The HLEC display maintains luminescence when stretched, rolled, folded and wrapped. Credit: Larson et al. / Science (2016)

Chris Larson and his colleagues at Cornell University, USA, report developing a hyperelastic, light-emitting capacitor (HLEC) and incorporating the device into an artificial skin that can stretch to more than 480 percent of its original size (Science, doi: 10.1126/science.aac5082). In addition to emitting different colors of light, the HLEC material also functions as a dielectric elastomer sensor (i.e., touch sensor) because of its parallel-plate capacitor construction. The researchers say that this new stretchy, electroluminescent skin could enable displays and robots that actively change their shape and color.

The HLEC—the technology that makes this artificial skin possible—is made up of five layers: an electroluminescent dielectric layer sandwiched between two hydrogel electrodes and encapsulated in a silicone matrix. The electroluminescent layer is a 1-mm-thick sheet of phosphor-powders and silicone. The hydrogel electrodes are made from aqueous lithium chloride and polyacrylamide. The silicone matrix contains ZnS doped with transition metals that emit different colors as electricity passes through it (e.g., blue in the presence of Cu; yellow in the presence of Mg).

A robot communication channel

The leader of the research team, Cornell assistant professor Rob Shepherd, noted in a press release that the new stretchable material “allows robots to change their color, and it also allows displays to change their shape.” That combination, he believes, could be important in contexts where humans and robots are interacting; fashioning a stretchable skin with tunable color that can be put on a robot expands the robot’s palette of options for feedback and communication.

The researchers demonstrated the new cephalopod-like skin on a soft-bodied robot. They covered the robot with three HLEC panels consisting of six layers: the top four layers contained transparent hydrogels sandwich an insulating elastomer sheet; and the bottom two layers contained pneumatic actuators that inflated and deflated, causing the robot to wriggle forward like a worm. A video of this demonstration can be viewed here:
 

Credit: Larson et al. / Science (2016)

Although the luminous efficacy of the HLEC panels is not as high as commercial AC powder electroluminescent devices, the authors say that it could be “greatly improved by tuning the materials system and device architecture.”