Cochlear implants, electronic devices that provide a sense of sound to individuals with severe hearing loss, have transformed the lives of over a million people globally, according to the National Institutes of Health. However, current implants rely on external components worn on the side of the head, limiting user activities and potentially discouraging adoption.
To address this, a collaborative team of researchers from MIT, Massachusetts Eye and Ear, Harvard Medical School, and Columbia University has developed a promising solution: an implantable microphone that rivals the performance of existing external hearing aid microphones. This innovation represents a significant step towards fully internal cochlear implants.
“It starts with the ear doctors who are with this every day of the week, 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,” says Jeffrey Lang, the Vitesse Professor of Electrical Engineering at MIT and co-senior author of the research paper.
Current cochlear implant microphones, positioned externally, miss out on the natural sound filtering and localization cues provided by the outer ear. While fully implantable microphones offer a solution, existing prototypes often struggle to capture soft sounds and a wide frequency range.
The team’s innovative microphone, dubbed the UmboMic, targets the umbo, a part of the middle ear that vibrates in a predictable inward and outward motion. This simplified movement pattern makes it easier to detect sound vibrations, even though the umbo moves only a few nanometers.
The UmboMic, a tiny sensor made from a biocompatible piezoelectric material called polyvinylidene difluoride (PVDF), measures approximately 3 millimeters by 3 millimeters, about the size of a grain of rice. When sound waves cause the umbo to vibrate, the PVDF layers in the UmboMic bend, generating electrical charges that are then measured by electrodes.
To ensure optimal performance, the team also developed a custom low-noise amplifier that boosts the signal while minimizing electronic noise. This amplifier, coupled with the UmboMic’s unique “PVDF sandwich” design, effectively cancels out electrical interference, further enhancing sound clarity.
Testing in human cadaver ear bones has shown the UmboMic to be highly effective within the intensity and frequency range of human speech. The device’s low noise floor allows it to discern even faint sounds from background noise.
Emma Wawrzynek, an electrical engineering and computer science graduate student at MIT and co-lead author of the paper, notes, “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.”
While significant hurdles remain, including long-term biocompatibility and surgical implantation techniques, the team is enthusiastic about the UmboMic’s potential. They are currently preparing for live animal studies to further investigate the device’s performance and are exploring ways to encapsulate the sensor for safe, long-term implantation.
Karl Grosh, professor of mechanical engineering at the University of Michigan, who was not involved in the research, comments, “The results in this paper show the necessary broad-band response and low noise needed to act as an acoustic sensor.
This result is surprising, because the bandwidth and noise floor are so competitive with the commercial hearing aid microphone. This performance shows the promise of the approach, which should inspire others to adopt this concept.”
The development of the UmboMic marks a significant leap towards fully internal cochlear implants, promising a future where individuals with hearing loss can experience sound with greater freedom and discretion.
This research was supported by the National Institutes of Health, the National Science Foundation, the Cloetta Foundation, and the Research Fund of the University of Basel.
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