Figure 1: A diagram of the mussel plaque and the chemical compositions of its glue.
Source Credit: AskNature (LINK)
When things need quick fixing, there are certain tools that we all turn to. One of them is glue. However, there are not only limits to the glue we use but also several problems. The current glues used in medical procedures are too weak or cannot hold in a wet environment. Even without these limitations, the glue is most likely unsuitable, even toxic, to be used in our body during medical procedures. In addition, glues can pollute the soil with toxic chemicals, damaging the fauna and flora in that area. Recognizing these problems, scientists strived to find an alternative. The solution, to many of their surprise, was closer than we thought.
Mussels, classified as bivalve mollusks, are marine organisms. Spending most of their lives in lakes and streams, mussels usually stick to surfaces and feed on planktons around them. What’s more fascinating, however, was that mussels are able to adhere to any surface, even underwater. This is in stark contrast with the current form of glue that we have, which cannot stick properly underwater. As a result, researchers began investigating the methods behind the mussels’ ability to maintain adhesiveness underwater.
Mussels extend thin fibers known as byssal threads from their shells and attach to surfaces using a strong adhesive. The end of each thread has a flat, circular surface called a plaque. This plaque secretes adhesive proteins with unique chemical structures. The most notable component among the proteins is the amino acid DOPA, also known as 3,4-dihydroxyphenylalanine. DOPA has a catechol functional group, which allows it to form strong bonds with the catechols on the adjacent molecules, as well as the metal molecules that compose most of the natural solid substrates. Another notable component is the ability of the catechol chains to drill into the surface instead of interacting with water molecules. This allows the adhesive to remain fixed even in the presence of water.
The research on mussel glues started decades ago, and hundreds of research papers have already been published regarding this topic. However, the reason why we are still not able to produce a glue that mimics mussels’ such behavior is that there is a more complicated mechanism behind the adhesive proteins than the mere assembly of proteins. Factors that must be considered are the salt concentrations, timing, and pressure, which are harder to control. Companies such as ACatechol are looking for ways to recreate these perfect conditions for the glues to be made and mass producing them for future uses.
Though not yet ready for commercial usage, a number of plans are already set up to bring this glue into effect. Among the various fields that aim to use mussel adhesion, most of the attention is concentrated in the medical field. Particularly in dentistry, dentists have struggled to attach dental crowns and dentures in the mouth because the inside of our mouths is always humid and wet. Most glues are not very effective in wet conditions and eventually fall off, leading to many problems for the patients. The development of “mussel glues” might have the potential to resolve this issue as it has the ability to stick even in water. In addition to dentistry, these glues can also be used in treating blood vessels or even delivering drugs to specific regions of our body. Medical researchers are also considering the possibility of using this glue for tissue adhesion, as it is not only safe for human bodies but also very effective.
Another factor that this glue can contribute to is the environment. Glues made from biotechnology are easily degradable, meaning that certain objects that were put together using this adhesion could be recycled despite having glue on it. Not only that, but it reduces the chance of polluting the soil like the glues created out of lethal chemicals. This is better for the environment.
Despite the challenges associated with the creation of these adhesives, some predict that these glues can be out in the market in just a few years. Although the glues have not yet been used in real-life settings, just from what was shown in the experiments by far, many people are expecting great results in the future.
Q&A:
Jiwon: At the end of your article, you mentioned that this glue was not only effective in the medical field, but also in protecting the environment. How can degradable glue allow for easier recycling—shouldn’t recycling be dependent on what the object is made of, not what the adhesive is composed of?
Yes, definitely in almost all cases recycling depends on the main object, not the glue. But there are sometimes objects that have to be recycled but can’t because of the glue on them, and I was pointing out how biomimetics glue would not have these problems.
Sally: You mentioned that companies like ACatechol are trying to recreate the factors, so is there anything we know about their progress? How are they trying to recreate timing, pressure, etc?
