Stealthy sea creatures inspire color-changing system

Many animals have the ability to change their appearance so that they can blend in with their environment and hide, or make themselves more visible. NCCR Bio-Inspired Materials Principal Investigator Professor Marco Lattuada and his group at the University of Fribourg are aiming to create materials that replicate this ability. Their project focuses specifically on the capacities of squids and octopuses, which can rapidly change the colors of their skin.

Of course, in order to mimic, one must first understand the principles and processes underlying the stimuli-responsive color-changing systems in the animals. Special cells called chromatophores play a central role in the color-changing systems of squids and octopuses. These cells contain color pigments in small pockets located in the center of the cells. The pockets are controlled by muscular contractions, and the animals are able to change their size, so that the colored pigments can be spread from a small point to a larger area. Because the skin contains cells with pigments of many different colors, the animals are able to change into an array of hues, rather than simply turning one color on and off.

Lattuada’s project seeks to develop materials that replicate this mechanism, or in other words, to create cells capable of changing the area occupied by colors. To achieve this, researchers dispersed colored magnetic nanoparticles in small droplets of water. In the absence of an external stimulus, the particles were homogeneously distributed in the droplets, giving them a brown appearance. When a magnet was placed close to the droplets, however, it attracted the particles, which accumulated on the side of the droplet nearer to the magnet. Much like in the natural model, the remaining portion of the droplets became colorless. While the size of the droplets remained the same, as in the chromatophores of squids and octopuses, the area occupied by the colored particles changed.

The next step was to embed these droplets in a polymer matrix. While the water droplets were fixed in their position, the magnetic color particles were still able to circulate throughout the water droplets. The scientists succeeded in making a colored polymer material that, when a magnet was approached, became clearer. “The material goes from dark to clear using a simple mechanism,” sums up Lattuada, adding, “because of its simplicity, it could be used in a variety of ways.”

Next, Lattuada hopes to extend this mechanism using different colorants in water, so that the material could change from dark to blue, for instance. He also states that “one could imagine doing this with other kinds of stimuli, such as electric fields, so that you have an electric activated system.” Further in the future, a big step would be to replicate the rest of the behavior of the animals – that is, the ability to not only change color, but to do so in response to the coloring of the immediate environment. Such a system, however, requires a much more sophisticated approach, as it would be necessary to first detect the other color before imitating it.

Possible applications for these color-changing materials include practical and aesthetic ones. They could be integrated into materials and used as a safety feature by recognizing properties of a system and signaling these with a change in color, for example in paper money. On a completely different note, it is also easy to imagine how such a technology could be used simply for the enjoyment of it. “Having something that can visually change color in response to stimuli could be a lot of fun in fashion, for example,” concludes Lattuada.

Reference: Isapour, G.; Lattuada, M. Bioinspired stimuli‐responsive color‐changing systems, Advanced  Materials, 2018, 30, 1707069