Mussel power

Many labs now mimic the protein chemistry used in mussel, oyster and barnacle adhesion in their hunt for glues that are super-strong, non-toxic and that stick under water or inside the body. Emma Davies reports.

Jonathan Wilker inhales the fresh scent of the ocean every time he enters his lab at Purdue University in landlocked Indiana, US. He grew up on the East Coast and has always been into all things marine. His lab now hosts bubbling tanks filled with mussels and oysters, which can be considered mini underwater glue factories. Although much is known about how the shellfish stick to surfaces underwater, vital chemical secrets still await unlocking.

Despite their many potential applications – especially in medicine and dentistry – bio-inspired glues have been slow to be adopted commercially. But a need for more sustainable products and a research shift towards simpler, more industry friendly, production processes mean commercial success may finally be on the horizon. A report from B2B analysts, MarketsandMarkets says the global bioadhesives market is projected to grow from $6bn in 2020 to $9.7bn by 2025, at a 10.0% CAGR (marketsandmarkets.com/Market-Reports/bioadhesive-market-16386893.html). This mainly comprises glues made from natural products, but some bio-inspired glues could fit this category.

Major glue companies are keeping a close eye on advances in bioinspired products. For example, German company Henkel, which makes several well-known glue brands, is involved in ‘scouting activities’ with academic partners, although it remains tight-lipped about them. The ‘key challenge’ for bioinspired products remains achieving performance at ‘competitive costs,’ according to a Henkel spokesperson.

Bio-inspired glues will add to a global adhesives and sealants market worth almost $60bn in 2020, according to Fortune Business Insights (fortunebusinessinsights.com/industry-reports/adhesives-and-sealants-market-101715). In the UK alone, the adhesives manufacturing industry is set to bring in revenues of over £472m in 2021, according to IBIS World. In a recent report, it found the industry to be ‘dynamic and characterised by innovation’, as well as driven by a shift towards environmentalism and sustainability. 

With adhesives, what’s good for one application might not be good for another. Every different polymer system that we make has different properties.
Jonathan Wilker Purdue University, Indiana, US

Cool as catechol

Glues and cements from mussels, oysters and barnacles have chemical and mechanical properties that allow them to hold fast in challenging environmental conditions, through fluctuating temperatures, changes in salinity, tidal forces and crashing waves. It’s little wonder that scientists have been inspired to replicate their chemistry in the lab.

Mussel adhesive chemistry is highly complex and there are still aspects that are not understood. In simple terms, mussels secrete proteins to create strong proteinaceous fibres called byssal threads, known to cooks as beards. At the ends of the threads are adhesive plaques that anchor to wet surfaces. A key constituent of the proteins is an amino acid called 3,4-dihydroxyphenylalanine (DOPA). This substance’s sticky magic lies in side-chains containing 1,2-dihydroxybenzene, otherwise known as catechol.

Catechol is involved both in reversible and irreversible crosslinking between different proteins. For example, catechol is oxidised by a mussel enzyme to form reactive quinone, which then takes part in covalent crosslinking with other protein residues, in effect curing the adhesives. Metal ions such as iron and vanadium also play a significant role in adhesion, chelating with DOPA’s catechol side chains to create metal-protein complexes.

Catechol chemistry lies at the heart of most bioinspired glue systems but efforts to translate it to technical and biomedical applications have been hampered by challenges in controlling the highly reactive DOPA. For example, the level of oxidation needs to be carefully balanced because some unoxidised DOPA resides are also needed for adhesion to different surfaces.

At Tufts University in Massachusetts, US, researchers have used their expertise in silk proteins to create a strong underwater adhesive inspired by both mussels and barnacles. Barnacle cement contains adhesive nanofibres made from proteins that form beta sheets, linked together through hydrogen bonding. Silk fibroin (SF) and barnacle cement have a very similar amino acid composition and share shape and bonding characteristics, according to Fiorenzo Omenetto, who leads the Tufts team and its SilkLab.

The researchers use polydopamine (PDA) as a catechol-bearing molecule and mix it with SF. They enhance adhesion strength by adding iron chloride as a curing agent. While the metal ions form complexes with catechol groups, as in mussels, the acidic conditions also cause SF to aggregate.

The PDA catechols provide sites for mussel-like crosslinking and iron chelation, while the polypeptide SF backbone provides a structural stability found in barnacle cement. In tests, the glue works in both wet and dry environments, comparing well with synthetic commercial glues. Combining SF and polydopamine (PDA) ‘brings together the best adhesive properties’ of both mussel and barnacle systems, the researchers report (Adv. Sci., doi: 10.1002/advs.202004786).

‘We have a lot of sticky states of silk, but mussels have a particular mechanism of adhesion and combining the two was really having two elements whose results were greater than the sum of their individual parts,’ says Omenetto.

SF has a ‘tactical advantage’ over other polymers because it uses water as a solvent, he says. As a result, as well as having an ‘entirely biological composition,’ the PDA-SF glue does not require synthetic steps or organic solvents. Next, the team wants to improve the glue system to give faster curing times and to extend its range of potential applications. This involves optimising the blends to achieve ‘just the right recipe of mussels and silk,’ says Omenetto.

