Invisible Cochlear Implants

Cochlear implants that electrically stimulate the auditory nerve have granted at least limited hearing to hundreds of thousands of people worldwide who otherwise would be totally deaf. Current devices, however, require that a transmitter about an inch in diameter be affixed to the skull, with a wire snaking down to a combined microphone and power source that looks like an oversized hearing aid.

Researchers at MIT’s Microsystems Technology Laboratories collaborated with physicians from Harvard Medical School and the Massachusetts Eye and Ear Infirmary to develop a new low-power signal-processing chip that could lead to a cochlear implant with no external hardware. It would be wirelessly recharged and would run for about eight hours per charge.

They also developed a prototype charger that plugs into an ordinary cell phone and can recharge the signal-processing chip in roughly two minutes.

“The idea with this design is that you could use a phone, with an adapter, to charge the cochlear implant, so you don’t have to be plugged in,” says Anantha Chandrakasan, a professor of electrical engineering and corresponding author on a paper by Marcus Yip, PhD ’13, presented at the International Solid-State Circuits Conference. “Or you could imagine a smart pillow, so you charge overnight, and the next day, it just functions.”

Existing cochlear implants use an external microphone to gather sound, but the new implant would use the natural microphone of the middle ear, which is almost always intact in cochlear-implant patients. Normally, 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 ones. The new device would employ a tiny sensor that detects the ossicles’ vibrations, relaying their signal to a microchip implanted in the ear. That microchip would convert it to an electrical signal and pass it on to an electrode in the cochlea.

Lowering the power requirements of the converter chip was the key to dispensing with the skull-mounted hardware. Among other innovations, Chandrakasan’s lab developed a new signal-generating circuit whose waveform—the basic electrical signal it emits—requires 20 to 30 percent less power to produce than those used in existing cochlear implants.

The researchers showed that the chip and sensor can pick up and process speech played into the middle ear of a human cadaver. They also tested the new waveform on four patients with cochlear implants and found that it did not compromise their ability to hear.