17 April 2020
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Positive news has been relatively hard to come by of late, with attention understandably focussed on the developing COVID-19 pandemic. However, good news is still out there, as shown by a story appearing across the national press highlighting exciting developments in the field of enzymatic depolymerisation of plastics (see The Guardian and The Telegraph). In this article, we take a look at the science and the patents behind the press reports.

Background

A circular economy is the target for any sustainable industry. Transforming ‘waste’ into ‘resource’ both reduces waste accumulation and increases resource productivity. (Read more on this topic in our earlier blog Circular Economy: The solution to plastic pollution?).

In the world of plastics, there has been a huge emphasis on developing new waste-to-resource transformation methods. The focus has primarily been on honing existing mechanical and chemical recycling methods, as discussed in our recent blogs Polymers Unzipped and New technologies rise to the challenge. While traditional mechanical methods are now better than ever, the quality of the recycled plastics is still not akin to virgin-plastics and polymers can only be recycled a finite number of times. New chemical recycling methods offer the opportunity to overcome this quality degradation issue by making polymers anew, although the field is still at a relatively early stage of commercialisation.

However, mechanical and chemical methodologies are not the only routes open to us – another option rich with promise and innovation is the use of biotechnology-based recycling methods.

Biological Depolymerisation

Everything from caterpillars and mealworms, to microbes and bacteria have been reported as capable of transforming plastic waste back into its constituent monomers. 

Whilst these various biological approaches to plastic recycling have shown interesting results on a small scale, the real challenge lies in making the processes commercially viable at large scale, and ideally superior to existing chemical or mechanical recycling techniques.

Some earlier efforts fall well short in this regard. For example, many biotechnological attempts were no better than mechanical recycling systems, merely shredding the plastic into fragments, as opposed to actively depolymerising the molecular chains. Other versions, while successfully depolymerising the plastic, transformed a significant proportion of the polymer into CO2 rather than useable monomer. Clearly, proliferation of micro-plastic fragments and needless CO2 release is not ideal from an environmental standpoint.

Nonetheless, some later systems did show promise. However, to be viable on a large scale the speed and efficiency of the depolymerisation needed to be improved.

In all of the biological approaches above, the key to unlocking plastics is enzymes present in the natural systems. It is not in itself surprising that natural enzymes can depolymerise certain plastics. On a molecular level, the ester bond found in, for example, polyethylene terephthalate (PET), is identical to that in lipids. However, in nature the lipids are relatively diffuse, so the ester bond is accessible to the enzyme. In contrast, commercial polymers have high levels of crystallinity, which is required for the mechanical properties that are needed for everyday use. This means that the polymer chains and ester bonds are tightly packed together, limiting access for the enzyme.

So, to increase the speed and efficiency of depolymerisation the polymer needs to be ‘unpacked’, thereby improving access to the ester bonds.

Recent developments - Carbios

The stories reported in the national press have focussed on innovations by Carbios, whose results set out in a recent paper in the journal Nature are trumpeted as being the most promising yet to achieving enzymatic plastic recycling at scale.

Founded in 2011, Carbios’ early work focussed on autonomously biodegradable plastics, with enzymes embedded throughout the polymer architecture, ready and waiting to break down the plastic after use. Embedding the enzymes does somewhat improve access to the ester bonds, but the approach is more suited to slow, in-situ degradation.

Furthermore, these products are not really adapted for large scale harvesting of the monomeric degradation products.

To achieve this, Carbios looked to refine known methods of ‘unpacking’ polymers. One approach was to pre-amorphise the plastic, thereby destroying the crystalline structure and ‘unpacking’ the polymer before enzyme addition. This improved the efficiency (i.e. the percentage of polymer depolymerised) but the depolymerisation was still relatively slow.

A more promising approach was to heat the polymer to its glass transition temperature, at which point the inaccessible crystalline structure transitions to a more accessible non-crystalline structure. The latter ‘unpacked’ structure improves access to the ester bonds for the enzyme, increasing degradation speed and efficiency. However, commercial plastics like PET have high glass transition temperatures at around 75 °C. At this temperature, most natural enzymes will rapidly denature and aggregate, rendering them ineffective.

Carbios’ recent innovation, involves modifying the depolymerase enzymes to resist this denaturing and aggregation at high temperatures, so that the enzymes can work on the ‘unpacked’ polymer. The modification appends sugars to the enzyme’s polypeptide backbone, creating a physical barrier to unravelling and aggregation.

This culminates in a process which, at 75 °C and just 0.3 wt.% enzyme, is said to yield at least 90 % PET depolymerisation in just 10 hours. The company says that this level of improvement in efficiency and speed makes large-scale enzymatic recycling commercially practical.

Patent Protection 

Research is an iterative process, and when considering patent protection for iterative inventions, it is important to give careful thought about how to build a patent portfolio alongside scientific developments.

For Carbios, their latest innovation stems from nearly 10 years of iterative development, starting with enzyme-impregnated plastics and evolving into the latest modified enzymes.

Although we are not involved with Carbios’ patent work, based on publicly available documents it can be seen that early patent applications were directed towards the ‘method’ of depolymerising plastics with enzymes and the first generation enzyme impregnated plastics discussed above. Generally, ‘method’ claims afford strong protection for the process itself, which can still hold significant value at early stages of development.

As research progressed, new patent applications have been directed to broad classes of enzymes. Such patent applications can prove hugely useful to protecting an area from competitors whilst innovation is still ongoing to identify the best enzyme candidate. These patent applications with ‘product’ claims are also largely easier to enforce than ‘method’ claims, as acquiring evidence that the exact method has been used can sometimes prove difficult. Product claims can also be offer protection against use of the enzyme in fields other than polymer depolymerisation.

More recent applications track with the technology advances discussed above, focusing on the enzyme’s thermostability and type (e.g. esterases), as well as refining the process claims.

Carbios’ patent portfolio is also augmented by several narrow patent applications, specific to peptide sequences and modifications, seemingly directed to their commercially promising enzymes. These highly specific applications are perhaps more likely to be granted than broader, genus applications, but have an obvious trade-off in claim breadth. 

Clearly, building a patent portfolio alongside iterative product development requires careful thought and planning. While filing applications early does have its advantages, these applications can also serve as prior art against later filings. Therefore, exhaustive disclosure at an early stage can hamper the patentability of later inventions. Furthermore, if the product in question is not yet market-ready, filing early may make the application relatively inflexible to adapt to changing product characteristics, as well as deprive the applicant of commercially useful patent term.

Conclusion

The widespread reports of Carbios’ work in the press shine a welcome light on enzymatic depolymerisation, and are testament to the increased interest of the general public in the circular plastic economy.

The scale of the world’s plastic waste problem means that huge opportunities exist to innovators able to develop a commercially scalable enzymatic depolymerisation system, particularly those capable of processing common plastics, such as PET. With this in mind, it is clear that innovators should be thinking at an early stage of development of how best to protect their technology, in case their lab bench curio of today turns into the workhorse enzyme of tomorrow.

Niles is a trainee patent attorney working in the chemistry field. Niles has an MChem degree in chemistry from the University of Oxford. His undergraduate research project was on the synthesis of novel perylene diimide containing macrocycles for anion recognition and sensing applications.
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