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. When light strikes the algae, they begin to photosynthesize. By absorbing carbon dioxide and fixing nitrogen in this process, the algae make the local pH more basic, making the responsive hydrogel swell. Conversely, when it becomes dark, the algae release carbon dioxide through respiration, returning the pH to the acidic side and the hydrogel shrinks.

The difference in the volumes of the two hydrogel layers controls the bending. According to the researchers, the pH-responsive hydrogel layer changed significantly with the algae’s activity. In acidic conditions, such as between pH 3 and 7, the layer was approximately 37 percent of its original weight, but it had quadruple its original weight at pH 11.

Other researchers have constructed soft robots that respond to pH, but typically the chemistry of all the surrounding liquid has to be altered. This is not the case for the algal bots. The photosynthetic activity has an effect that’s much more local, meaning it would be better suited to applications where minimal environmental impact is essential.

These aren’t, however, the speediest of robots. The chemical shift begins quickly, about 20 seconds after light is applied and about 10 seconds after it becomes dark, but it takes several hours for the hydrogel to bend significantly.

After their first proof of concept, the research team constructed several more algal devices. One was a four-fingered gripper that closes when exposed to light. It successfully grasped and lifted a hollow metal sphere, releasing it when the light was turned off. Another hook-shaped gripper could pick up and put down a rubber ring. These items were extremely light, weighing far less than a small paperclip, but the arms were able to grasp them with great dexterity.