Plastic-eating enzymes
Enzymes, microbes and such can develop the ability to break down any substance they find in their close environment, including plastics. They are now being screened and tailored to depolymerise PET plastics, polyester textiles and fabrics made from polyamide.
It is called destructive metabolism. From the Greek metabole, meaning change, it is a form of metabolism that produces energy and waste matter. This is, roughly, the biological process at work when enzymes metabolise or break down synthetics into their original building blocks. And change is also an apt term for this biorecycling category, whose technology is arguably one of the most ‘advanced’ of the many so-called ‘advanced recycling’ technologies in development and implementation.
The technology is still in its infancy, but it is taking its first steps. Since 2021, French company Carbios has been operating a pilot biorecycling plant in Clermont-Ferrand and the construction of an industrial scale facility is under way in Longlaville, with Indorama Ventures. Its projected capacity will be 50,000 tonnes of PET waste per year, when finished. A good step forward. But, when the company first began research in enzymatic recycling in 2011, the mandate set by company founder, Philippe Pouletty, was to develop the technology for ten different polymers. Two of the ten have been achieved. In addition to PET, the company’s Carbiolice division has developed an enzyme-based additive that makes PLA compostable, not quite recyclable, but still an end-of-life solution. Each polymer, of course, has a different chemical structure, and will require a different type of organism to break it down. As Carbios’ chief scientific officer, Dr Alain Marty, told WSA at the inauguration of the pilot plant in 2021, the ester bonds of polyesters are relatively easy to cleave, as could be the amide bonds of polyamides. But the carbon-to-carbon bonds of polyethylene (PE) and polypropylene (PP) are very difficult to separate.
Enzymes at work
In the development of enzymatic recycling processes, the first and critical step is to identify the organisms that have an affinity with a polymer. This can be done by placing various plastics or synthetics in various natural environments, including soil, fresh or salt water, and letting nature do her thing. When a biofilm forms on the material, the agent responsible for this sign of biodegradation can then be singled out as a possible candidate. Although artificial intelligence and machine learning have sped up the process of screening enzymes, this is not the end of the recruiting process. The enzymes need to withstand various conditions, remain highly efficient and ideally fast. This research stage can be a long process of tweaking and tailoring to achieve the desired skills. The stability of the enzymes, the rate at which they break down the plastic and temperature at which they do so in the vats are some of many parameters that need to be resolved. It took Carbios several years to engineer a specific ‘PETase’ strain capable of operating in temperatures around 100°C.
Enzymes may be powerful biological catalysts, but thermoplastics are sturdily built materials that do not naturally or easily break down. The second stage of research into these new biotech recycling methods is thus to make the plastic polymer more readily digestible. This can involve various chemical and mechanical preparatory phases to alter the structure of the polymer into a more amorphous form and increasing its surface area to make it more accessible to the action of the enzymes.
Once these two sides of the equation are solved, proponents of these technologies believe that the highly targeted action of enzymes gives them an advantage over other forms of recycling. An extensive scientific article written by the Carbios team, and published in Chemical Reviews this year, ‘Enzymes’ power for plastics degradation’, presents a critical overview of all research published to date on the enzymatic degradation of all types of plastics. It includes a summary of the promise of enzymes as “highly efficient biocatalysts” for their ability to operate in milder reaction conditions (temperature, pH and pressure), their high selectivity, and low environmental impact (less energy consumption and waste generation).
Catalysts for change
Carbios is not the only company betting on biorecycling. Protein Evolution, based in New Haven, Connecticut, is developing a biorecycling process for polyester and Samsara Eco, a start-up based in Canberra, has successfully identified polyamide-metabolising enzymes. The first presented a parka made from recycled polyester designed by British designer Stella McCartney, an early investor and partner, for the COP28 climate conference in Dubai. The latter recently announced that its partner, Lululemon, had made the first samples of garments from its biorecycled nylon.
Founded in 2021 by Connor Lynn and Dr Jonathan Rothberg, Protein Evolution has developed a biorecycling process that breaks down polyester textiles into terephthalic acid (PTA) and mono-ethylene glycol (MEG), as does the Carbios process. These can then be converted into 100% recycled polyester that is “indistinguishable from oil-derived polyester, but with a far lower carbon footprint,” says Connor Lynn, chief business officer. He adds: “Our Biopure process can handle a variety of traditionally tough-to-recycle materials that involve dyes, mixed materials and dense weaves.” The proprietary technology has been validated in an initial prototype system and is currently transitioning to an industrial demonstration facility, he says.
The start-up uses artificial intelligence to identify enzymes that have the desired plastic-degrading properties across vast biotech databases. This accelerates the discovery process and reduces the time from initial concept to potential enzyme. “Additionally, through our machine-learning models, we can predict the activity of enzymes under different conditions or modifications. This predictive capability is crucial for understanding how changes in enzyme structure might impact function, allowing our engineers to design enzymes with specific characteristics without extensive trial and error,” says Mr Lynn. AI, he adds, helps predict which mutations are likely to result in beneficial properties (streamlining the enzyme evolution process), as well as to create entirely new ‘enzymes’ that do not yet exist in nature. The company is not stopping at polyester and is also pursuing research into enzymes capable of breaking down polyamide and polyurethane. It has recently teamed up with Basecamp Research, a UK firm specialising in mapping biodiversity data for AI biodesign, to do so. “We expect to have promising enzyme candidates for new applications in the second half of 2024,” says Mr Lynn.
In February 2024, Canadian athleisure brand Lululemon presented the first samples of clothing made from enzymatically recycled nylon 6.6, a long-sleeve top for its Swiftly Tech range. The commercial version of this item is made in a blend of 50% nylon, 44% recycled polyester, 3% elastane and 3% X-static nylon. According to the press release, 90% of the nylon content is biorecycled using Samsara Eco’s patent-pending technology.
Also founded in 2021, the start-up is backed by the Australian National University (ANU), retail group Woolworths and Main Sequence, the deep-tech venture arm of national research centre CSIRO. It says it draws its technology from an extensive library of plastic-eating enzymes. Until now, the textile-to-textile recycling of nylon 6.6 has been near impossible, as company founder and CEO, Paul Riley, points out. “We’ve started with nylon 6.6, but this sets the trajectory of what’s possible for recycling across a range of industries as we continue expanding our library of plastic-eating enzymes.” Samsara Eco is preparing to scale its technology as it sets up a new R&D centre in New South Wales, which “will serve as a key milestone as we move towards our goal recycling 1.5 million tonnes of plastic per annum by 2030,” he tells WSA.
Mechanics of metabolism
Carbios, as mentioned, has turned its attention to identifying enzymes capable of depolymerising polyamides and polyolefins, both polyethylene (the world’s most produced plastic) and polypropylene. These three thermoplastics currently have few recycling pathways, the company noted in a strategic update. The French firm says it has vastly accelerated its enzyme optimisation techniques through the use of microfluidics, which enable it to screen millions of enzymes a day, compared with a few thousand a week using conventional technologies.
It can therefore be expected that progress in enzyme screening and optimisation will, at some point, make it possible to biologically break down all types of synthetics and plastics. Like any new technology, it won’t happen overnight. Change, like metabolism, can take time.
The Carbios demonstration plant in Clermont-Ferrand has been running since 2021.
CREDIT: Carbios-SkotchProdAgency
