Civil engineers may one day be able tell how healthy a building is with a quick visual check. Combining organic materials that react to ultraviolet light with a stretchable, rubber-like material, Princeton researchers have designed a new kind of sensory skin for monitoring structural integrity.

The material, called polydimethylsiloxane (PDMS), is made to be wavy on the micro-scale (one-millionth of a meter). Then it is coated with a layer of organic molecules that give off a particular color of light when a UV beam shines on them. This color is in partly determined by the precise spacing of the micro-ridges in the material — the rows of ridges make a diffraction grating which reflects the light so as to amplify the signal, an effect known as lasing. So, when the material is stretched out due to something like strain or corrosion on the building, the structural change is literally reflected in a color change. Moreover, the the diffraction grating is so small that treated PDMS can show signs of distress at miniscule scales, much smaller than where any crack would be visible.

The prototype isn't quite ready; as of now the organic molecule coating begins to shred off when the material is stretched too thin. But the researchers are hopeful that this type of building skin could one day be a useful technology for civil engineers. And the lack of electrical wiring, fiber optics, or other expensive sensory equipment to achieve the same job makes the new material economically-appealing too. Engineers examining a building might even be able to do so from afar, since all they need to do is shoot a UV light at a surface and measure the emitted color.

"There's really a critical need to develop better sensors that can be applied to infrastructure systems," University of Michigan professor of civil and environmental engineering Jerome Lynch tells MIT's Technology Review. If this all works out, in the future we might really know when buildings are feeling “blue.”