A future cochlear implant with no exterior hardware required
February 13, 2014
A new low-power signal-processing chip that could lead to a cochlear implant that does not require external devices has been developed by researchers at MIT’s Microsystems Technology Laboratory (MTL), together with physicians from Harvard Medical School and the Massachusetts Eye and Ear Infirmary (MEEI).
The chip uses the natural microphone of the middle ear rather than a skull-mounted microphone. The implant would be wirelessly recharged and would run for about eight hours on each charge.
Existing cochlear implants require patients to have a disk-shaped transmitter about an inch in diameter affixed to their skull. That device inductively links with a matching device implanted under the skin, which attaches to wire that connects to the cochlea.
There’s also a microphone/power source that looks like an oversized hearing aid mounted around the patient’s ear.
Here’s a video from MED-EL showing how existing cochlear implants work:
‘Middle-ear implant’ design
The MIT researchers’ new design removes all of the external components and exploits the mechanism of a different design: a “middle-ear implant.” Delicate bones in the middle ear, known as ossicles, convey the vibrations of the eardrum to the cochlea — the small, spiral chamber in the inner ear that converts acoustic signals to electrical.
In patients with middle-ear implants, the cochlea is still functional, but one of the ossicles (the staples) doesn’t vibrate with enough force to stimulate the auditory nerve. The existing middle-ear implant consists of a tiny sensor that detects the ossicles’ vibrations and an actuator that helps drive the stapes accordingly.
The new device would use the same type of piezoelectric sensor, but the signal it generates would travel instead to a microchip implanted inside the ear, which would then convert to an optimized electrical signal and pass it on to an electrode in the cochlea.
Lowering the power requirements of the converter chip by 20 to 30 percent was the key to dispensing with the skull-mounted hardware.
“It’s very cool,” says Lawrence Lustig, director of the Cochlear Implant Center at the University of California at San Francisco. “There’s a much greater stigma of having a hearing loss than there is of having a visual loss. So people would be very keen on losing the externals for that reason alone. But then there’s also the added functional benefit of not having to take it off when you’re near water or worrying about components getting lost or broken or stolen. So there are some important practical considerations as well.”
Lustig points out that the new cochlear implant would require a more-complex surgery than existing implants do. “A current cochlear-implant operation takes an hour, hour and a half,” he says. “My guess is that the first surgeries will take three to four hours.” But he doubts that that would be much of an obstacle to adoption. “As we get better and better and better, that time will shorten,” he says. “And three to four hours is still a relatively straightforward operation. I don’t anticipate putting a lot of extra risk into the procedure.”
The researchers describe their chip in a paper they’re presenting this week at the International Solid-State Circuits Conference. The paper’s lead author — Marcus Yip, who completed his PhD at MIT last fall — and his colleagues Rui Jin and Nathan Ickes, both in MIT’s Department of Electrical Engineering and Computer Science, will also exhibit a prototype charger that plugs into an ordinary cell phone and can recharge the signal-processing chip in roughly two minutes.