Implantable microphone could lead to fully internal cochlear implants

According to the National Institutes of Health, cochlear implants, small electronic devices that can restore hearing to people who are deaf or hard of hearing, have helped improve the hearing of more than one million people worldwide.

However, cochlear implants are currently only partially implanted and rely on external hardware that is usually located on the side of the head. These features limit users, who cannot, for example, swim, exercise or sleep while wearing an external unit, and may cause others to forgo the implant altogether.

On the way to a fully internalized cochlear implant, a multidisciplinary team of researchers from MIT, Massachusetts Eye and Ear, Harvard Medical School, and Columbia University has created an implantable microphone that works as well as commercial external hearing aid microphones. The microphone remains one of the biggest obstacles to the adoption of a fully internalized cochlear implant.

This tiny microphone, a sensor made of biocompatible piezoelectric material, measures microscopic movements on the underside of the eardrum. Piezoelectric materials generate an electrical charge when they are compressed or stretched. To maximize the device’s performance, the team also developed a low-noise amplifier that boosts the signal while minimizing noise from the electronics.

While many challenges will need to be overcome before this type of microphone can be used with a cochlear implant, the team plans to continue refining and testing the prototype, which builds on work begun at MIT and Mass Eye and Ear more than a decade ago.

“It starts with ENT doctors who are out there every day of the week trying to improve people’s hearing, recognizing the need and bringing it to us. Without that collaborative teamwork, we wouldn’t be where we are today,” says Jeffrey Lang, a Vitesse professor of electrical engineering, a member of the Research Laboratory of Electronics (RLE) and co-author of a senior paper on the microphone.

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

Overcoming the Implant Impasse

Cochlear implant microphones are typically placed on the side of the head, meaning users cannot benefit from the noise filtering or sound localization cues provided by the structure of the outer ear.

Fully implantable microphones offer many advantages. However, most devices currently in development that sense sound under the skin or movement of the bones in the middle ear can have difficulty capturing quiet sounds and wide frequencies.

For the new microphone, the team focused on a part of the middle ear called the hump. The hump vibrates in one direction (in and out), making it easier to sense these simple movements.

Although the hump has the greatest range of motion of the middle ear ossicles, it only moves a few nanometers. Developing a device to measure such tiny vibrations comes with its own challenges.

Furthermore, any implantable sensor must be biocompatible and resistant to the body’s moist, dynamic environment without causing harm, which limits the range of materials that can be used.

“Our goal is for the surgeon to implant 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 processes occurring in the ear,” Wawrzynek says.

Thanks to a thoughtful engineering approach, the team managed to overcome these challenges.

They created UmboMic, a 3-by-3-millimeter triangular motion sensor made of two layers of a biocompatible piezoelectric material called polyvinylidene fluoride (PVDF). The PVDF layers are placed on either side of a flexible printed circuit board (PCB), creating a microphone about the size of a grain of rice and 200 micrometers thick. (The average human hair is about 100 micrometers thick.)

The narrow tip of the UmboMic would be placed at the hump. When the hump vibrates and presses on the piezoelectric material, the PVDF layers bend and generate electrical charges that are measured by electrodes in the PCB layer.

Increasing efficiency

The team used a “PVDF sandwich” design to reduce noise. When the sensor is bent, one layer of PVDF generates a positive charge, the other a negative charge. Electrical interference adds to both equally, so the difference in charge cancels out the noise.

Using PVDF offers many advantages, but the material makes manufacturing particularly difficult. PVDF loses its piezoelectric properties when exposed to temperatures above about 80 degrees Celsius, but very high temperatures are needed to evaporate and deposit titanium, another biocompatible material, onto the sensor. Wawrzynek solved this problem by gradually depositing titanium and using a heat sink to cool the PVDF.

But developing the sensor was only half the battle—the umbo’s oscillations are so tiny that the team had to amplify the signal without introducing too much noise. When they couldn’t find a suitable low-noise amplifier that also used very little power, they built their own.

With both prototypes in place, the researchers tested the UmboMic on human cadaveric ear bones and found that it had solid performance across the range of human speech intensity and frequency. The microphone and amplifier together also have a low noise floor, meaning they can distinguish very quiet sounds from the general noise floor.

“One of the things we observed that was really interesting is that the frequency response of the sensor depends on the anatomy of the ear that we’re experimenting on, because the hump moves slightly differently in different people’s ears,” Wawrzynek says.

The researchers are preparing to begin studies in live animals to further investigate this discovery. These experiments will also help them determine how UmboMic responds to implantation.

They are also investigating ways to encapsulate the sensor so it can remain safely in the body for up to 10 years but still be flexible enough to pick up vibrations. Implants are often encapsulated in titanium, which would be too stiff for the UmboMic. They also plan to explore methods of mounting the UmboMic that won’t introduce vibrations.

“The results in this paper demonstrate the necessary wideband response and low noise floor needed to function as an acoustic sensor. This result is surprising because the bandwidth and noise floor are so competitive with commercial hearing aid microphones. This performance shows the promise of the approach, which should inspire others to adopt the concept. I would expect that smaller sensing elements and lower-power electronics will be needed in next-generation devices to improve ease of implantation and battery life issues,” says Karl Grosh, a professor of mechanical engineering at the University of Michigan, who was not involved in the work.

This research was funded in part by the National Institutes of Health, the National Science Foundation, the Cloetta Foundation in Zurich, Switzerland, and the Research Fund of the University of Basel, Switzerland.