It's entirely possible that the future of wearable computing looks more like Saran Wrap than Google Glass. Already, scientists have developed circuits you can wear like temporary tattoo and thin, flexible silicon-based devices that dissolve in water. Now, researchers at the University of Tokyo say they have created electronics thin and flexible enough to be considered "imperceptible." “There are other groups that are working on these things,” says Martin Kaltenbrenner, first author of the Nature paper, published July 25, that describes the group’s work. “But I would like to say we are able to make the largest, most flexible and thinnest devices.”
Their first prototype, a sub-featherweight touch sensor, is 30 times lighter than printer paper and one-fifth the thickness of sandwich wrap. In a video the group released, Takao Someya, a researcher on the team, puts it this way: “People cannot feel the existence of this device.”
The foundation for their device is polyethylene napthalate (PEN) foil, which resembles industrial-grade kitchen wrap. One micron thick, the foil comes in rolls 10 cm wide and 10 kilometers long. Electronics manufacturers use it to build billions of commodity capacitors per year. As a result, it’s cheap, and that’s a good thing—it should, in principle, make flexible electronics simple to manufacture. The catch is, on a microscopic scale it is shockingly rough, like a nanoscale mountain range. “On top of that you need to build a device that is 100 nanometers thick,” Kaltenbrenner says. “It’s not so easy.”
But Kaltenbrenner and his colleagues found that if you apply the layers of electronics in just the right way, PEN’s roughness becomes an unexpected advantage: metal sticks to it very well. As a result, Kaltenbrenner’s electronics are not just thin, they’re also durable, capable of performing perfectly well after being crushed into a ball.
To build their prototype touch sensor, the researchers took a 4.8-square-centimeter piece of PEN foil and applied an integrated-circuits layer and a layer containing 144 tactile circuits. “It came down to 1 to 2 microns thick with everything, the whole package,” Kaltenbrenner says. “That in turn makes it a lot more flexible than anything out that’s out there.”
Imperceptibly thin circuits could have obvious medical applications—monitoring systems, wearable medical instruments, and so on. It could be used as robotic skin. But if Kaltenbrenner and his colleagues are correct in their prognostication—“Imperceptible electronic foils may be as common in the future as plastic wrap is today,” they write in the abstract for the Nature paper—the technology could have applications that are hard to imagine today. “It’s pretty much like a roll of kitchen wrap, but it has electronic functionality,” Kaltenbrenner says. “You can functionalize pretty much any object you can think of, because you just wrap it in the foil.”