When people want camouflage, they change their clothes. When a cuttlefish wants to blend into the background of the seafloor, it changes its skin.
Taking inspiration from the cuttlefish and other animals that can control the pigments and optical properties of their skins for camouflage, scientists at the University of Bristol have created soft, artificial skin and muscles that function similarly.
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The advance could lead to clothing that automatically changes colors to blend in with a background or transfers heat to balance body temperatures. The technology could also help create visual displays that have more color and optical options than today's screens and monitors.
"Artificial muscles are new, soft materials that when you put electricity across them, they move in some way,” said Jonathan Rossiter, a professor of engineering at Bristol with a focus on biomimetics. "They bend or the twist or they get a bit larger or a bit smaller."
To develop the artificial muscles, Rossiter and his colleagues looked at several biological mechanisms responsible for color-shifting in cuttlefish, squid and octopus.
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In cephalopods, like cuttlefish and squid, sacs on the surface of the skin that are filled with dark, granular particles help the animals change color. When the muscles surrounding each sac contract, they stretch the sacs and create much larger dark spots. The color change happens fast, although the area of the color is limited by how far the skin can stretch.
"The cephalopod looks at its environment to decide on an appropriate body pattern," explained Lydia Mäthger, a marine biologist at Woods Hole, who is not a part of the Bristol team.
Mäthger explained that cells in the skin called chromatophores create the brown, red and yellow tones in body patterns, but cephalopods use other cells, like light reflectors, to create greens and blues.
In a separate family, the Zebrafish use a completely different mechanism to change the color of their skin. They have a reservoir of dark liquid several layers under their skin. They squeeze their muscles and that squeezes the liquid up to the surface, changing the color of their skin.
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Using this knowledge, Rossiter and his team designed artificial skin and muscles made from elastic polymers that respond to an electrical stimulus. Taking inspiration from the cuttlefish, they developed artificial cephalopod chromatophores and muscles. The muscles contract when stimulated, stretching out a sac of dark material -- in this case made of carbon grease. When the electric impulse stops, the material relaxes and the dark, chromatophore-like spot shrinks.
In the mechanism modeled on the Zebrafish, dark fluid is held in a reserve. The stimulating electricity squeezes the artificial muscle around the reserve, pushing the fluid to flow into an empty space in the material, creating a superficial color change. When the stimulus stops, the liquid flows back into its reserve.
"The only limit is how much fluid you’ve got and how much of an area at the top you can push the fluid out into," Rossiter said. “The visual effect that you can get from the Zebrafish type of liquid-based chromatophore is really quite large. You can go from really tiny spots to really big areas.”
The downside is that the Zebrafish chromatophore is a much slower mechanism than the cephalopod mechanism because it's built on fluid movement.
“The muscles in the cephalopods can activate much quicker. That’s why cuttlefish can have these really nice patterns emerging across their skin in real time,” Rossiter said.
Rossiter explains than while quick color change would be ideal for an instantly changing camouflage skin, the slower change has its own potential. The fluid movement they designed from the Zebrafish could be used for thermal regulation materials. For example, if the fluid reservoir is near the skin of a hot person or a hot engine, it could be released to the surface of the artificial skin to transfer the heat out and cool the person or engine.
“This is a really cool way of camouflage,” Mäthger said, explaining that this research has only developed the first step in biomimetic camouflage. “They’ve only done this for a few, but in cephalopod skin there are thousands. It’s lots more complicated, but this is a first step. If you want to reproduce cephalopod skin, you don’t just need chromatophores.”
So far, Rossiter’s research team has only built artificial chromatophores with one color, moving from lighter to darker or darker to lighter. However, he explained that there is no reason that they couldn’t use more colors and create more complicated patterns. Unlike the color displays we are used to, that emit light, chromatophores reflect light.
“We are not restricted to this light-emitting stuff. They could be iridescent. They could polarize light. They could do something else with light. The kinds of things we can’t do with TVs and we can’t do with monitors, we should be able to with these,” Rossiter said.