A push-pull doll has connecting strings that, when pulled taut, make the toy stand stiff. But when these strings are released, the toy’s “limbs” go limp. Using the same string-tension principle that controls a doll, researchers have developed a new type of metamaterial, one that is designed to have properties that promise advanced capabilities.

Published in Materials HorizonsThe UCLA study showcases the new lightweight metamaterial, which is equipped with motor-driven or self-actuating strings strung through interlocking cone-shaped beads. When activated, the strings tighten, causing the nested chain of bead particles to jam and straighten into a line, making the material stiff while maintaining its overall structure.

The research also revealed the material’s versatile qualities, which could lead to its eventual application in soft robotics or other reconfigurable structures:

• The tension level in the cords can “tune” the stiffness of the resulting structure — a fully tensioned state provides the strongest, stiffest level, but incremental changes in the tension of the cords allow the structure to bend while still providing strength. The key is the precise geometry of the nesting cones and the friction between them.
• Structures using the design can collapse and stiffen over and over again, making it useful for long-term designs that require repeated movement. The material also offers easier transportation and storage when in its unexpanded, flaccid state.
• After use, the material shows pronounced adjustability: it becomes more than 35 times stiffer and its damping capacity changes by 50%.
• The metamaterial could be designed to self-activate, using artificial tendons to activate the shape without human control.

“Our metamaterial opens up new possibilities and shows great potential for integration into robotics, reconfigurable structures, and aerospace engineering,” said corresponding author and UCLA Samueli School of Engineering postdoctoral researcher Wenzhong Yan. “For example, built with this material, a self-deploying soft robot could calibrate the stiffness of its limbs to adapt to different terrains for optimal movement while maintaining its body structure. The sturdy metamaterial could also help a robot lift, push, or pull objects.”

“The general concept of contractile cord metamaterials opens up intriguing possibilities for embedding mechanical intelligence into robots and other devices,” Yan said.

A 12-second video of the metamaterial in action is available here, via the UCLA Samueli YouTube channel.

The paper’s lead authors are Ankur Mehta, an associate professor of electrical and computer engineering at UCLA Samueli and director of the Laboratory for Embedded Machines and Ubiquitous Robots, of which Yan is a member, and Jonathan Hopkins, a professor of mechanical and aerospace engineering who directs UCLA’s Flexible Research Group.

According to the researchers, potential applications for the material also include self-assembling shelters with shells that enclose a collapsible scaffolding. It could also serve as a compact shock absorber with programmable damping capabilities for vehicles driving through harsh environments.

“Looking into the future, there is still a lot to explore in terms of customization and personalization by changing the size and shape of the beads, as well as the way they connect to each other,” said Mehta, who is also an associate professor of mechanical and aerospace engineering at UCLA.

While previous research has focused on contractile cords, this paper takes a closer look at the mechanical properties of such a system, including ideal shapes for bead alignment, self-assembly, and the ability to tune the system to maintain its overall framework.

Other authors of the paper include Talmage Jones and Ryan Lee, UCLA mechanical engineering graduate students (both in Hopkins’ lab), and Christopher Jawetz, a Georgia Institute of Technology graduate student who worked on the research as a member of Hopkins’ lab while he was an undergraduate in aerospace engineering at UCLA.

The research was funded by the Office of Naval Research and the Defense Advanced Research Projects Agency, with additional support from the Air Force Office of Scientific Research and computing and storage services from the UCLA Office of Advanced Research Computing.