Modular Adhesives and Energy-Dissipating Materials
BACKGROUND: Natural materials are renowned for their strength and toughness. Spider dragline silk has a breakage energy per unit weight two orders of magnitude greater than high tensile steel, and is representative of many other strong natural fibers. The abalone shell, a composite of calcium carbonate plates sandwiched between organic material, is 3,000 times more fracture resistant than a single crystal of the pure mineral. The organic component, comprising just a few percent of the composite by weight, is thought to hold the key to nacre's (or mother-of-pearl's) fracture toughness.
DESCRIPTION: Scientists at the University of California have discovered the mechanism by which the protein, Lustrin A, acts to hold the mineral plates in abalone shells together. This protein acts as a modular adhesive, affording it a unique ability to resist an applied force. Initially, the modular adhesive stretches, like an elastic adhesive. As the force reaches a significant fraction of the force required to break a bond, one of the modules unfolds. At this point, the protein again acts like an elastic adhesive and the process repeats until all modules have unfolded and finally the bond breaks.
APPLICATIONS: This modular elongation mechanism may prove to be quite general for conveying toughness to natural fibers and adhesives, including dragline silk. Although this behavior was discovered in a protein, it can be extended to human-made fibers and adhesives, such as those composed of block copolymers. It has applications ranging from armor to artificial tendons.
ADVANTAGES: This newly discovered mechanism enables the development of materials with unique characteristics, including:
* Adhesives or fibers that are not subject to catastrophic failure (i.e. breaking the molecular backbone of the adhesive or its attachments to the surface being glued);
* Adhesives or fibers that can "heal" or refold once the force is reduced;
* Adhesives or fibers designed to suit a specific need. For instance, modules that unravel at different forces could be used to produce adhesives that yield easily at first and then become more and more rigid;
* Adhesives used to pull surfaces together. Surfaces could be glued under conditions where the modules are unfolded and then the modules could be caused to enter the folded state;
* Smoothed force-verse-extension curves achieved using modules that unfold in multiple steps.
US 6,376,636 [MORE INFO
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