A multidisciplinary team at MIT, Massachusetts Eye and Ear, Harvard Medical School, and Columbia University has demonstrated a proof-of-concept for a new biocompatible audio sensor that outperforms conventional cochlear microphones. Such a development could revolutionize cochlear implants by making them fully implantable.
Both sides of the prototype, UmboMic, for the implantable microphone. Image (modified) used courtesy of MIT
The new device performs twice as well as current external cochlear microphones and promises a better quality of life for users by not requiring external components. The microphone includes a piezoelectric sensor to measure tiny movements on the underside of the eardrum and a high-performance, low-noise amplifier (LNA).
The Fully Implantable UmboMic
UmboMic is a polyvinylidene difluoride (PVDF)-based biocompatible microphone that attaches to the eardrum at the middle ear section called the umbo. The umbo is a small conical structure in the tympanic membrane.
Anatomy of the tympanic membrane (middle ear). Image used courtesy of ResearchGate
The tympanic membrane separates the outer from the inner ear, with the ear canal opening to the outside on one side and the auditory bones on the other.
While the umbo delivers the most movement of any section of the ear, its movement is still quite small—in the nanometer range. The means that a sensor must be extremely sensitive and will require significant amplification. The team was unable to find a workable off-the-shelf amplifier solution, so they had to develop their own high-performance differential LNA.
The sensor is constructed of symmetrical layers deposited on both sides of a flex PCB core. It’s built up and trimmed post-fabrication and sputter coated with an aluminum ground/shield. The result is a highly sensitive differential sensor with excellent biocompatibility characteristics. The completed sensor can then be implanted into the umbo.
Proof of concept biocompatible piezoelectric sensor. Image used courtesy of MIT
The microphone demonstrated a performance of “32.3 dB SPL over the frequency range 100 Hz to 7 kHz, good linearity, and a flat frequency response to within 10 dB from about 100 Hz to 6 kHz.” Typical human hearing has a flat frequency response in the 100 Hz to 4 kHz range.
A Brief History of Cochlear Implants
Cochlear implants give some level of hearing to individuals with severe hearing loss due to inner ear damage. Conventional hearing aids amplify sounds and deliver higher-level sound to the ear canal in audio form using a small speaker. People who have significant inner ear damage, however, may not have any sensitivity to audio or may have so little that the brain cannot interpret even a highly amplified signal.
A cochlear implant has an external microphone and sound processor in a detachable arrangement connected with an RF link to an implanted electrode. The electrode is placed into the cochlea, the snail-shaped inner ear, bypassing the mechanical components of the ear and directly stimulating the cochlear nerve. By directly stimulating the nerve, signals can get to the brain’s audio processing center regardless of the condition of the inner ear.
Cochlear ear implant. Image used courtesy of the Mayo Clinic
While the first cochlear implants were invented in the late 1950s and early 1960s, a practical implant was not invented until the late 1970s. In 1977, engineer Adam Kissiah leveraged his NASA instrumentation engineering experience to patent a usable cochlear implant, which he sold for commercialization. That same year, Ingeborg Hochmair and Erwin Hochmair commercialized and implanted the first device at the Technical University of Vienna in Austria.
While the cochlear implant revolutionized treatment for certain types of extreme hearing loss, it has its limitations. The primary disadvantage of today’s cochlear implant is the internal/external construction architecture. The microphone and amplifier reside outside the body, externally attached to the head near the ear. Users of the implant must remove the external components during some activities, such as swimming, exercising, or sleeping.
UmboMic would eliminate the need for such external components and would take advantage of the sound-channeling effects of the normal ear structure. This would improve a user’s ability to sense the direction and location of incoming sounds.
Next Steps for the UmboMic
UmboMic has only been tested in cadaver ears so far. The researchers must test the microphone in a living host to verify its biocompatibility and robustness for an implant with a ten-plus-year lifespan. The 3 mm x 3 mm device and the LNA will need further size reductions. With further development, however, this technology could lead to the most natural hearing restoration yet.
