Food digestion points the way to more efficient plastic recycling

The way nature processes old proteins to create new ones has led NCCR Bio-Inspired Materials researchers at Lausanne’s Federal Institute of Technology (EPFL) to develop a potentially more efficient approach to plastic recycling.

More than 400 million tons of plastics are produced every year, and dozens of them end up in the natural environment, making plastic pollution one of the most pressing global issues. The NCCR scientists decided to tackle the problem by investigating how nature degrades proteins. These molecules, much like plastic, are polymers made by several building blocks, or monomers. “Proteins are the most abundant polymers on earth, and they’re sustainable,” explains NCCR Principal Investigator Prof. Francesco Stellacci, who co-led the research at EPFL’s School of Engineering. Unlike synthetic polymers, proteins are sequence-defined polymers, which means that the sequence of their building blocks is essential — as it determines a protein’s structure and function — and is dictated by the genetic instructions ‘written’ in the DNA.

When we digest food, proteins break up into their constituent parts. Then, cells put these building blocks back together in different orders to form new proteins, depending on the cell’s specific needs. To try to replicate this natural cycle outside a living cell, Stellacci and his colleagues selected specific proteins and chopped them up into individual building blocks. Then, they put the building blocks into a system containing the cellular machinery that translates genetic instructions into proteins. Using this approach, the researchers were able to transform silk and a mixture of hormones into a specific protein that emits fluorescence when exposed to light. The findings were published in the journal Advanced Materials.

“This is a great nature-inspired approach to recycling, where you take a random mixture of the starting material, digest it in its components, and build from it what you need,” Stellacci says. He notes that proteins produced with this approach have the same quality of a newly made protein, whereas plastic loses some of its quality every time it is recycled in a conventional way. The study is a proof of concept that a similar recycling approach could be used also for plastic. The idea, according to the researcher, is that in the future, plastic polymers should also be sequence-defined, just like proteins. They would then be degraded in a ‘maxi stomach’, which would break them into single monomers. Finally, a computer would instruct a polymer-making machine about the type of polymer to produce. “One day it may be a soft polymer to make clothes, the day after it may be a hard polymer to make chairs, pens or computers,” Stellacci says.

However, he adds, it will take years — if not decades — before this approach can be applied in the real world. For one, every cell has a polymer-making machine, called the ribosome, that is able to translate genetic instructions into new proteins. “For synthetic polymers, we don’t have a ribosome,” Stellacci points out. And to make plastic recycling sustainable, he adds, we would need ribosomes on industrial scales. “The challenges are huge.” Next, the researchers hope to scale up their approach to recycle grams of synthetic polymers into materials. “Now, we are doing this at microgram level,” the NCCR PI explains. Despite all the challenges, this protein-inspired approach represents a new way of looking at recycling, he says. “The dream is that we’ll be able to throw a bunch of old plastics together and transform them into plastic that is as good as new plastic and is different every day.”

Reference: Giaveri, S.; Schmitt, A. M.; Roset Julià, L.; Scamarcio, V.; Murello, A.; Cheng, S.; Menin, L.; Ortiz, D.; Patiny, L.; Bolisetty, S.; Mezzenga, R.; Maerkl, S. J.; Stellacci, F. Nature-Inspired Circular-Economy Recycling for Proteins: Proof of Concept. Advanced Materials 2021, 33 (44), 2104581.https://doi.org/10.1002/adma.202104581

Author: Giorgia Guglielmi