Led by a team at the University of California San Diego, the research has developed a biodegradable form of thermoplastic polyurethane (TPU), a melt-processable thermoplastic elastomer found in footwear, floor mats, cushions and memory foam.
The TPU is filled with bacterial spores that, when exposed to nutrients present in compost, germinate and break down the material at the end of its life cycle. The work is detailed in Nature Communications.
The biodegradable TPU was made with bacterial spores from a strain of Bacillus subtilis that breaks down plastic polymer materials.
“It’s an inherent property of these bacteria,” said study co-senior author Jon Pokorski, a nanoengineering professor at the UC San Diego Jacobs School of Engineering and co-lead of the university’s Materials Research Science and Engineering Center (MRSEC). “We took a few strains and evaluated their ability to use TPUs as a sole carbon source, then picked the one that grew the best.”
The researchers used bacterial spores, a dormant form of bacteria, due to their resistance to harsh environmental conditions. Furthermore, bacterial spores have a protective protein shield that enables bacteria to survive in a vegetative state.
To make the biodegradable plastic, the researchers fed Bacillus subtilis spores and TPU pellets into a plastic extruder. The ingredients were mixed and melted at 135oC, then extruded as thin strips of plastic.
To assess the material’s biodegradability, the strips were placed in microbially active and sterile compost environments. The compost setups were maintained at 37oC with a relative humidity ranging from 44 to 55 per cent. Water and other nutrients in the compost triggered germination of the spores within the plastic strips, which reached 90 per cent degradation within five months.
“What’s remarkable is that our material breaks down even without the presence of additional microbes,” Pokorski said in a statement. “Chances are, most of these plastics will likely not end up in microbially rich composting facilities. This ability to self-degrade in a microbe-free environment makes our technology more versatile.”
According to UC San Diego, the researchers used adaptive laboratory evolution to create a strain of bacterial spores that is resilient to extrusion temperatures. The process involves growing the spores, subjecting them to extreme temperatures for escalating periods of time, and allowing them to mutate. The strains that survive this process are then isolated and put through the cycle again.
“We continually evolved the cells over and over again until we arrived at a strain that is optimised to tolerate the heat,” said study co-senior author Adam Feist, a bioengineering research scientist at the UC San Diego Jacobs School of Engineering. “It’s amazing how well this process of bacterial evolution and selection worked for this purpose.”
The current study is focused on producing smaller lab-scale quantities to understand feasibility, but the researchers are working on optimising the approach for use at an industrial scale. Ongoing efforts include scaling up production to kilogram quantities, evolving the bacteria to break down plastic materials faster, and exploring other types of plastics.
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