Cochlear implants, tiny electronic devices that provide a sense of sound to individuals who are deaf or hard of hearing, have enhanced the hearing of over a million people globally, according to the National Institutes of Health.
However, current cochlear implants are only partially implanted and rely on external hardware typically placed on the side of the head. These external components limit users, preventing activities like swimming, exercising, or sleeping while wearing the external unit, and may even deter some from opting for the implant altogether.
In pursuit of 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 hearing aid microphones. The microphone has been a significant barrier to achieving a fully internalized cochlear implant.
This small microphone, a sensor made from a biocompatible piezoelectric material, measures minute movements beneath the eardrum. Piezoelectric materials generate an electric charge when compressed or stretched. To enhance the device’s performance, the team also developed a low-noise amplifier that boosts the signal while minimizing electronic noise.
Although many challenges remain before such a microphone can be used with a cochlear implant, the collaborative team is eager to further refine and test this prototype, building on work initiated at MIT and Mass Eye and Ear over a decade ago.
« This effort begins with the otologists who deal with this issue daily, striving to improve people’s hearing, recognizing a need, and bringing it to us. Without this team collaboration, we wouldn’t be where we are today, » says Jeffrey Lang, Vitesse Professor of Electrical Engineering, member of the Research Laboratory of Electronics (RLE), and co-lead author of a paper on the microphone.
Lang’s co-authors include lead authors Emma Wawrzynek, a graduate student in Electrical Engineering and Computer Science (EECS), and Aaron Yeiser SM ’21; John Zhang, a graduate student in Mechanical Engineering; Lukas Graf and Christopher McHugh from Mass Eye and Ear; Ioannis Kymissis, Kenneth Brayer Professor of Electrical Engineering at Columbia; Elizabeth S. Olson, Professor of Biomedical Engineering and Auditory Biophysics at Columbia; and co-lead author Hideko Heidi Nakajima, Associate Professor of Otolaryngology and Head and Neck Surgery at Harvard Medical School and Mass Eye and Ear. The research is published today in the Journal of Micromechanics and Microengineering.
Overcoming an Implantation Impasse
Cochlear implant microphones are typically placed on the side of the head, meaning users cannot benefit from the noise filtering and sound localization provided by the outer ear’s structure.
Fully implantable microphones offer numerous advantages. However, most devices currently in development, which detect sounds under the skin or movements of the middle ear bones, struggle to capture faint sounds and wide frequencies.
For the new microphone, the team targeted a part of the middle ear called the umbo. The umbo vibrates in a unidirectional manner (inward and outward), making it easier to detect these simple movements.
Although the umbo has the greatest range of motion among the middle ear bones, it only moves a few nanometers. Developing a device to measure such tiny vibrations presents its own challenges.
Additionally, any implantable sensor must be biocompatible and able to withstand the body’s moist and dynamic environment without causing harm, limiting the materials that can be used.
« Our goal is for a surgeon to implant this device simultaneously with the cochlear implant and the internalized processor, optimizing the surgery while navigating around the internal ear structures without disrupting any processes, » says Wawrzynek.
Through meticulous engineering, the team overcame these challenges.
They created the UmboMic, a triangular motion sensor measuring 3 millimeters by 3 millimeters, composed of two layers of a biocompatible piezoelectric material called polyvinylidene difluoride (PVDF). These PVDF 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 would be placed against the umbo. When the umbo vibrates and pushes against the piezoelectric material, the PVDF layers bend and generate electric charges, which are measured by electrodes in the PCB layer.
Amplifying Performance
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, so taking the difference between the charges cancels out the noise.
Using PVDF offers many benefits, but this material makes fabrication particularly challenging. PVDF loses its piezoelectric properties when exposed to temperatures above about 80 degrees Celsius, but very high temperatures are needed to vaporize and deposit titanium, another biocompatible material, on the sensor. Wawrzynek circumvented this issue by gradually depositing the titanium and using a heat sink to cool the PVDF.
But developing the sensor was only half the battle: umbo vibrations are so minimal that the team needed to amplify the signal without introducing too much noise. When they couldn’t find a suitable low-noise, low-power amplifier, they built their own.
Once both prototypes were in place, the researchers tested the UmboMic on human ear bones from cadavers and found it performed robustly within the intensity and frequency range of human speech. The microphone and amplifier also have low background noise, meaning they can distinguish very faint sounds from the overall noise level.
« One interesting finding was that the sensor’s frequency response is influenced by the ear anatomy we experiment on, as the umbo moves slightly differently in different people’s ears, » says Wawrzynek.
The researchers are preparing to launch studies on living animals to further explore this discovery. These experiments will also help determine how the UmboMic responds to implantation.
Additionally, they are investigating ways to encapsulate the sensor so it can safely remain in the body for up to 10 years while remaining flexible enough to detect 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 paper show the broadband response and low noise necessary to act as an acoustic sensor. This outcome is surprising, as the bandwidth and background noise are very competitive with commercial hearing aid microphones. This performance shows the promise of the approach, which should inspire others to adopt this concept. I would expect smaller sensing elements and lower-power electronics to be needed for next-generation devices to improve implantation ease and battery life issues, » says Karl Grosh, Professor of Mechanical Engineering at the University of Michigan, who was not involved in this work.
This research was 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 in Switzerland.
