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Algenesis submerged shoes made with its biodegradable polyurethane foam in the Pacific Ocean to demonstrate their decomposition.
The more people hear or read about microplastics turning up in air, drinking water, placentas, and brains, the more they want solutions. “It becomes very personal,” says Shannon Pinc, senior circular economy manager at NatureWorks, a Minnesota-based company that makes polylactic acid (PLA), one of the most common compostable plastics.
NatureWorks and other makers of biodegradable plastic are putting forward their products as one of those solutions.
But even biodegradable plastics shed microplastics, though they won’t persist indefinitely. They will continue to break down and eventually become food for microbes. While some experts emphasize that this is an unambiguous improvement on conventional plastics, which will stick around for hundreds of years, others stress that the degree of improvement depends enormously on what these polymers are used for and where they wind up at the end of their lives.
Richard Thompson, director of the Marine Institute at the University of Plymouth and coiner of the term “microplastic” in 2004, is one of those urging caution. “We need to be careful that we don’t spread the sphere of benefit wider than actually it should be,” he says.
There are many types of biodegradable plastic, but the ones synthesized on the largest scale currently are PLA, polybutylene adipate terephthalate (PBAT), and polybutylene succinate (PBS). Polyhydroxyalkanoates (PHAs) are a smaller but rapidly growing part of the market. And researchers are continuing to work on developing new materials and improving established ones. For example, the California-based start-up Algenesis is working on creating biodegradable polyester polyurethanes using compounds derived from algae and plants.
What all these materials have in common is that their monomers are linked together by chemical bonds that are frequently found in nature and can be broken relatively easily by water or enzymes. Often, that means ester bonds, but amide or carbamate bonds can also work. Many biodegradable plastics, including PLA, are also biobased, but some, such as PBAT, are petrochemical in origin.
Anytime microplastics are being generated, we want those to be transient.
Ryan Simkovsky, chief technology officer, Algenesis
The source of the carbon in a plastic and the fate of the carbon are entirely separate conversations, says Michael Sander, an environmental chemist at the Swiss Federal Institute of Technology (ETH), Zurich. And when it comes to questions about end-of-life outcomes and microplastics, only the latter really matters.
For a polymer to be considered truly biodegradable, it has to be fully disassembled and microbially converted to carbon dioxide, water (or methane, in anaerobic conditions), and biomass. Measuring the release of CO2 is the most direct way to check if decomposition is happening, says Marissa Tessman, a senior research scientist at Algenesis. Techniques such as electron microscopy or mass spectrometry can also help scientists monitor the degradation process but don’t directly measure how complete it is.
It’s important to verify complete degradation because “biodegradable doesn’t mean microplastic-free,” says Xinfeng Wei, a scientist at KTH Royal Institute of Technology who studies microplastics and sustainable polymers. In fact, in the short term, the opposite is often true. In a study published in 2021, Wei and his colleagues found that PBAT released more microplastics than polyethylene in simulated ocean conditions (Water Res. 2021, DOI: 10.1016/j.watres.2021.117123).
There are simply more ways to chop up the backbones of biodegradable polymers, Wei says. They’re designed to undergo hydrolysis easily, and the photochemical and mechanical bond-breaking processes that chew up conventional polymers also act on biodegradable ones.
But Sander doesn’t think that the short-term release of biodegradable microplastics is necessarily a reason to worry. On a chemical level, he says, these particles are “fundamentally different from nonbiodegradable microplastics” because they can continue to degrade. Microbes can’t easily take up and metabolize molecules larger than about 1,500 g/mol, so the polymer first has to be chopped up into smaller units.
The perspective that microparticles from biodegradable polymers aren’t of concern assumes that they’re not going to stick around long enough to cause problems.
As long as there’s some water available to facilitate hydrolysis, PLA will stay in the microplastic size range for a shorter time than conventional plastic, NatureWorks’s Pinc says. She points to a metastudy commissioned by the Dutch trade organization Holland Bioplastics that says all PLA will continually break down by hydrolysis in the environment. The actual timeline of degradation varies enormously depending on conditions, however.
