You were never meant to see the strings holding up the props in cheesy old sci-fi movies, but researchers at North Carolina State University have developed a soft robot that doesn’t hide the fact that it needs a string to get around. Soft robots are being selected for more and more applications these days due to their inherent flexibility that makes them safer for working in close quarters with humans, and more easily able to fit into tight spaces. But the flexibility of these robots also makes them very difficult to control with precision.
In an effort to overcome these problems, the researchers have proposed a very unusual soft robot design that zips along a string. As long as there is a string stretched out along its path of travel it can move with extreme precision, and it is capable of carrying up to 12 times its own weight and climbing up very steep inclines. They are not easily tripped up, either — differences in the thickness of the string and obstacles like knots will not even slow them down.
An infrared image of a robot moving along fishing line (📷: F. Qi et al.)
Each robot is made of a ring of ribbon-like liquid crystal elastomers that have been twisted up. A few of the twists go around the string or wire that the robot is to travel along. The material is heat sensitive, and when it is warmed up with an infrared light, the differences between warm and cool areas will cause it to twist. As the ring rolls, its rotation is converted into a linear motion via screw theory.
This track-based approach offers a clever solution to one of soft robotics’ biggest challenges — path predictability. While most soft robots move freely through air, water, or across surfaces — requiring sophisticated feedback systems and control algorithms — this robot is inherently constrained by its track. That constraint turns out to be an advantage in this case, significantly reducing the complexity of motion planning and enabling the robot to move autonomously with minimal external input.
Despite being on a track, the robot is not limited to moving in a straight line. In tests, the robot successfully traveled along complex paths — including spirals, curves, and even dynamically shifting routes — all with no onboard electronics or moving parts. The light source stays stationary and constant, allowing the robot to rely entirely on its material properties and the geometric relationship with the track for motion.
The team envisions a range of possible uses, from small-scale transport in manufacturing or medicine to future space or environmental monitoring systems, where robots may need to follow predefined aerial paths without complex navigation systems. The researchers are also exploring ways to power the robot using ambient energy sources like sunlight, broadening its potential for sustainable operation in diverse environments.