Biological camouflage

Camouflage is a method of concealment that allows otherwise visible animals, military vehicles, or other objects to remain unnoticed by blending with their environment. Examples include a leopard's spotted coat, the battledress of a modern soldier and a leaf-mimic butterfly. Researchers from the University of Bristol have created artificial muscles that can be transformed at the flick of a switch to mimic the remarkable camouflaging abilities of organisms such as the squid and zebrafish. They demonstrate two individual transforming mechanisms that they believe could be used in smart clothing to trigger camouflaging tricks similar to those seen in nature. The study was published May 2 in IOP Publishing’s journal Bioinspiration and Biomimetics, and is accompanied by a video (www.youtube.com/watch?v=W2CgtJU3ckY) showing the camouflaging in action. "We have taken inspiration from nature's designs and exploited the same methods to turn our artificial muscles into striking visual effects," said lead author of the study Dr Jonathan Rossiter, Senior Lecturer in the Department of Engineering Mathematics. The soft, stretchy, artificial muscles are based on specialist cells called chromatophores that are found in amphibians, fish, reptiles and cephalopods, and contain pigments of colors that are responsible for the animals’ remarkable color-changing effects. The color changes in these organisms can be triggered by changes in mood, temperature, stress or something visible in the environment, and can be used for camouflage, communication or attracting a mate. Two types of artificial chromatophores were created in the study: the first based on a mechanism adopted by a squid and the second based on a rather different mechanism adopted by zebrafish. A typical color-changing cell in a squid has a central sac containing granules of pigment. The sac is surrounded by a series of muscles and when the cell is ready to change color, the brain sends a signal to the muscles and they contract. The contracting muscles make the central sacs expand, generating the optical effect which makes the squid look like it is changing color. The skin is covered in these cells, which enable the squid to change color to suit its surroundings, making it practically invisible. The fast expansion of these muscles was mimicked using dielectric elastomers (DE) 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. In contrast, the cells in the zebrafish contain a small reservoir of black pigmented fluid that, when activated, travels to the skin surface and spreads out, much like the spilling of black ink. The natural dark spots on the surface of the zebrafish therefore appear to get bigger and the desired optical effect is achieved. The changes are usually driven by hormones. 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.