Making underwater adhesives, biomimicry in action

Have you ever thought about repairing your leaky faucet without shutting off your home water supply? Or fixing your pool without emptying it first? Or restoring your boat directly at the marina? It may seem surprising, but despite all the commercial adhesives available at home improvement retailers, finding a high-standard adhesive that can stick underwater is still a huge challenge.
Indeed, most commercially available glues require application onto dry and clean surfaces to be considered effective. Well-known adhesives like Super Glue or Crazy Glue, which are both cyanoacrylate-based glues, use the humidity of the air to initiate their curing processes and stick. When these glues are used underwater, they cure too quickly, forming a goopy solid. This relegates them to poorer adhesives that are difficult to apply underwater. Other glues, such as Gorilla Glue, are not so sensitive to water. Nevertheless, when they are applied on wet surfaces, they demonstrate deceptive adhesive properties. A thin layer of water molecules always remains at the interface between the bonding surface and the glue, regardless of the manner and force by which the glue may be applied. This water layer acts as a lubricant and prevents the adhesive from effectively sticking onto the object's surface, leading to premature failure of the adhesive.
Removing this thin lubricant layer of water from an object immersed in water may seem to be an unsolvable problem for a scientist confined to their laboratory. However, if such a scientist decides to venture outside, perhaps on a field trip to our shorelines, they would be able to observe a wide variety of animals that can efficiently and durably stick underwater, such as mussels on a reef. Mussels produce several adhesive proteins to attach to the reef underwater. These proteins contain an unusual amino acid, 3,4-dihydroxyphenylalanine, which exhibits a pendant catechol group. Catechols have been identified as the main responsible agent for underwater adhesion. Using this chemical and applying a brushing movement to deposit their protein adhesives, mussels can remove the thin layer of water on the reef and stick, despite being below the Ocean.
Directly implementing the mussel protein adhesives in our daily lives as an underwater glue would be impossible. Mussels cannot produce as many adhesive proteins as we would need, and their collection, handling, and storage would be a highly complicated and costly process. Therefore, at Purdue University, we have been developing several glues containing the catechol group of the mussels. One of these glues is a biomimetic polymer that is composed of styrene and vinyl catechol monomer units. As we have observed prior, the catechol units promote underwater adhesion, while the styrene units are included to ease the processability and handling of the glue, optimize its mechanical properties, and enhance its resistance to water degradation. Indeed, polymers based on styrene units are widely employed in our daily lives, with a worldwide annual production of several million tons. When tested underwater, this novel biomimetic adhesive polymer shows adhesion over three times higher than any other conventional commercial glues on different surfaces. An effective underwater adhesive has been found!
Furthermore, efforts have been carried out in the laboratory to simplify the production of this polymeric glue. Among these, we have decided to use a polymerization in suspension protocol. This process consists of dispersing the vinyl catechol and styrene monomeric units in water using a vigorous stirring blade. Such strong stirring induces the formation of small micrometer-sized droplets where the chemical reaction occurs, allowing the formation of our biomimetic glue. This polymerization technique possesses several advantages: it limits the use of harmful organic solvents and replaces them with water, it reduces the risk of reaction runaways, and it is estimated to reduce the final price of the polymer production by 38%. Such efforts have proven the feasibility of the production of our biomimetic polymeric adhesive on an industrial scale and bring us closer to being able to purchase an adhesive in a home improvement retailer that will stick underwater.
More broadly, this research highlights how biomimicry, or Nature-inspired product design, is a delightful and multidisciplinary procedure aiming to solve daily life issues. It encompasses diverse areas of science, starting from the biological study of animals and plants, the chemical design of new innovative materials, and the engineering approach to scale up the production of such materials.
About the Author:
Alexandre Lancelot is an international postdoctoral researcher at Purdue University since 2021. He earned his PhD in Chemistry at the University of Zaragoza in Spain. In 2023, he was granted a Marie Sklodowska-Curie Actions fellowship to conduct investigation of new adhesives to be used in the biomedical field. His research focuses on synthesizing new materials to be used for unconventional applications and/or innovative therapies.
April 08, 2024