We all know the plastic soup, the polluted beaches and the turtles with plastic bags in their stomachs, but what not everyone knows is that there is also a lot of plastic on the seabed between coral and under rocks. To clear that up, small robots have been developed.

It is very difficult to get hold of those small plastic particles as they nestle in every nook and cranny on the bottom of seas, rivers and ports. Chinese researchers have therefore developed a light-activated fishing robot that swims around and picks up and takes microplastics with it, the scientists write in scientific journal Nano Letters from ACS.

Pearl

The uniqueness of the robots, which look like fish, is the material used. Previously, hydrogels and elastomers were used, but these are easily damaged on the rough seabed. A better material would be mother-of-pearl, the researchers write. This is strong and flexible and is on the inside of shells. Mother-of-pearl consists of several layers of calcium carbonate with a layer of silky proteins in between. This structure makes the material both strong and resilient.

Inspired by this, researcher Xinxing Zhang and his colleagues wanted to develop a similar structure for the flexible fishing robots, which is durable and flexible. The researchers combined β-cyclodextrin molecules and sulphonated graphene, creating very strong, thin layers. These were alternated with layers of a mixture of polyurethane (PU) and latex. Like mother-of-pearl, the material of the fishing robot was built up layer by layer. The robot is 15 millimeters long. Quickly turning an infrared laser on and off at the fish’s tail causes it to move and propel the robot.

The fishing robot in action. Photo: Nano Letters 2022

As fast as phytoplankton

The robot can move at 2.67 body lengths per second. That is much faster than existing fishing robots and about as fast as phytoplankton. The researchers showed that the swimming fish robot adsorbed microplastics several times and could thus take them elsewhere, where they were cleaned up. The material also has a self-healing effect if it is damaged. According to the researchers, the durability and speed of the fishing robot make it suitable for monitoring the seabed and for carrying plastic that it encounters along its way, especially in harsh sea conditions.

Whether it is actually a solution for the plastic on the seabed remains to be seen. Professor Bart De Schutter of TU Delft tells Scientias.nl that scalability will be difficult. “You need a lot of robots to really clean up a lot of microplastics.”

SeaClear Project

He himself is working with a team from TU Delft, among others, on a different method for collecting plastic on the seabed in a fully autonomous way. The Chinese project can be ‘a nice addition’ to this, according to De Schutter. His SeaClear project focuses on larger pieces of plastic. “We are trying to clean it up before it breaks down into microplastics.”

The project consists of a mothership and two underwater robots. One is a reconnaissance robot that scans the seabed for plastic and the other cleans up waste, such as bottles and cans. A drone searches for plastic from the air, provided the water is clear enough. The underwater robot then cleans up the plastic that the drone detects. “The underwater robot has grippers with a grid structure, so that the plastic is retained, but fish and other sea creatures can swim out again,” explains De Schutter.

cloudy water

The robot brings the plastic to a container that is also on the bottom and sends out signals so that the robot can find it. “There has already been a test in Dubrovnik, where the water is very clear and the drone was therefore able to detect a lot of plastic from the air,” says the professor. “In addition, the system was recently tested in Hamburg, where the water is very turbid. Because of the mud, visibility was sometimes only a few centimeters at most. The sonar was supposed to help detect the plastic, but the system turned out to be especially suitable for metals in this type of water,” he explains. “That is why we are now looking for a solution to be able to clean up plastic even in cloudy water. A combination of methods is being considered.” The SeaClear team also conducted further research in Hamburg into the extent to which the underwater robot could find the container in which to put the plastic.

Tourist areas

The SeaClear project is virtually unique. There is one other project that actually cleans up really big junk, but that requires human intervention, and now there are the Chinese fishing robots, which are targeting the microplastics. “Ultimately, the system must be rented out to tourist areas and ports, so that they can be cleaned again,” says De Schutter, who explains that the last tests received a lot of attention from the media and other interested parties. “There will be another test in Marseille and then demonstrations in Hamburg and Dubrovnik.” And then the underwater robots can really get to work.

How much plastic is there in the sea?

Estimates of the amount of plastic in the sea vary. National Geographic writes that the plastic waste that ends up in the world’s oceans every year will triple to 29 million tons per year by 2040. In 2015, there would be approximately 150 million tons of plastic in seawater. That would increase to 600 million tons by 2040. The oceans alone contain 26 to 66 million tons of plastic. 94 percent of it is on the bottom. The World Wildlife Fund states that an amount of plastic ends up in the sea every hour, which you can use to fill eleven swimming pools.