Auxetics are materials that have a negative Poisson's ratio. When stretched, they become thicker perpendicular to the applied force. This occurs due to their hinge-like structures, which flex when stretched. Auxetic materials can be single molecules or a particular structure of macroscopic matter. Such materials are expected to have mechanical properties such as high energy absorption and fracture resistance. Auxetics may be useful in applications such as body armor, packing material, knee and elbow pads, robust shock absorbing material, and sponge mops.
Auxetics can be illustrated with an inelastic string wound around an elastic cord. When the ends of the structure are pulled apart, the inelastic string 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, or auxesis, meaning "increase" (noun). This terminology was coined by Professor Ken Evans of the University of Exeter.
The earliest published example of a synthetic auxetic material was in Science in 1987, entitled "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.
Examples of auxetic materials include:
- Certain rocks and minerals
- Living bone tissue (although this is only suspected)
- Specific variants of polytetrafluorethylene polymers such as Gore-Tex
- Paper, all types. If a paper is stretched in an in-plane direction it will expand in its thickness direction due to its network structure.
- Tailored structures designed to exhibit special designed Poisson's ratios.
- Acoustic metamaterials
- Mechanical metamaterials
- Zetix, a type of auxetic material commercially manufactured
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