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Mussel Glue
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13623 |
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Section : |
NATURAL SCIENCE
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| Issue
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4 / 1988 |
945 Words |
| Author
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Herbert Waite
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Adhesives have become an integral part of our lives from the cradle to the grave. As infants we wear disposable diapers with tape closures, and as corpses our lips are sealed by undertakers with cyanoacrylate glues. Increasingly in between, everything we use is being adhesively bonded. Packaging for food, bathroom tiles, plywood, floor coverings, paperback books, press-on nails, dentures, and toys are just a few examples. Plans for adhesively bonded cars and passenger jets are only a few years away. Adhesives have the ability to join dissimilar materials and allow better stress distribution in bonded joints because the points of contact between two surfaces are not limited to a rivet or screw here and there but to millions of molecular interactions between the adhesives and the bonded surfaces.
If adhesives are really that superior, why aren't they being used for everything? The reason is simply that the current generation of synthetic adhesives has performance limits. A common limitation in everyday adhesive use is the presence of moisture. Water forms a fine boundary layer between the glue and its intended bonding surface, and thus subverts the bonding process. This subversion is obvious when moisture prevents the adhesion of a bandage to a cut finger; it is more subtle when water gradually creeps under paint on cars or into the seals of NASA's booster rockets.
Enter the common mussel. Mussels spend their lives surrounded by seawater, yet remain strongly attached to each other and to rocks. Attachment is by way of a byssus (flax), which is a bundle of fine golden threads. The ancient Greeks wove byssal threads into fabrics which only the wealthiest could afford. Each of these threads is, in fact, an extracorporeal tendon originating inside the mussel in a retractor muscle and projected onto a solid surface. By drawing itself up on these tendinous threads, the mussel can control the tension of attachment. It is at the very tips of the threads that underwater adhesion is best demonstrated, however. The tips end in flattened pads called plaques, where bonding between the threads and underlying surfaces occurs.
Each thread is formed in a groove along the ventral surface of the foot of the mussel. This fascinating process can be observed by anyone with a modicum of patience. Simply pluck a mussel off the rocks, pull off as much of the old byssus as possible, then place the mussel in a glass bowl of cool seawater and wait. First the two valves of the shell open slightly, then the foot extends very gingerly from the gap and begins an exploratory process of feeling around to determine the accessibility and texture of nearby surfaces. Once it has found a satisfactory spot on the glass, the mussel firmly places the tip of its foot onto the glass and becomes quite still. Through the glass, one can observe a milky white substance being exuded from a dimple near the tip of the foot. This is the nascent adhesive plaque. The thread meanwhile is being molded in the groove of the foot. When the assembly is complete in one to two minutes, the foot gracefully lifts free of the thread and plaque and retreats into the shell or continues making more threads.
Viewed more analytically, the simple beauty of a mussel spinning byssal threads reveals a highly complicated chemical process. The milky white substance in the plaque is a two-part adhesive composed of a resin and a hardener. These are sprayed or spread as a foam onto the surface by the foot. As the hardener progressively acts on the resin, the foam changes in toughness
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