Cochlear implants, small electronic devices that can provide a sense of sound to people who are deaf or hard of hearing, have improved hearing for more than a million people worldwide, according to the National Institutes of Health.

However, current cochlear implants are only partially implanted and rely on external hardware that typically sits on the side of the head. These components limit users, who cannot, for example, swim, exercise, or sleep while wearing the external unit, leading some people to abandon the implants.

On the path to creating a fully internal cochlear implant, a multidisciplinary team of researchers from MIT, Massachusetts Eye and Ear, Harvard Medical School, and Columbia University has developed an implantable microphone that performs as well as commercial external microphones for hearing aids. The microphone remains one of the biggest hurdles to adopting a fully internal cochlear implant.

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

Although there are still many challenges to overcome before such a microphone can be used with a cochlear implant, the collaborative team looks forward to further refining and testing this prototype, which builds on work started at MIT and Mass Eye and Ear over a decade ago.

Addressing implant issues
Cochlear implant microphones are usually placed on the side of the head, meaning users cannot take advantage of the noise filtering and sound orientation provided by the structure of the outer ear.

Fully implantable microphones offer many advantages. But most devices currently under development that detect sound beneath the skin or middle ear bone movements struggle to capture quiet sounds and broad frequencies.

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

Although the umbo has the greatest range of motion among the middle ear bones, it moves only a few nanometers. Developing a device that can measure such tiny vibrations presents its own challenges.

Additionally, any implantable sensor must be biocompatible and able to withstand the body's moist, dynamic environment without causing harm, limiting the materials that can be used.

Maximizing performance
With careful engineering, the team overcame these challenges. They created UmboMic, a triangular, 3-millimeter by 3-millimeter motion sensor made of two layers of biocompatible piezoelectric material called polyvinylidene difluoride (PVDF). These PVDF layers are placed on either side of a flexible printed circuit board (PCB), forming a rice-grain-sized microphone 200 micrometers thick. (The average human hair is about 100 micrometers thick.)

The narrow tip of the UmboMic would be positioned against the umbo. When the umbo vibrates and presses on the piezoelectric material, the PVDF layers bend and create electric charges, which are measured by electrodes in the PCB layer.

The team used a “PVDF sandwich” design to reduce noise. When the sensor bends, one PVDF layer produces a positive charge, and the other a negative charge. Electrical interference adds equally to both layers, so taking the difference between the charges cancels out the noise.

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

But developing the sensor was only half the battle—the umbo’s vibrations 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 uses very little power, they built their own.

With both prototypes in place, the researchers tested the UmboMic on cadaveric human ear bones and found it to have robust performance within the intensity and frequency range of human speech. The microphone and amplifier together also have low noise levels, meaning they can distinguish very quiet sounds from the overall noise level.

One interesting thing they noticed is that the sensor’s frequency response is affected by the ear anatomy they experiment on, as the umbo moves slightly differently in different people.

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

Additionally, they are exploring ways to encapsulate the sensor so it can remain in the body safely for up to 10 years while still being flexible enough to capture vibrations. Implants are often packaged in titanium, which would be too rigid for the UmboMic. They also plan to explore mounting methods for the UmboMic that won’t introduce vibrations.

The results of this work demonstrate the broadband response and low noise level needed to function as an acoustic sensor. These results are surprising because the bandwidth and noise level are so competitive with commercial hearing aid microphones. This performance shows the promise of the approach, which should encourage others to adopt this concept. I expect that smaller sensor elements and lower-power electronics will be needed for the next generations of devices to improve implantation ease and battery life.

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

Source: Massachusetts Institute of Technology