Scientific Method / Science & Exploration

Researchers make color-shifting surface modeled on squid

Devices that parallel biological materials make for one-second color changes.


Materials scientists may have many clever tricks up their sleeves, but with a several-billion-year head start, evolution has already beaten them to the creation of some amazing materials. One of the most astonishing natural systems is the skin of cephalopods such as squid, octopus, and cuttlefish. These animals can sense their background and control tiny structures that can radically change their skin color in a matter of seconds, allowing amazing feats of camouflage.

Now, an international consortium of researchers has decided that if you can't beat nature, you can at least copy it. The researchers have developed a flexible structure that can sense ambient conditions and adjust its color to match them. At the moment, it only works in black and white, but it's a start.

The color-shifting structures in the skin of cephalopods, termed chromatophores, rely on three layers to rapidly alter coloration. The top is a pigmented structure with muscles that allow it to expand and contract, altering the amount of light of different wavelengths that get absorbed. Below that is a reflective layer, which can also be actively altered by the nervous system. Below that is a passive white pigment that reflects incidental light. By varying the behavior of the two overlying layers, the animals are able to radically alter their visual appearance.

To mimic this system, the researchers needed to add a couple more layers, but they followed the same principles. The top layer has a polymer with a dye embedded in it. A relative of chemicals called fluorans, the dye reversibly changes color based on the temperature: above 47 degrees Celsius and it's black, below that and it's transparent. Just below that, the researchers placed a layer of silver, which acts similarly to the reflective layer in squid.

The use of silver allows this layer to transmit heat to the dye from a heating element that lies below it, which provides control over whether the dye is clear or black. The whole stack is created as a square with the corners notched out; photodiodes in these corners allow the device to sense ambient light, either coming from above the device or below it.

Enlarge / The authors, many of whom are at the University of Illinois, show off the device's ability with their school's abbreviation.

The authors could embed arrays of these stacks in a polymer. All the component materials were thin and flexible—even the silicon heating element was made ultrathin so it could tolerate being bent—allowing the array to be flexible. The polymer also kept the heating element from altering the coloring of neighboring elements.

The end result is a flexible surface that, depending on the lighting conditions, can change the pattern of light and dark pixels on its surface without the intervention of any control logic. All it needs is power for its individual components.

The authors showed that the surface is able to switch the colors of individual pixels in as little as a second, and when placed over grids with various light patterns shining through them, it would modify its coloration to match them. As mentioned, your average cuttlefish still easily outperforms this system in terms of the diversity of colors it can create. But the cuttlefish is a bit slower, relies on a nervous system to control its coloration, and has the advantage of millions of years of fine-tuning. The authors will undoubtedly produce an improved version of their device in just a few years.

PNAS, 2014. DOI: 10.1073/pnas.1410494111  (About DOIs).

Expand full story

You must to comment.