Flexible micro-LEDs lay the foundation for a new generation of wearable technology

　　LED is the abbreviation of “Light Emitting Diode”, which is translated into “light-emitting diode” in Chinese. It is a semiconductor device that can convert electrical energy into light energy.

　　LEDs are used in a wide range of products from brake lights to billboards. They are thin, energy-efficient and can emit a variety of light types, making them ideal components for backlighting and displays in electronic devices.

　　Micro LEDs, which can be as small as 2 microns and bundled into any size, have higher resolution than other LEDs. This size makes them ideal for small devices such as smart watches, but they can also be bundled together to work in flat-screen TVs and other larger displays. However, LEDs of all sizes are fragile and can usually only be used on flat surfaces.

　　Innovation

　　To meet the demand for bendable, wearable electronics, researchers at The University of Texas at Dallas and their international colleagues have developed a new method for creating micro-LEDs that can be folded, twisted, cut and attached to different surfaces.

　　Flexible micro-LEDs can be twisted (left) or folded (right). (Image credit: University of Texas at Dallas)

　　The research, published online in June in the journal Science Advances, paves the way for a new generation of flexible wearable technology.

　　technology

　　"The great thing about this work is that we created a removable LED that can be mounted on almost any surface," said Moon Kim, a professor of materials science and engineering at UT Dallas and the study's corresponding author. "You can transfer it to your clothes or even rubber, which is the main idea. Even if you crumple it, it will still work. If you cut it open, you can still use half of the LED."

　　Researchers in the Erik Jonsson School of Engineering and Computer Science and the College of Natural Sciences and Mathematics helped develop the flexible LED through a technique called remote epitaxy, which involves growing a thin layer of LED crystals on a sapphire wafer, or substrate.

　　Normally, the LEDs would remain attached to the wafer. To make it removable, the researchers added a nonstick layer to the substrate, which acts similarly to the way parchment paper protects a baking sheet, making it easier to remove cookies, for example. The added layer, made of a single-atom-thick sheet of graphene, prevents the LED crystals from sticking to the wafer.

　　Professor Kim, who oversaw the physical analysis of the LEDs using a scanning/transmission electron microscope with atomic-level resolution at the UT Dallas Nano Characterization Facility, said: "Graphene does not form a chemical bond with the LED material, so it adds a layer that allows us to peel the LEDs off the wafer and stick them to any surface."

　　Colleagues in South Korea conducted laboratory tests by gluing LEDs to curved surfaces and to materials that were then twisted, bent and crumpled. In another demonstration, they glued LEDs to the legs of a Lego figure in different leg positions.

　　Kim said bending and cutting will not affect the quality and electronic properties of the LED.

　　value

　　The bendable LED has a variety of possible uses, including flexible lighting, clothing, and wearable biomedical devices. From a manufacturing perspective, this fabrication technique offers another advantage: Because the LED can be removed without damaging the underlying wafer substrate, the wafer can be reused.

　　“You can use one substrate multiple times and it will have the same functionality,” Kim said.

　　In ongoing studies, the researchers are also applying this manufacturing technique to other types of materials.

　　“This is very exciting,” Kim said. “This method is not limited to just one material. It works for all kinds of materials.”