Auxetics are materials with a negative Poisson ratio - when you stretch them, they get fatter instead of thinner. One might wonder how this is even physically possible. The answer is in their underlying structure. Imagine a foam made out of millions of tiny bow-tie shaped cells, connected to one another. If you pull on the sides of the material, the bow ties expand into squares, expanding on the transverse plane as well as the plane parallel to the stretching action. Because the phenomenon is caused by the macrostructure or microstructure of the material and not the chemical composition of the material itself, many common materials can be put in auxetic arrangements, though materials that are flexible and stretchy work best.

The whole auxetics field is relatively new. The concept of materials with a negative Poisson ratio was first published in Science in 1987 by Rod Lakes of the University of Iowa, who continues to be a leader in the nascent field. The term "auxetic" was not used to refer to these materials until around 1991, and was derived from the Greek auxetikos, which means "that which tends to increase."

Auxetic materials are not natural. No known biological examples exist. The first auxetics were foams with specifically engineered 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% or so before shredding due to the stretching force. With more advanced auxetics, structured on the molecular level, more impressive expansion may be possible.

Proposals for the use of auxetics are fairly wide in scope, though few implementations have actually been created. Auxetics used in small medical probes could be used to dilate blood vessels. Because auxetics expand so readily, they would also be ideal filters, capable of catching many foreign particles in their macrostructure. Unlike traditional filters, they could remain small and very compact when not in use.

Threading auxetic fibers through composites could allow for strength improvements, the tendency to expand under stretching stress helping keep the overall structure of the composite together, particularly composites with materials that have a tendency to slide past each other. Many of the potential applications for auxetics probably haven't even been dreamt up yet, because the entire field is so new. But rest assured that they will be in the news over the coming decades, serving as an important newcomer to the science of materials.