Researchers from Caltech (California) and the ETH (Eidgenössische Technische Hochschule, Zurich, Switzerland) have developed robots that can propel themselves without requiring motors, servos or a power supply. Instead, these unique devices paddle through water using a material that deforms with temperature changes.

The material is the motor

Through this research, the boundary between the material and what it is used for becomes blurred. In the self-propelled devices the material itself makes the machine functional. The system is based on strips of a flexible polymer that are curled up in the cold state and stretch out in the warm state. The strip is attached such that it operates a switch inside the robot; the switch, in turn, is connected to a paddle with which the robot propels itself using a 'rowing' motion.

Bistable element

The switch is a so-called bistable element – a component that has two stable states. In this case the element is built from strips of elastic material which, under pressure from the temperature-sensitive polymer, jumps from one state to the other. The research, which has been published in the Proceedings of the National Academy of Sciences, builds on earlier research where chains of bistable elements were used to transfer signals and to build a kind of logic gate.


At the moment these bistable elements have to be reset manually once they have changed shape and released their energy. The team is now researching methods to construct bistable elements in such a way that they will reset themselves when the water temperature changes again – as a result they would, in theory, continue to paddle around forever, as long as the water temperature continues to change.