“Biodegradable” plastics exist, but they’re not so great at biodegrading. These polymers often take months or years to decompose, and even then can form potentially harmful microplastics. Now scientists have been able to speed up the process by encasing plastic-chomping enzymes in a protective coating and incorporating the resulting nanoparticles into the plastic as it’s made (Nature, 2021, DOI: 10.1038/s41586-021-03408-3). Exposure to humidity and temperatures of 40 to 60 °C unleashes the enzymes, which decompose the polymers into monomer to trimer units in hours to days.
Polycaprolactone (PCL) and polylactic acid (PLA) are both biodegradable plastics, used for food containers, biomedical applications, and biodegradable dog poop bags. However these polymers only degrade readily at the high temperatures found in industrial composting facilities. Ting Xu, a chemical engineer at the University of California, Berkeley, and her team from Lawrence Berkeley National Laboratory, the University of Massachusetts, and UC Berkeley wanted to make it possible for consumers to degrade these materials at home.
The team’s new material incorporates nanoparticles built like a chemical ravioli, stuffed with either lipase or proteinase K, and wrapped in a polymer made from a mix of methacrylic methylene esters. This coating protects the enzymes within from the high temperatures needed to melt and extrude plastics to form sheets and other shapes. The team adds less than 2% of the nanoparticle by weight, so the particles don’t interfere with the chemical or mechanical properties of the plastics, Xu says.
It’s only when the scientists expose the plastics to humidity and either heat or UV light that the protecting layer breaks down, releasing the enzymes inside. Depending on the polymer and temperature of the test, the enzyme broke down up to 98% of the polymers in as little as 30 h. The resulting lactic acids can be washed down the drain or added to garden soil, Xu says.
Most plastics are made up of both crystalline and amorphous parts, Xu says. In typical biodegradable plastics, the polymer-eating microorganisms found in compost piles degrade the amorphous parts but can’t squeeze into the crystalline parts. As a result, the materials aren’t fully broken down, leaving bits of crystalline microplastic, Xu says. By embedding the enzymes within the plastic, the enzymes are in a better position to access the crystalline parts and degrade the polymers completely. The key point is that you can disperse the enzymes at the nanoscopic level, which is the same scale as the individual polymer chains, Xu says. This dispersion increases the enzymes’ availability, she says. Most of the enzyme is going to be close enough to the polymer chains to catalyze their breakdown, “so you don’t need to add too much,” Xu says.
“There have been prior attempts to embed enzymes in plastic for the purpose of degrading them at the end of their life. They failed,” says Julia A. Kornfield, a chemical engineer at the California Institute of Technology. “I considered the approach to be a dead end until I read this research,” she says. Compared with previous attempts to use enzymes to break down plastics, Xu’s nanoparticles disperse much more finely in the plastic,“like the difference between the size of a tennis ball and a human hair,” Kornfield says.
Xu and her team envision people being able to decompose these plastics in their homes, either in tubs of warm water or in backyard compost piles. They would also be compatible with larger scale municipal compost services. Xu’s former student and coauthor on the new work Aaron Hall has spun off a start-up called Intropic Materials to commercially develop these plastics.
This story was updated on April 23, 2021, to correct the description of the material coating the enzyme nanoparticles. The coating was made from a mix of methacrylic methylene esters, not methacrylates as was originally stated.