A team of engineers at North Carolina State University has developed a new design for thermal actuators that could be used to create rapid movement in soft robotic devices.
‘Using thermal actuation is not new for soft robots, but the biggest challenge for soft thermal actuators was that they were relatively slow – and we’ve made them fast,’ said Yong Zhu, the Andrew A Adams distinguished professor of mechanical and aerospace engineering at NC State.
Actuators create motion in a device by converting energy into work. ‘What makes this new actuator design work is a structure with a bi-stable design,’ said NC State PhD student Shuang Wu, who led the research. ‘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 that is also stable.’
The new thermal actuator is made of a bi-stable material the shape of which is dictated by temperature. The researchers created the actuator by layering two materials on top of each other, with silver nanowires sandwiched between them. The two materials have different coefficients of thermal expansion, which means that they expand at different rates as they heat up. In practical terms, this means that the structure bends when it’s heated.
This layered material is then shaped into a design that gives it a default curvature in a single direction. When a voltage is applied to the silver nanowires, the material heats up. Once it reaches a critical temperature, the material snaps into the new default shape, bending rapidly in the opposite direction.
When the voltage is removed, the temperature decreases. Once it passes another, lower, critical temperature, the material rapidly snaps back to its previous default shape. By applying current to the nanowires in a regular pattern, the material can be made to snap back and forth between the two default shapes.
To demonstrate the technique, the researchers created two prototypes – one that emulates the snapping behaviour of a Venus flytrap and another that can ‘crawl’ at more than one body length per second.
‘Potential applications range from biomedical applications to prosthetic devices to high-end manufacturing,’ Zhu says. ‘Any application in which you would want to be able to move quickly, but also want to avoid rigid materials and conventional robotics.’
The team is now working on developing sensor and control mechanisms that could more fully automate the actuation process. ‘We’re also interested in exploring other possible materials, so that we can fine-tune the thermal and mechanical properties,’ Zhu says. ‘This could allow us to tailor both actuator speed and force.’
The research has been published in the Soft Robotics.