Researchers develop ‘bacteria-loaded’ biodegradable TPU

San Diego, California – Researchers led by the University of California San Diego (UCSD) have developed a biodegradable thermoplastic polyurethane (TPU) that can be readily broken down at the end-of-life stage.

The material is “filled with bacterial spores that, when exposed to nutrients present in compost, germinate and break down the elastomer, said UCSD in a paper published 30 April in Nature Communications.

Suitable for soft yet durable applications, such as footwear, floor mats, cushions and memory foam, the TPU incorporates bacterial spores from a strain of Bacillus subtilis.

The bacteria have an inherent capability to break down polymer materials, according to study co-senior author Jon Pokorski, a nanoengineering professor at UCSD.

“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,” he explained.

The US researchers used bacterial spores, a dormant form of bacteria, due to their resistance to harsh environmental conditions.

Unlike fungal spores, which serve a reproductive role, bacterial spores have a protective protein shield that enables bacteria to survive while in a vegetative state.

To make the biodegradable polymer, the research team fed Bacillus subtilis spores and TPU pellets into an extruder.

The ingredients were mixed and melted at 135 degrees Celsius, then extruded to produce thin strips, according to the UCSD report.

To assess the material’s biodegradability, the strips were placed in both microbially active and sterile compost environments.

The compost setups were maintained at 37 degrees Celsius with a relative humidity ranging from 44% to 55%.

Water and other nutrients in the compost triggered germination of the spores within the polymer strips, which reached 90% degradation within five months.

The TPU material, Pokorski noted, breaks down “even without the presence of additional microbes.”

As most of such polymers will likely not end up in microbial-rich composting facilities, the ability to ‘self-degrade’ makes the technology “more versatile,” the research co-author added.

The spores also serve as a strengthening filler,  Pokorski noting that the resulting TPU has significantly enhanced mechanical strength and greater stretchability.

The addition of spores, he commented, “pushes the mechanical properties beyond known limitations where there was previously a trade-off between tensile strength and stretchability.”

While the researchers still need to further study how the material degrades, they expect that any lingering bacterial spores are “likely harmless.”

“Bacillus subtilis is a strain used in probiotics and is generally regarded as safe to humans and animals and can even be beneficial to plant health,” UCSD added.

The bacterial spores used in the study were “evolutionary engineered” to survive the high temperatures necessary for TPU extrusion.

The process involves growing the spores, subjecting them to extreme temperatures for escalating periods of time, and allowing them to naturally mutate.

“We continually evolved the cells... 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 UCSD.

Whiole the current study focused on producing smaller lab-scale quantities to understand feasibility, the team is now working to optimise the approach for use at an industrial scale.

Efforts include scaling up production to kilogramme quantities, evolving the bacteria to break down polymer materials faster, and exploring other types of materials beyond TPU.