Leaves of a plant shift and turn toward the sunlight throughout the day. Some manmade materials can mimic this slow but steady reaction to light energy, usually triggered by lasers or focused ambient light. Researchers discovered a way to speed up this effect enough that its performance can compete against electrical and pneumatic systems.
The team wanted to create machines where light is the only source of energy and direction. While some movement and actuation were achieved with light-driven polymers, it was too slow of a response to be practical. When the polymer sheet is flat, the light animates it slowly, curving or curling over time. The researchers found that by forming the polymer into a curved shape like a shell, the bending action happened much more quickly and generated more torque.
The researchers used a photoresponsive azobenzene-functionalized liquid crystalline polymer (ALCP) film that is 50 micrometers thick and several millimeters in width and length. A shelllike geometry was created by confining this material along its edges to create a curve. Shining light on this geometry folds the shell at a crease that spontaneously nucleates. This folding occurs within tens of milliseconds and generates torque densities of up to 10 newton-meters per kilogram (10 Nm/kg). The light-driven response is magnified by about three orders of magnitude in comparison to the material that was flat.
This approach towards scaling up the performance of light-driven polymers could enable the design of fully untethered soft robots with numerous technological applications.