Many aquatic animals spend much of their lives stuck to surfaces that can include rocks, ships or even whales. Limpets and sea stars, for example, use a form of adhesion that allows them to move on the surface they have colonised, but which makes them very hard to remove from that surface. Barnacles and mussels, meanwhile, produce fully-fledged underwater glues, known as bioadhesives; once they attach to a surface, these animals spend the rest of their lives stationary. Researchers have been aware of the strength of these glues for some time, but attempts to create artificial substances that rival their underwater adhesive power have been largely unsuccessful – until now.
Dr Timothy Lu and his colleagues at the Massachusetts Institute of Technology have created a novel glue based on byssus, the bioadhesive produced by the Mediterranean mussel (Mytilus galloprovincialis). Their results, recently published in the journal Nature Nanotechnology, show that the substance is 50% stronger than any bio-inspired, protein-based underwater glue produced so far.
To achieve such strong adhesion, Dr Lu and his team used two proteins from the mussel’s foot, which the animal uses to shoot byssus into any crevice it wants to attach to. The researchers inserted the genes that encode the production of these proteins into Escherichia coli bacteria, which are themselves known for the sticky fibres they produce to adhere to our intestines. As a result, the bacteria produced a ‘hybrid’ glue, although they did not survive the process.
Mussel adhesives and E. coli adhesives use different types of chemicals. Byssus is based on a material called L-3,4-dihydroxyphenylalanine, or DOPA, which is thought to be responsible for the water resistance and fast-acting nature of this glue. E. coli, on the other hand, produce amyloids, fibre-like structures that are notable for their self-assembly: no external guidance is needed for the molecules to arrange themselves into networks that grow denser and larger as long as more raw material is supplied. A combination of these two types of adhesives, the researchers reasoned, would be particularly potent.
Despite using mussel proteins, there were no aquatic creatures in the lab at any point. The scientists simply ordered the DNA sequences responsible for producing the proteins from an online database. This allowed them to experiment with many different proteins, mixing and matching them to find out what they do – and to identify the ones that produced the best adhesives when inserted into E. coli.
The two proteins Dr Lu and his colleagues used are just a few of the many that play a role in producing byssus. We do not yet know what each mussel protein does, and using just two of them yielded an imperfect adhesive. For example, while the glue worked well underwater, it was much less effective when in direct contact with air. We know that some barnacles and mussels live in intertidal areas that are only submerged part of the time, so it must be possible to produce adhesives that work on land using mussel proteins – we just haven’t found the right ones yet.
Dr Lu notes that it is possible to extract the adhesive substance from the E. coli without killing them. He is currently working to introduce this feature, which would result in a ‘living’ glue with self-repairing properties: the bacteria could continuously produce more molecules to add to the framework. Repairing damage in ships’ hulls is just one example of the possibilities of such an adhesive.
Despite its imperfections, Dr Lu believes that there will be plenty of interest in the new adhesive if he and his team manage to produce it in larger quantities. While it would not replace cheap, generic superglue, a bioadhesive that works well in wet conditions could become an invaluable tool for surgeons, allowing them to seal wounds more easily and less intrusively than with stitches.
Much research remains to be done to unlock the secrets of mussel glue and how best to combine the proteins involved in producing it. E. coli have proven a useful basis for producing the substance – in fact, Dr Lu and his colleagues have used them to produce a range of substances in previous research. They have taken an important step towards making a strong, commercially viable underwater glue – and it may only be a matter of time until scientists discover an adhesive that can out-stick mussels themselves…
Zhong, C., Gurry, T., Cheng, A., Downey, J., Deng, Z., Stultz, C., & Lu, T. (2014). Strong underwater adhesives made by self-assembling multi-protein nanofibres Nature Nanotechnology, 9 (10), 858-866 DOI: 10.1038/nnano.2014.199