Self-folding soft robots inspired by origami

Researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Caltech Division of Engineering and Applied Science have developed soft robotic systems, inspired by origami, that can move and change shape in response to external stimuli — a breakthrough that paves the way for fully untethered soft robots.

Using materials known as liquid crystal elastomers that change shape when exposed to heat, the research team 3D printed two types of soft hinges that fold at different temperatures and thus can be programmed to fold in a specific order. Their work was described in the journal Science Robotics.

“Using hinges makes it easier to program robotic functions and control how a robot will change shape,” said Caltech graduate student Connor McMahan, co-first author on the paper. “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.”

Co-first author Arda Kotikian, a graduate student at SEAS, added, “With our method of 3D 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.”

To demonstrate this method, the team built several soft devices — including an untethered soft robot (the ‘Rollbot’) that begins as a flat sheet, about 8 cm long and 4 cm wide. When placed on a hot surface, about 200°C, one set of hinges folds and the robot curls into a pentagonal wheel. Another set of hinges is embedded on each of the five sides of the wheel. A hinge folds when in contact with the hot surface, propelling the wheel to turn to the next side, where the next hinge folds. As they roll off the hot surface, the hinges unfold and are ready for the next cycle.

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

McMahan added, “Many existing soft robots require a tether to external power and control systems or are limited by the amount of force they can exert. These active hinges are useful because they allow soft robots to operate in environments where tethers are impractical and to lift objects many times heavier than the hinges.”

Another device, when placed in a hot environment, can fold into a compact shape resembling a paper clip and unfold itself when cooled. As noted by Kotikian, “All we need to do is expose the structures to specific temperature environments, and they will respond according to how we programmed the hinges.”

While this research only focused on temperature responses, liquid crystal elastomers can also be programmed to respond to light, pH, humidity and other external stimuli.

Image credit: ©stock.adobe.com/au/phonlamaiphoto

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