By liberating tiny LEDs from their rigid backbones, researchers hope to produce a new class of biological sensors.
An international team of researchers has devised a way to embed tiny light emitters and light sensors into stretchable, bendable, twistable sheets. The flexible systems might someday find use as implanted sensors to keep tabs on biological processes.
Many approaches to developing flexible electronics have targeted a class of light emitters known as organic LEDs, which can be assembled using electrically conductive polymers and deposited on bendable plastic substrates. Conventional, inorganic LEDs are a more mature technology, with their own distinct advantages, but they are generally tethered to semiconductor wafers that limit their elasticity.
Now researchers have bridged the gap between organic and inorganic LEDs by harnessing the light of conventional electronics in an elastic system with biomedical potential. “The applications we’re interested in mostly include interfaces with the human body,” says John Rogers, a materials scientist at the University of Illinois at Urbana–Champaign and a co-author of a paper published online October 17 in Nature Materials describing the advance. For some biological applications, he adds, a conventional LED’s brightness, reliable operation and suitability for waterproof implementation make it a more attractive option than an organic LED.
Rogers and his colleagues printed an interlaced array of tiny light-emitting diodes, or LEDs, on a rigid wafer, then dissolved the top layer of the substrate to release a thin network of LEDs that can be transferred to a flexible, waterproof polymer sheet. “We can lift off from the wafer just the active layers,” Rogers says, describing the process as a rubber stamp that picks up the LED array as if it were solid ink. Each LED is just 100 microns across (about the width of a human hair) and 2.5 microns thick—a micron is one millionth of a meter—and is connected to its neighbors by serpentine strands that can accommodate the deformation of stretching and twisting.
As a demonstration of the technology the researchers put LED arrays through any number of experimental implementations. They deposited LEDs on aluminum foil, the leaf of a tree, and a sheet of paper; they wrapped arrays around nylon thread and tied it in a knot; and they distended LED arrays by inflating the polymer substrate or stretching it over the tip of a pencil or the head of a cotton swab. “Eventually the students just got tired” of devising new tests for the light-emitting sheets, Rogers says. “There was nothing that we tried that we couldn’t do.”
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