Researchers have developed a new propulsion concept for swimming robots that exploits temperature fluctuations in the water for propulsion without the need for an engine, propellant, or power supply.
As a proof-of-concept study, the researchers developed a 7.5-centimeter mini-submarine equipped with paddles that were fabricated entirely using a multi-material 3D printer. The paddles are actuated using a bistable propulsion element triggered by two shape memory polymer strips. Designed to expand in warm water, the polymer strips power the robot by acting like muscles. If the water in which the mini-submarine floats is heated, the expansion of the muscles causes the bistable element to quickly snap, triggering a paddle stroke. The directional motion, force, and timing of the paddle strokes are precisely defined by the robot's geometry and material.
At present, each actuating element can execute a single paddle stroke and must then be reprogrammed manually; however, it is possible to fabricate complex swimming robots with multiple actuators. A mini-submarine was made that can paddle forward with one stroke, release its “cargo” (a coin), and then navigate back to the starting point with a second paddle stroke in the opposite direction, all by sensing changes in temperature of the water. Varying the geometry of the polymer muscles allowed the scientists to define the sequence at which the paddle stroke is triggered: thin polymer strips heat up faster in warm water and therefore respond faster than thicker ones.
A potential development would be using polymers that do not react to the water temperature, but to other environmental factors such as the acidity or salinity of the water.