Researchers have created a polymer "Chinese lantern" that can snap into more than a dozen curved, three-dimensional shapes by compressing or twisting the original structure. This rapid shape-shifting behavior can be controlled remotely using a magnetic field, allowing the structure to be used for a variety of applications.

The corresponding research paper, "Reprogrammable snapping morphogenesis in freestanding ribbon-cluster meta-units via stored elastic energy," is published in Nature Materials.

The paper was co-authored by Caizhi Zhou and Haitao Qing, both Ph.D. students at NC State; and by Yinding Chi, a former Ph.D. student at NC State who is now a postdoctoral researcher at Penn.

The basic lantern object is made by cutting a polymer sheet into a diamond-like parallelogram shape, then cutting a row of parallel lines across the center of each sheet. This creates a row of identical ribbons connected by a solid strip of material at the top and bottom of the sheet. By connecting the left and right ends of the solid strips at top and bottom, the polymer sheet forms a three-dimensional shape resembling a roughly spherical Chinese lantern.

"This basic shape is, by itself, bistable," says Jie Yin, corresponding author of a paper on the work and a professor of mechanical and aerospace engineering at North Carolina State University. "In other words, it has two stable forms. It is stable in its lantern shape, of course. But if you compress the structure, pushing down from the top, it will slowly begin to deform until it reaches a critical point, at which point it snaps into a second stable shape that resembles a spinning top.

"In the spinning-top shape, the structure has stored all of the energy you used to compress it. So, once you begin to pull up on the structure, you will reach a point where all of that energy is released at once, causing it to snap back into the lantern shape very quickly."

"We found that we could create many additional shapes by applying a twist to the shape, by folding the solid strips at the top or bottom of the lantern in or out, or any combination of those things," says Yaoye Hong, first author of the paper and a former Ph.D. student at NC State who is now a postdoctoral researcher at the University of Pennsylvania.

"Each of these variations is also multistable. Some can snap back and forth between two stable states. One has four stable states, depending on whether you're compressing the structure, twisting the structure, or compressing and twisting the structure simultaneously."

By attaching a thin magnetic film to the solid strip at the bottom of the structure, the researchers were able to compress or twist the structures remotely, using a magnetic field. They then demonstrated several applications that made use of snapping between two stable shapes. These applications included a noninvasive gripper for grasping fish; a filter that opened and closed to control the flow of water; and a compact shape that rapidly expanded into a tall shape to open a collapsed tube.

The researchers also developed a mathematical model that captures the way in which different angles in the structure control both the shape of each variation and the amount of energy that is stored in each stable state.

"This model allows us to program the shape we want to create, how stable it is, and how powerful it can be when stored potential energy is allowed to snap into kinetic energy," says Hong. "And all of those things are critical for creating shapes that can perform desired applications."

"Moving forward, these lantern units can be assembled into 2D and 3D architectures for broad applications in shape-morphing mechanical metamaterials and robotics," says Yin. "We will be exploring that."