July 21, 2022
With bans as early as 2025, BASF and MIT are searching for a biodegradable substitute.
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Microplastics, tiny particles of plastic now found worldwide in the air, water, and soil, are increasingly recognized as a severe pollution threat and are now found in the bloodstreams of animals and people. Some microplastics are intentionally added to various products, including agricultural chemicals, paints, cosmetics, and detergents. According to the European Chemicals Agency, this substance amounts to an estimated 50,000 tons annually in the European Union alone. The EU has already declared that these added, nonbiodegradable microplastics must be eliminated by 2025. So the search is on for suitable replacements, which do not currently exist.
Now, a chemical company BASF and the Massachusetts Institute of Technology scientists have developed a silk-based system that could provide an inexpensive and easily manufactured substitute. The new process is described in a paper in the journal Small.
The microplastics widely used in industrial products generally protect some specific active ingredient (or ingredients) from being degraded by exposure to air or moisture until they are needed. They provide a slow release of the active ingredient for a targeted period and minimize adverse effects to its surroundings. For example, vitamins are often delivered as microcapsules packed into a pill or capsule, and pesticides and herbicides are similarly enveloped. But the materials used today for such microencapsulation are plastics that persist in the environment for a long time. Until now, no practical, economical substitute has been available to biodegrade naturally.
Much of the burden of environmental microplastics comes from other sources, such as the degradation over time of larger plastic objects such as bottles and packaging and the wear of car tires. Each of these sources may require its kind of solutions for reducing its spread, says Bernedetto Marelli, a co-author of the study and an MIT professor of civil and environmental engineering. The European Chemical Agency has estimated that the intentionally added microplastics represent approximately 10-15% of the total amount in the environment. Still, this source may be relatively easy to address using this nature-based biodegradable replacement.
"We cannot solve the whole microplastics problem with one solution that fits them all," Marelli says. "Ten percent of a big number is still a big number, and we'll solve climate change and pollution of the world one percent at a time."
Unlike the high-quality silk threads used for delicate fabrics, the silk protein used in the new alternative material is widely available and less expensive, Liu says. While silkworm cocoons must be painstakingly unwound to produce the fine threads needed for fabric, for this use, non-textile-quality cocoons can be used, and the silk fibers can be dissolved using a scalable water-based process. The processing is so simple and tunable that the resulting material can be adapted to work on existing manufacturing equipment, potentially providing a simple "drop-in" solution using existing factories.
Silk is considered safe for food or medical use, as it is non-toxic and degrades naturally in the body. In lab tests, the researchers demonstrated that the silk-based coating material could be used in existing, standard spray-based manufacturing equipment to make a water-soluble micro-capsule herbicide product. This method was then tested in a greenhouse on a corn crop. The test showed it worked even better than an existing commercial product, inflicting less damage to the plants, says Muchun Liu, an MIT postdoc who is the study's lead author.
While other groups have proposed biodegradable materials that may work at a small laboratory scale, Marelli says, there is a "strong need" for such a material to work well commercially without sacrificing performance.
Liu explains that the secret to making the material compatible with existing equipment is in the silk material's tunability. By precisely adjusting the polymer chain arrangements of silk materials and the addition of a surfactant, it is possible to fine-tune the properties of the resulting coatings once they dry out and harden. The material can be hydrophobic (water-repelling) even though it is made and processed in a water solution. Or it can be hydrophilic (water-attracting) or anywhere in between, and for a given application, it can be made to match the characteristics of the material it is being used to replace.
The new method can use low-grade silk that is unusable for fabrics and large quantities of which are currently discarded because they have no significant uses, Liu says. It can also use discarded silk fabric, diverting that material from being disposed of in landfills.
Currently, 90% of the world's silk production takes place in China, Marelli says, but that's mainly because China has perfected the production of the high-quality silk threads needed for fabrics. But because this process uses bulk silk and does not need that quality, production could quickly be ramped up in other parts of the world to meet local demand if this process becomes widely used.
This process "represents a potentially highly significant advance in active ingredient delivery for a range of industries, particularly agriculture," says Jason White, director of the Connecticut Agricultural Experiment Station. He was not associated with the research. "Given the current and future challenges related to food insecurity, agricultural production, and a changing climate, novel strategies such as this are greatly needed."
The research team also included Pierre-Eric Millard, Ophelie Zeyons, Henning Urch, Douglas Findley, and Rupert Konradi from the BASF corporation in Germany. The U.S. BASF supported the work through the Northeast Research Alliance (NORA).
This article was originally published in MIT News.
TRIZ thoughts: Ideality – Low-grade silk cocoons are cheap, readily available, and a biodegradable resource that can replace non- biodegradable microplastics.