A new, low-power signal-processing chip that could be used to develop a novel cochlear implant that would require no external hardware has been developed by researchers at the Massachusetts Institute of Technology (MIT), along with physicians from Harvard Medical School and the Massachusetts Eye and Ear Infirmary (MEEI). The implant – which would use the natural microphone of the inner ear rather than a skull-mounted sensor – could be wirelessly recharged and could run for about eight hours on each charge.

A cochlear implant is a surgically implanted medical device that provides a sense of sound to a person who is profoundly deaf or severely hearing-impaired as a result of damage to the sensory hair cells in their cochleae. Hundreds of thousands of people worldwide have benefited from such devices since they were first implanted in adults in 1984.

Easy charging

Unfortunately, current implants are bulky and unattractive. "Today's cochlear implants rely on an external behind-the-ear component to house an external microphone and power source, which can be cumbersome, has limited usage in water and can be aesthetically unappealing," says Anantha Chandrakasan of the Department of Electrical Engineering and Computer Science at MIT, who led the new work. "The idea with this design is that you could use a phone, with an adaptor, to charge the cochlear implant, so you don't have to be plugged in," he adds.

The new implant is an adaptation of existing devices that vibrate bone structures in the middle ear that are normally intact in patients using cochlear implants. The inner ear has three bones, or "ossicles", which conduct sound vibrations to the inner ear and to the cochlea – the small, spiral chamber that converts acoustic signals to electrical signals that are then detected by the brain. Patients with middle-ear implants suffer because one of the ossicles – the stapes – does not vibrate strongly enough to stimulate the auditory nerve. The new implant therefore has a tiny sensor that detects the ossicles' vibrations and an actuator that helps drive the stapes.

Chandrakasan and his team envisage that the new device would use a similar type of sensor, but the signal it generates would travel to a microchip implanted in the ear, which in turn would convert it to an electrical signal and pass it on to an electrode in the cochlea. The team has now made a prototype of this chip, along with a prototype charger that plugs into an ordinary mobile phone and can recharge the signal-processing chip in about two minutes. The findings and prototype were presented by Marcus Yip, the lead author of the paper, at the International Solid-State Circuits Conference held in San Francisco, US, last week.

Specialized waveform

One key procedure that let the team dispense with the bulky apparatus was cutting the power requirements of the converter chip. Chandrakasan's lab specializes in low-power chips, and so the researchers could apply techniques they have perfected over the years, such as tailoring the arrangement of low-power filters and amplifiers to the precise acoustic properties of the incoming signal.

But one novel feature of the chip is the new signal-generating circuit that slashes its power consumption by an additional 20–30%. It does this by using a new waveform – the basic electrical signal emitted by the chip that is modulated to encode acoustic information. Based on previous research into simulated nerve fibres, the waveform was adapted to make the device more power-efficient while still offering enough stimulation of the auditory nerve. Two of Chandrakasan's collaborators at MEEI – Konstantina Stankovic and Don Eddington – tested the waveform on four patients who already had cochlear implants, and found that it did not affect their ability to hear and that the results were on a par with the team's computational modelling.

While the device has not been implanted in any patients yet, the sensor was tested in the ears of human cadavers to show that the sensor and the microchip could pick up speech signals played into the ear of the corpse. Conventional cochlear implants are approved by the US Food and Drug Administration for one-year-old children, so the team envisions that its implants could benefit toddlers too. Stankovic told physicsworld.com that the added advantage of a fully implantable implant for children is that they could "play without worrying about the external component falling, breaking or being uncomfortable".

The research will be published in the proceedings of the IEEE International Solid-State Circuits Conference.