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The Naked Ear

A fully implantable hearing aid is showing promise.
December 18, 2007

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.

The Carina, a fully implantable hearing aid from Boulder-based Otologics, is in clinical trials.

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?

Carina, fully implantable hearing device
$20,000 upon FDA approval

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.


  • Watch Michael Chorost explain how his cochlear implant works.

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.

The fourth component was a vibrating piston in the middle ear, secured by four titanium bolts screwed to the skull. This was what actually delivered the sound. The middle ear consists of three tiny bones that conduct vibrations from the eardrum to the inner ear. The piston moved the bones more forcefully than the eardrum would, so it acted as an amplifier.

I peered at the skull, feeling like ­Hamlet contemplating a high-tech Yorick. The Carina was a strange-looking gadget. Big, too: at about five inches long, it stretched from behind the ear to just behind the ­temple. The surgery would involve opening a skin flap, drilling into the skull to counter­sink the components, and then drilling into the middle ear to install the piston. A lot of hardware to get into place.

“How long does the battery last?” I asked.

Each charge was good for about a day, Conn told me. The battery could go through enough charge cycles to last at least five years, and possibly 10 or more.

“And when the battery can’t hold a charge anymore?” I asked.

When that happened, Conn told me, the entire device, except for the piston, would be replaced. The microphone, coil, and processor had many connections to each other, so they had to be hermetically sealed together to keep body fluids out. The piston had only two connections, though, so a seal could be maintained between them. In any case, the surgery would be simple. Pull out the old unit. Snap in the new one.

“Wow,” I said to Yorick.

The $20,000 Question
Otologics arranged for me to speak with a Carina user in Hamburg, Germany, a 25-year-old medical student named Veronika Koch. I called her from the company’s conference room, aware that the situation was full of acoustical land mines. A totally deaf American was going to speak to a mostly deaf German through a speakerphone and across a language barrier.

But we understood each other with very little trouble. Veronika said she loved having a fully implanted device: “You don’t have to think about it. That’s the most important thing. When it was turned on, it was one of the most beautiful experiences I ever had. Nothing touching my ear. That natural feeling of hearing–it’s just beautiful.”

I asked her how the Carina sounded. “Sound quality is one of the biggest advantages,” she said. “Speech quality is good and more natural, and music is very beautiful.”

Veronika was clearly pleased. But I knew that Otologics would have given me its star patient. That’s why the FDA puts new products through clinical trials–to get an objective look at their performance. What do the Carina’s clinical trials show?

The device is now in phase II trials. (Phase I trials, conducted on a small group, test for safety; phase II, which involve a slightly larger group, tests for effectiveness; and phase III assesses both safety and effectiveness in a large group.) Results of the phase I trial had been ambiguous. In controlled hearing tests, its 20 subjects had scored somewhat worse with the Carina than they had with their own hearing aids, particularly in their ability to hear soft sounds. In a written survey to measure subjective impressions of the device, on the other hand, subjects had said, consistently, that they heard better with the Carina.

But still, the company wanted to see whether the phase II patients could get better results on the tests. They theorized that if the surgeons put the microphone in a specific spot behind the ear where there were fewer scalp muscles, soft sounds would not be masked so much.

So far, the phase II study has enrolled only 12 of the 70 to 80 users it needs, so its results are preliminary. But Herman ­Jenkins, the primary investigator, told me that the new microphone placement seems to be working. The phase II patients can hear a 3,000-hertz tone (a common frequency in speech) at a volume of 37 decibels, whereas the phase I subjects could hear it only at 55 decibels. This is a significant improvement; 37 decibels is about the ambient sound level of a library, whereas 55 decibels is the approximate level of conversational speech. And nine of the patients got word recognition scores that averaged 82 percent, statistically matching the 84 percent they got with their conventional hearing aids.

It would be a great achievement if an implanted hearing aid could match a conventional one. The question would then become, At four times the cost, is it worth it? Neither a high-end conventional aid, at $5,000, nor the Carina, at $20,000, is covered by insurance. And of course, if the patient got two, the price would double. (Surgeons currently implant the aids in only one ear to minimize risk, but once the devices prove themselves, patients may opt for two.)

Otologics hoped that the military, at any rate, would think it worth the cost. Jim Easter, the company’s director of business development, explained to me that military jets are much louder than they used to be. Ear protection helps only a little; the whole skull vibrates. Pilots and ground crew are going deaf in alarming numbers.

A conventional aid, Easter said, might be okay for a desk jockey, but not for a pilot who has to wear headgear, execute high-G maneuvers, and possibly end up in the water. And not for a crew member who sweats like crazy on a hot flight deck. He thought the military would like a device that went inside the body and stayed there.

And what about me, with my decidedly one-G writer’s life? Could the technology be used in cochlear implants?

The main challenge, Conn said, would be to substitute an electrode array in the inner ear for the piston the Carina uses in the middle ear. It might be possible to create a detachable electrode that would stay in place when the unit needed to be replaced, but that would require maintaining a seal with as many as 30 separate connections. Still, Conn thought it could be done.

On the way home, I thought about the pros and cons of the Carina. Four times the price. Surgery–and not just once, but every five or 10 years. On the other hand, quite possibly better hearing. Being able to hear while swimming, sleeping, and showering. Having a body that looked normal–felt normal. If I were a hearing-aid user, would I do it?

I’d want to see good results over a longer period of time first: the complete FDA testing and findings. I’d want to see patients doing well with the device for a while after it hit the market. And I’d need to have a spare 20 grand lying around.

But given all that, the answer is yes, I probably would.

Michael Chorost is the author of Rebuilt: How Becoming Part Computer Made Me More Human.

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