The forces from an explosive blast or a head-on tackle can shake a person’s head, causing damage that might trigger long-term neurodegeneration. To help doctors determine whether a patient might suffer from this type of traumatic brain injury, materials scientists at the University of Pennsylvania have developed a thin polymeric film that changes color when struck with forces similar to those produced during football games or combat. The material is lightweight and doesn’t require any power to function, so the developers think it could be affixed to helmets without burdening football players or military personnel as they work. To design the sensor, Shu Yang and her team relied on their experience with photonic crystals - materials with ordered nanostructures that interact with light to produce color. The researchers thought that such a crystal could serve as a force sensor, because a collision would compress these nanostructures, changing their shapes and, in turn, their color.