A team of researchers at MIT has developed an inexpensive biodegradable system based on silk that can be easily manufactured and used to replace microplastics added to agricultural products, paints and cosmetics.
Microplastics, tiny particles of plastic that are now found worldwide in the air, water and soil, are increasingly being seen as a serious pollution threat. They have been found in the bloodstream of people and other animals.
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According to the European Chemicals Agency, an estimated 50,000 tons of microplastics are intentionally added to products each year in the EU alone. The EU has declared that these added, non-biodegradable microplastics must be eliminated by 2025; however, there are currently no suitable alternatives.
The microplastics widely used in industrial products generally protect a specific active ingredient or ingredients from being degraded by exposure to air or moisture. They provide a slow release of the active ingredient for a targeted period of time and minimise adverse effects to its surroundings. At present, the materials used for such microencapsulation are plastics that persist in the environment for a long time. Until now, there has been no practical, economical substitute available that would biodegrade naturally.
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According to MIT postdoctoral researcher Muchun Liu, the silk protein used in the new material is widely available and less expensive than the high-quality silk threads used for fine fabrics. 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 fibres can simply be dissolved using a scalable water-based process. The processing is so simple and tuneable 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 to be 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 standard water-soluble microencapsulated herbicide product, which was then tested in a greenhouse on a corn crop. The test showed that it worked even better than an existing commercial product, causing less damage to the plants.
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While other research groups have proposed degradable encapsulation materials that may work at a small laboratory scale, ‘there is a strong need to achieve encapsulation of high-content actives to open the door to commercial use,’ said Benedetto Marelli, a professor of civil and environmental engineering at MIT. ‘The only way to have an impact is where we can not only replace a synthetic polymer with a biodegradable counterpart, but also achieve performance that is the same, if not better.’
The secret to making the material compatible with existing equipment, Liu explained, is in the tunability of the silk material. By precisely adjusting the polymer-chain arrangements of silk materials and addition of a surfactant, it’s 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’s 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’s being used to replace.
In order to arrive at a practical solution, Liu had to develop a way of freezing the forming droplets of encapsulated materials as they were forming, to study the formation process in detail. She did this using a special spray-freezing system, and was able to observe exactly how the encapsulation works in order to control it better. Some of the encapsulated ‘payload’ materials are water-soluble and some are not, and they interact in different ways with the coating material.
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‘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 said. 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.
The new method can make use of low-grade silk that is unusable for fabrics, large quantities of which are currently discarded because they have no significant uses, Liu said. It can also use discarded silk fabric, potentially diverting material from landfill.
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Details of the new process have been published in Small.