The molecules normally emit visible red light, but when the rubber surface is stretched or compressed, it alters the color of light that is emitted. By stretching the rubber 2.2 percent of its length, the researchers could change the color of light. A light detector would notice a difference of about five nanometers between the starting and ending wavelengths of emitted light. This could correlate to tiny changes in strain within a structure, explains Wagner. “It’s highly sensitive, and that’s the advantage,” he says. “In many cases, structural or civil engineers would like to see incipient failure, not a visible crack; and they’d like to have a sensor capable of that sensitive measurement.”
Optically pumping the stretchable laser skin could be an advantage for the system. It could reduce the cost of installation, because it wouldn’t require wires. It would also mean that an engineer could stand at a distance from a structure, shine ultraviolet light onto the surface of the sensing skin to detect tiny changes in strain.
The concept could “fill a critical niche in structural health,” says Lynch. “The approach seems novel, and it’s interesting what kind of results the technology could yield when deployed in the real world.” Lynch is developing large-area sensing skins that rely on layers of carbon nanotubes and other organic molecules to sense strain, cracking, and corrosion, among other defects.
Wagner says that his prototype still needs to be fine-tuned. While the PDMS sheets can stretch a great distance, the organic layers sheer off when they’re extended too far. Fixing this problem will likely come down to testing different types of light-emitting molecules and finding a way to better affix them to the PDMS. “We know the experiments to do,” he says. “We just haven’t found the magic recipe yet.”