Strain-induced color changes in biomimetic materials

Taking a clue from jellyfish and squids that quickly alter their appearance via muscle-controlled morphology changes in their bodies’ surface structures, researchers have designed polymeric materials that change appearance reversibly in response to mechanically induced folds and deformations (Nat. Commun. 2016, DOI: 10.1038/ncomms11802). The animals exploit their color-changing abilities for camouflage and safety purposes. Scientists could use the new synthetic materials for those applications or as mechanical sensors, optical switches, and color-changing smart windows. The researchers, led by Luyi Sun of the University of Connecticut, made several types of so-called mechanochromic materials by depositing a transparent rigid film made from polyvinyl alcohol and a silicate compound on a flexible polydimethylsiloxane (PDMS) substrate. Stretching the simple bilayer material by 40% changes its appearance markedly yet reversibly from transparent to opaque. The optical changes result from stretch-induced microscopic cracks and folds that trap and scatter light. The team varied the material design to induce other optical effects. For example, they bonded an ultraviolet-shielding film to PDMS doped with a variety of fluorophores. Stretches and strains as small as 5% caused these materials to quickly change colors or change from nonluminescent to highly luminescent.