To build the lantern, the team began with a thin polymer sheet cut into a diamond-shaped parallelogram. They then sliced a series of evenly spaced lines through the center of the sheet, forming parallel ribbons connected by solid strips of material at the top and bottom. When the ends of these top and bottom strips are joined, the sheet naturally folds into a round, lantern-like shape.

"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."

The researchers also gave the lanterns magnetic control by attaching a thin magnetic film to the bottom strip. This allowed them to remotely twist or compress the structures using a magnetic field. They demonstrated several possible uses for the design, including a gentle magnetic gripper that can catch and release fish without harm, a flow-control filter that opens and closes underwater, and a compact shape that suddenly extends upward to reopen a collapsed tube. A video of the experiment is available below the article.

To better understand and predict the lantern's behavior, the team also created a mathematical model showing how the geometry of each angle affects both the final shape and how much elastic energy is stored in each stable configuration.

"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."

The paper, "Reprogrammable snapping morphogenesis in freestanding ribbon-cluster meta-units via stored elastic energy," was published on Oct. 10 in the journalNature 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.

This work was done with support from the National Science Foundation under grants 2005374, 2369274 and 2445551.

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