MIT researchers develop silk capsules to replace microplastics

MIT researchers have developed silk-based substitute for microplastics.
Christopher McFadden

In material science news, an exciting development from a collaboration of researchers may have solved the growing problem of microplastics in the environment.

Microplastics, microscopic particles of plastic found in the air, water, and soil, are a severe pollution problem and have been identified in the bloodstreams of animals and people worldwide.

In some circumstances, according to the European Chemicals Agency, some microplastics are purposely introduced into agricultural chemicals, paints, cosmetics, and detergents. To this end, the European Union (EU) has mandated that nonbiodegradable microplastics be removed by 2025.

microplastics are a big problem
Source: pcess609/iStock

Thus the search is on for viable substitutes.

After years of research, researchers at the Massachusetts Institute of Technology (MIT), and other scientists, have devised a silk-based device that could be cheap and easy to make. The novel approach is described in a study by MIT written by postdoc Muchun Liu, MIT professor Benedetto Marelli, and five others at BASF in Germany and the U.S.

Why are microplastics added to products? 

Microplastics used in industrial items shield active chemicals from air and moisture until needed. They slow-release the active component over time to minimize side effects. Pesticides and herbicides are also encased in microcapsules. Microencapsulation today uses durable polymers.

Until now, no viable, affordable biodegradable replacement has ever been developed.

Much of the environmental microplastics burden originates from bottle and container deterioration and tire wear. Marelli said each source might need its method to reduce spread. The European Chemical Agency estimates that purposefully added microplastics account for 10-15 percent of the overall amount in the environment. Still, this source may be easier to eliminate using this biodegradable substitute, he says.

microplastics environment
Source: Placebo365/iStock

“We cannot solve the whole microplastics problem with one solution that fits them all,” Marelli explained. “Ten percent of a big number is still a big number. … We’ll solve climate change and pollution of the world one percent at a time," he added.

Silk might be the perfect alternative to microplastics

According to Liu, the silk protein utilized in the new alternative material is readily available and less expensive. For this usage, non-textile-quality cocoons can be used, and the silk fibers can be dissolved using a scalable water-based technique. The technique is so simple and customizable that the produced material may be adapted to current manufacturing equipment, giving a "drop-in" solution using existing factories.

Silk is harmless and degrades naturally in the body, making it suitable for food or medical use. In lab testing, the researchers showed that the silk-based coating material could be utilized in existing spray-based manufacturing machinery to manufacture a water-soluble microencapsulated herbicide product, which was then tested on a corn crop in a greenhouse. Liu believes it worked better than a commercial treatment, causing less plant damage.

Other researchers have proposed degradable encapsulating materials that may work in the lab, but Marelli thinks high-content actives are needed for commercial application. The only way to have an impact is to replace a synthetic polymer with a biodegradable one while maintaining or improving performance.

Liu says the silk's tunability makes it compatible with existing equipment. By altering the polymer chain configurations of silk materials and adding a surfactant, coating characteristics can be fine-tuned. The material can be hydrophobic (water-repelling) even though it is manufactured and processed in water, hydrophilic (water-attracting), or anywhere in between. It can be made to match the qualities of the material it is replacing.

microplastics bad
 Source: pcess609/iStock

Liu had to design a mechanism to freeze developing droplets of encapsulated materials to examine their creation. She used a spray-freezing technology to examine encapsulation and better manage it. Some encapsulated "payload" elements, such as insecticides, nutrients, or enzymes, are water-soluble and interact differently with the covering.

“To encapsulate different materials, we have to study how the polymer chains interact and whether they are compatible with different active materials in suspension,” she explained. 

In a solution, payload and coating are sprayed. As droplets develop, the payload is encased in a coated shell, whether it's synthetic plastic or silk.

Liu claims the new process can utilize low-grade silk that is usually wasted since it has no uses. It can also employ used silk, diverting it from landfills.

China produces 90 percent of the world's silk because it has perfected high-quality silk threads, adds Marelli. Because this approach employs bulk silk and doesn't require that grade, production could simply be ramped up in other regions of the world to satisfy local demand, he says.

"This elegant and clever study describes a sustainable and biodegradable silk-based replacement for microplastic encapsulants, which are a pressing environmental challenge,” says Alon Gorodetsky, an associate professor of chemical and biomolecular engineering at the University of California at Irvine, who was not associated with this research.

silk production
Scanning Electron Microscope (SEM) images of silk-based encapsulants for various applications. Source: Muchun Liu et al/MIT News

“The modularity of the described materials and the scalability of the manufacturing processes are key advantages that portend well for translation to real-world applications,” he added.

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, who also was not associated with this work. “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 and in the U.S. BASF supported the work through the Northeast Research Alliance (NORA).

Study abstract:

"There is a compelling need across several industries to substitute non-degradable, intentionally added microplastics with biodegradable alternatives. Nonetheless, stringent performance criteria in actives’ controlled release and manufacturing at a scale of emerging materials hinder the replacement of polymers used for microplastics fabrication with circular ones. Here, the authors demonstrate that active microencapsulation in a structural protein such as silk fibroin can be achieved by modulating protein protonation and chain relaxation at the point of material assembly. Silk fibroin micelles’ size is tuned from several to hundreds of nanometers, enabling the manufacturing—by retrofitting spray drying and spray freeze drying techniques—of microcapsules with tunable morphology and structure, that is, hollow-spongy, hollow-smooth, hollow crumpled matrices, and hollow crumpled multi-domain. Microcapsules degradation kinetics and sustained release of soluble and insoluble payloads typically used in cosmetic and agriculture applications are controlled by modulating fibroin's beta-sheet content from 20% to near 40%. Ultraviolet-visible studies indicate that burst release of a commonly used herbicide (i.e., saflufenacil) significantly decreases from 25% to 0.8% via silk fibroin microencapsulation. As a proof-of-concept for agrochemicals applications, a 6-day greenhouse trial demonstrates that saflufenacil delivered on corn plants via silk microcapsules reduces crop injury when compared to the non-encapsulated version."

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