture vibrations. While titanium packaging would be too rigid for the UmboMic, the team is exploring alternative methods for mounting the device without introducing vibrations.

In summary, the collaborative effort has resulted in a promising step towards fully internal cochlear implants, leveraging innovative technology to improve the quality of life for those with hearing impairments.

By Impact Lab