North Carolina State University researchers developed a ring-shaped soft robot that can crawl across surfaces when subjected to high temperatures or infrared light. The researchers demonstrated that these “ringbots” could move a tiny payload across the surface — in the air or under the water — and also pass through a gap smaller than their ring size.
The ringbots are constructed of liquid crystal elastomers in the shape of a bracelet-like looping ribbon. The segment of the ribbon touching the surface compresses while the portion exposed to the air does not when the ringbot is placed on a surface that is at least 55 °C. (131 °F) hotter than the surrounding air. The ribbon begins to roll as a result of this.
When investigators shine infrared light on the ringbot, the piece of the ribbon that is exposed to the light contracts, while the portion that is covered from the light does not. This causes a rolling motion in the ribbon.
This means, in practical terms, that the crawling ringbot moves from the bottom up when placed on a hot surface. Yet, when exposed to infrared light, the movement begins from the top down.
One of the factors driving this constant motion is the fact that ringbots are bistable, which means they have two shapes when at rest. If the ribbon starts to twist, it will either return to its initial shape or advance into the other bistable state.
It is similar to a rubber bracelet in the shape of a ribbon. The bracelet will snap back to its original shape if individuals fold two ends forward slightly and then let go. However, if they fold the ends over far enough, it will snap over, turning the bracelet inside out.
In the case of the ringbots, the “folding” is accomplished by exposing the elastomer to continual heat or infrared light, making it contract and rotate. This will presumably make the ring robot dance in place if it is symmetrical.
But by engineering the shape of the loop, so that one side of the loop is permanently twisted, the structure is asymmetrical. This means that the loop is exposed to the heat or infrared light unevenly, which causes the soft robot to move laterally across the surface.
Jie Yin, Study Corresponding Author and Associate Professor, Mechanical and Aerospace Engineering, North Carolina State University
When mounted on a hot surface, the crawling ringbot begins to pull itself forward. However, when subjected to infrared light, the crawling ringbot advances (compare the front-wheel drive versus rear-wheel drive).
The ringbots demonstrated the ability to pull a tiny weight and function in both ambient air and underwater environments.
The scientists also proved that a ringbot’s body shape could be adapted to fit through a tight location that was more than 30% narrower than the ringbot’s diameter. When the gap becomes too small for the soft robot to pass through, it redirects itself to move away from it.
This is a fundamental advance, not something designed with a specific application in mind. We are demonstrating what can be accomplished when ‘physical intelligence’ is engineered into the material and the design of the structure itself, allowing it to move and navigate space without computational input.
Yao Zhao, Postdoctoral Researcher, North Carolina State University
The research was funded by the National Science Foundation under grants 2005374, 2126072, and 2026622.
Zhao, Y., et al. (2022) Self-sustained Snapping Drives Autonomous Dancing and Motion in Free-standing Wavy Rings. Advanced Materials. doi.org/10.1002/adma.202207372.