In the Future, Your City Could Change Colors Like an Octopus
Buildings cloaked in color-changing skins are close to reality.
Image: David Robert Bliwas / Flickr
Most of us were born and will die a certain color, but octopuses are masters of their hue, changing from transparent to shades of red, pink, purple and blue by stretching and relaxing their skin. If we could unlock their secret and wrap our buildings in octopus skin, then city skylines might shimmer a spectrum of colors and opacities as the sun waxed and waned.
"Octopus skin can switch from transparent to opaque naturally. Our interest has been to figure out how, and to reproduce the effect in a simple and cost effective way," Shu Yang, a materials scientist at the University of Pennsylvania, told me.
Octopuses evolved their color-changing ability for camouflage. But Yang believes the same trick could revolutionize energy efficiency in buildings, allowing structures to heat up or cool off by altering their transparency according to the ambient temperature and sunlight. That's the motivation behind Yang's latest invention: A synthetic skin that, like the octopus, changes both color and opacity when stretched.
"If temperatures are low, we'd like buildings that are transparent so they can harness more sunlight," Yang told me. "But if the sunlight is strong, we'd like those buildings to become opaque or reflective. The octopus offered us a template."
The secret to octopus skin has to do with the unusual way in which the animals produce color. While most creatures get their color from pigments—molecules that absorb certain wavelengths of light while reflecting others—color can also be built by bending and scattering light within molecular lattices. So-called "structural color" is the basis for some of nature's most dazzling displays, including iridescent butterfly wings and shimmering peacock feathers. Unlike pigment-based colors, which always reflect the same wavelengths of light, structural colors can be tuned by modifying the nanoscale architecture that creates them.
Cephalopods are masters of disguise, often harnessing both structural color and pigments to modify their cloak based on the brightness of their environment. Take Japetella heathi, a small octopus that lightens or darkens in a blink to avoid predators. When sunlight streams in from above, it passes through creature's skin unimpeded, rendering the octopus transparent. When a predator shines a bioluminescent searchlight on them, these animals will stretch out, enlarging pigmented proteins and rendering themselves opaque.
Video of Japetella heathi changing color
Yang, a prolific innovator with over 35 patents, often draws inspiration from nature when designing new materials. Last year, her lab designed and fabricated a spray coating that mimics the iridescence and water-repellant properties of butterfly wings. Octopus skin has been her latest challenge.
"People have fabricated color-changing materials before, but most of these systems use expensive liquid crystal polymers or electrochromic materials that require electricity to operate and are complicated to implement," Yang told me. "What we're talking about is making skin-like materials that can switch from transparent to colored just by stretching, which requires little energy."
After several years of experimenting, the Yang lab has apparently accomplished exactly that. Their synthetic octopus skin consists of a stretchy, rubbery film coated in light-bending silica nanoparticles, similar to those used in the cosmetics industry. In its "resting" state, the material is highly transparent. But when stretched, cracks and wrinkles appear in the nanoparticle coating, diffusing light and rendering the skin opaque.
What's more, the specific wavelength of light reflected can be tuned by changing the nanoparticle size, Yang says. This offers the aesthetic advantage that skins can take on any color under the sun, but it also gives the designer additional control when it comes to thermal regulation. For instance, a material that can be adjusted to reflect infrared light—the heat radiation emitted by everything from people to buildings to the Earth itself—may be able to keep its cool naturally, even on the hottest summer days.
Yang, who recently filed a patent for the material, hopes that her stretchy, colorful skin will soon makes its way onto buildings across the world. She imagines that the building skin could be applied to windows and manually adjusted like blinds, darkening and coloring itself at the pull of a switch. A more energy-intensive option would be to couple the building's skin with sensors that monitor changes in light intensity and cause it to stretch automatically.
Beyond decorating buildings, synthetic octopus skin might be used for displays, allowing screens to change color depending on the ambient light. The military might be interested in the skins for camouflage. There may even be security applications, as Yang and her team demonstrated by embedding the letters "P-E-N-N" onto a sample of the stuff. The letters, only visible when the skin was stretched, suggest an entirely new way to store private information.
The ability to change one's color has evolved to serve myriad purposes in nature. Likewise, the applications for synthetic octopus skin appear to be only as limited as our imaginations. But certainly, that the future is looking pretty colorful right now.