According to foreign media reports, engineers at Tufts University have developed a new type of glue inspired by the sticky substances secreted by barnacles and mussels. They report their findings in the journal Advanced Science. Starting with silkworm fibroin, they were able to replicate key features of barnacle and mussel "glue," including protein filaments, chemical cross-links and iron bonds. The result is a strong, non-toxic glue that works just as well underwater as in dry conditions, and is stronger than most synthetic glue products on the market today.
"The composites we created not only survive underwater better than most adhesives today," said Fiorenzo Omenetto, the Frank C. It works better and it achieves this strength with less material. Also, since the material is made from extracted biological resources and the chemical composition is benign - taken from nature, synthesis is largely avoided steps or using volatile solvents -- it could also have advantages in manufacturing."
Silklab's "Glue Group" focuses on several key elements that are replicated in aquatic adhesives. Mussels secrete long sticky filaments called byssus. These secretions form polymers that embed into the surface and chemically cross-link to strengthen the bond. These protein polymers consist of long chains of amino acids, including a type of dihydroxyphenylalanine (DOPA), a catechol-containing amino acid that can cross-link with other chains. The mussels have added another special ingredient - an iron complex - to strengthen the cohesion of the paractenoid.
Barnacles secrete a powerful cement made of proteins that form polymers that anchor to surfaces. Proteins in barnacle cement polymers fold their amino acid chains into beta sheets -- a herringbone arrangement that provides a flat surface and plenty of opportunity to interact with the next protein in the polymer or surface to which the polymer filaments attach Forms strong hydrogen bonds.
Inspired by all these molecular binding tricks used in nature, Omenetto's team set out to replicate them, using their expertise in the chemistry of silk fibrin extracted from silkworm cocoons. Silk cellulose shares many shape and adhesive properties with barnacle cementin, including the ability to assemble large beta-sheet surfaces. The researchers added polydopamine -- a random polymer of dopamine that presents cross-linked catechols along its length, much like the cohesive filaments that mussels use to cross-link them. Adhesion strength was significantly improved by curing the adhesive with ferric chloride, which ensured the binding of the entire catechol, just as they do in natural mussel adhesives.
Marco Lo Presti, a postdoctoral scholar in Omenetto's lab and an author of the study, said: "The combination of filamentous cellulose, polydopamine and iron brings about the same level of bonding and cross-linking that makes these barnacles and mussels adhesive So strong. We ended up with an adhesive that even looked like its natural counterpart under the microscope."
Obtaining the correct mix of silk cellulose, polydopamine, and ferric ion-curing acidic conditions is essential for the adhesive to cure and work underwater, reaching a strength of 2.4 MPa when resisting shear forces. This is better than most existing experimental and commercial adhesives, and only slightly lower than the strongest underwater adhesive, which is 2.8 MPa. However, the added advantage of this adhesive is that it is non-toxic, consists of all-natural materials, and requires only 1-2 milligrams per square inch to achieve the bond -- that's just a few drops.
"The combination of possible safety, conservative use of materials and exceptional strength suggests potential utility in many industrial and marine applications, and could even be adapted for consumer-facing applications such as model building and home use," from Barry University Aldo Moro's study collaborator Gianluca Farinola, an adjunct professor of biomedical engineering at Tufts University, said. Omenetto added: "The fact that we have used silk cellulose as a biocompatible material for medical purposes is driving us to explore these applications as well."