To help doctors determine whether a patient might have experienced a traumatic brain injury, materials scientists have developed a thin polymeric film that changes color when struck with forces similar to those produced during football games or combat. The sensor is basically litmus paper for forces, said Shu Yang of the University of Pennsylvania. Yang and her team designed the sensor as a photonic crystal—a material with ordered nanostructures that interact with light to produce color. They built it by dipping a silicon wafer in a suspension of silica nanoparticles. The particles self-assembled into an ordered crystal on the wafer. The team then added a thermoplastic, allowing the polymer to fill in around the particles and solidify. After dissolving away the silica with acid, the researchers were left with an ordered array of voids where the particles once were. This material starts out orange-red. An applied force compresses these voids, shrinking their widths and shifting the color of the overall material, first to green and then to purple and blue.