Researchers at Japan's National Institute for Materials Science—or NIMS for short—have taken some inspiration from insects like beetles and butterflies to create a stretchable rubber-like material that changes colour under stress. As the material is stretched, it moves through a spectrum of shimmering colours that can be used to visually identify how much stress is being applied, and when it might be near its breaking point.
The colourful wings of some beetles and butterflies have a similar shimmering appearance to them that's a result of how the surface of their wings reflect light at varying wavelengths at the microscopic level. So Tsutomu Sawada and Hiroshi Fudoji, who work at the NIMS' Photonic Materials Unit, recreated that behaviour with a rubber sheet by infusing it with nanoparticles that were sporadically arranged like crystals so they would reflect light in certain wavelengths.
As the rubber sheet is stretched, it becomes thinner and those embedded particles get closer together changing the wavelength of light they reflect, which in turn results in a different colour appearing. Since very few people are interested in stretchy colour-changing rubber trousers, the researchers at NIMS instead seeing it as being useful for doing easier visual inspections of public buildings. Certain parts of a bridge, for example, could have a layer of the material applied and as it moved and shifted over time the change in colours could be documented to give engineers an idea of just how much stress it has to deal with.