Embedded enzymes make for compostable plastics that break down in days
Plastic is a major environmental issue, since most of it doesn’t break down easily – and even when it does, it usually forms microplastic pieces that pose their own problems. A new type of compostable plastic is embedded with enzymes that, when triggered, quickly break the material down to its constituent molecules.
The vast majority of plastic ends up in landfill or worse, the environment. Even those well-meaning materials that are biodegradable can often only break down so far, leaving microplastic particles that accumulate in the ocean and cause health problems for animals and humans.
An emerging solution focuses on enzymes, nature’s little catalysts that degrade organic matter. By design, plastic normally resists this kind of degradation – after all, you don’t want your soup dissolving its container before you can eat your lunch. But in recent years scientists have identified bacteria that can gobble up plastic, isolated the enzymes they use to do it, and engineered them to be even more efficient at the task.
Previously the plastics had to be specifically treated with these enzymes, perhaps as part of a recycling facility, but that means that any that still made their way into the environment would cause all the usual havoc. So for the new study, the researchers developed a way to embed small amounts of commercial enzymes directly into plastic itself, so they would break down on their own, no matter where they end up.
“If you have the enzyme only on the surface of the plastic, it would just etch down very slowly,” says Ting Xu, senior author of the study. “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.”
That obviously poses its own challenges – we’re back to the problem of plastics potentially breaking down during storage or use. So the researchers also threw in an enzyme protectant called four-monomer random heteropolymer (RHP), which disperses the enzymes several nanometers apart.
The result is a plastic that will stay stable during regular use, but will begin to break down only when exposed to compost soil or hot water. In tests, the team found that soaking the plastic in water at room temperature for three months didn’t make it degrade – that process is only triggered when the heat is cranked up a little.
They added an enzyme called BC-lipase to PCL plastic, and another called proteinase K into PLA plastic. The PCL broke down completely within two days at 40 °C (104 °F), while the PLA degraded within six days at 50 °C (122 °F). And rather than producing microplastics, the materials degrade into harmless lactic acid.
The team has already filed a patent application for the new degradable plastic, and founded a startup to help commercialize it.
The research was published in the journal Nature.