Innovative biological adhesive

Like silkworms, butterflies and spiders, caddisfly larvae produce silk threads, but they do so underwater, not on land. Now, researchers at the University of Utah have discovered why their silk threads are sticky in an aqueous environment, and how this information may be valuable for the future development of surgical adhesives.

Like silkworms, butterflies and spiders, caddisfly larvae produce silk threads, but they do so underwater, not on land. Now, researchers at the University of Utah have discovered why their silk threads are sticky in an aqueous environment, and how this information may be valuable for the future development of surgical tape.

"These insect silk threads may one day be useful as a medical bioadhesive for the adhesion of moist tissues," says Russell Stewart, professor of bioengineering and lead author of the paper detailing the chemical and structural properties of the threads.

"I imagine it as a kind of wet sticky bandages (plasters) that could be used in surgeries - like using a piece of tape to fuse cuts instead of stitches," adds the researcher. "Gluing objects together under water is not an easy task. Have you ever tried to stick a band-aid on your body under the stream of shower water? This insect has been able to do this for close to two hundred million years." The findings of this study will soon be published in the scientific journal Biomacromolecules.

There are thousands of species of this insect around the world and they are part of the series called Trichoptera, which is close to the Lepidoptera series, which also includes moths and butterflies that produce dry silk threads. Since these insects are used as food for trout fish (trout, or your name in Hebrew), experienced fishermen use them as fish baits.

Some species go through the stages of molting under water, and build around themselves a kind of cylinder-shaped protective box composed of sticky silk fibers and grains of stone or sand. Other species use silk fibers, small pieces of wood and pieces of leaves.

In each of the larvae, the head and four legs protrude outside this box. Most often, the shape of the box is like a cone, since it grows with the development of the larva that matures inside it. Eventually the larvae molt and seal the box as they develop into the adult form, then they hatch out of it.

These underwater insects, terrestrial butterflies as well as various types of moths diverged from their ancient silk-producing ancestors about two hundred million years ago. Insects of this type are found throughout the world in reservoirs ranging from those that flow strongly to stagnant swamps. "The ability of these insects to successfully inhabit a variety of aquatic niches is closely related to their original use of underwater silk threads to build complex structures for protection and food capture," the new study explains.

Caddisflies belong to a sub-group called Brachycentrus echo, in which the insects wrap themselves in a box and carry it around while they search for food. Some of these insects build a stationary escape niche for protection in the form of a dome stuck to an underwater rock, where there is a silk-fiber network to capture water-borne food.

The research team examined natural adhesives, including a glue created in ocean water between high tide and low tide by the sandcastle worm. In this material lies the ability to be used as an adhesive for fusing fractures in small bones. The lead researcher became interested in the sticky silk fibers of this insect family after another researcher showed him several cylindrical larval boxes.

"We looked inside the box with a microscope and identified the silk fibers stretched between the grains of the stone and realized that this is really fascinating," he says. "When I returned home I put on my fishing boots and wandered among the mountain streams in search of these insect larvae." "There is a fascinating variety of these insects and their adhesives are able to bind a variety of underwater surfaces: soft and hard, organic and inorganic. If we can copy this glue it will be useful for a wide variety of tissue types."

The larvae of these insects emit silk fibers from an organ known as a spinneret. Two silk glands fit together in it so that the resulting glue looks like a double strip with a seam in the center. The caterpillar weaves this sticky web back and forth around grains of sand, sticks or pieces of leaves to obtain the finished roll.

The researchers raised these larvae in an aquarium and replaced the natural granules with glass beads. The larvae expanded their boxes with the beads, which were glued together by wet silk fibers. The researchers severed several beads to obtain clean samples of silk. They examined the silk using several methods, including scanning electron microscopy, which showed how the silk fibers bind together the glass beads inside the protective box. "It's like using transparent adhesive tape inside a box to strengthen and stabilize its shape," explains the researcher. "More than anything else it reminds me of adhesive paper - one that works in the water." The researchers next intend to measure the strength of these silk fibers. "Individual threads are not very strong by themselves, but weaving dozens of them together makes them so. If we can duplicate this composition and create an adhesive from it, the grip strength will increase significantly."

The current research included a detailed examination of the chemical and structure of the silk fibers, and showed the similarity to the product of silkworms used in the textile industry and even to spider webs, but with an adaptation that allows them to function underwater. The researcher says that his intention was to characterize the sticky silk fibers "in order to try and reproduce them in the laboratory" so that their synthetic version could be used as an adhesive for surgeries. He discovered that the silk of these insects is a fiber composed of large proteins called fibroin in which about a fifth of the amino acids are of the serine type. The key difference between dry silk fibers, which originate from silkworms and butterflies, and the wet fibers, which originate from insects, is that the amino acids of the serine type in the wet form contain phosphate groups that were added to them during the creation of the protein.

"Phosphorine groups are known as adhesion promoters and are used in dental accessories such as crowns or fillings," notes the researcher. "They are also found in water-based resin paints, where they increase the adhesion of these paints. The paint industry discovered this only recently, while these insects have been doing it for about one hundred and fifty million years."

The phosphorylation groups attached to serine are negatively charged while other amino acids in the protein are positively charged. The researchers discovered that this fact is a major factor in the ability to produce silk fibers underwater. The protein chains - each of which has alternating regions of negative charge and positive charge - line up parallel to each other as the regions of opposite charge are attracted to each other respectively. "Imagine these chains side by side, when they oscillate so that the minus and plus areas face each other and assemble the silk fibers with a large amount of such proteins in a single fiber," explains the researcher. "It's true that we can't make shirts out of them, but maybe we can make a sticky band out of them for a wet environment."

"These fibroin proteins, which make up the silk fibers, are water-soluble due to the electrical charges present in them. A comparison of the amino acids from three other strains of these insects showed that there is a lot of similarity between them and implies that the phosphorylation groups are also responsible for the existence of wet silk fibers. The researchers add and say that these fibers also have a common denominator with the adhesive substance of the sand-palace worms - both have proteins that contain many phosphorus groups that cause the receipt of charged amino acids. The researcher points out that the ability to produce active adhesives in an aquatic environment has already been identified in four living systems: sand castle worms, snails, sea cucumbers and the family of insects in question - and they all came to this solution independently.

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