A polymer-eating enzyme integrated into plastic could help solve significant pollution problems by fully composting PLAs.
Whilst biodegradable plastics are often hailed as a solution to Earth’s growing plastic pollution crisis, these materials come with significant disadvantages such as the fact that they do not break down completely and can often contaminate other plastics.
These qualities represent significant issues for recycling and mean that compostable plastics made from polylactic acid (PLA)— commonly found in plastic bags, plastic utensils, and container lids — usually end up sitting in landfills. This means these compostables hang around the environment just like non-biodegradable plastics and thus represent as much as a threat to the environment.
Fortunately, new research points towards a potential solution.
A team of scientists from the University of California, Berkeley, led by Ting Xu, a professor of materials science, engineering and chemistry, have pioneered a mechanism to break down compostable plastics much more easily and completely. The process requires nothing more than heat and water and means the plastics can decompose in a matter of weeks.
“People are now prepared to move into biodegradable polymers for single-use plastics, but if it turns out that it creates more problems than it’s worth, then the policy might revert back. We are basically saying that we are on the right track. We can solve this continuing problem of single-use plastics not being biodegradable.”
Ting Xu, University of California, Berkeley
The team’s research is published in the latest edition of the journal Nature.
This Plastic Will Self-Destruct
The team’s method involves working plastic-eating enzymes into plastic as it is being made. The team placed billions of these nanoparticles in plastic resin beads, which form the starting point of all plastic production.
These enzymes are protected from unraveling and becoming inactive by a simple polymer coating - the real innovation of this mechanism - which is also fully biodegradable. This coating is shed when the enzyme is exposed to water and heat.
The enzyme is then free to start consuming the plastic polymer that surrounds it — breaking it down into its constituent building blocks. For PLA this means turning it into lactic acids which can then feed microbes in surrounding soils.
The team found that the modified PLA fibers degraded within about a week at room temperature. higher temperatures meant more rapid degradation, with standard industrial composting temperatures — about 170 ⁰ C causing the plastics to break down within six days. Xu believes that this is because the higher temperatures allow the enzymes to move
Whilst water and heat are the key elements to kickstarting the degradation process, happily, the team found the modified plastics cope quite well with dampness and even emersion in lower temperature water.
“It turns out that composting is not enough — people want to compost in their home without getting their hands dirty, they want to compost in water,” Xu adds. “So, that is what we tried to see. We used warm tap water. Just warm it up to the right temperature, then put it in, and we see in a few days it disappears.”
Xu demonstrated the viability of hiding an enzyme away in a plastic or other material until the conditions are right for them to be unleashed back in 2018. She and her team built an enzyme capable of degrading toxic organophosphate chemicals — like those found in insecticides and chemical warfare agents — into fiber mats. The enzyme remained hidden and inert until the mat was exposed to the right chemical conditions.
No Plastic Left Behind
The solution developed by the researchers also tackles a plastics problem that has only recently shown itself to be a major ecological hazard. The team says that in addition to breaking down plastics, their mechanism also eliminates microplastics.
These tiny fragments of plastic — less than 5mm in length — are a by-product of many plastic degradation processes and have been found to widely pollute marine environments. Recently researchers found that these microplastics had even become airborne, thus building up in remote areas and presenting a significant risk to wildlife and human health.²
Excitingly, Xu says that the mechanism developed by her and her Berkeley team could also be adapted to work with other plastics — particularly those that are polyesters.
This could result in the development of plastic containers that are compostable and can replace non-degradable polyethylene. Xu also points to significant advantages that make the plastic ideal for use in waste management and even by the military in applications like sensing, decontamination, and self-healing materials.
The self-degradation mechanism could even be used in a glue that can hold devices and electronics together briefly, before dissolving and allowing the parts to be reassembled and used as a different device.
“It is good for millennials to think about this and start a conversation that will change the way we interface with Earth. Look at all the wasted stuff we throw away: clothing, shoes, electronics like cellphones and computers. We are taking things from the earth at a faster rate than we can return them. Don’t go back to Earth to mine for these materials, but mine whatever you have, and then convert it to something else.”
Ting Xu, University of California, Berkeley
¹ Xu. T., DelRe. C., Jiang. Y., et al, , ‘Near-complete depolymerization of polyesters with nano-dispersed enzymes,’ Nature, [https://doi.org/10.1038/s41586-021-03408-3]
² Brahney. J., Mahowald. N., Prank. M., et al, , ‘Constraining the atmospheric limb of the plastic cycle,’ Proceedings of the National Academy of Science, [https://doi.org/10.1073/pnas.2020719118]