Researchers have created a two-dimensional, shape-changing sheet that moves autonomously in a reactant-filled fluid. The integrated system utilizes a chemical reaction to activate the fluid motion that simultaneously transports a flexible object and “sculpts” its shape, all occurring autonomously.
This self-propulsion and reconfiguration was achieved by introducing a coating of catalysts on the flexible sheet that is roughly the width of a human hair. The addition of reactants to the surrounding fluid initiates both the sheet's motion and the changes of its form. Further, by placing different catalysts on specific areas of the sheet and controlling the amount and type of reactants in the fluid, a useful cascade of catalytic reactions was created where one catalyst breaks down an associated chemical, which then becomes a reactant for the next of the set of catalytic reactions. Adding different reactants and designing appropriate configurations of the sheet allows for a variety of actions — in this study, enwrapping an object, making a flapping motion, and tumbling over obstacles on a surface.
A microfluidic device that contains the active sheets can perform vital functions such as shuttling cargo; grabbing a soft, delicate object; or creeping along to clean a surface. The flexible micro-machines convert chemical energy into spontaneous reconfiguration and movement, enabling them to accomplish a variety of useful jobs.
If the sheet is cut into the shape of a four-petal flower and placed on the surface of a microfluidic device, the chemistry of the petals can be “programmed” to open and close individually, creating gates that perform logic operations as well as generate particular fluid flows to transport particles throughout the device. The petals can trap a microscopic ball and hold it for a finite time, then initiate a new chemical reaction on a different set of petals so that the ball moves between them in a chemically directed game of catch.
Experiments were conducted with the placement of the catalyst on different parts of the sheet to create specific motions. In one experiment, placing the catalyst on just the body of the sheet, rather than the head and tail, triggered a creeping movement similar to the movement of an inchworm. In another realization, when obstacles were placed in front of the coated sheet, it would tumble over the obstacle and continue moving, allowing it to traverse a bumpy terrain.
The research provides insight into how chemistry can drive autonomous, spontaneous actuation and locomotion in microfluidic devices. The next step is to explore microfabrication by using the interaction and self-organization of multiple sheets to bring them together into specific architectures designed to perform complex, coordinated functions. Experimenting with different stimuli such as heat and light can enable the design of mobile, 3D micro-machines that adapt their shape and action to changes in the environment — a vital step in creating the next generation of soft robotic devices.