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Recently we took a look at mussel anatomy in our mussel dissection video (which you can find here!), where we briefly mentioned that mussels can attach themselves to rocks or other solid surfaces. Mussels are sessile organisms and spend most of their lives stuck to a substrate. But mussels also live in water! If you’ve ever tried to use a strip of wet tape or glue down something underwater, you’ll know very well that water and adhesives are generally not a good mix. Most of our manmade adhesives are ineffective in watery or wet conditions because water molecules are polar. These polar water molecules form a thin layer that coats the surface and prevents contact between the surface and the adhesive. This is a huge problem because water is present in many biological systems. When we want to use adhesives in tissue repair, drug delivery, or biomedical devices, many of our existing synthetic adhesives fail or are ineffective.
Fortunately for us, mussels have already cracked the code of wet bonding through millions of years of evolution. Mussels outperform basically every type of synthetic glue we have in harsh, underwater conditions. So how do they do it? Mussels have evolved to be able to attach to a substrate even amidst pounding waves and constant watery conditions. Their byssus, or foot, secretes hair-like threads that cling to rocks. These threads are strong, flexible, and attach to rocks using a powerful water-resistant adhesive that the byssus also secretes.
The secret to the glue that mussels secrete is a rare amino acid called DOPA (3,4-dihydroxyphenylalanine). DOPA is not one of the 20 amino acids used in typical proteins, but makes up 20%-30% of the proteins found in mussel adhesives. Along with another amino acid, Lysine, DOPA plays a crucial role in binding the mussel to the substrate underwater. So what makes DOPA so special? Researchers have found that the most important compound within DOPA that gives the mussel its adhesion properties is a chemical compound group called catechols. While most adhesives are ineffective in water because the water forms a film between the adhesive and the surface, the catechol groups have a special talent for “drilling down” through the water molecules and binding onto surfaces.
Researchers have taken advantage of these catechol groups in various ways. One group from the University of Akron created a polymer with three key parts - a compound to form the liquid base, a segment containing a catechol, and a group that links similar groups together when exposed to ultraviolet light. To test the glue, they squeezed it onto glass slides underwater, then activated it by shining ultraviolet light on it. The glass slides stuck together with an adhesive strength comparable to that of mussel adhesive proteins.
(Adhesive being applied to a glass plate underwater)
Another group of scientists from Purdue University took a slightly different approach, creating a polymer called poly(catechol-styrene), which contains around 22% DOPA and approximately 78% styrene. The researchers state that “Poly(catechol-styrene) is looking to be, possibly, one of the strongest underwater adhesives found to date.” It is 17 times stronger than even the natural adhesive produced by mussels.
Mussel adhesive proteins have even been used in immunotherapy. Immunotherapy is a cancer treatment that works by activating our immune cells to attack cancer cells in the body. However, it often fails to be effective because the antibodies that train the immune cells are rapidly carried away from the target area by blood flow. Scientists at the Pohang University of Science & Technology have devised a way to counteract this with mussel adhesive proteins. Their novel immunotherapy platform called imuGlue connect mussel adhesive proteins to antibodies in order to effectively deliver the antibodies to the target area. imuGlue is especially effective because it doesn’t lose its properties in wet conditions like body fluid- or blood-rich environments and at mucous surfaces.
Water-resistant adhesives are still a subject of active research, and new discoveries are being made every day! So the next time you eat a mussel, consider taking a second to thank the mussel for its invaluable contributions to materials science and biomedical studies.
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