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 bring high performances to robots, e.g., high-speed locomotion, adaptive sensing, fast grasping, etc. 

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

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 geometry configurations, dimensions, materials, and actuation methods for various robotic applications. 

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

The reported bistable structure was fabricated by folding a sheet material to a specific crease pattern and 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 was 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 could get more and more delicate. That is how the researchers achieved a large range of adjustable trigger forces of the proposed controllable bistable structure. 

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 can be tuned to 0.1% of its maximum value, while the lifted weight difference exceeds 10⁷ times 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 weight 110g cannot break its energy barrier." Said Dr. LI. 

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 exceeding 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 leads a way to future design in robotics, biomedical engineering, architecture, and kinetic art."