Engineers at Tufts University have developed new type of glue inspired by the incredible sticking power of some common underwater crustaceans and molluscs. Starting with the fibrous silk protein harvested from silkworms, the team replicated key features of barnacle cement and mussel adhesive, including protein filaments, chemical crosslinking, and iron bonding. They generated 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 team is also considering the potential use of the new glue for biomedical applications.

“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, PhD, Frank C. Doble Professor of Engineering at Tufts School of Engineering, director of the Tufts Silklab where the material was created. “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.”

Omenetto is corresponding author of the team’s published study, in Advanced Science, which is titled “Bioinspired Biomaterial Composite for All-Water-based High-Performance Adhesives.”

Anyone who has tried to chip a mussel off a seawall or a barnacle off the bottom of a boat will understand that nature can teach us a great deal about how to make powerful adhesives. “Marine glues and cements are materials with unique chemical and mechanical properties that allow functional performance in challenging environmental conditions such as broad temperature ranges (-20 to +40°C), chemical variations through fluctuating salinity and moisture, mechanical stressors such as tides, waves or currents, and the exposure to a whole host of hungry and opportunistic micro-organisms,” the authors wrote.

Mussels—molluscs of the genus Mytilus—secrete long sticky filaments known as 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, dihydroxyphenylalanine (DOPA), a catechol-bearing amino acid 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—Cirripedia species crustaceans—meanwhile, secrete a strong cement that is made of proteins that form into polymers, which anchor onto surfaces. The proteins in barnacle cement polymers fold their amino acid chains into beta sheets— a zig-zag arrangement that presents flat surfaces and plenty of opportunities to form 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 Silklab “glue crew” team set to work replicating them, and drawing on their expertise with the chemistry of silk fibroin (SF) protein extracted from the cocoon of silkworms. “This approach seeks to recapitulate the two distinct mechanisms that underpin the adhesion properties of the Mytilus and Cirripedia, with the former secreting sticky proteinaceous filaments called byssus while the latter produces a strong proteic cement to ensure anchoring,” the team wrote.

Silk fibroin shares many of the shape and bonding characteristics of the barnacle cement proteins, including the ability to assemble large beta sheet surfaces. In fact, SF and barnacle cement are very similar in terms of amino acid composition, the team noted, and they also share a common evolutionary origin, “ … since SF is a fibrous protein-polymer spun by many different types of animals, primarily arthropods, and characterized by extraordinary mechanical properties, such as high tensile strength and extensibility, driven by silk’s molecular assembly, as well as biological compatibility.”

To create their super-strong adhesive from the silk protein starting point, the researchers added polydopamine—a random polymer of dopamine which presents cross-linking catechols along its length, much like the mussels use to cross-link their bonding filaments. The adhesion strength was then significantly enhanced by curing the adhesive with iron chloride. “The choice of FeCl3 as a curing agent is inspired by the adhesion mechanism exhibited by mussels that accumulate Fe3+ and use it as a cross-linker between catechol units,” the investigators explained.

Model airplane assembled with silk-based glue [Marco Lo Presti, Tufts University]
Marco Lo Presti, PhD, post-doctoral scholar in Omenetto’s lab and first author of the study, further commented, “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. We ended up with an adhesive that even looks like its natural counterpart under the microscope.”

Getting the right blend of silk fibroin, polydopamine, and acidic conditions of curing with iron ions was critical to enabling the adhesive to set and work underwater, reaching strengths of 2.4 MPa (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 2.8 MPa. It also requires only 1–2 mgs per square inch to achieve that bond—that’s just a few drops. As the scientists wrote in their paper, their resulting adhesive “ … has properties that rival or exceed natural and synthetic systems with the underwater adhesion strength exceeding, in certain cases, adhesion performance rates in dry conditions all while using small (1–2 mg) quantities of adhesive.”

The new glue has the added advantage of being non-toxic, composed of all-natural materials. “The preparation and application of the blend employ benign chemistries that require no synthetic steps nor any use of organic solvents from the extraction process from raw materials all the way to adhesive application,” the team continued.

“We present here a bioinspired adhesive composite that combines both adhesion mechanisms of mussels and barnacles through a blend of silk, polydopamine, and Fe3+ ions in an entirely organic, nontoxic water-based formulation … The composite shows remarkable adhesive properties both in dry and wet conditions, favorably comparing to synthetic commercial glues and other adhesives based on natural polymers, with performance comparable to the best underwater adhesives with the additional advantage of having an entirely biological composition that requires no synthetic procedures or processing.”

“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 such as model building and household use,” said Gianluca Farinola, PhD, a collaborator on the study from the University of Bari Aldo Moro, 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.

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