Light, temperature, pH, moisture, and the types of microbes all play important roles in determining how quickly a plastic object—say, a to-go cup—might go from holding your iced latte to becoming a feast for bacteria. So what breaks down easily in a compost facility may not fare well in cold ocean water.
Credit: Shutterstock
Biodegradable mulch films are designed to degrade in the soil within 2 years.
If biodegradable plastics end up in conditions that are not conducive to full degradation, “you’re not going to achieve the benefits that you’re perhaps wanting, and you could indeed end up with microplastics” that are quite persistent despite not being technically permanent, says the University of Plymouth’s Thompson.
PLA typically needs industrial composting at elevated temperatures to hasten its decomposition. This has garnered some criticism for the material. Pinc says that’s a limited view because the alternative is usually a plastic that will never degrade, though she agrees that alternative materials like PLA should be held to a high standard for safety and sustainability.
In the open environment, PLA will break down into microplastics over the course of a few months to a few years, then slowly depolymerize and become metabolized over another several years (Sci. Total Environ. 2023, DOI: 10.1016/j.scitotenv.2023.165025). PHAs, meanwhile, decompose much faster in a wider range of environments, including the ocean. It takes weeks to months for a PHA object to break into microplastics, and days or weeks to achieve full biodegradation.
It’s important to look at the overall object rather than just the polymer, says Francesca De Falco, a materials scientist at the University of Plymouth working with Thompson. Additives introduced during manufacture, or even an item’s shape, can influence how it breaks down. On top of that, lab tests can tell you only so much about how the process will work in the real world, she adds.
The fact that degradation is fostered in some environments and hindered in others is something to take advantage of in products, says Ryan Simkovsky, Algenesis’s chief technology officer. For example, the firm uses its biodegradable polyurethane to make foam for shoe soles. That foam has to hold up to everyday use: nobody would buy the shoes if it didn’t. But it’s still biodegradable. “The trigger is getting it into the right environment,” Simkovsky says. The shoes will start to decompose if they’re buried in soil—or tied to the underside of a pier in La Jolla, California.
Apparel and shoes made from synthetic polymers are inevitably going to shed microplastics as a result of mechanical wear and tear, so making them out of biodegradable materials, as Algenesis is doing, has benefits. “Anytime microplastics are being generated, we want those to be transient,” Simkovsky says.
To show that Algenesis’s polyurethane wouldn’t form persistent microplastics, its scientists ground the material into tiny pieces and monitored them under soil-composting conditions (Sci. Rep. 2024, DOI: 10.1038/s41598-024-56492-6). They found that the particles disappeared within 200 days, and carbon dioxide evolution experiments showed that 75% of the polyurethane carbon was converted to CO2 within 45 days.
The most beneficial applications for biodegradable plastics are ones in which there’s a high probability of plastic leakage into the environment, says Sander, who worked on a 2020 scientific advice report that helped inform the European Union’s approach to these materials. For example, it makes sense if plastic mulch films for crops can ultimately become food for soil microbes. And given the high incidence of lost plastic fishing lines and nets to the environment, it’s prudent to make them from polymers like PHA that won’t persist forever in the ocean.
Many biodegradable plastics on the market go into packaging or food service items such as cups and cutlery, however. The idea is to make greener replacements for petrochemical-based, nonbiodegradable single-use plastics. But that isn’t truly solving the core problem, which is that “the more [plastic] you’ve got in the environment, the more likely you are to realize concentrations that are going to cause harm,” Thompson says. “We shouldn’t be designing plastic single-use items with the idea that the environment is the place we dispose of them.”
Making sure that biodegradable plastics are disposed of correctly is essential for maximizing their green potential. But even if people have access to compost facilities that accept plastic, well-meaning consumers often don’t know how to properly sort plastic products. That confusion leads to mismanagement, and “mismanagement only creates more plastic pollution,” De Falco says.
If we make and discard biodegradable single-use plastics more quickly than they decompose, that’s still a net accumulation of plastic waste—and microplastics, Thompson says. The solution is “not about designing a plastic that’s safe to be littered,” he adds. Better to avoid creating the litter in the first place.
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