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A hearing aid is a straightforward device. Its microphone collects sound, its electronics amplify it, its tiny loudspeaker sends the sound into a tube placed in the ear canal, and the power comes from a disposable battery. There’s just one problem: people hate hearing aids. They get lost. They’re hard to wear while sleeping. They mustn’t get wet. They get chewed up by the dog. They’re awkward during sex.

I don’t have a hearing aid. But I do have a cochlear implant. Cochlear implants are for people who are so deaf that even the most powerful hearing aids won’t help. A processor worn on my ear collects sound and digitizes it, then transmits it by radio to a receiver embedded in my skull. The receiver sends pulses to electrodes attached to my auditory nerves.

It should be called a cochlear semi-implant, really, because half of it is on the outside. It lets me hear, which is great, but it has the same disadvantages as hearing aids. For starters, I have to assemble myself in the morning–literally. But more than that, my cochlear implant feels like something decidedly attached to me. Naturally, I would love to have a body that’s whole and complete in itself. A body that could plunge into the water without sacrificing the ability to hear friends’ laughter when it emerged.

So far, no one’s built a fully implantable cochlear implant. But two fully implantable hearing aids are now in clinical trials (that is to say, they are considered investigational by the U.S. Food and Drug Administration and are not yet approved for commercial sale). One, the Esteem, is built by Envoy Medical of St. Paul, MN. The other, from Otologics of Boulder, CO, is called the Carina. Hopes are high that they will be the first successful devices of their kind. Making such things is a challenge. Where does the microphone go? How is the amplified sound sent into the ear? What’s the power source? And how can it be kept in the body without leaking?

I was curious to know whether the new devices worked as well as conventional hearing aids. I was even more curious to know whether the tech­nology could be applied to cochlear implants. ­Otologics was game to show me its work.

Lucky Yorick
At Otologics, Brian Conn, the engineering director, brought out a skull with the company’s device bolted onto it. I realized after a queasy moment that it was a real skull.

The device didn’t look like a hearing aid. There were four connected pieces designed to be countersunk into the skull.

The first piece, the microphone, sat behind the outer ear. The sensitivity of a microphone drops by a factor of 10 when it’s buried under skin, so to compensate, the microphone had a surface area 10 times as big as a hearing aid’s. It was about the size of a fingernail. Its output went to the biggest component, the processing unit. Its shell also contained a rechargeable lithium-ion battery.

The battery was recharged, Conn told me, by the third component: an inductive coil. An inductive coil converts radio waves into electricity. For an hour or two a day, the user puts a small radio transmitter up against the coil. Since both the coil and the transmitter have magnets in them, they stick together through the skin. The patient can walk around wearing the charging unit until the battery is full.

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Credit: Peter Belanger

Tagged: Biomedicine, implant, electronic gadgets, hearing

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