A Shape-Shifting Metamaterial Inspired by Push Puppets

Andrew Corselli

Inside a push puppet, there are connecting cords that, when pulled taught, will make the toy stand stiff. But by loosening these cords, the “limbs” of the toy will go limp. Using the same cord tension-based principle that controls a puppet, UCLA researchers have developed a new type of metamaterial, a material engineered to possess properties with applications for soft robotics, reconfigurable architectures, and space engineering.

Published in Materials Horizons, the UCLA study demonstrates the new lightweight metamaterial, which is outfitted with either motor-driven or self-actuating cords that are threaded through interlocking cone-tipped beads. When activated, the cords are pulled tight, causing the nesting chain of bead particles to jam and straighten into a line, making the material turn stiff while maintaining its overall structure.

The study also unveiled the material’s versatile qualities that could lead to its eventual incorporation into soft robotics or other reconfigurable structures:

• The level of tension in the cords can “tune” the resulting structure’s stiffness — a fully taut state offers the strongest and stiffest level, but incremental changes in the cords’ tension allow the structure to flex while still offering strength. The key is the precision geometry of the nesting cones and the friction between them.
• Structures that use the design can collapse and stiffen over and over again, making them useful for long-lasting designs that require repeated movements. The material also offers easier transportation and storage when in its undeployed, limp state.
• After deployment, the material exhibits pronounced tunability, becoming more than 35 times stiffer and changing its damping capability by 50 percent.
• The metamaterial could be designed to self-actuate, through artificial tendons that trigger the shape without human control.

“Our metamaterial enables new capabilities, showing great potential for its incorporation into robotics, reconfigurable structures, and space engineering,” said Corresponding Author Wenzhong Yan, UCLA Samueli School of Engineering Postdoctoral Scholar. “Built with this material, a self-deployable soft robot, for example, could calibrate its limbs’ stiffness to accommodate different terrains for optimal movement while retaining its body structure. The sturdy metamaterial could also help a robot lift, push or pull objects.”

“The general concept of contracting-cord metamaterials opens up intriguing possibilities on how to build mechanical intelligence into robots and other devices,” Yan said.

According to the researchers, potential applications of the material also include self-assembling shelters with shells that encapsulate a collapsible scaffolding. It could also serve as a compact shock absorber with programmable dampening capabilities for vehicles moving through rough environments.

“Looking ahead, there’s a vast space to explore in tailoring and customizing capabilities by altering the size and shape of the beads, as well as how they are connected,” said Senior Author Ankur Mehta, UCLA Samueli Associate Professor of Electrical and Computer Engineering and Director of the Laboratory for Embedded Machines and Ubiquitous Robots, of which Yan is a member.

While previous research has explored contracting cords, this paper has delved into the mechanical properties of such a system, including the ideal shapes for bead alignment, self-assembly and the ability to be tuned to hold their overall framework.

Here is an exclusive Tech Briefs interview, edited for length and clarity, with Yan and Mehta.

Tech Briefs: What was the biggest technical challenge you faced while developing this metamaterial?

Yan: The biggest technical challenge was designing the geometry of the beads. We needed to ensure precise alignment during assembly while also allowing for a wide range of stiffness when tension is applied.

Tech Briefs: What was the catalyst for your work?

Yan: The project arose from the limitations of conventional particle jamming, which typically requires bulky vacuum pumps and lacks shape-changing capabilities without additional modules, leading to cumbersome systems. By leveraging the simple mechanism of push puppets, we developed a method using contracting cords to achieve particle jamming in a more compact and efficient way.

Tech Briefs: Can you explain in simple terms how it works?

Yan: This project introduces a new material that can change its shape and stiffness. It’s made of beads threaded on a string, similar to a necklace. When the string is tightened, the beads are pressed together, forming a set shape and becoming stiff. Loosening the string allows the beads to move apart, making the material soft and flexible. This makes it ideal for applications where materials need to change shape and stiffness, such as in soft robots or adaptable structures.

Tech Briefs: What are your next steps? Do you have plans for further research/work/etc.?

Mehta: With this work, we've only just skimmed the surface in the world of deployable metamaterials. What happens when we vary the shapes of the beads along the string? Or assemble strings into more complex shapes? What kinds of non-computational intelligence can we structurally program into our metamaterials? These questions and more are what we hope to answer.

Tech Briefs: Do you have any updates you can share?

Yan: We are currently developing robots and machines made from this material. The adaptability of the material, which can change shape and stiffness, allows these robots and machines to function effectively in dynamic environments.

Tech Briefs: Do you have any advice for engineers/researchers aiming to bring their ideas to fruition, broadly speaking?

Yan: My advice is to focus on early prototyping and continuous iteration. Start building and testing your ideas as soon as possible, and don’t hesitate to create initial prototypes. Seek feedback from a wide range of people — peers, potential users, and those outside your field — to refine your ideas and identify potential challenges. Remain open-minded and adaptable, as turning an idea into reality often requires flexibility, a willingness to learn, and the ability to adjust your approach.

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