Engineers Reuse 19th-Century Photographic Technique to Create Color-Changing Stretch Films | MIT News

Imagine stretching a piece of film to reveal a hidden message. Or check the color of an armband to assess muscle mass. Or wear a swimsuit that changes color as you do tricks. Such chameleon-like, color-changing materials could be on the horizon, thanks to a photographic technique that has been resurrected and repurposed by engineers at MIT.

Applying a 19th century color photography technique to modern holographic materials, an MIT team has printed large-scale images onto elastic materials that, when stretched, can transform their color, reflecting different wavelengths when the material is stretched.

Researchers have produced stretch films printed with detailed flower bouquets that change from warm hues 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’s results provide the first scalable manufacturing technique to produce large-scale, detailed materials with “structural color” – color resulting from a material’s microscopic structure, rather than chemical additives or colorants.

“Scaling up these materials is not trivial, because you have to control these structures at the nanoscale,” says Benjamin Miller, a graduate student in MIT’s Department of Mechanical Engineering. “Now that we’ve cleared this scaling hurdle, we can explore questions like: can we use this material to create robotic skin that has a human-like sense of touch? Are we creating touch-enabled devices for things like virtual augmented reality or medical training? That’s a big space we’re looking at now.

The team’s results appear today in Natural materials. Miller’s co-authors are MIT undergraduate student Helen Liu and Mathias Kolle, associate professor of mechanical engineering at MIT.

Hologram Random

Kolle’s group develops optical materials inspired by nature. Researchers have studied the properties of light reflection in mollusc shells, butterfly wings and other iridescent organisms, which appear to shimmer and change color due to microscopic surface structures. These structures are angled and layered to reflect light like miniature colored mirrors, or what engineers call Bragg reflectors.

Groups, including that of Kolle, have sought to replicate this natural, structural color in materials using a variety of techniques. Some efforts have produced small samples with precise nanoscale structures, while others have generated larger samples, but with less optical precision.

As the team writes, “an approach that offers both [microscale control and scalability] remains elusive, despite several potential high-impact applications.

As he pondered how to solve this challenge, Miller visited the MIT museum, where a curator told him about an exhibit on holography, a technique that produces three-dimensional images by superimposing two light beams onto a physical material.

“I realized what they’re doing in holography is kind of the same thing nature is doing with structural color, Miller says.

This visit prompted him to learn about holography and its history, which took him back to the late 1800s, and about Lippmann photography – an early color photography technique invented by the French physicist- Luxembourgian Gabriel Lippmann, who went on to win the Nobel Prize in Physics for the technique.

Lippmann generated color photos by first placing a mirror behind a very thin transparent emulsion – a material he concocted from tiny light-sensitive grains. He exposed the configuration to a beam of light, which the mirror reflected back through the emulsion. The interference of incoming and outgoing light waves stimulated the grains of the emulsion to reconfigure their position, like many tiny mirrors, and to reflect the pattern and wavelength of the exposure light.

Using this technique, Lippmann projected structurally colored images of flowers and other scenes onto his emulsions, although the process was laborious. This involved making the emulsions by hand and waiting days for the material to be sufficiently exposed to light. Due to these limitations, the technique has largely faded into history.

A modern touch

Miller wondered if, combined with modern holographic materials, Lippmann’s photography could be accelerated to produce large-scale, structurally colored materials. Like Lippmann emulsions, today’s holographic materials are made up 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 reactive that it’s possible to do this technique on a short timescale just with a projector,” Kolle notes.

In their new study, the team glued an elastic, transparent holographic film to a mirror-like reflective surface (in this case, aluminum foil). The researchers then placed a standard projector several feet away from the film and projected images onto each sample, including Lippman-style bouquets.

As they suspected, the films produced large, detailed images in minutes, rather than days, vividly reproducing the colors of the original images.

They then peeled the film off the mirror and glued it to a black elastic silicone backing to support it. They stretched the film and observed the change in colors – a consequence of the structural color of the material: as the material stretches and thins, its nanoscale structures reconfigure to reflect slightly different wavelengths , for example by changing from red to blue.

The team discovered that the color of the film is very sensitive to voltage. After making an entirely red film, they glued it to a silicone support of variable thickness. Where the backing was thinnest, the film remained red, while the thicker sections stretched the film, causing it to turn blue.

Likewise, they found that squeezing various objects into samples of red film left detailed green imprints, caused, for example, by the seeds of a strawberry and the wrinkles of a fingerprint.

Interestingly, they could also project hidden images, tilting the film at an angle 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. For example, green light used during material exposure and development would lead to reflection of red light, and exposure to red light would result in structures that reflect infrared – a wavelength that does not is not visible to humans. When the material is stretched, this otherwise invisible image changes color to reveal itself as red.

“You could encode messages this way,” Kolle explains.

Overall, the team’s technique is the first to allow large-scale projection of detailed, structurally colored materials.

“The beauty of this work is that they have developed a simple but extremely efficient way to produce large-area photonic structures,” says Sylvia Vignolini, professor of chemistry and biomaterials at the University of Cambridge, who n was not involved in the study. “This technique could be a game-changer for coatings and packaging, as well as clothing.”

Indeed, Kolle notes that the new color-changing materials are easily integrated into textiles.

“Lippmann’s materials wouldn’t even allow him to produce a Speedo,” he says. “Now we could do a full leotard.”

Beyond fashion and textiles, the team is exploring applications such as color-changing bandages, to be used to monitor bandage pressure levels when treating conditions such as venous ulcers and certain lymphatic disorders. .

This research was supported, in part, by the Gillian Reny Stepping Strong Center for Trauma Innovation at Brigham and Women’s Hospital, the National Science Foundation, the MIT Deshpande Center for Technological Innovation, Samsung, and the MIT ME MathWorks Seed Fund.

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