Auxetics

Auxetics are materials that become thicker perpendicularly to the applied force when stretched. They have a negative Poisson's ratio, that is, they get thicker and not thinner when stretched. Materials of this sort are expected to have interesting mechanical properties such as high energy absorption and fracture resistance which may be useful in applications such as packing material, knee and elbow pads, robust shock absorbing material, and sponge mops.

The term auxetic is derived from the Greek word αυξητικός (auxetikos) which means, that which tends to increase and has its root in the word αύξησις (auxesis), meaning increase . Auxetic materials can be single molecules, or a particular structure of macroscopic matter.

The earliest published example of a synthetic auxetic material was in Science in 1987, "Foam structures with a negative Poisson's ratio" by R.S. Lakes of University of Iowa. The use of the word Auxetic to refer to this property probably began in 1991.^{[citation needed]}

Scientist have known about auxetic materials for over 100 years, but have only recently given them special attention.

Typically, auxetic materials are of low density, which is necessary to allow the hinge-like areas of the auxtic microstructures to flex. ref>http://space.newscientist.com/article/mg15420854.200-a-stretch-of-the-imagination.html</ref>

Examples of auxetic materials include:

The skin covering cows' teats.
Certain rocks and minerals
Living bone tissue (although this is only suspected).
Specific variants of polytetrafluorethylene polymers

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