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EPFL engineers pioneer a novel method for recycling plastics

Two EPFL engineers have developed a revolutionary new method for combating plastic pollution by harnessing the inner workings of proteins. Their method is the first of its kind. As a result, what happened? Plastic recycling is being approached from a completely different perspective.

Every human being consumes approximately 30 kg of plastic per year on average. Given that the average global life expectancy is currently approximately 70 years, each person will discard approximately two metric tons of plastic over the course of his or her life. When you multiply that figure by the total number of people on the planet – which is constantly increasing – the result is mind-boggling. Given this, Francesco Stellacci, a full professor at EPFL's School of Engineering and the director of the Supramolecular Nanomaterials and Interfaces Laboratory, began considering whether there was a way to solve the problem of used plastics and recycle them more effectively. Stellacci established a collaboration with Prof. Sebastian J. Maerkl in the Bioengineering Institute at EPFL, and the two decided to co-advise a PhD student, Simone Giaveri. The team's conclusions, which are based on scientific research, were published in the journal Advanced Materials.

Following a review of the existing plastic recycling options available, the engineers decided to devise a completely new method of recycling plastic. We must either stockpile or bury the residue left behind by the degradation process when we use biodegradable plastics. The more land that is allocated for this, the less land that is available for farming, and there are environmental consequences to consider as bio-degradation products change the ecosystem of the area,” says Stellacci. “The more land that is allocated for this, the less land that is available for farming.” So, how do we come up with a comprehensive solution to the problem of plastics recycling that will work for everyone? It is possible that nature itself will provide part of the solution.

Proteins are one of the most important organic compounds that make up the structure of our world. Proteins, like DNA, are members of the polymer family; proteins are long chains of molecules, or monomers, known as amino acids, that are linked together. “A protein can be thought of as a string of pearls, with each pearl representing an amino acid. A different color is assigned to each pearl, and the color sequence determines the string structure and, as a result, the properties of the pearls. The amino acids that make up protein chains are broken down in the natural world, and cells reassemble these amino acids to form new proteins, which is analogous to creating new strings of pearls with a different color sequence, according to Professor Giaveri.

The first step in replicating this natural cycle in the lab was made by Giaveri, who attempted to do so without the use of living organisms. “We chose proteins and subdivided them into amino acids,” says the researcher. He continues, "We then placed the amino acids into a cell-free biological system, which assembled the amino acids back into new proteins with entirely different structures and applications." For example, Giaveri and Stellacci were successful in converting silk into a protein that can be used in biomedical technology applications. “Most importantly, when proteins are broken down and reassembled in this manner, the quality of the proteins produced is identical to the quality of a newly synthesized protein. Indeed, you are constructing something entirely new,” Stellacci observes.

How the process of protein assembly relates to plastic recycling, you might wonder. Because both compounds are polymers, the mechanisms that occur naturally in proteins could be applied to plastics as well, which would be a significant advance. Stellacci cautions that, while this analogy may appear to be promising, the development of such methods will not happen overnight. A fundamentally different frame of mind will be required. Polymers are strings of pearls, but synthetic polymers are made mostly of pearls of the same color, and when the color is different, the order in which the pearls are strung does not matter. Polymers are also used in jewelry. Furthermore, we do not have an efficient method of assembling synthetic polymers from different color pearls in a way that allows us to control the sequence in which they are assembled.” Nevertheless, he would like to highlight the fact that this new approach to plastic recycling appears to be the only one that appears to adhere to the circular economy's guiding principle. In the future, pushing upcycling to its logical conclusion will entail combining a large number of different objects and recycling the resulting mixture to produce a different new material every day. Nature already does this,” he says in his conclusion.

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