Microplastics are the breakdown products of the plastic objects we use in everyday life – wrappings, packaging, bottles, synthetic clothing, and kitchen ware, for example. None of these plastics is biodegradable but, in the presence of sunlight and normal environmental conditions, they perish into small fragments that find their way into the oceans, rivers, foods, drinking water and even the blood of humans and other animals. They are one of the world’s biggest environmental problems because, once dispersed, they are very difficult to remove.
Scientists have previously proposed the use of soft robots to collect up these microscopic particles of plastic, but soft robots are commonly made of hydrogels or silicone rubber, which are mechanically inferior and easily damaged in ocean conditions. In a new study, published by the American Chemical Society in the journal Nano Letters, scientists from China and Germany have used nanotechnology to develop a light-activated fish robot that is made of composite sheets of graphene and can ‘swim’ around quickly and remove microplastics from the environment.
“It is of great significance to develop a robot to accurately collect and sample detrimental microplastic pollutants from the aquatic environment,” said Yuyan Wang, a researcher at the Polymer Research Institute of Sichuan University and one of the lead authors on the study. “To the best of our knowledge, this is the first example of such soft robots.”
The experts used a substance called nacre, also commonly known as mother-of-pearl, upon which to base the design of a new material. Nacre is strong and flexible, and is found on the inside surfaces of clam shells. Nacre layers have a microscopic gradient, going from one side with lots of calcium carbonate to the other side with mostly a silk protein filler. Inspired by this substance, Xinxing Zhang, also from the Polymer Research Institute, Sichuan University, and colleagues wanted to try a similar type of gradient structure to create a durable and bendable material for soft robots.
The team created this new material by layering various microscopic sheets of molecules according to nacre’s specific chemical gradient. They developed an elastomer actuator with sulfonated graphene-based gradient nanostructures that produced a stretchy, flexible material which is tough and can tolerate high temperatures.
From this material, the researchers made robo-fish that can bend and twist, and even pull along a weight of up to 5kg. Most importantly, the bionic fish can absorb nearby free-floating bits of microplastics because the organic dyes, antibiotics, and heavy metals in the microplastics have strong chemical bonds and electrostatic interactions with the fish’s materials. In this way, the particles are attracted to the surface of the fish and are then absorbed and removed from the water.
“After the robot collects the microplastics in the water, the researchers can further analyze the composition and physiological toxicity of the microplastics,” said Wang.
The tiny fish robot is only 15mm long (about half-an-inch) but it responds to a near-infrared laser light by flapping its tail and ‘swimming’ forward. It can move at 2.67 body lengths per second, a speed that is about the same as that of active phytoplankton moving in water. At this stage, the robo-fish only functions on the surface of the water, where it would be able to absorb floating microplastics but not those deeper down in the water column.
In addition, the newly developed nanocomposite material from which the robots are made is able to heal itself, said Wang, who specializes in the development of self-healing materials. This means the robot fish can heal itself back to 89 percent of its former ability and continue to absorb pollutants, even if it is damaged or torn – which could happen if it goes looking for microplastics in rocky environments or in rough waters.
At this stage, the fish robot is just a proof of concept, Wang notes, and much more research is needed – especially into how this technology could be deployed in the real world. For example, Wang’s team will soon be working on more functionally complex fish robots that can go below the water surface in search of microplastics in the water column. Still, this bionic design could inspire other, similar projects, Wang said. “I think nanotechnology holds great promise for trace adsorption, collection, and detection of pollutants, improving intervention efficiency while reducing operating costs.”