Infrastructure protectionUltrathin radios enable flexible structural-health monitoring system
Currently, engineers can use single-point sensors or fiber optic strips to detect structural problems, but the devices can collect data over relatively small spaces. The problem is that many failures develop over large areas and cannot be detect that at an early stage. The 2007 collapse of a highway bridge in Minneapolis, for example, developed over a gusset plate with an area of several square meters, far too large for current monitoring systems to practically survey. Researchers have developed ultrathin radios which can be embedded directly on plastic sheets, which can be applied to walls and other structures. The innovation could be used for new devices ranging from an invisible communications system inside buildings to sophisticated, flexible structural health monitoring system for use on bridges, buildings, roads, pipelines, and other structures.
Using a modern twist on a technology developed in the 1920s, researchers at Princeton University have embedded ultrathin radios directly on plastic sheets, which can be applied to walls and other structures. The innovation could serve as the basis for new devices ranging from an invisible communications system inside buildings to sophisticated structural monitors for bridges and roads.
“We originally built this for energy management in a smart building,” said Naveen Verma, an assistant professor of electrical engineering and one of the project’s principal researchers.
“Temperature sensors and occupancy sensors communicate with a central management system using distributed radio arrays that are patterned on wallpaper.”
A Princeton University release reports that the plastic sheets are as thin as wallpaper and can be painted without diminishing their function. They are also flexible and can be applied to irregular surfaces such as bridge decks or supporting columns. And they can be self-powered; solar cells on the plastic sheets supply electricity to the radios.
Patterning circuits on plastic, a relatively new idea, is challenging because plastic tends to melt or deform at the high temperatures used to create circuitry. In recent years, researchers have developed techniques to avoid damaging the plastic. These methods, however, required some alterations that lower the performance of electronic components, such as transistors, that are critical to the operation of complex devices likes radio transmitters.
“Radios have been a real challenge,” Verma said.
Radios require a relatively high frequency to operate, and that has been impractical for plastic-based electronics. Take, for example, the transistors developed by two collaborators on this project: Sigurd Wagner, a professor of electrical engineering and an expert on the use of flexible materials in electronics; and James Sturm, the Stephen R. Forrest Professor in Electrical Engineering and director of the Princeton Institute for the Science and Technology of Materials.
In a 2008 research paper, “Highly Stable Amorphous-silicon Thin-film Transistors on Clear Plastic,” the pair described developing a method for printing circuits without melting the underlying plastic. Sturm and Wagner developed a method and a new kind of circuit material that reduced that temperature from about 1,000 degrees Celsius to 300 degrees.
At that temperature, plastic worked well. To create the working circuitry, however, Sturm and Wagner had to alter some of the properties of the components. That spelled trouble for would-be radio designers.
