Engineers at MIT have developed pressure-sensing photonic fibers woven into a compression bandage. As the bandage is stretched, the fibers change color. [Image: Courtesy of MIT / Creative Commons]
Compression bandages are commonly used to apply pressure to a patient’s limbs in order to improve blood flow and minimize swelling. Different conditions require different amounts of pressure, but there is currently no method for measuring applied pressure during compression therapy. Now, a team of researchers from the Massachusetts Institute of Technology (MIT), USA, has engineered biomimetic optomechanical fibers that change color in response to strain and could be used as pressure sensors in compression bandages (Adv. Healthc. Mater., doi: 10.1002/adhm.201800293).
Inspired by the structural coloration that creates the vibrant, metallic-blue hues of Margaritaria nobilis berries, the color of the MIT photonic fiber is dependent on how light reflects off of its internal periodic structure. The fiber’s color can be tuned in a predictable way by changing its shape (for example, when it is stretched). The team believes its stretchable optomechanical fibers could someday be stitched into standard compression bandages, making it easier for healthcare providers to deliver the optimal amount of pressure for a patient’s specific condition.
Biomimetic design for optomechanical sensors
The structural colors found in nature are not created with dyes or pigments; instead, they arise from the interference of light scattered at different wavelengths by nanostructures in the material. The interfering light is seen as a color by the viewer—and the color can change depending on the viewing angle or in response to a change in the material’s shape (imagine a swirly, iridescent soap bubble).
The MIT team translated the blueprint for naturally occurring structural colors into a design for pressure-sensing optomechanical fibers. The fibers have a periodic structure consisting of two thin, transparent layers of polydimethylsiloxane (PDMS) and polystyrene-polyisoprene triblock polymer (PSPI), rolled 30 to 60 times around a black PDMS fiber core. (The resulting fiber is about 10 times as thick as a human hair.) The PDMS/PSPI “jelly-roll” acts as a Bragg reflector, strongly reflecting incoming visible light in a narrow range of wavelengths.
The color of the light reflected off the periodically stacked fiber depends on the nanostructure of each of the layers. When the fiber is stretched, the layer period decreases, which changes the color of the fiber from red (no strain) to orange, yellow, green and finally blue (maximum strain). The researchers note that it is possible to design fibers with different reflection peaks, to create a pressure-sensitive fiber that shifts in color from blue to red with increasing strain.
Quantitative tests to measure the fibers’ optomechanical properties showed that they can be stretched to more than twice their length, and after 10,000 stretching cycles, the fibers still produce consistent and highly visible colors.
Demonstrating fiber sensors for pressure detection
The researchers created a short movie clip showing the color-changing fiber in action. [View clip] [MIT press office]
For demonstrations of pressure sensing during compression therapy, the MIT researchers sewed their stretchable, optomechanical fibers along the length of standard compression bandages. They also created a chart that matched the fibers’ color to the amount of pressure generated by the bandage.
About 12 untrained student volunteers were used to test the photonic compression bandage’s effectiveness. The students were given two commonly used compression-bandage systems, as well as the MIT photonic compression bandage and its accompanying color chart. Results from this demonstration showed that students using the photonic compression bandage and the color chart were more likely to apply optimal pressure than when using the other two traditional compression-bandage systems.
While the results of this preliminary study are encouraging, the researchers say that they need to find a more cost-effective way to produce their photonic-fiber-threaded bandages to make them practical for medical textiles.