Some single-use plastics have been replaced with biodegradable options in recent years, but even those aren’t fully compostable. Polymer scientist Ting Xu knows that because when she picks up composted soil from her parents’ garden, it is often littered with plastic bits that haven’t fully degraded, she tells Carmen Drahl at Science News.
For more than a decade, Xu has researched how plastic could be created with enzymes that break down the stubborn material. Now, a paper published on April 21 in the journal Nature describes a new plastic material that degrades by up to 98 percent after less than a week in damp composting soil. The plastic itself has a sprinkling of polymer-munching enzymes mixed in that are activated by heat and moisture to degrade the plastic from the inside.
The goal is to create truly compostable plastics that can replace the single-use plastics that have become especially common amid the Covid-19 pandemic. “We want this to be in every grocery store,” says Xu to Science News.
Only a few kinds of plastic, labeled as types one and two, are reliably recyclable. A 2015 study showed just nine percent of plastics in the world are recycled—most plastics wind up in landfills or scattered across the globe as pollution. The recent introduction of biodegradable plastics offered promise to rid the world of some debris, but these materials require specific processing to fully break down. If standard biodegradable plastics don’t reach an industrial composting facility, they won’t fully degrade.
"Under other conditions such as soil or marine environments, these materials often display a similar durability as their conventional fossil-fuel-based counterparts, causing significant environmental damage and pollution," says Queensland University of Technology materials scientist Hendrik Frisch, who was not involved in the new study, to Gemma Conroy at ABC Science.
The new plastic has enzymes embedded in it that have been individually wrapped with four-part nanoparticles. The nanoparticles prevent the enzymes from falling apart while they wait to go to work. The wrapped enzymes are mixed with polymer beads early in the plastic-forming process. The end material includes thin film pieces and thick plastic filaments.
The enzymes don’t alter the plastic’s usual properties—the film is as strong and flexible as standard plastic bags. But when the material is immersed in warm water, or damp soil, the enzymes’ polymer coating falls away and the enzymes become activated. Because the enzymes are embedded throughout the material itself, and not added later, they can thoroughly degrade it.
“If you have the enzyme only on the surface of the plastic, it would just etch down very slowly,” says Xu in a statement. “You want it distributed nanoscopically everywhere so that, essentially, each of them just needs to eat away their polymer neighbors, and then the whole material disintegrates.”
One of the plastics tested in the new study, called PLA, is commonly used in single-use food packaging. But with the addition of the embedded enzymes, the plastic was degraded into its molecular parts after just six days at about 120 degrees Fahrenheit. The enzymes break the PLA down into lactic acid, which microbes in the soil can use as food.
Frisch tells ABC Science that the researchers have more work to do to show whether the enzymes could be applied to other kinds of plastic. But for now, Xu plans to patent the technology and support a co-author in commercializing it.
"Enzymes are really just catalysts evolved by nature to carry out reactions," says Xu to ABC Science. "If you want to get a material to become a part of nature, we should go with what nature has already developed.”