Pressure-Sensing Fibers Change Color When Stretched

The new material could make compression bandages more effective and easier to use

Animated gif of the color-changing fiber.
Gif: MIT/Univeristy of Tokyo/U.S. Air Force Research Laboratory/Harvard University
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There’s something special about the sheen of rainbow colors on oil droplets spattered on wet pavement, or on the surface of soap bubbles rising in the air.

Those colorful swirls are not the result of pigment or dye, but of light reflecting off the interfaces between the layers of water, oil or soap, and air. That effect, called “optical interference,” is also what makes the feathers of peacocks and wings of butterflies so dazzling.

Now, a team at the Massachusetts Institute of Technology has harnessed this phenomenon to produce cool, color-changing fibers that respond to pressure. The new material, the creators say, could be used to measure force exerted on large and small areas: This could be myriad things including air pushing on an airplane’s wings or cells exerting force on their environment. But to start, the team is testing the fibers in compression bandages, the medical wrappings that help stimulate healing in certain conditions such as venous ulcers.

“I was surprised, when we talked to doctors, that there is not a [convenient tool] to tell you what the pressure is underneath these bandages,” says study author Mathias Kolle, an assistant professor in mechanical engineering at MIT. “We thought there might be a direct link between fiber color and bandage pressure if we incorporated fibers into bandages, and that worked really well.” In a small proof-of-principle study, the color of the fibers accurately predicted the amount of pressure a bandage applied to the skin beneath it.

Like other smart bandages or e-skin, the material is flexible and sensitive, yet it stands apart because it requires no electronic components or power source.

For now, the thin fibers—each about 10 times the width of a human hair—are only 6 inches long and require a fairly intensive manufacturing process. First, the researchers dissolve commonly used transparent rubbers in solvents, then pour them onto a spinning wheel. The wheel flings off most of the solution, leaving behind a thin coating. Individual coating layers are then stacked and rolled up, like a Little Debbie Swiss Roll. Light reflects off the interfaces between the layers and interacts to create colors in the visible spectrum.

With a bit of calculation, the team can change the thickness of the layers to produce whatever color tuning they desire: yellow to green, or orange to blue, for example. They can also directly correlate colors with the amount of strain, or pressure, on the fiber.

The color is bright—achieving about 95 percent reflection, with just a bit of scattering, says Kolle—and does not fade. That is, as long as you don’t scratch it: The team is working to add a protective coating that doesn’t interfere with the optical effect but protects the layers from damage.

To put the material to use, the team stitched the fibers into standard compression bandages, created a color chart indicating how hues correspond with pressures, and asked a dozen student volunteers to apply the bandage to each other’s legs. Students applied the photonic fiber bandage with more accurate pressure than they could with a plain bandage or with a commercially available bandage employing a different sensing mechanism, the authors report.

Next, the team hopes to produce longer fibers in sufficient quantities to test their bandages in clinical trials at a local hospital.

Oh, and did we mention that athletic apparel manufacturers have expressed interest in the new material? Kolle says that with a long enough spool of the fibers, it should be feasible to make color-changing sneakers, or t-shirts that gauge muscle strain during workouts. I can see the advertising slogan now: Wear the rainbow.

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