If you've ever tried to pull a clam off a sea wall or a barnacle from the bottom of a boat, you'll understand that there is a lot we can learn from nature about making high-performance adhesives. Tufts University engineers took note of this and are reporting today in Advanced Science magazine about a new type of glue inspired by these stubbornly adherent creatures. Using fibrous silk protein extracted from silkworms, they were able to replicate key features of the barnacles and mussel glue, including protein filaments, chemical crosslinking, and iron bonding. The result is a powerful, non-toxic adhesive that cures and works just as well underwater as it does in the dry conditions and it is stronger than most of the synthetic adhesive products currently available in the market. “The joint we developed is not only more efficient under water than most adhesives available today but it also achieves this strength with much less quantity of material,” said Fiorenzo Omenetto, Frank C. Double Professor of engineering at the Tufts School of Engineering, director of the Tufts Silklab where the material was created, and corresponding author of the study. “And because the material is made from extracted biological sources, the chemicals are harmless as it is drawn from nature and largely without involving any synthetic steps or the use of volatile solvents, it could also have advantages in production” he further added.
The Silklab “glue team” paid attention to many key elements for synthesizing aquatic glue. Mussels secrete long, sticky threads called byssus. These secreted byssus form polymers that embed themselves into surfaces and chemically cross-link to strengthen the bond. The protein polymers are made up of long amino acid chains, including dihydroxyphenylalanine (DOPA), a catechol-containing amino acid that can cross-link with the other chains. The mussels add another special ingredient, the iron complexes that reinforce the cohesive power of the byssus.
Barnacles secrete a strong cement made from proteins that form into polymers that anchor to surfaces. The proteins in barnacle cement polymers fold their amino acid chains into beta sheets which is a zig-zag arrangement that offers flat surfaces and opportunity to form strong hydrogen bonds with the subsequent polymer protein or to the surface to which the polymer filament is adhering.
Inspired by all of these molecular bonding strategies that nature uses, the Omenetto team is set to work to replicate them. They are also drawing on their expertise with the chemistry of silk fibroin protein extracted from silkworm’s cocoon. Silk fibroin share many of the shape and bonding properties of the barnacle cement proteins, including the ability to build up large surfaces of beta-sheets. The researchers added poly dopamine, a random polymer of dopamine that presents crosslinking catechols along its length, much like the clams use to crosslink their bonding filaments. The adhesive strength is significantly enhanced by the curing the adhesive with iron chloride, which secures the bonds with the catechols, as is the case with natural mussel adhesives.