Chameleons are well known for their abilities to change colors to camouflage themselves, communicate, and control their temperature.
Drawing inspiration from chameleon skin, researchers have developed a flexible film that changes color in response to stretching, pressure, or humidity. Video Credit: American Chemical Society.
Although researchers have made attempts to reproduce these color-changing properties for anti-counterfeiting measures, electronic displays, and stealth technologies, the developed materials have restrictions.
Currently, scientists have designed a flexible film that modifies color in reaction to humidity, pressure or stretching. The findings of the study have been reported in ACS Applied Materials & Interfaces.
Chameleons can alter the way light gets reflected from guanine crystals under the surface by relaxing or tensing their skin. Thus, they produced the so-called structural coloration. Such structural colors are distinct from the pigments that provide several other creatures their shades.
Researchers have simulated the crystalline nanostructures of chameleon skin in different color-changing materials, but they are generally hard to make or based on non-renewable petroleum resources. On the other hand, cellulose nanocrystals are renewable and have the ability to self-assemble into a film with iridescent structural colors.
But these films are normally delicate and, unlike chameleon skin, cannot be expanded without breaking. Fei Song, Yu-Zhong Wang, and their collaborators wished to design a highly flexible film using cellulose nanocrystals that changes color upon stretching.
The team added a polymer known as PEGDA to enhance the cellulose nanocrystals’ flexibility and utilized UV light to crosslink the polymer to the rod-shaped nanocrystals, thus synthesizing films with bright iridescent colors varying from blue to red, based on the amount of PEGDA.
The films were flexible and strong, extending up to 39% of their actual length before they finally broke. When stretched, the color of one film slowly shifted from red to green, and then again changed back when relaxed.
The researchers believe that this is the first time stretching- and relaxing-induced, reversible structural color variations—bright and visible to the naked eye—have been achieved for cellulose nanocrystal materials.
Moreover, the film also changed its color with humidity and pressure, thus enabling the researchers to display or hide writing created by an inkless pen. The new bio-based smart skin could be used in encryption, strain sensing, and anti-counterfeiting measures.
This study was financially supported by the National Natural Science Foundation of China, the Science and Technology Fund for Distinguished Young Scholars of Sichuan Province, the State Key Laboratory of Polymer Materials Engineering, and the Fundamental Research Funds for the Central Universities.
Zhang, Z.-L., et al. (2020) Chameleon-Inspired Variable Coloration Enabled by a Highly Flexible Photonic Cellulose Film. ACS Applied Materials & Interfaces. doi.org/10.1021/acsami.0c13551.