Researchers at Princeton University have built a new type of sensor that could help engineers quickly assess the health of a building or bridge. The sensor is an organic laser, deposited on a sheet of rubber: when it's stretched—by the formation of a crack, for instance—the color of light it emits changes.
Making waves: An atomic force microscopy image shows ripples on the surface of a rubbery material called PDMS. The ripples affect the color of light emitted when the material is stretched.
"The idea came from the notion that perhaps it's possible to cover large structures like bridges with a skin that you can use to detect deformation of the structure from a distance," says Sigurd Wagner, professor of electrical engineering at Princeton University, who developed the stretchable laser sensor with and Patrick Görrn, a researcher at Princeton. The work was published last month in Advanced Materials.
For more than a decade, researchers have explored ways to make dense arrays of sensors capable of covering large areas. Sensing skins are especially intriguing to civil engineers, who know the importance of detecting damage in infrastructure so that disasters like the 2007 collapse of a bridge in Minneapolis can be averted. "There's really a critical need to develop better sensors that can be applied to infrastructure systems," says Jerome Lynch, professor of civil and environmental engineering at the University of Michigan.
Traditional strain sensors simply measure stress along a particular line. One such sensor is a wire that changes resistivity when it's under strain. Another type is an optical fiber that indicates strain when light injected at one end is scattered by a defect in the structure. "But the problem is if the damage occurs between the sensors—it's difficult to detect," says Branko Glisic, a professor of civil and environmental engineering at Princeton who was not directly involved with the project. A stretchable laser could solve this problem by covering more area than wires or fiber optics. To make the device, a sheet of stretchable material called polydimethylsiloxane (PDMS) was specially prepared so that it had a wavy surface. Next, the researchers spun a liquid mixture of organic molecules onto the wavy surface. When an ultraviolet laser is shone on the organic layer (a method of powering a laser called optical pumping), it stimulates the emission of photons from the organic molecules. Lasing occurs because the wavy surface acts as a diffraction grating, reflecting the light between the waves, effectively amplifying the signal.
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."
By Kate Greene
From Technology Review