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About 36 million Americans suffer from some type of hearing loss. However, only a fifth of those who could benefit from a hearing aid actually wear one, according to the National Institute on Deafness and Other Communication Disorders. MIT engineer Douglas Hart, SM ‘85, believes that percentage could be boosted if there were a better way to fit hearing aids to people’s ears.

“A lot of people are likely walking around with hearing aids that don’t fit, because they don’t know what they’re supposed to feel like,” says Hart, a professor of mechanical engineering. He has patented a new way of using 3-D imaging technology to make a model of the ear canal–a process that is much faster, easier, and more accurate than the technique used now.

Currently, patients who need a hearing aid usually have to spend about an hour with an audiologist, who fills the patient’s ear canal with a gooey silicone substance. After about 15 minutes, the gel hardens into a mold that is removed from the ear and shipped to a hearing-aid manufacturer, who scans the mold and builds a custom-fitted hearing aid using a 3-D printer.

With this method, it can be difficult to achieve a tight seal between the hearing aid and the patient’s ear canal, because the ear canal moves as the jaw does.

The new MIT system, however, takes this movement into account. A stretchy polymer membrane bearing a tiny fiber-optic camera is inserted into the ear and inflated to take the shape of the ear canal. The balloon­like membrane is then filled with a fluorescent dye, and the camera photographs the dye to record the ear canal’s shape. The entire process takes only a minute or two. Because the camera captures 3-D images so quickly, it can measure how much the surface of the ear canal deforms when the pressure changes and how the canal shape changes when the wearer chews or talks.

The hearing aids made using Hart’s fitting process sit deeper in the ear canal and seal well, greatly improving sound quality, battery life, and comfort. The accurate fit also prevents feedback between the microphone and the receiver, which can produce squealing sounds that annoy the wearer and anyone standing nearby.

The new technology follows a recently commercialized 3-D scanning system that Hart developed to replace the silicone molds traditionally used to make impressions for dental crowns and bridges. While he was working on that system, hearing-aid manufacturers approached him to see what he could do to improve their fitting process.

Hart patented the hearing-aid fitting system in January and has founded a company called Lantos Technologies in the hope of bringing the innovation to market. He plans to market the technology to hearing-aid manufacturers first, but he believes it could also be useful to build fitted earphones for MP3 music players or earplugs for military personnel and other people who work in noisy environments.

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Credit: Federico Frigerio

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