The world has produced over nine billion tons of plastic since the 1950s, with another 400 million tons added each year. Plastic is everywhere because it’s cheap, durable, and versatile. You can use plastic to make anything from cups and forks to eyeglasses and electronics. It’s hard to find any consumer good that doesn’t contain at least some plastic.
However, the same things that make plastic so appealing also make it the bane of the environment. It’s just too durable for its own good, requiring centuries to fully decompose. And even then, it doesn’t disappear — it just gets smaller and smaller, turning into an ocean of microplastics.
Luckily, most people are aware that plastic pollution is really bad for the environment. This is why we have recycling policies. However, less than 10% of plastic in the United States is recycled, with this percentage even lower in other nations.
In reaction to these challenges, biodegradable plastics have been advertised as the solution to the plastic pollution problem bedeviling the world. These biodegradable and compostable plastics can be broken down into water, carbon dioxide, and biomass by microorganisms — but only under the right conditions.
The right conditions for compost
In reality, today’s “compostable” plastic bags, utensils and cup lids often fall short of their mission. They frequently fail to break down during typical composting. In the process, they contaminate other recyclable plastics, creating headaches for recyclers. Most compostable plastics, made primarily of a type of polyester known as polylactic acid, or PLA, end up in landfills devoid of oxygen and last as long as forever plastics.
Researchers at the University of California, San Diego have now put a new spin on biodegradable plastics. And this change may finally usher in a new class of guilt-free plastics (one can only hope). They have pioneered a new type of thermoplastic polyurethane (TPU) that is not only durable but also truly biodegradable. TPU is common in products ranging from footwear to memory foam, making it a staple in various industries.
The innovative twist introduced by the UC San Diego team involves embedding bacterial spores within the plastic, which activate and break down the material when exposed to nutrients found in compost environments. These harmless spores are essentially dormant bacteria, ready to wake up when the time is right — such as when you’ve decided that your plastic product has reached the end of its lifetime.
This research utilizes Bacillus subtilis — a bacterium known for its ability to decompose 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.
“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.”
From years to months
Unlike fungal spores, bacterial spores have a protective protein wrapping, allowing them to survive in a vegetative state without nutrients. This shield also enables the bacteria to withstand the high-temperature environment during the plastic manufacturing process, which is why they can be embedded inside bulk plastic.
The process involves feeding Bacillus subtilis spores and TPU pellets into a plastic extruder, which combines and melts them at 135 degrees Celsius into thin plastic strips.
In an experiment meant to evaluate how well the material breaks down, scientists placed the strips in two types of compost environments. One environment teemed with microbes and the other was sterile. They kept both setups at 37 °C and maintained the humidity between 44% and 55%. The presence of water and nutrients in the compost activated spores embedded in the plastic strips, leading to a 90% degradation of the material within five months.
“What’s remarkable is that our material breaks down even without the presence of additional microbes,” said Pokorski. “Chances are, most of these plastics will likely not end up in microbially rich composting facilities. So, this ability to self-degrade in a microbe-free environment makes our technology more versatile.”
The benefits of bacteria
The research team continues to refine this technology. Currently their goals are to scale up production and explore the degradation of other types of commercial plastics. Another notable feature is that the presence of the bacterial spores enhances the durability and stretchability of the plastic. This mirrors the way rebar reinforces concrete, which could make the material attractive for a broader range of applications.
“Both of these properties are greatly improved just by adding the spores,” said Pokorski. “This is great because the addition of spores pushes the mechanical properties beyond known limitations where there was previously a trade off between tensile strength and stretchability.”
“There are many different kinds of commercial plastics that end up in the environment — TPU is just one of them,” said study co-senior author Adam Feist, a bioengineering research scientist at the UC San Diego Jacobs School of Engineering. “One of our next steps is to broaden the scope of biodegradable materials we can make with this technology.”
The findings appeared in the journal Nature Communications.
