Compression therapy is a form of treatment for patients who have venous ulcers and other conditions that make it hard for veins to return blood from the lower extremities. Compression bandages wrapped around the limb tightly can stimulate blood flow, but it is hard to tell if a bandage is applying optimal pressure for different conditions.
The MIT-developed bandage uses pressure-sensing photonics fibers created by the reseearchers. The fibers are then woven into a typical compression bandage and as the bandage stretches, the fibers change color. Caregivers can use a color chart to stretch the bandage until it matches the color of a desired pressure before wrapping it for treatment. The photonics fibers are essentially a pressure sensor.
“Getting the pressure right is critical in treating many medical conditions including venous ulcers, which affect several hundred thousand patients in the U.S. each year,” Mathias Kolle, assistant professor of mechanical engineering at MIT, said in a press release. “These fibers can provide information about the pressure that the bandage exerts. We can design them so that for a specific desired pressure, the fibers reflect an easily distinguished color.”
Each of the photonics fibers is about 10 times the diameter of human hair. The fiber is made from ultrathin layers of transparent rubber materials that were rolled up to make a jelly-roll-type structure. Each layer is only a few hundred nanometers thick.
Because of the rolled-up configuration, the light reflects off of each interface between the individual layers. The consistent thickness allows the reflections to interact and strengthen some colors in the visible spectrum. This is what makes the fiber appear as a certain color, depending on how thick the layers of the fiber are.
“Structural color is really neat because you can get brighter, stronger colors than with inks or dyes just by using particular arrangements of transparent materials,” Joseph Sandt, one of the researchers on the project, said. “These colors persist as long as the structure is maintained.”
The design of the fibers also relies on optical interference, which is when light that is reflected from a periodic stack of thin, transparent layers is able to produce vibrant colors that rely on the geometric parameters and material composition of the stack. Optical interference is what makes colorful swirls in soap bubbles and oily puddles and gives peacocks their colors.
“My interest has always been in taking interesting structural elements that lie at the origin of nature’s most dazzling light manipulation strategies, to try recreating and employing them in useful applications,” Kolle said.
Kolle was inspired by work from Pete Vukusic, a professor of biophontonics at the University of Exeter,on a tropical plant that makes shiny blue berries. The skin of the fruits have cells with a periodic cellulose structure that light can reflect off of to give the fruit its metallic blue color. Using that knowledge, Kolle and Vukusic tried to replicate the fruit’s photonics architecture to make a synthetic material.
Together, the two researchers were able to make multilayered fibers from stretchable materials and thought that the fibers would change the thickness of each layer, allowing them to run the colors of the fibers.
Kolle and his group of MIT researchers improved the design of the phontonic fibers to make them from layers of commonly used and widely available transparent rubbers that are wrapped around highly stretchable fiber cores. Sandt then made each layer using spin-coating. He also formed two layers on top of a water-soluble film on a silicon wafer to make the fiber, then submerged the wafer in water to dissolve the water-soluble layer. The end result was two rubbery layers floating on the water’s surface. The final product is carefully rolled around a black rubber fiber to make the final colorful photonics fiber.
“If you want a fiber to go from yellow to green, or blue, we can say, ‘This is how we have to lay out the fiber to give us this kind of [color] trajectory,'” Kolle said. “This is powerful because you might want to have something that reflects red to show a dangerously high strain, or green for ‘ok.’ We have that capacity.”
[Image from MIT]The research team stitched the fibers onto a compression bandage that could be used to determine the pressure the bandage was generating. They tested the new color-changing bandage and found that the bandage woven with photonic fibers gave the clearest pressure feedback when compared to a commercially-available bandage.Researchers are now working on scaling up the fiber fabrication process to make them longer, such as meters or kilometers at a time.
“Currently, the fibers are costly, mostly because of the labor that goes into making them,” Kolle said. “The materials themselves are not worth much. If we could reel out kilometers of these fibers with relatively little work, then they would be dirt cheap.”
The research was published in the journal Advanced Healthcare Materials and was supported in part by the National Science Foundation and the MIT department of mechanical engineering.