Researchers have developed a biodegradable system based on silk 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 recognized as a serious pollution threat, and have been found
in the bloodstream of animals and people around the world.
Some of these microplastics are intentionally added to a variety of products, including
agricultural chemicals, paints, cosmetics, and detergents — amounting to an estimated
50,000 tons a year in the European Union alone, according to the European Chemicals Agency.
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 team of scientists at MIT and elsewhere has developed a system based on silk 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 from being degraded by exposure to air or moisture, until the time they are
needed.
They provide a slow release of the active ingredient for a targeted period of time and
minimize adverse effects to its surroundings.
For example, vitamins are often delivered in the form of 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, there has been no practical, economical substitute available that would 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 from the wear of
car tires.
Each of these sources may require its own kind of solutions for reducing its spread.
The European Chemical Agency has estimated that the intentionally added microplastics
represent approximately 10-15 percent of the total amount in the environment, but this
source may be relatively easy to address using this nature-based biodegradable replacement.
Unlike the high-quality silk threads used for fine fabrics, the silk protein used in
the new alternative material is widely available and less expensive.
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
simply 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 recognized as safe for food or medical use, as it is nontoxic 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 it worked even better than an existing commercial product, inflicting
less damage to the plants.
While other 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.
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, is in the tunability
of the silk material.
By precisely adjusting the polymer chain arrangements of silk materials and 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.
In order to arrive at a practical solution, the researchers had to develop a way of freezing
the forming droplets of encapsulated materials as they were forming, to study the formation
process in detail.
The researcher 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, whether they be pesticides or nutrients or
enzymes, are water-soluble and some are not, and they interact in different ways with the
coating material.
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, and large quantities
of which are currently discarded because they have no significant uses.
It can also use used, discarded silk fabric, diverting that material from being disposed
of in landfills.
Currently, 90 percent of the world’s silk production takes place in China, but that’s
largely because China has perfected the production of the high-quality silk threads needed for
fabrics.
But because this process uses bulk silk and has no need for that level of quality, production
could easily be ramped up in other parts of the world to meet local demand if this process
becomes widely used.