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RESEARCHERS at the University of Utah, US, have discovered why caddisfly larvae’s silk is sticky underwater, which could lead to the development of surgical adhesives.

Russell Stewart, an associate professor of bioengineering at Utah, and Ching Shuen Wang, a member of his laboratory team, wanted to characterise caddisfly larva silk as part of wider research looking at natural adhesives, including that of the sandcastle worm, a marine organism which forms a protective casing by ‘gluing’ sand grains together. Aquatic caddisfly larvae similarly form a protective tube out of whatever debris is nearby, be it grit, plant debris, or in the case of Stewart’s captive larvae, small glass beads.

Initial examination of wild caddisfly larvae using a scanning electron microscope revealed that the individual fragments making up their tubes were effectively joined together by tapes of silk. Two glands in the body of each larva release their products into the spinneret, which extrudes the long ribbon-like silk. Once the captive larvae had formed tubes from the glass beads, samples of clean silk were removed and analysed by several methods including gel electrophoresis, amino acid analysis and tandem mass spectroscopy.

The serine (an amino acid) groups in the fibroin proteins making up the silk are quite heavily phosphorylated and carry a negative charge. These link with positively-charged arginine molecules (another amino acid) in the protein chain. Alternating positive and negative regions within the protein chains attract each other even underwater, resulting in the stickiness. Stewart notes that serine groups in silk spun by the larvae of the terrestrial silkworm moth are not phosphorylated. As silkworms are terrestrial, Stewart believes that it is the phosphate groups which are responsible for the adhesive qualities, and also are the reason the silk is insoluble in water.

Stewart and Wang hope that the discovery could result in the development of what he calls a “wet band-aid”, in other words an adhesive which could be used in surgery instead of sutures.

"There's just a fascinating diversity of these insects. Their adhesive is able to bond to a wide range of surfaces underwater: soft and hard, organic and inorganic. If we could copy this adhesive it would be useful on a wide range of tissue types," says Stewart.

The research will be published in Biomacromolecules.