Tiny fish-shaped robot ‘swims’ around picking up microplastics

A light-activated fish-shaped robot collects microplastics as it swims. Photo Credit: American Chemistry Council/Adapted from Nano Letters 2022, DOI: 10.1021/acs.nanolett.2c01375

Microplastics are found nearly everywhere on the planet and can be harmful in lots of ways, one of which is to animals if they’re ingested. Making matters worse, it’s extremely hard to remove such tiny particles from the environment, especially once they settle into nooks and crannies at the bottom of waterways.

Now, a team of Chinese scientists from Sichuan University in southwest China have created a light-activated fish robot that “swims” around quickly, picking up and removing microplastics from the environment.

The application improves on traditional hydrogel and elastomer materials used for soft robots, which can be damaged easily in aquatic environments. Looking at a different material called mother-of-pearl, also known as nacre, which is strong and flexible, the Sichuan research team tried using a similar type of gradient structure to create a durable and bendable material for soft robots. This natural substance is found on the inside surface of clam shells, and its layers have a microscopic gradient, going from one side with lots of calcium carbonate mineral-polymer composites to the other side with mostly a silk protein filler.

So, the researchers then linked β-cyclodextrin molecules to sulfonated graphene, creating composite nanosheets. Then solutions of the nanosheets were incorporated with different concentrations into polyurethane latex mixtures. After a layer-by-layer assembly method, an ordered concentration gradient of the nanocomposites was created through the material, and the team formed a tiny fish robot that was 15 mm (about half an inch) long. Rapidly turning a near-infrared light laser on and off at a fish’s tail caused it to flap, propelling the robot forward – moving up to 2.67 body lengths per second – a speed that’s faster than previously reported for other soft swimming robots and that’s about the same speed as active phytoplankton moving in the water.

The researchers showed that the swimming fish robot could repeatedly adsorb nearby polystyrene microplastics and transport them elsewhere, and the material could also heal itself after being cut, still maintaining its ability to adsorb microplastics. The durability and speed of the fish robot could allow it to be used for monitoring microplastics and other pollutants in harsh aquatic environments, the researchers believe.