In October 2021, a US team led by Xuanhe Zhao and Hyunwoo Yuk from the Massachusetts Institute of Technology, and Christoph Nabzdyk from the Mayo Clinic in Rochester reported making a surgical paste inspired by a different aspect of barnacle glue (Nature Biomed. Eng., doi: 10.1038/s41551-021-00769-y). They were attracted to the host of lipids that barnacles use to clean and prime surfaces before applying their cement. The bioadhesive can seal tissues in under 15s, say the researchers. The paste mimics barnacle systems by suspending microparticles made from polyacrylic acid and chitosan, a naturally occurring sugar, in oil. The oil cleans the tissue site while the polymers rapidly crosslink and stick. The researchers hope that the paste may help to treat severe bleeding. In tests, the paste formed a bond that was tougher than sealant pastes used by surgeons. The team has also run tests on anaesthetised live animals, sealing small cuts in heart chamber muscles.

Simple solutions

At Purdue University, Wilker has found that specialised chemical synthesis can be a significant barrier to commercialisation. ‘We are chemists, we have a synthetic lab. For us to do a two-step polymer synthesis is not a big deal,’ he says. However, companies wanting to try out a new glue product are less inclined to take on the synthesis without knowing whether the product would suit their needs, he says.

His lab has been working on high-performance adhesive systems that do not require specialised chemical synthesis, using proteins and catechol chemistry. They obtain proteins and catechol from different sources and combine them without needing to chemically modify the protein by attaching catechol synthetically. ‘If you put the right things together with the right conditions, you can still get some of the performance,’ he says. The adhesive systems are ‘working really well,’ leading to an uptick in commercial interest, he adds.

One of the systems uses a protein called zein, which comes from corn and is a waste product of bioethanol production. The team adds naturally occurring plant phenolics as a source of catechol. ‘If we combine them in just the right way, we can actually make adhesives that are fully bio-based and sustainably sourced, without any chemical synthesis,’ says Wilker. In tests, the glue has an adhesion strength comparable with super glue. ‘It’s one of those things that once you figure it out, it looks super easy but there’s a lot of testing to get to that point,’ he says.

One of the most promising adhesive systems to emerge from Wilker’s group is also one of the first that his team worked on 20 years ago, based on polystyrene with added catechol groups. ‘As sometimes happens in science, the one that we were working on first is turning out to have a lot of nice aspects with regard to low toxicity, strong bonding when it’s dry and strong bonding when it’s wet,’ he says. ‘Conceptually it’s the simplest reduction of the complexities of how mussels make their proteins and stick together.’

The polystyrene system is the basis of a start-up company called Mussel Polymers (MPI). (musselpolymers.com). With labs in Bethlehem, Pennsylvania, MPI has licensed a base technology from Purdue, with Wilker as lead scientist. MPI’s main focus is on adhesive bonding in wet environments, including marine and biomedical applications. But it is also looking at other applications, including cosmetics.

Wilker’s lab is also working on glues made from corn-derived polylactic acid (PLA), again with added catechol groups. ‘With adhesives, what’s good for one application might not be good for another,’ he says. ‘Every different polymer system that we make has different properties.’

The polystyrene system is very strong, but the PLA system may allow de-bonding at the end of a product’s life. Being able to disassemble glued items at the end of their lives is becoming increasingly important as the circular economy moves up the agenda. Currently, a staggering number of commercial products end up in landfill because their adhesive content makes them difficult if not impossible to recycle.

For their PLA system, Wilker and his team have been able to create adhesives that bond very strongly but that can also be tuned as to how quickly or slowly they debond. ‘Chemically there are several knobs that you can turn,’ he says. These knobs include changing the percentage of monomer with catechol groups as well as tweaking the metal crosslinking agents.

In 2020, researchers from Michigan Technological University, US, found that it is possible to deactivate a catechol-containing adhesive using electrochemical oxidation (J. Am. Chem. Soc., doi: 10.1021/jacs.9b11266). A team led by Bruce Lee reported that applying a 9V charge for one minute completely deactivated the adhesive.

For Omenetto, ‘glues happen to be one part of a very big picture that is deeply rooted in sustainability, deeply rooted in circular materials and in materials that minimise long-term impact on the planet.’

Despite being drawn down a commercial path, Wilker is as hooked on the fundamental science and what exactly goes on in the tanks in his lab as he was 20 years ago. He still wants to understand more about natural marine processes, especially in oysters.

‘There’s a lot we don’t understand,’ he says. His team has been studying how the oysters find surfaces and attach to them. ‘One of the cool things we found is that actually the animals have two different adhesive systems. When they are larvae, they have a ‘pre-made’ adhesive ready to go. Within about 48 hours of hitting a surface they shoot out the adhesive and attach temporarily to the surface. That’s their cue to undergo metamorphosis from a larva to a juvenile. Then at that point they switch to a totally different adhesive system.’

In the ocean, oysters cement together to form huge reef structures, which can be 10m deep and 100km long. These structures have a huge impact on the health of marine ecosystems, dissipating storm surge energy and holding sand in place, but as many as 98% of native reefs have been lost worldwide, says Wilker.

It is possible to grow larval oysters, but most don’t survive in the sea. ‘If we knew how the oysters actually produce their adhesives and cements and how they construct their reefs we would probably be more effective at reintroducing them to the ocean,’ he says.

When he started in the field, Wilker didn’t have a master plan. ‘It started out as: this might be cool. Let’s just try it, see what happens.’ Now it’s a ‘very exciting time’ for the field, he believes. And it’s still cool.