Shape-Shifting Structures: The Future of Robotic Innovation

Researchers in China have developed an ultra-tunable bistable structure with programmable energy barriers and trigger forces. The structures can be customized in various geometric configurations, dimensions, materials, and actuation methods for use in robotic applications. By reshaping the structure from the metastable state to any intermediate state, the energy barrier decreases, enabling smaller external stimulations to trigger fast snap-through. The team demonstrated the tunability of the structure with various prototypes, including a robotic flytrap, grippers, a jumper, a swimmer, a thermal switch, and a sorting system. This work could lead to advances in robotics, biomedical engineering, architecture, and kinetic art. (Abstract fractal art representing shape-shifting structures.)

Ultra-tunable Bistable Structures Developed for Universal Robotic Applications

Chinese researchers have developed an ultra-tunable bistable structure with customizable features for robotic applications, providing adjustable trigger forces and demonstrating potential uses in a range of fields.

Bistable structures in nature are unparalleled for their fast response and force amplification even with the minutest physical stimulation. Harnessing bistability and instability to rapidly release the stored energy in bistable structures could improve robot performance in several areas, e.g., high-speed locomotion, adaptive sensing, and fast grasping.

However, current works on bistable structures mainly focus on their stable states, while intermediate states with a large range of tunable energy barriers are missing from current studies.

Recently, a research team led by Dr. LI Yingtian from the Shenzhen Institute of Advanced Technology (SIAT) of the Chinese Academy of Sciences has proposed a type of ultra-tunable bistable structure with programable energy barriers and trigger forces of orders of magnitude differences. The structures can also be customized with varied geometric configurations, dimensions, materials, and actuation methods for various robotic applications.

This work was published in the journal Cell Reports Physical Science on April 18.

Schematic of the proposed ultra-tunable bistable structure. Credit: LI Yingtian)

The reported bistable structure was fabricated by folding a sheet material to a specific crease pattern. It possesses a stable state, a metastable state, and enormous intermediate states. When the bistable structure transitions from its metastable state to the stable state, there exists a critical point, where the stored strain energy reaches its maximum value, and the fast snap-through starts.

In this work, the enormous intermediate states with programmable energy barriers before the bistable structure reaches its critical point were reported.

By reshaping the structure from the metastable state to any intermediate state, the energy barrier decreases, meaning that smaller external stimulations are required to trigger the fast snap-through of the bistable structures. As the energy barrier keeps decreasing, the required external stimulation gets more and more delicate. That is how the researchers achieved a large range of adjustable trigger forces for the proposed controllable bistable structure.

Demonstration of ultra-sensitive force detection and fast response properties. The proposed structure can be triggered by a droplet and flying bees when adjusted to intermediate states with super-low energy barriers. Credit: LI Yingtian

To demonstrate the tunibility of the proposed structure, the researchers conducted a series of experiments and illustrated that the trigger force of a single structure could be tuned to 0.1% of its maximum value, while the lifted weight difference was 10⁷ times greater using grippers fabricated by the proposed structures with different design parameters.

“We can tune the structure to an ultra-sensitive state so that it will respond to a minute stimulation as gentle as a touch of a flying bee, while we could also set the structure to an insensitive state that even a steal ball weighing 110g could not break its energy barrier,” said Dr. LI.

A robotic flytrap. The ultra-sensitive “pistil” can respond to the soft touch of a flying bee in 10 ms, and then the “lobes” can close themselves to trap bees and then reopen to set them free. Credit: LI Yingtian

To validate the potentials of the structure in diverse applications, various prototypes were developed, including a robotic flytrap, grippers, a jumper, a swimmer, a thermal switch, and a sorting system. The prototypes demonstrate that the robotic flytrap with a sensitive “pistil” can be triggered by physical stimulation in 10 ms; the bistable catcher can capture a high-speed (10 m/s) table tennis ball; and the minimal jumper reaches a height more than 24 times of its body height, etc.

“We are happy to find out our proposed structure could be used in such a wide range of applications, which demonstrates superior performances,” said Dr. LI. “This work could broaden the frontiers of bistable structure design and lead a way to future designs in robotics, biomedical engineering, architecture, and kinetic art.”

Reference: “Ultra-tunable bistable structures for universal robotic applications” by Yongkang Jiang, Yingtian Li, Ke Liu, Hongying Zhang, Xin Tong, Diansheng Chen, Lei Wang and Jamie Paik, 18 April 2023, Cell Reports Physical Science.
DOI: 10.1016/j.xcrp.2023.101365