Imagine stretching a piece of film to reveal a hidden message.
Or checking an arm band’s color to gauge muscle mass.
Or sporting a swimsuit that changes hue as you do laps.
Such chameleon-like, color-shifting materials could be on the horizon, thanks to a photographic
technique that’s been resurrected and repurposed by MIT engineers.
By 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 as the material is strained.
The researchers produced stretchy films printed with detailed flower bouquets that morph from
warm to cooler shades when the films are stretched.
They 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 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.
The team’s results appear in the journal Nature Materials.
The research team 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, which appear to shimmer and shift their color due to microscopic
surface structures.
These structures are angled and layered to reflect light like miniature colored mirrors,
or what engineers refer to as Bragg reflectors.
The researchers 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.
An approach that offers both microscale control and scalability remains elusive, despite several
potential high-impact applications.
While puzzling over how to resolve this challenge, researcher Benjamin Miller happened to visit
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.
Milller says that he realized what they do in holography is kind of the same thing that
nature does with structural color.
That visit spurred him to read up on holography and its history, which led him back to the
late 1800s, and 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 prepared from tiny light-sensitive grains.
He exposed the setup to a beam of light, which the mirror reflected back through the emulsion.
The interference of the incoming and outgoing light waves stimulated the emulsion’s grains
to reconfigure their position, like many tiny mirrors, and reflect the pattern and wavelength
of the exposing light.
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.
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.
As they suspected, 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 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.
For instance, green light used during material exposure and development would lead to red
light being reflected, and red light exposure would give structures that reflect infrared
— a wavelength that is not visible to humans.
When the material is stretched, this otherwise invisible image changes color to reveal itself
in red.
The researcher says that we could encode messages in this way.
Overall, the team’s technique is the first to enable large-scale projection of detailed,
structurally colored materials.
This technique could be game-changing for coatings and packaging, and also for wearables.
The researcher notes that the new color-changing materials are easily integrated into textiles.
Beyond fashion and textiles, the team is exploring applications such as color-changing bandages,
for use in monitoring bandage pressure levels when treating conditions such as venous ulcers
and certain lymphatic disorders.