What if a robot could be squishy? Soft, flexible robots could be useful for a wide variety of applications, including artificial muscles and wearable devices—or really any application where a robot must be especially delicate.

This is what has fueled development of “soft actuators” that can flex like a bicep and have a fine-tuned sense of touch as in the human hand. Like your stereotypical robot, soft actuators convert energy—such as electricity, heat, or light—into mechanical motion, but they look nothing like the metal and wire constructs that the word “robot” most often evokes.

One variety of these soft robots, known as bioactuators, utilizes the functions of living cells to generate motion. These biological machines have attracted particular attention in recent years, and now a research team at Delft University of Technology in the Netherlands has developed a soft robotic arm colonized by algae on its surface, and when the microbes photosynthesize, the arm moves.

“We didn’t start with a specific application. Our goal has always been to bring the research of living materials to new levels,” Marie-Eve Aubin-Tam, who led the research, explained in a press release. “We wanted to explore what becomes possible when living cells are integrated into functional materials.”

The tiny robot arms—about three-quarters of an inch long and about as thick as a credit card—are constructed of two layers of hydrogel that the algal cells live on . One of these layers responds to changes in pH, which are mediated by the metabolism of the algae.

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