04 Aug 2022
Technique opens route to pressure-monitoring bandages, shade-shifting fabrics, and touch-sensing robots.MIT (Massachusetts Institute of Technology) has printed large-scale images onto elastic materials that when stretched can transform their color, reflecting different wavelengths as the material is strained.
The researchers have produced stretchy films printed with detailed flower bouquets that morph from warm to cooler shades when the films are stretched. They have also printed films that reveal the imprint of objects such as a strawberry, a coin, and a fingerprint.
The team says its results “provide the first scalable manufacturing technique for producing detailed, large-scale materials with structural color” — color that arises as a consequence of a material’s microscopic structure, rather than from chemical additives or dyes.
“Scaling these materials is not trivial, because you need to control these structures at the nanoscale,” says Benjamin Miller, a graduate student in MIT’s Department of Mechanical Engineering. “Now we can explore questions like: can we use this material to make robotic skin that has a human-like sense of touch? And can we create touch-sensing devices for things like virtual augmented reality or medical training?”
The work is described in Nature Materials.
Miller’s co-authors are MIT undergraduate Helen Liu, and Mathias Kolle, associate professor of mechanical engineering at MIT. Kolle’s group develops optical materials that are inspired by nature. The researchers have studied the light-reflecting properties in mollusc shells, butterfly wings, and other iridescent organisms. These structures are angled and layered to reflect light like miniature colored mirrors, or Bragg reflectors.
The Nature Materials paper states, “an approach that offers both [microscale control and scalability] remains elusive, despite several potential high-impact applications.”
Coincidentally, Miller had visited the MIT Museum, where a curator talked him through an exhibit on holography, a technique that produces three-dimensional images by superimposing two light beams onto a physical material. “I realized what they do in holography is kind of the same thing that nature does with structural color,” said Miller. That visit inspired him to research holography and its history, including Lippmann photography, an early color photography technique invented by Franco-Luxembourgish physicist Gabriel Lippmann, who later won the Nobel Prize in Physics for the technique.
Lippmann generated color photos by first setting a mirror behind a very thin, transparent emulsion — a material that he concocted from tiny light-sensitive grains. He exposed the setup to a beam of light, which the mirror reflected back through the emulsion. Using this technique, Lippmann projected structurally colored images of flowers and other scenes onto his emulsions, though the process was laborious. It involved hand-crafting the emulsions and waiting for days for the material to be sufficiently exposed to light. Because of these limitations, the technique largely faded into history.
Miller wondered if, paired with modern, holographic materials, Lippmann photography could be sped up to produce large-scale, structurally colored materials. Like Lippmann’s emulsions, current holographic materials consist of light-sensitive molecules that, when exposed to incoming photons, can cross-link to form colored mirrors.
“The chemistries of these modern holographic materials are now so responsive that it’s possible to do this technique on a short timescale simply with a projector,” said Kolle.
In their new study, the team adhered elastic, transparent holographic film onto a reflective, mirror-like surface (in this case, a sheet of aluminum). The researchers then placed an off-the-shelf projector several feet from the film and projected images onto each sample, including Lippman-esque bouquets.
The films produced large, detailed images within several minutes, rather than days, vividly reproducing the colors in the original images. They then peeled the film away from the mirror and stuck it to a black elastic silicone backing for support.
They stretched the film and observed the colors change — a consequence of the material’s structural color: When the material stretches and thins out, its its nanoscale structures reconfigure to reflect slightly different wavelengths, for instance, changing from red to blue.
The team found the film’s color is highly sensitive to strain. After producing an entirely red film, they adhered it to a silicone backing that varied in thickness. Where the backing was thinnest, the film remained red, whereas thicker sections strained the film, causing it to turn blue. Similarly, they found that pressing various objects into samples of red film left detailed green imprints, caused by, say, the seeds of a strawberry and the wrinkles of a fingerprint.
Interestingly, they could also project hidden images, by tilting the film at an angle with respect to the incoming light when creating the colored mirrors. This tilt essentially caused the material’s nanostructures to reflect a red-shifted spectrum of light.
Overall, the MIT team’s technique is the first to enable large-scale projection of detailed, structurally colored materials. “The beauty of this work is the fact that they have developed a simple yet extremely effective way to produce large-area photonic structures,” said Sylvia Vignolini, professor of chemistry and bio-materials at the University of Cambridge, who was not involved in the study.
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