Engineers use 19th-century photographic techniques to create stretchable, color-changing films

Imagine taking a piece of film and stretching it to reveal a secret message. Or use the hue of a bracelet as a measure of muscle density, or wear a bathing suit that changes color as you do laps in the pool. Now engineers at Massachusetts of Technology (MIT) have revived and repurposed a 19th-century photographic technique in hopes of creating these kinds of chameleon-like, color-changing materials.

The team was able to do this thanks to the photographic techniques invented by Gabriel Lippmann. The Luxembourg inventor invented this method in 1891 to take color photos using a mirror and a special emulsion. When light reflects off the mirror, it leaves an image on the emulsion. In 1908 Lippmann received the Nobel Prize in Physics for this work.

Previously, scientists have studied the microscopic surface structures of iridescent organisms such as mollusk shells, butterfly wings and other iridescent organisms to understand how they reflect light and create the illusion of shimmering and changing colors. These structures are angled and layered to reflect light like miniature colored mirrors or what engineers call Bragg reflectors.

By combining Lippmann’s techniques with modern holographic materials, the MIT group successfully printed large format images onto stretchable materials that change color when stretched, such as They also printed films showing the imprint of objects such as a strawberry, a coin, and a fingerprint.

The team’s findings, published in natural materialsrepresent the first scalable manufacturing technology for producing detailed, large-area materials with “textured color,” where the color is the result of the microscopic structure of the material rather than the addition of chemical additives or dyes.

“The scaling of these materials is non-trivial because you have to control these structures at the nanoscale,” said Benjamin Miller, a graduate student in MIT’s Department of Mechanical Engineering. “Now that we’ve cleared that scaling hurdle, 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 develop touch-enabled devices for things like virtual augmented reality or medical training? It’s a big space that we’re looking at now.”

Miller questioned whether or not the production of large-scale, structurally colored materials could be accelerated by combining Lippmann photography with contemporary holographic materials. Today’s holographic materials, such as Lippmann’s emulsions, are composed of light-sensitive molecules that crosslink to form colored mirrors in response to incident photons.

Miller and co. first placed the sample on sheets of aluminum, which have a surface resembling a mirror, and then projected images onto the sample. Then they removed it and placed it on a black silicone pad to keep it in place. As the material was stretched, the nanoscale structures changed to reflect different wavelengths, some of which are invisible to the human eye.

Intriguingly, by angling the film in relation to the incoming light, they were able to project hidden images alongside the colored mirrors. Because of this tilt, the nanostructures in the material reflected light with a red-shift. Exposed and developed with green light, materials would reflect red; exposed to red light and developed would yield infrared-reflecting structures invisible to the human eye. As the material is stretched, a red image that was previously invisible becomes visible.

The method developed by the group is the first of its kind, making it possible to project intricately colored structural materials on a large scale.

“The beauty of this work is the fact that they have developed a simple but extremely effective method for fabricating large-scale photonic structures,” says Sylvia Vignolini, professor of chemistry and biomaterials at the University of Cambridge, who was not involved in the study. “This technique could be game-changing for coatings and packaging, but also for wearables.”

Researchers are now investigating potential applications beyond fashion and textiles, such as B. Color changing bandages that can be used to monitor bandage pressure levels in the treatment of conditions such as venous ulcers and certain lymphatic disorders.