The quick degradation works well with municipal composting, which typically takes 60 to 90 days to turn food and plant waste into usable compost. Industrial composting at high temperatures takes less time, but the modified polyesters also break down faster at these temperatures.
Xu suspects that higher temperatures make the enshrouded enzyme move around more, allowing it to more quickly find the end of a polymer chain and chew it up and then move on to the next chain. The RHP-wrapped enzymes also tend to bind near the ends of polymer chains, keeping the enzymes near their targets.
The modified polyesters do not degrade at lower temperatures or during brief periods of dampness, she said. A polyester shirt made with this process would withstand sweat and washing at moderate temperatures, for example. Soaking in water for three months at room temperature did not cause the plastic to degrade.
Soaking in lukewarm water does lead to degradation, as she and her team demonstrated. "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," she said. "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 is developing RHP-wrapped enzymes that can degrade other types of polyester plastic, but she also is modifying the RHPs so that the degradation can be programmed to stop at a specified point and not completely destroy the material. This might be useful if the plastic were to be remelted and turned into new plastic.
"These results provide a foundation for the rational design of polymeric materials that could degrade over relatively short timescales, which could provide significant advantages for Army logistics related to waste management," said Stephanie McElhinny, Ph.D., program manager with the Army Research Office. "More broadly, these results provide insight into strategies for the incorporation of active biomolecules into solid-state materials, which could have implications for a variety of future Army capabilities, including sensing, decontamination and self-healing materials."
Xu said that programmed degradation could be the key to recycling many objects. Imagine, she said, using biodegradable glue to assemble computer circuits or even entire phones or electronics, then, when you’re done with them, dissolving the glue so that the devices fall apart and all the pieces can be reused.
"It is good for millennials to think about this and start a conversation that will change the way we interface with Earth," Xu said. "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."