Polymer gels can move rather snappily

Imagine a rubber band capable of snapping itself many times over, or a small robot that can jump up a set of stairs propelled by nothing more than its own energy. Researchers at the University of Massachusetts (UMass) Amherst have discovered how to make materials that snap and reset themselves, relying solely on energy flow from their environment. This discovery, reported in a paper in Nature Materials, may prove useful for industries that want to source movement sustainably, from toys to robotics, and is expected to further inform our understanding of how the natural world fuels some types of movement. Al Crosby, a professor of polymer science and engineering in the College of Natural Sciences at UMass Amherst, and Yongjin Kim, a graduate student in Crosby's group, along with visiting student researcher Jay Van den Berg from the Delft University of Technology in the Netherlands, uncovered the physics during a mundane experiment that involved watching a gel strip dry. The researchers observed that when the long, elastic gel strip lost internal liquid due to evaporation, the strip moved. Most movements were slow, but every so often, they sped up. These faster movements were snap instabilities that continued to occur as the liquid evaporated further. Additional studies revealed that the shape of the material mattered and that the strips could reset themselves to continue their movements. "Many plants and animals, especially small ones, use special parts that act like springs and latches to help them move really fast, much faster than animals with muscles alone," explains Crosby. "Plants like the Venus flytraps are good examples of this kind of movement, as are grasshoppers and trap-jaw ants in the animal world. Snap instabilities are one way that nature combines a spring and a latch and are increasingly used to create fast movements in small robots and other devices, as well as toys like rubber poppers. However, most of these snapping devices need a motor or a human hand to keep moving. With this discovery, there could be various applications that won't require batteries or motors to fuel movement." Kim explains that after learning the essential physics from the drying strips, the team experimented with different shapes to find the ones most likely to react in expected ways and to move repeatedly without any motors or hands resetting them. The team even showed that the reshaped strips could do work, such as climb a set of stairs on their own. "These lessons demonstrate how materials can generate powerful movement by harnessing interactions with their environment, such as through evaporation, and they are important for designing new robots, especially at small sizes where it's difficult to have motors, batteries or other energy sources," says Crosby. These latest results from Crosby and his group are part of a larger multidisciplinary university research initiative funded by the Army Research Office, an element of the US Army Combat Capabilities Development Command's Army Research Laboratory. Led by Sheila Patek, professor of biology at Duke University, this initiative aims to uncover many similar mechanisms in fast-moving biological organisms and translate them into new engineered devices. "This work is part of a larger multidisciplinary effort that seeks to understand biological and engineered impulsive systems that will lay the foundations for scalable methods for generating forces for mechanical action and energy storing structures and materials," says Ralph Anthenien, branch chief, Army Research Office. "The work will have myriad possible future applications in actuation and motive systems for the Army and DoD." This story is adapted from material from the University of Massachusetts Amherst, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.