Most soft robots rely on an external power source and control in order to operate. This causes them to remain tethered to off-board systems or become equipped with hard components. Researchers from Harvard John A Paulson School of Engineering and Applied Sciences (SEAS) have teamed up with Caltech to develop a soft robotic system that can move and change shape when it comes in contact with external stimuli. Researchers named the system “Rollbot” and it has the potential to change the way we use fully untethered soft robots in the future. Their studies were recently published in Science Robotics.


The device can fold into a shape when placed on a heated surface. (Image Credit: Kotikian et al., Sci. Robot. 4, eaax7044 (2019))


Inspiration for the Rollbot came from an origami, which enabled the researchers to come up with ideas to develop multifunctional soft robots. Through sequential folds, origami can be made into many different shapes and functionalities within a single structure. The team used liquid crystal elastomers that can change shape through heat exposure. Researchers also 3D-printed two different types of soft hinges that can fold when exposed to different temperatures. It can also be programmed to fold in a specific order, like an origami’s folding pattern to create a shape. 


"With our method of 3-D printing active hinges, we have full programmability over-temperature response, the amount of torque the hinges can exert, their bending angle, and fold orientation. Our fabrication method facilitates integrating these active components with other materials," said Arda Kotikian, a graduate student at SEAS and the Graduate School of Arts and Sciences and co-first author of the paper.


The Rollbot starts as a flat sheet, measuring 4 centimeters wide and 8 centimeters long, curling into a pentagonal wheel when added onto a heated surface, with temperatures reaching 200°C. Every hinge embedded on all sides of the wheel folds when it touches the surface, causing the wheel to turn over to the next side. When the hinges move off the surface by rolling away from it, they unfold and can be used for the next cycle. "Using hinges makes it easier to program robotic functions and control how a robot will change shape. Instead of having the entire body of a soft robot deform in ways that can be difficult to predict, you only need to program how a few small regions of your structure will respond to changes in temperature," said Connor McMahan, a graduate student at Caltech and co-first author of the paper.


Researchers also developed a device that can fold into a shape, imitating a paperclip when exposed to heat. When the device cools off, it unfolds itself back into its original shape.


Even though this research focuses on soft robots and temperature responses, liquid crystal elastomers can be programmed to respond to other factors, like pH, light, humidity, and other external stimuli.


Rollbot (above) is shown in its printed form and is shown in its folded form (below). (Image Credit: Kotikian et al., Sci. Robot. 4, eaax7044 (2019))


"This works demonstrates how the combination of responsive polymers in an architected composite can lead to materials with self-actuation in response to different stimuli. In the future, such materials can be programmed to perform ever more complex tasks, blurring the boundaries between materials and robots," said Chiara Daraio, Professor of Mechanical Engineering and Applied Physics at Caltech and co-lead author of the study.