CHAMELEONS ARE famous for their ability to change colour. But their reputation exceeds the reality. Chameleons can not, in fact, take on any colour they touch. And although their colour-shifting abilities are useful for camouflage, they are used mostly to regulate body temperature (darker shades absorb more energy from the sun), or to communicate with other chameleons (brighter colours seem to signify aggression).
Nonetheless, chameleons have inspired Hyeonseok Kim and his colleagues at Seoul National University to create a new kind of artificial camouflage. In a paper published in Nature Communications, Dr Kim describes how their “artificial chameleon skin” can detect the colour of the environment in which it is placed, and mimic it to blend in.
Real chameleon skin is made up of two layers of specialised cells called iridophores, which are filled with pigments and tiny crystals made of guanine, an organic chemical that also serves as one of the four letters of DNA. Their colour-shifting capabilities are mechanical, rather than chemical. Relaxing or exciting the skin changes the distances between neighbouring crystals. That affects the wavelength—and therefore the colour—of the light that is reflected.
The artificial stuff, by contrast, relies on a layer of thermochromic liquid crystals to perform its magic. As the name suggests, thermochromic materials change colour depending on their temperature. Like real chameleon skin, it is the structural arrangement of the crystals that determine the colour of light which is reflected.
By itself, that is not much use for camouflage, since the crystals would simply take on the temperature of their surroundings. Dr Kim’s team have therefore set the crystals within a three-dimensional lattice of tiny silver wires that act as heaters. The wires, in turn, are connected to sensors that detect the background colour. These signal to the wires to turn on or off, heating or cooling the crystals and producing the desired colour. The wires take longer to cool down than to heat up, meaning colour changes that require cooling take longer. Still, the system works: the colour changes take around half a second on average, appearing almost instantaneous to the human eye. The resulting skin is thin, flexible and sturdy, meaning it can be applied to almost any object, or even worn as clothing.
Dr Kim hopes that his team’s invention could find uses from camouflaging soldiers to creating striking, colour-changing architecture. For now, though, the technology remains in its infancy. One priority is to improve the artificial skin’s ability to match complex patterns and textures. That would allow objects to fade from view in front of more than just monochromatic expanses of colour. The use of a temperature-driven colour system also raises questions about how to hide from infrared sensors, which are common in modern armies. Whether this invention becomes the next big thing in artificial camouflage remains to be seen—though, if it does work, perhaps it will not be seen at all.