Microplastics are found nearly everywhere on Earth; they’re pervasive on land, in the air, and in aquatic environments. And once they’re out there in the environment it can be extremely difficult to remove them, especially if they settle into nooks and crannies at the bottom of waterways.
But researchers think that small, flexible, self-propelled robots could do the job. Materials engineers have created a tiny fish-shaped soft robot that “swims” around picking up and removing microplastics, according to a new study in Nano Letters.
The soft robot, just 15mm long, is propelled by turning a near-infrared laser rapidly on and off, causing the fish’s tail to flap – moving 2.67 body lengths per second. That’s faster than has been previously reported for other soft swimming robots and is about the same speed as active phytoplankton moving in water. It can also be steered by changing the direction of the scanning light on its tail.
Because it’s so durable and can swim as fast as it does (for how small it is), the researchers say that it could be used for monitoring microplastics and other pollutants in the hard-to-reach places of complex aquatic environments.
Instead of being made out of the traditional materials used for soft robots – hydrogels and silicone rubber – that can be damaged easily in aquatic environments, this one was made from a material that takes inspiration from the microscopic gradients found in mother of pearl (also called nacre).
The researchers made what are known as composite nanosheets – two-dimensional nanostructures of β-cyclodextrin molecules embedded in a sulfonated graphene sheet. Solutions of the nanosheets were then incorporated in different concentrations into polyurethane mixtures to form a gradient.
the composite material. (c) Graphical representation of the multiple hydrogen-bonding interactions between polyurethane chains and cyclodextrin-sulfonated graphene sheets. Credit: Adapted from Nano Letters 2022, DOI 10.1021acs.nanolett.2c01375
The structure of the material includes multiple sites where microplastic pollutants can be absorbed onto its surface. This way the fish robot can repeatedly adsorb nearby polystyrene microplastics and transport them elsewhere.
Since the process is reversible, the microplastics can be subsequently removed, but even while fully loaded the soft robot can move at speeds of 1.93 body lengths per second.
It also has elastic properties and can be stretched to 20 times its original length before returning to normal after 30 minutes. And, while already durable, the material can also heal itself – with 89% efficiency – by about 12 hours after being cut.
This is possible because of the multiple hydrogen bonds linking the nanosheets and polyurethane matrix together. These attractive forces disperse at high temperatures, or when the material is stretched or cut, but gradually reform again at room temperature.
“Generally, the engineering principle behind our robot design can be potentially useful in guiding integrated robots designed for executing other functions,” the authors conclude.