It's relatively easy to get glue to stick in dry conditions, but it's much harder to get it to stay glued underwater. Still, a new bio-based glue not only works underwater but also gets stronger when submerged.
The nontoxic adhesive was developed by Gudrun Schmidt, an associate professor at Purdue University in Indiana, and his colleagues. It is made primarily from zein (a protein extracted from corn) and tannins (derived from the galls of oak tree bark).
Sandwiching the glue between two objects and holding them under water will initially create a thin skin. Just prick it with your finger or something similar to break this thin layer of skin. In this way, surrounding water can enter the glue and increase its bonding strength. Adhesion is greatest when the water temperature is approximately 30ºC (86ºF).
While the exact cause of this reaction isn't entirely clear, Schmidt points out that tannic acid is the primary cause of adhesion to surfaces, and that the acid's molecules have similarities to those in the natural glue mussels use to adhere to rocks underwater. The entire gluing process is no different than preparing a hard-boiled egg.
Schmidt told us: "When you drop a raw egg into warm water, a distinct skin forms around the egg, while the inside is still raw. If the water is neither too hot nor too cold, the skin around the egg is so thin that it can be easily broken with the tip of a fork […] If you now squeeze a hard-boiled egg between two slices of bread, then you've more or less done the job of sandwiching a glob of glue between two substrates."
The analogy goes a step further, if you heat a sandwich, the eggs will harden and hold the two slices of bread together.
Schmidt added that the glue is easy to make outside the laboratory from cheap, sustainable ingredients. It could eventually have applications in areas such as construction, biomedical/dental surgery, and even coral reef restoration.
A paper on the research was recently published in the journal ACS Applied Materials and Interfaces.