Scientists at Northwestern University have developed a polymer that protects the enzyme PETase which breaks down polyester but has been largely unusable until now as it fails at high temperatures.

The researchers are hopeful the enzyme could provide the solution for removing microplastics from rivers and oceans.

Monica Olvera de la Cruz, the senior author of the paper, explained that in recycling, plastic is heated up, broken down and then rebuilt into weaker, poorer quality plastics. But in upcycling, breaking down polymers into their fundamental components sometimes allows them to become even sturdier plastics than they were before.

de la Cruz’s team hoped to create as green an upcycling process as possible; one that didn’t create pollutants but instead removed them. Using an enzyme that can be synthesised in a lab, the researchers developed a process without using other solvents that can be used repeatedly.

“People have discovered an enzyme — a bacteria that eats polyester to survive and converts it into monomeric units,” de la Cruz said. “But they haven’t been able to use it because it breaks down at a certain temperature. Our idea was to build polymers capable of encapsulating the enzyme to protect its structure, so that it can continue to function outside of living cells and in the lab at sufficiently high temperatures to be able to break down PET.”

In the study, the team designed a polymer and the conditions needed to effectively protect the enzyme (called PETase), so that when the structure was heated, the PETase wouldn’t unravel and become ineffective. The polymer consists of a hydrophobic (water-repelling) backbone and highly specific concentrations of its three components, calculated by first author and Ph.D. student Curt Waltmann, to specifically interact with active sites on the enzyme.

Waltmann found that too much negative charge in the polymer meant the enzyme would dissolve in the water, and the polymer would not cover enough of the enzyme surface to protect it. He had to be careful also not to put too many hydrophobic components into the polymer such that the polymer wouldn’t wrap into itself instead of wrapping around the PETase surface.

After the polymer was synthesised using a technique called free-radical polymerisation, which rapidly links monomers together, it was mixed with chemically synthesised enzymes.

“We found that if you put the complex of the polymer with the enzyme together, and close to a plastic, and then you heat it up slightly, the enzyme was able to break it down into small, monomeric units,” de la Cruz said. “In addition to operating in an environment like where it could clean microplastics, our method has protected against high-temperature degradation, and one student was able to do the testing.”

By finding a way to protect the enzyme from heat, the team opened many doors for the research community. The team has its sights set on encapsulating entire microplastics in the structure, then making an aggregate of microplastics with these enzymes.

The US Department of Energy has an initiative to fund polymer upcycling projects that would reduce plastic waste, and de la Cruz says her project will contribute to moving the initiative forward.

“You can make a new polymer with the monomeric units,” de la Cruz said. “These are dangerous things that are bad for our health. We don’t need to make more. You can reuse the ones already here to make an equally good plastic  — or better.”

Research carried out by Groningen University, The Netherlands Organization for Applied Scientific Research, and Plymouth Marine Laboratory, found nylon and polyester negatively affected the growth and repair of airway tissue.

Earlier this year, Patagonia and Samsung announced a collaborative project on a new washing machine that cleans apparel while minimising the impact of microplastics.