Mussel-inspired polymer glue sticks to wet surfaces

For decades, scientists have tried to make surgical glues modeled after adhesive proteins that mussels use to cling to underwater surfaces. But making a practical glue that can be applied to wet tissues and cured within the body has been tricky. Now, researchers report a biodegradable adhesive that sticks to wet surfaces in minutes and, unlike previous glues, works under the body’s range of saltiness and acidity (ACS Nano 2020, DOI: 10.1021/acsnano.0c02396).

Medical glues are challenging to make because they need to attach to tissues dampened with, say, blood or sweat. “When tissue is soft and wet, it’s very difficult to spread materials on it and stick well,” says study coauthor Abraham Joy, a chemist at the University of Akron.

Mussels and sandcastle worms cling to underwater surfaces by secreting a mix of oppositely charged adhesive proteins. Electrostatic interaction between the charged proteins creates a coacervate, a suspension of insoluble protein droplets in water. The dense suspension spreads and sticks to surfaces. A weak molecular attraction keeps the proteins in place until cross-links can form among the proteins, as well as between the proteins and the surface, which strengthen and harden the adhesive.

Researchers have tried to mimic this strategy by creating mixes of oppositely charged polymers that can coalesce to form droplets that separate from water. But these adhesives rely on “very precise conditions of charge, salt, and solution pH,” says Jonathan J. Wilker, a chemist at Purdue University who was not involved in the work. “Once the solution conditions change even a little, the [complex] can be lost.”

So Joy, Ali Dhinojwala of the University of Akron, and their colleagues made a polymer adhesive that doesn’t rely on electrostatic interactions between charged polymers. Instead they start with a polymer with three key parts: a diethanolamide that helps to form a coacervate; a segment containing a catechol, an aromatic diol that plays a key surface-adhesion role in mussel proteins; and a coumarin group that can link to similar groups on other polymer strands via click chemistry when exposed to ultraviolet light.

The team made a solution of the polymer, triggered coacervate formation by warming it, and then filtered out the coacervate, which serves as the glue. To test the glue, they squeezed it onto glass slides submerged in water. After pressing the slides together for 15 min, they shined UV light on the glue for 10 min, cross-linking it to form an elastic adhesive. The adhesive works in solutions with pH 3–12 and 0–1 M salt concentration. The glass slides stuck together with an adhesive strength of 100 kPa, which is comparable to that of mussel adhesive proteins.

“The system in this new study is an exciting one,” Wilker says. The charge-neutral system gives impressive underwater adhesion, he adds.

“This work represents an important advance in underwater adhesives,” says Phillip B. Messersmith, a materials scientist and engineer at the University of California, Berkeley. So far, however, the researchers have only used it on glass, so they need much more testing to confirm that it would work in medical situations. “It will be interesting to see how this material performs on wet tissues,” he says.