Soft-robots snap into action with thermal actuators

Thermally actuated soft robots are not new, but they have been relatively slow. “We’ve made them fast,” said Yong Zhu, corresponding author a paper detailing the work in Soft Robotics. Zhu is also the Andrew A. Adams Distinguished Professor of Mechanical and Aerospace Engineering at NC State.

“What makes this new actuator design work is a structure with a bi-stable design,” said Shuang Wu, first author of the paper and a Ph.D. student at NC State. “Think of a snap hair clip. It’s stable until you apply a certain amount of energy [by bending it over], and then it snaps into a different shape – which is also stable.”

New technique prints silver nanowires that stretch and flex

To make the material, the researchers layered two materials on top of each other, with silver nanowires in the middle. The two materials have different coefficients of thermal expansion, so they expand at different rates as they heat up.

This layered material was then shaped into a design that gives it a default curvature in one direction. When voltage is applied to the silver nanowires, the material heats up, making it bend in the other direction. When a certain temperature is reached – the critical temperature – the material snaps into its new default shape, curving up rapidly. When the voltage is removed, the temperature goes back down. Once it cools past another critical temperature, the material snaps back to its previous default shape, curving down rapidly. By applying current to the nanowires in a regular pattern, the material can be made to snap back and forth.

To demonstrate the technique, the researchers created two prototypes. One of the prototypes emulates the snapping behaviour of a Venus flytrap, while the other is a ‘crawler’ capable of moving more than one body length per second.

“Potential applications range from biomedical applications to prosthetic devices to high-end manufacturing,” Zhu said in a statement. “Any application in which you’d want to be able to move quickly, but also want to avoid rigid materials and conventional robotics.”

Next steps include developing sensor and control mechanisms that could more fully automate the actuation process, allowing it to operate more efficiently than purely manual controls.

“We’re also interested in exploring other possible materials, so that we could fine-tune the thermal and mechanical properties,” Zhu said. “This could allow us to tailor both actuator speed and force.”