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Bacteria-sized bots break down plastic in their path

Researchers at the University of Chemistry and Technology in Prague have developed self-driving microrobots that can swim, cling to and break down plastics.

Microplastics are pieces of plastic less than half an inch long, often contained in toiletries or polyester clothing waste.

They pose a particularly serious threat to marine ecosystems and can absorb heavy metals and pollutants, while also traveling through food chains to humans with largely unknown impacts on human health.

They are considered a particularly troublesome part of the plastic waste problem because they are difficult to remove from the environment.

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

Left alone, it can take hundreds of years for microplastics to completely degrade. Catalysts activated by sunlight can accelerate this degradation (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-powered microrobots” that hunt for and break down pieces of plastic.

Previously, scientists and engineers have proposed an energy-efficient process for removing plastics in the environment using catalysts that produce highly reactive compounds under sunlight, which help these polymers break down quickly.

However, bringing these small plastic flakes into contact with the catalysts has proved a serious challenge, usually requiring pretreatments or bulky mechanical stirrers that cannot be easily scaled up to usable sizes.

Martin Pumera and his colleagues at the University of Chemistry and Technology wanted to create a sunlight-powered catalyst that actively moves toward, clings to and dismantles microplastics.

To convert a catalytic material into solar-powered microrobots, the researchers created star-shaped particles of bismuth vanadate just micrometers wide and evenly coated the particles with magnetic iron oxide.

These tiny devices can swim through a maze of channels, thanks to their built-in photocatalytic and magnetic properties, interacting with bits of plastic along their path. It may even be possible to precisely control the devices using a magnetic field in the channels.

The researchers found that under visible light, the microrobots adhered strongly to four common types of plastic, specifically polylactic acid and polycaprolactone.

This process, which would otherwise require mechanical stirring, was triggered by local nanoscale self-stirring effects, encouraging more interaction with the microplastics.

After exposing pieces of the four plastics covered with the catalyst in a dilute hydrogen peroxide solution for seven days, they saw that the plastic lost three percent of its mass and developed a pitted surface texture.

Small molecules and components of the plastics remained in the solution. The researchers hope that these self-propelled robotic catalysts will be a step towards active systems for capturing and breaking down microplastics in hard-to-reach locations.

“[This study] has demonstrated for the first time the possibility of efficient degradation of ultra-small plastic particles in confined complex spaces, which could influence research into microplastic treatments, with the ultimate goal of reducing microplastics as an emerging threat to humans and marine ecosystems,” the researchers wrote.