Light Where the Sun Don't Shine

Biocompatible optical fibers could deliver light inside human tissue

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An optical fiber made mostly of water could be used to zap tumors with light-activated drugs, continuously monitor a patient’s health, or trigger genetically modified cells with a laser beam, according to the scientist developing the fiber.

“We can reinvent optical components with biologically compatible materials,” says Seok-Hyun (Andy) Yun, an associate professor of biomedical optics at the Wellman Center for Photomedicine at Massachusetts General Hospital and Harvard Medical School. He presented the results of his recent work at the 2015 IEEE Photonics Conference in Reston, Va., earlier this month.

Yun and his colleagues have created a thin optical fiber from a hydrogel made of polyethylene glycol-diacrylate. They pour a solution of the material into a thin, transparent tube then cure it with ultraviolet light. Once they’ve formed the fiber, they coat it with alginate, a natural polymer derived from brown seaweed and commonly used in wound-healing dressings. The coating acts as a cladding similar to that used in typical silica fibers; the difference between the index of refraction of the two materials keeps the light traveling down the fiber instead of escaping through its transparent surface.

The fiber can carry blue-green laser light (wavelengths around 492 nanometers) for about 10 centimeters, a distance sufficient to reach from the surface of the skin to various organs. Visible light can normally only penetrate tissue to depths of a few millimeters, limiting the use of optical techniques to natural cavities such as the gastrointestinal tract.

Because the hydrogel is 80 to 90 percent water and very porous, it’s easy to dope it with various drugs that could be delivered to where they’re needed and then turned on with a beam of light. The fiber, which could carry molecules that would act as sensors for glucose, pH levels, or various biomarkers, would fluoresce in the presence of a target molecule. Yun’s team has even shown that they can infuse the fibers with genetically modified cells that stimulate insulin production in mice when the fibers are hit with a laser beam.

Clinical use of the fiber would probably require approval of the U.S. Food and Drug Administration (FDA), Yun says, but he’s using materials on the FDA’s list of items generally recognized as safe. One of the ingredients, for example, is the same one used to make biodegradable sutures.

Yun says his fiber has two advantages over conventional silica fibers. It’s much more flexible. “You can insert it like a needle into the body, with less worry about fiber breakages,” he says. And it’s biocompatible, so it could be left in for days or weeks with no immune reaction. One use he imagines is long-term monitoring of organ transplants. And even if a fiber were to get stuck, it would eventually be harmlessly absorbed by the body. It’s possible, by manipulating the chemistry of the fiber, to make it biodegrade in less than an hour or have it last for months, says Yun.

                

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