Fish cells inspire color-changing clothes

British researchers have mimicked the camouflaging abilities of creatures such as squid and zebrafish, in research that could help soldiers stay out of sight.

A University of Bristol team has created artificial muscles that can change color at the flick of a switch, and which could in future be used in ‘smart clothing’.

“We have taken inspiration from nature’s designs and exploited the same methods to turn our artificial muscles into striking visual effects,” says Dr Jonathan Rossiter of the Department of Engineering Mathematics.

The soft, stretchy, artificial muscles are based on specialist, pigment-containing cells called chromatophores that are found in amphibians, fish, reptiles and cephalopods.

The team created two types of artificial chromatophores: one based on a mechanism adopted by a squid and the second on a rather different method used by zebrafish.  

A typical color-changing cell in a squid has a central sac containing granules of pigment and surrounded by a series of muscles. When the cell is ready to change color, the brain sends a signal to the muscles and they contract. This makes the central sacs expand, generating the optical effect which makes the squid appear to change color.

The team copied this effect using dielectric elastomers (DEs) – smart materials, usually made of a polymer, which are connected to an electric circuit and expand when a voltage is applied. They return to their original shape when they are short-circuited.

Mimicking the zebrafish took a different technique. Zebrafish cells contain a small reservoir of black fluid that, when activated, travels to the skin surface and spreads out, broadening the natural dark spots on the fish’s skin.

The zebrafish cells were mimicked using two glass microscope slides sandwiching a silicone layer. Two pumps, made from flexible DEs, were positioned on both sides of the slide and were connected to the central system with silicone tubes; one pumping opaque white spirit, the other a mixture of black ink and water.

“Our artificial chromatophores are both scalable and adaptable and can be made into an artificial compliant skin which can stretch and deform, yet still operate effectively,” says Rossiter.

“This means they can be used in many environments where conventional ‘hard’ technologies would be dangerous, for example at the physical interface with humans, such as smart clothing.”