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No motor, no battery, no problem

Osama R. Bilal's picture

New robot concept uses responsive materials to swim through water

When we think of robots, we normally think of motors and batteries and electronics, but engineers at Caltech and ETH Zurich have created a swimming robot that's powered by material deformation. That means it's made using materials that change shape in response to temperature swings, and that deformation pushes the device through water. 

The work blurs the boundary between materials and robots. In the self-propelled devices, the material itself makes the machine function. "Our examples show that we can use structured materials that deform in response to environmental cues, to control and propel robots," says Daraio, professor of mechanical engineering and applied physics in Caltech's Division of Engineering and Applied Science, and corresponding author of the paper.

The new creation is also powered by heat, thanks to a temperature-sensitive polymer. When cool this material is curled, but once it warms up it straightens. The team harnessed these abilities by making bistable switches out of polymer strips, building them into the robot body and connecting them to oars. When warmed up, the polymer expands and in turn flicks the oar to propel the robot forwards. 

That makes for a robot that can move without needing any kind of motor or battery. And since thicker strips of the polymer take longer to heat up, it can be fine-tuned to perform a set of movements in a specific order, using different thicknesses. So, for example, picture a swimming robot with two paddles and a little claw on the front. With the polymer switches of different thicknesses made to fire at different times, the robot can be made to swim forwards, open the claw to release a payload, then paddle back to where it started. The team managed to demonstrate just this scenario.

"Combining simple motions together, we were able to embed programming into the material to carry out a sequence of complex behaviors," says Osama R. Bilal, co-first author of the study. In the future, more functionalities and responsivities can be added, for example using polymers that respond to other environmental cues, like pH or salinity. Future versions of the robots could contain chemical spills or, on a smaller scale, deliver drugs.

Currently, when the bistable elements snap and release their energy, they must be manually reset in order to work again. Next, the team plans to explore ways to redesign the bistable elements so that they are self-resetting when water temperature shifts again—making them potentially capable of swimming on indefinitely, so long as water temperature keeps fluctuating.

The research was published in the journal Proceedings of the National Academy of Sciences ( The robot can be seen in action in the video:

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