There are no details from ACatechol or any companies with their progress. However, there is still further research going on by multiple organizations and researchers worldwide. There are still many unknown factors about wet adhesion that are yet to be discovered.
Xavier: What are some specific medical operations and treatments that can be improved with the addition of this adhesive that sticks to wet surfaces?
One specific example is tissue adhesion. When there is damage in the tissues, the ends have to be held close together so that the wound heals more easily and also to prevent any biological fluids from leaking out. With the current mechanism, there are several limits and downsides: surgeons have a hard time using it and sometimes patients feel excessive pain. Such problems could reduce with the use of a biodegradable adhesive.
Wooseok: Are there any disadvantages / weaknesses / downsides / cautions that are important? E.g. ‘the organic aspect of the glue makes it weak against certain environments
One disadvantage is definitely the fact that this adhesion is complicated. Even though research on this topic began decades ago, new discoveries are still arising. Not only that but as I mentioned in my article, simply getting the right chemical composition isn’t the end, as there are other factors to be considered.
Josh: Nice explanation of why this is relevant even today. However, is the glue economically viable, even if the right physical conditions were found? What kind of materials would we have to synthesize and how cost-efficient would the process be?
There is a question of how cost-efficient this process will be. Though this research is being funded by several private institutions or even hospitals, the time and effort that would need to be put in will cost some money. However, I think the fact that corporations have formed contracts and plan to commercialize this glue implies that this glue can be economically viable.
Eric: What evolutionary advantages did mussels get by having the ability to stick so well to surfaces underwater?
Mussels got the advantage of not being washed away by the currents. Since mussels are not organisms that can move around freely, they need to be able to firmly hold themselves in place. The stronger the adhesion the mussels had, the more they had the ability to feed on planktons in their positions.
John: DOPA is a molecule often administered to patients with Parkinson’s disease, and thus is a valuable, not to mention quite expensive product. Will this substance really be marketable in the adhesive industry?
Yes; in fact, the founder of the ACatechol corporation, Dr. Kollbe Ahn, invented a way to efficiently synthesize DOPA. Using this method, I think it is possible to create the adhesive economically.
Citations
AskNature Team. “AskNature - Biological Strategy - Sticky Proteins Serve as Glue.” AskNature Sticky Proteins Serve as Glue Comments, asknature.org/strategy/sticky-proteins-serve-as-glue/.
Venere-Purdue, Emil. “Mussels Inspire Glue That Sticks despite Water.” Futurity, 13 Mar. 2017, www.futurity.org/mussels-glue-adhesives-1377312-2/.
Service, Purdue News. “Shellfish Chemistry Combined with Polymer to Create New Biodegradable Adhesive.” Purdue University News, www.purdue.edu/newsroom/releases/2017/Q1/shellfish-chemistry-combined-with-polymer-to-create-new-biodegradable-adhesive.html.
Patel, Prachi. “Mussel-Inspired Polymer Glue Sticks to Wet Surfaces.” C&EN, cen.acs.org/materials/adhesives/Mussel-inspired-polymer-glue-sticks/98/web/2020/07.
Mian, Shabeer Ahmad, and Younas Khan. “The Adhesion Mechanism of Marine Mussel Foot Protein: Adsorption of L-Dopa on - and -Cristobalite Silica Using Density Functional Theory.” Journal of Chemistry, Hindawi, 15 Jan. 2017, www.hindawi.com/journals/jchem/2017/8756519/.
“Solvent-Based Adhesives & The Environment.” Sure Tack Systems, 31 May 2018, suretacksystems.com/2016/11/are-solvent-based-adhesives-bad-for-environment/#:~:text=Soil%20Pollution,comes%20into%20contact%20with%20it.
Guo, Qi, et al. “Recent Progress in Synthesis and Application of Mussel-Inspired Adhesives.” Nanoscale, The Royal Society of Chemistry, 23 Dec. 2019, pubs.rsc.org/en/content/articlelanding/2020/nr/c9nr09780e#!divAbstract.
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