Author: Bernadette Ballantyne
Microplastics are everywhere. From the air we breathe, to the water we drink, to the food we eat.
These tiny plastic particles defined as less than 5mm in diameter have infiltrated even the most remote of locations from Arctic Ice to the middle of the Pacific Ocean. The effect of this on ecosystems is potentially catastrophic. So catastrophic that the EU is about to ban the deliberate addition of microplastics into any new products.
Banning microplastics
Microplastics are released from car tyres as friction leaves plastic residue behind when we drive but they are also intentionally added to many products, for example in cosmetics, fertilisers, paints and artificial football pitches. “We estimate here in ECHA that about 45,000t is emitted into Europe every year from these intentionally added microplastics,” says Bjorn Hansen, executive director of the European Chemicals Agency (ECHA) in a statement on their recent microplastics study. The organisation was commissioned to conduct a risk analysis of these intentionally added microplastics by the European Commission as part of its European Plastics Strategy. “Echa has concluded that there is a risk to the European environment by using microplastics in products. And we have also concluded that the most effective means to address that risk is to ban microplastics in such uses.”
However the reason that these polluting microplastics are being used is because they serve a useful function. In laundry detergent for example they release the fresh smelling odours when clothes are pulled from wardrobes. In medicines they can release other chemicals over time creating a more effective drug. So if these are to be banned we need alternatives. And one company in Cambridgeshire in the UK thinks it has found it.
Spiderman
Professor Tuomas Knowles, at the University of Cambridge is one of the world’s leaders in protein biophysics, and he has spent years studying the way that natural proteins hold together in materials such as spider silk. His research has enabled scientists to re-engineered plant based proteins to create a natural alternative to plastic that has comparable mechanical properties. Just as importantly these new plastic alternatives are also 100 percent biodegradable meaning that they won’t end up polluting the planet. “What we do is we sort of really try and understand the fundamental principles that govern protein activity in nature,” says Professor Knowles
“He asked himself some time ago, about 15 years ago, the question, how does a spider make silk and silk is just a protein material, “explains Simon Hombersley, the CEO of Xampla, which is the company created to develop and commercialise this technology. “And the spider takes a fly very energy efficiently, and converts it into one of the strongest materials on earth.”
Professor Knowles explains further: “So the spider really requires minimal energy input, so effectively to fly. And that’s enough to produce both energy and the raw materials to form this amazing sort of engineering structure.” The engineering structure is its web which has incredible strength and flexibility. “So we were really fascinated by trying to understand the fundamental principles by which spider, but more generally, nature, the subsequent really, really high performance materials are made.”
Understanding bonding
Tuomas says that what’s interesting from a molecular sciences point of view is the fact that often in materials chemistry, when scientists and engineers think about making strong materials, they often fall back to sort of really strong bonding between within the molecules and between the between the molecules. But what’s remarkable about protein based materials such as silk is that actually, the bonding between the molecules is relatively weak. So it’s hydrogen bonding.”
Hydrogen bonds are created through the electrostatic forces acting between particles. On their own hydrogen bonds are much weaker than covalent or ionic bonds, but as a regularly arranged network with lots of bonds they become incredibly strong. Importantly these individual connections don’t require lots of energy to break them down meaning that they are easily biodegradable, which is something we will get to later.
Usually material strength relies on covalent bonds, which are the strongest types of connection.
In this kind of bonding the electrons that whizz around the outside of the nucleus of an atom connect with an electron from another atom forming a pair that is bonded together. Where atoms share more than two electron pairs the bonding is incredibly strong. In fact this is the secret behind the incredible hardness of diamond, whose carbon atoms share 4 electron pairs with other atoms.
Conversely hydrogen bonds don’t use electron sharing and rely on other interactions between atoms. It is here where the team at the University of Cambridge focussed their research. “We spent years trying to understand these principals and we really came to the conclusion that actually, what gave this really high sort of mechanical performance was the fact that there was a very regular array of these hydrogen bonding networks really effectively between every single molecule in this in this peptide, so really a backbone hydrogen bond. And it was really that sort of basic motif, which allowed nature to generate these really strong materials out of proteins and peptides,” says Tuomas.
