Auxetics

Auxetics are materials that, when stretched, become thicker perpendicularly to the applied force, that is, they have a negative Poisson's ratio. This occurs because they contain hinge-like structures which flex when stretched. Such materials are expected to have interesting mechanical properties such as high energy absorption and fracture resistance. This may be useful in applications such as body armor, packing material, knee and elbow pads, robust shock absorbing material, and sponge mops.

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Auxetics can be illustrated with an inelastic string wound around an elastic cord. When the ends of the structure are pulled apart, the inelastic straightens while the elastic cord stretches and winds around it, increasing the structure's effective volume.

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

Scientists have known about auxetic materials for over 100 years but have only recently given them special attention. 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 from the University of Iowa. The use of the word auxetic to refer to this property probably began in 1991.^{[citation needed]}

Typically, auxetic materials have low density, which is necessary to allow the hinge-like areas of the auxetic microstructures to flex.^[1]

Examples of auxetic materials include:^{[citation needed]}

• The skin covering the teats on cows
• Certain rocks and minerals
• Living bone tissue (although this is only suspected)
• Specific variants of polytetrafluorethylene polymers such as Gore-Tex