Silk offers environmentally friendly solution to microplastics problem

As microplastics continue polluting our planet’s seas and oceans, ubiquitous in their use from cosmetics to agricultural chemicals, a team of scientists offer a way out.

A team of international scientists devised a silk-based system that could offer an inexpensive and easily manufactured microplastics substitute. The MIT scientists, led by the researchers, describe their process in a paper published in the journal Small.

Microplastics used in today’s products generally serve the purpose of protection of an active ingredient or ingredients from interacting with air or moisture before they are meant to be released. They ensure “a slow release of the active ingredient for a targeted period of time, and minimise negative effects to its surroundings,” a news release notes.

Among the products benefitting from microplastics’ ability for timed release are vitamins, delivered in the form of microcapsules packed into a pill or capsule, and pesticides and herbicides that slowly release deadly toxins to the environment. All of these products use microplastics that do not naturally biodegrade and remain in nature as pollutants for as long as decades. Until now, there were no naturally biodegradable microplastics substitutes that also met the criteria of being practical and inexpensive.

In small doses

“Microplastics are small plastic pieces less than five millimetres long which can be harmful to our ocean and aquatic life,” explains the US National Ocean Service. 

Microplastics have infiltrated the air, water and soil worldwide and “pos[e] a potential threat to aquatic life,” not to mention human beings and other terrestrials. They have even been found in the bloodstream of animals and people.

According to a 2015 factsheet by the United Nations Environment Programme (UNEP), “Since their appearance in cosmetics 50 years ago, plastics have become widespread in cosmetic and personal care formulations and have replaced natural options.” In addition to cosmetics, microplastics can be found in agricultural chemicals and paints.

The same report notes that “A total amount of 4360 tonnes of microplastic beads were used in 2012 across all European Union countries plus Norway and Switzerland according to a survey by Cosmetics Europe, focusing on the use of microplastic beads, with polyethylene beads representing 93 percent of the total amount equaling 4037 tonnes.”

The European Chemicals Agency has determined that “Each year around 42 000 tonnes of microplastics end up in the environment when products containing them are used.” Moreover, there is also microplastics that emerge from the wear and tear on regular-sized plastics: “In addition, the releases of unintentionally formed microplastics (when larger pieces of plastic wear and tear) are estimated to be around 176 000 tonnes a year to the European surface waters.”

The European Union (EU) has declared that the added microplastics must be eliminated by 2025, spurring companies and individuals to search for suitable replacements that do not take a toll on nature.

While it is a fact that a big portion of microplastics that pollute the environment originate from other sources, such as the degradation over time of larger plastic objects such as bottles and packaging, and from the wear of car tires.

MIT professor of civil and environmental engineering Benedetto Marelli says each source may require its own kind of solutions to reduce its spread. 

The European Chemical Agency has estimated that the intentionally added microplastics (excluding the breakdown of larger plastics) represent about 10 to 15 percent of the total amount in the environment, but this source may be “relatively easy to address” using the silk-based biodegradable replacement, Marelli notes.

“We cannot solve the whole microplastics problem with one solution that fits them all,” he says. “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 plus side of this approach is that the silk needed to replace intentionally added microplastics does not have to be the high-quality silk that is used for fine fabrics. It is “widely available and less expensive,” MIT postdoc Muchun Liu says.

For high-quality textiles, silkworm cocoons must be unwound without breakage; yet as replacement for microplastics, non-textile-quality cocoons can be used. The inferior quality cocoons can be harvested using existing manufacturing equipment, and can be produced worldwide, Marelli says, instead of relying on just China, which is the origin of much of textile-quality silk.

Being non-toxic, silk is safe for food and medical use, degrading naturally in the body. The researchers used standard spray-based manufacturing equipment to produce a water-soluble microencapsulated herbicide in lab conditions. After testing in a greenhouse on a corn crop, they found that it worked even better than an existing commercial product, “inflicting less damage” to the plants, Liu says.

Liu explains that silk is a flexible-use material whose polymer chain arrangements can be adjusted before a surfactant is added, offering different types of coatings once they dry out and harden: the coatings can be hydrophilic (water-attracting) or hydrophobic (water-repelling) regardless of its production process which involves the silk fibres being teased out in a water solution.

Liu observed the droplets of encapsulated materials as they formed, saying, “To encapsulate different materials, we have to study how the polymer chains interact and whether they are compatible with different active materials in suspension.” 

According to the researchers, the new method can utilise low-grade silk that is subpar and cannot be used for fabrics, as well as discarded silk or fabric left over from silk garment production.