Auxetics are materials that expand when stretched due to their underlying structure, not chemical composition. They were first published in 1987 and are not found in nature. Potential applications include medical probes, filters, and improving composite strength.
Auxetics are materials that have a negative Poisson’s ratio: when stretched, they get fatter instead of thinner. This is possible due to their underlying structure. One could imagine a foam made up of millions of tiny bowtie-shaped cells, connected to each other. If someone pulls on the sides of the material, the bow ties expand into squares, expanding in the transverse plane and the plane parallel to the stretching action. This phenomenon is caused by the macrostructure or microstructure of the material and not the chemical composition of the material itself, so many common materials can be put into auxetic arrangements, although flexible and elastic materials work better.
The concept of materials with a negative Poisson’s ratio was first published in the journal Science in 1987 by Rod Lakes of the University of Iowa who was a leader in the nascent field. The term “auxetic” was not used to refer to these materials until about 1991. It comes from the Greek word auxetikos, which means “that which tends to increase”.
No natural examples
Auxetic materials are not natural and there are no known biological examples. The first auxetics were foams with specially designed microstructures. Depending on the size of the air gaps in the microstructure, the auxetic effect in these materials can be more or less extreme. Most auxetic foams expand by a factor of about 30 percent or so before crushing due to the force of stretching. With more advanced molecularly structured auxetics, a more impressive expansion may be possible.
Potential applications
Proposals for the use of auxetics have been fairly broad in scope, although few implementations have actually been created as of 2011. The auxetics used in small medical probes could be used to dilate blood vessels. These materials expand so easily that they would also be ideal filters, capturing many foreign particles in their macrostructure. Unlike traditional filters, they can stay small and compact when not in use.
Threading auxetic fibers through composites could allow for strength improvements, with a tendency to expand under elongation stress which helps hold the overall composite structure together. This is especially true for composites made up of materials that have a tendency to slide over each other. Many other potential applications for auxetics are yet to be developed, although the list is long and shows great promise in many fields.
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