Delft researchers put living microalgae on a two-layer hydrogel strip: photosynthesis raises the pH and bends it one way, and in the dark the algae’s own respiration bends it back.

Most soft robots are driven by something unglamorous: a pump, a compressed air line, a heating element, a battery. The actuator may be a gentle silicone finger, but behind it sits hardware that needs power and plumbing, and that is usually what stops such devices from going anywhere on their own.

A team at Delft University of Technology has built one whose power source is alive. Writing in Nature Communications, they describe a soft actuator that bends when light falls on it and bends back when the light goes off, driven by microalgae living on its surface.

“Our proof-of-concept actuator is a green and soft strip made of two different hydrogel layers,” said Professor Marie-Eve Aubin-Tam, who led the research. “It is about 2 cm long and 1 mm thick. When exposed to changing light conditions, the strip can bend back and forth.”

How photosynthesis becomes movement

The mechanism is a chain of three steps, and each one is simple.

When light falls on the strip, the algae photosynthesise. Photosynthesis consumes dissolved carbon dioxide, and removing carbon dioxide from water raises its pH — the familiar chemistry of a planted aquarium, where pH climbs through a sunny afternoon.

The pH change is what the material responds to. “Since the two hydrogel layers respond differently to pH changes, they swell to different degrees, causing the strip to bend,” said Fan Jiang, a PhD candidate at Delft and the study’s first author.

A bilayer that bends because its two halves swell unequally is a well-established trick in soft robotics and in nature — it is how a pine cone opens as it dries. What is unusual here is the signal: the layers are not responding to heat or solvent but to a chemical change generated by organisms sitting on top of them.

The return stroke is the part that makes it a cycle rather than a one-way actuator. In the dark the algae metabolise their stored fuel, releasing carbon dioxide, which lowers the pH again. The strip begins bending the opposite way within 10 seconds of the light going off.

That the reverse motion is also biological is what distinguishes this from a light-driven material that simply relaxes when illumination stops. Both directions are powered by the algae’s metabolism; the light only determines which way the metabolism is running.

What it can pick up

The demonstrations were deliberately physical. The actuator hooked and lifted a rubber ring, and grasped a small metal ball.

Lifting a metal ball with a two-centimetre hydrogel strip is a better test than it sounds, because hydrogels are mostly water and not notably strong. The release gives no force or payload figures, which is the most significant omission: without them there is no way to judge whether this is a mechanism that could be scaled toward useful work or a demonstration that happens to manage a bead.

The timing is the other obvious constraint. Ten seconds to begin reversing is slow by the standards of any actuator with a motor in it, and the rate is set by how fast algae can change the chemistry of the water around them. That is not a parameter engineering can push very far.

Not starting from an application

Aubin-Tam is candid that the work was not aimed at a product. She says the team did not begin with a specific application in mind, the goal having been something else — and the release’s suggested uses, light-responsive blinds and agricultural devices, read as plausible destinations rather than targets.

The reason for the interest is the organism. “Microalgae are plantlike organisms that are largely self-sustaining,” she said.

That is the serious claim underneath the novelty. An actuator whose energy comes from ambient light via a self-maintaining organism needs no battery, no fuel line and no recharging — it needs light, water and the occasional nutrient. For devices meant to sit outdoors for long periods doing something slow and occasional, which describes a great deal of agricultural and environmental monitoring equipment, that trade is attractive in a way it is not for a robot that has to move quickly.

The qualifier in “largely self-sustaining” is doing work, though. Algae need nutrients, they are vulnerable to contamination by other organisms, and a culture can crash. A device that depends on keeping a specific population alive has a failure mode that a battery does not, and the release does not say how long these actuators kept working.

Watching metabolism happen

Aubin-Tam offers a motive that is not about applications at all. “It is also just exciting to directly visualise the metabolic activity in microorganisms to be honest,” she said.

There is something to that beyond charm. The strip is a mechanical readout of photosynthesis and respiration, converting a chemical process that normally requires instruments to detect into a visible bend. As a demonstration object it makes an invisible metabolism legible, which is a use in itself.

The broader field this belongs to — sometimes called biohybrid robotics — has produced swimming devices driven by heart muscle cells and walkers driven by engineered muscle tissue, most of them requiring careful incubation and lasting days. Plant-type cells are hardier than animal ones, and an actuator that runs on light rather than on a nutrient broth is a step toward something that could survive outside a laboratory.

The release does not name the team’s other members and does not state the study’s funding sources.

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Notes

1. Jiang, F., et al. Nature Communications, 2026. DOI: 10.1038/s41467-026-78653-z.
2. Delft University of Technology. “A soft robot arm powered by algae and light.” EurekAlert!, 22 October 2026. https://www.eurekalert.org/news-releases/1148382.

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