Researchers stick to natural approach for novel adhesive

If you have ever tried to chip a mussel off a seawall or a barnacle off the bottom of a boat, you will understand that we could learn a great deal from nature about how to make powerful adhesives. Engineers at Tufts University have taken note, and now report, in a paper in Advanced Science, a new type of glue inspired by those stubbornly adherent crustaceans.

Starting with the fibrous silk protein harvested from silkworms, the engineers were able to replicate key features of barnacle and mussel glue, including protein filaments, chemical crosslinking and iron bonding. The result is a powerful non-toxic glue that sets and works as well underwater as it does in dry conditions, and is stronger than most synthetic glue products now on the market.

"The composite we created works not only better underwater than most adhesives available today, it achieves that strength with much smaller quantities of material," said Fiorenzo Omenetto, professor of engineering at Tufts School of Engineering, director of the Tufts Silklab (where the material was created) and corresponding author of the paper. "And because the material is made from extracted biological sources, and the chemistries are benign – drawn from nature and largely avoiding synthetic steps or the use of volatile solvents – it could have advantages in manufacturing as well."

The Silklab 'glue crew' focused on replicating several key elements in their aquatic adhesives. Mussels secrete long sticky filaments called byssus. These secretions form polymers, which embed into surfaces and chemically cross-link to strengthen the bond. The protein polymers are made up of long chains of amino acids, including one, a catechol-bearing amino acid known as dihydroxyphenylalanine (DOPA), that can cross-link with the other chains. The mussels add another special ingredient – iron complexes – that reinforce the cohesive strength of the byssus.

Barnacles secrete a strong cement made of proteins that form into polymers that anchor onto surfaces. The proteins in these barnacle cement polymers fold their amino acid chains into beta sheets – a zig-zag arrangement that provides flat surfaces and plenty of opportunities for forming strong hydrogen bonds to the next protein in the polymer, or to the surface to which the polymer filament is attaching.

Inspired by these molecular bonding tricks used by nature, Omenetto's team set to work replicating them. To do so, they drew on their expertise with the chemistry of silk fibroin protein extracted from the cocoon of silkworms. Silk fibroin shares many of the shape and bonding characteristics of the barnacle cement proteins, including the ability to assemble large beta-sheet surfaces.

To the silk fibroin protein, the researchers added polydopamine – a random polymer of dopamine with cross-linking catechols along its length, much like the protein polymers used by mussels to cross-link their bonding filaments. Finally, they significantly enhanced the adhesion strength by curing the adhesive with iron chloride, which secures bonds across the catechols, just like the iron complexes in natural mussel adhesives.

"The combination of silk fibroin, polydopamine and iron brings together the same hierarchy of bonding and cross-linking that makes these barnacle and mussel adhesives so strong," said Marco Lo Presti, a post-doctoral scholar in Omenetto's lab and first author of the paper. "We ended up with an adhesive that even looks like its natural counterpart under the microscope."

Finding the right blend of silk fibroin, polydopamine and acidic conditions for curing with iron ions was critical for getting the adhesive to set and work underwater, where it reached strengths of 2.4MPa (megapascals; about 350 pounds per square inch) when resisting shear forces. That's better than most existing experimental and commercial adhesives, and only slightly lower than the strongest underwater adhesive, at 2.8MPa. Yet this adhesive has the added advantage of being non-toxic and composed of all-natural materials, and requires only 1–2mg per square inch to achieve that bond – just a few drops.

"The combination of likely safety, conservative use of material and superior strength suggests potential utility for many industrial and marine applications and could even be suitable for consumer-oriented applications such as model building and household use," said Gianluca Farinola, a collaborator on the study from the University of Bari Aldo Moro in Italy, and an adjunct professor of biomedical engineering at Tufts.

"The fact that we have already used silk fibroin as a biocompatible material for medical use is leading us to explore those applications as well," added Omenetto.

This story is adapted from material from Tufts University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.