A motorless mini-submarine has been created by researchers from ETH Zurich and the California Institute of Technology. The 3D-printed vessel moves using a new propulsion concept designed for swimming robots, enabling it to paddle without engine power.

By responding to temperature changes in water, the robot moves its paddles back and forth, gently — albeit briefly and awkwardly — propelling itself forward.

“Imitating the motion of rowing boats, frogs or water striders, the swimming robot propels itself forward by driving its oars back,” Kristina Shea, an ETH Zurich professor of engineering design who led the project, told Digital Trends.

Rather than relying on an electric motor that connects to a power source, the robot’s oars are activated using a similar mechanism used in snap-on bracelets. This activation is triggered by a “shape memory polymer” strips, which act like muscles.

“A shape memory polymer is a type of plastic that becomes compliant when heated,” Shea explained. “In the heated condition, it can be ‘programmed’ from its permanent shape to a secondary shape. This secondary shape becomes stable when the shape memory polymer is cooled down, yet it does not lose its ‘memory’ of its permanent shape. When it is reheated, it recovers its permanent shape.”

In recovering its permanent shape, force is transferred to an internal mechanism, causing the oars to snap back and drive the robot forward. The shape memory polymer is designed to expand in water, so when the water is heated, they serve as something like muscles for the machine.

The machine is still very primitive, capable of performing just a single paddle stroke and small tasks like dropping a coin. But by demonstrating that this propulsion method is feasible, the researchers hope to develop more complex techniques.

“The main takeaway from our work is that we have developed a new and promising means of propulsion that is fully 3D printed, tune-able, and works without an external power source,” Shea said in a statement.

Among these possible applications, Shea and her team envision similar systems providing low-cost power sources for ocean exploration. In addition to responses to changes in water temperature, these future versions may be sensitive to things like acidity and salinity.

A paper detailing the study was published this week in the journal Proceedings of the National Academy of Sciences.