Bacteria-sized bots break down plastics in their path

Microplastics are fragments of plastic less than half a centimetre long, often contained in toiletries or shedding from polyester clothing. They pose a particularly serious threat to marine ecosystems and can absorb heavy metals and pollutants, whilst also travelling up food chains to humans with largely unknown impacts on human health. They are considered a particularly tricky element of the plastic waste problem due to the difficulty of removing them from the environment.

They are present in virtually every corner of the Earth, with recent studies identifying microplastics on top of the Alps, in Antarctic ice and at the depths of the oceans.

Left alone, microplastics can take hundreds of years to completely degrade. Catalysts activated by sunlight can speed up this breakdown (sunlight-driven photocatalysis) but getting these compounds to interact with microplastics is a challenge. In this proof-of-concept study, Czech researchers developed “intelligent visible-light-driven microrobots” which hunt pieces of plastic and break them down.

Previously, scientists and engineers have proposed a low-energy process of removing plastics in the environment using catalysts which produce highly reactive compounds under sunlight, which help break down these polymers quickly. However, putting these tiny flakes of plastic into contact with the catalysts has proved a serious challenge, usually requiring pre-treatments or bulky mechanical stirrers which cannot be scaled up to useful sizes by any easy means.

Martin Pumera and his colleagues at the University of Chemistry and Technology wanted to create a sunlight-propelled catalyst which actively moves towards microplastics, latches onto them and dismantles them. In order to transform a catalytic material into solar-powered microrobots, the researchers created star-shaped particles of bismuth vanadate just micrometres wide and evenly coated the particles with magnetic iron oxide. These tiny devices can swim down a maze of channels, thanks to their built-in photocatalytic and magnetic properties, and interact with pieces of plastic along their path. It may even be possible to precisely control the devices using a magnetic field inside the channels.

The researchers found that under visible light the microrobots strongly stuck to four common types of plastic, particularly polylactic acid and polycaprolactone. This process, which otherwise would require mechanical stirring, was caused by local self-stirring effects at the nanoscale, encouraging more interaction with the microplastics.

After illuminating pieces of the four plastics covered with the catalyst for seven days in a dilute hydrogen peroxide solution, they saw that the plastic lost three per cent of its mass and developed a pitted surface texture. Small molecules and components of the plastics were left behind in the solution.

The researchers hope that these self-propelled robotic catalysts will be a step towards active systems for capturing and degrading microplastics in tricky-to-reach locations.

“[This study] has shown for the first time the possibility of efficient degradation of ultrasmall plastic particles in confined complex spaces, which can impact research on microplastic treatments, with the final goal of diminishing microplastics as an emergent threat for humans and marine ecosystems,” the researchers wrote.