Next steps
Once this process was understood the next step was to take other natural materials not related to silk, and get them to assemble into new structures which have high mechanical ability. Simon acknowledges that to date there have been many other breakthroughs in the use of natural materials but each have significant challenges. Starch based natural polymers are highly brittle for example and alginate polymers derived from seaweed have limited mechanical strength and water resistance. Even cellulose based plastic alternatives need chemical addition to perform as a plastic would, meaning that they then no longer biodegrade. “Plant polysaccharides, which are the main materials that you find in plastic replacements they’re very easy to work with and they’re commonly available, and they’re low cost but they lack performance, they lack structural strength. So to be useful, you have to chemically crosslink them, which then means their end of life is affected, they don’t degrade when they environment, they’re no longer natural,” says Simon. “What we can do is engineer plant proteins without any chronic chemical cross linking, and only using the hydrogen bonding the intermolecular hydrogen bonding to create strong new materials.”
Critical to all of this molecular engineering was working out how to get these natural proteins to self-assemble and create these networks of hydrogen bonds without the addition of any harmful chemicals. The answer came from the way that the spider spins its silk. “So the way the spider does that is does this to length scales really. So there’s a there’s a molecular length scale, a nanometre length scale, where what the spider does is, is through using shear flow, it manages to align these molecules such that they can form the strong hydrogen bonding networks, that’s almost sort of a self-assembly step,” says Tuomas. The spider basically creates conditions in the spinning duct where the molecules have ideal conditions to come together and lock into place. “And then there’s sort of a micro scale micro to macro scale step where the spinning ducts really give us the thread, the overall shape to the structure. So this is really a multiscale process.”
Essentially the team have replicated the processes which happen inside a spider’s spinning duct getting the plant proteins to self-assemble into these dense hydrogen bonded structures that are technically Beta sheet rich. Beta sheets are the secondary structure formed when these long chains of particles connect with each other using hydrogen bonding that Tuomas says can be thought of almost like a zip being fastened.
Creating new structures
Simon and Tuomas describe their product as supramolecular engineered proteins and these can be moulded into whatever shape users require. Microcapsules to replace those being banned by the EU is one key product under development but also films, which don’t biodegrade and are not recycled.
Demand for plastic alternatives is expected to grow and with consumers, legislators and regulators all moving in the same direction Xampla is not struggling to find investors. To date the firm has raised over £8m in finance from leading investors worldwide to enable the company to begin commercial production. “I think it’s unusual for a material science company, which is essentially what Xampla is, to have such a force of consumer and regulatory drivers on its side,” says Simon. “So this is a material that’s emerging from the science base at absolutely the perfect time. Even the fossil fuel majors recognise that we need to move to a world where we’re making these materials out in a more sustainable way.
This is an interesting point, because using natural proteins means that the feedstock for Xampla will be an agricultural one. Simon says this is an opportunity to move away from the energy and transportation hungry supply model that dominates global plastics. “One thing that I’m personally quite passionate about is that the plastics industry was a 20th century construct and the value chain is bonkers, you know, drilling oil in Saudi shipping it to Korea to be cracked, we turned into nurdles in the states to come back to the UK to be made into a plastic film to wrap some tomatoes in Spain, which come back to Britain and then get thrown away. This is madness.”
For the 21st century he says that we have to think locally, meaning locally grown plants, turned into locally used plastic, that can then locally biodegrade. Peas have proven to be a successful option so far and are appropriate for food grade material but other options are also being investigated. “The next step we’re taking is into non-food sources. So things that are coming currently to animal feed. And we’re particularly looking at agricultural co-products, so essentially waste streams from existing materials out there.”
Simon says that pretty much every plant has got protein in and that Xampla is able to make protein materials from any plant. “So when we think locally, we can start looking at cassava in Africa, or rice in Asia, we can start working with local crops to build local plastics in in that particular region.”
Of course it would have to be cost effective and there would have to be sufficient local demand for Xampla’s products. With legislation forthcoming in the EU this has clearly helped the business case, but moving away from single use plastics in general is a global movement. The race is on to find new ways of carrying fragrances in washing detergent or enabling controlled release of chemicals in medicines or for delivering nutrients or seeds in agriculture. “However you look at it, growing plants and turning them into useful materials is better than drilling oil, cracking it into plastic and leaving it to pollute the planet for a couple of thousand years.”
