MIT Study: Discarded Silk Can Replace Microplastics

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These scanning electron microscope images show silk-coated microcapsules containing vitamin C, at different scales of detail. On the left, and top center, samples made by spray drying, a method already widely used in industry. On the right and at bottom center, samples made by ultrasonic spray freeze drying, a method used by the researchers to reveal greater detail of the process involved. Credit: SEM images by Muchun Liu, edited by MIT News

Microplastics are not new to our world and our oceans, but as climate change and other environmental concerns become more pressing, they have undoubtedly become a focus as of late. A lot of microplastic pollution comes from the breakdown of larger plastic products, like bottles and packaging, but manufacturers also intentionally add their own.

For decades, microplastics have been added to everyday products like laundry detergent, toothpaste, chemicals, paints and more—amounting to an estimated 50,000 tons a year in the European Union alone. In fact, the EU has declared that these intentionally added microplastics—which comprise 10 to 15 percent of the total amount—must be eliminated by 2025.

In new research, scientists at MIT, in collaboration with the chemical company BASF, have taken the first step toward development of an inexpensive and easily manufactured substitute.

“We cannot solve the whole microplastics problem with one solution that fits them all,” said study co-author Benedetto Marelli, professor of civil and environmental engineering at MIT. “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.”

The new process takes advantage of silk—but not the high-quality kind that is used for fabric. Textile-quality silk requires silkworm cocoons to be meticulously unraveled, which takes time, money and resources. Whereas, the non-textile-quality cocoons used by Marelli and his team simply dissolve in a scalable water-based process. Currently, large quantities of “low-grade silk” are discarded since they are considered unusable for fabrics—but they would absolutely work within the context of this new process.

For the study, published in Small, the researchers tested the silk coating material in existing spray-based manufacturing equipment by creating a water-soluble microencapsulated herbicide product. When the product was sprayed on corn in a greenhouse, not only did it work as well as existing products, but it also significantly reduced crop damage.

Fortunately, the new silk coating could replace microplastics for more than just chemicals. In fact, the researchers discovered that the silk is so tunable that they can change properties of the resulting coating once it dries and hardens. For example, the material can be hydrophobic even though it is made in a water solution, or it can be hydrophilic—or anywhere in between. It can even be made to match the characteristics of the material it is being used to replace.

“To encapsulate different materials, we have to study how the polymer chains interact and whether they are compatible with different active materials in suspension,” said co-author and MIT postdoc Muchun Liu. “The payload material and the coating material are mixed together in a solution and then sprayed. As droplets form, the payload tends to be embedded in a shell of the coating material, whether that’s the original synthetic plastic or the new silk material.”

Future work will investigate the effects of microcapsule size, wall thickness, and payload content on release profiles, together with the influence of other payload types, says the research team. For now, the process can be readily applied in real-world manufacturing—as demonstrated in the study potentially providing a “drop in” solution for existing factories.

Lab products used in this environmental research:

  • Merlin High-resolution SEM- Zeiss
  • UV-1600PC spectrophotometer- VWR
  • Spectrum 65 with ATR accessory- PerkinElmer
  • Zetasizer Nano-ZS- Malvern Panalytical
  • Milli-Q- MilliporeSigma
  • Mini spray dryer SD-18A- LABFREEZ Instruments
  • ECHO ultrasonication generator- Sono-tek
  • PARSource HID grow lights- Parsource Lighting Solutions
  • New-Tech Spray Chamber- New-Tech

 

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