Stretching the boundaries of materials design

When you stretch a material, it normally becomes thinner.  Take the case of a rubber elastic band.  Most materials behave like this.  Yet, a small number of materials can become thicker when they are stretched! (see Fig. A) An example is the tendons which connect muscle to bone. These materials are known as ‘auxetics’, from the Greek auxetos, meaning ‘that which may be increased’.

This behaviour is found in several naturally occurring materials such as ceramics (e.g. natrolite and alpha-cristobalite), metals (e.g. arsenic and cadmium) and biological systems (e.g. cat skin, salamander skin and cow teat skin) among others.  It is also possible to manufacture certain auxetic materials including foams and yarns. 

While not common, this behaviour is especially interesting since the resulting materials have unique and superior mechanical properties. For example, they can bend into a dome-shaped structure and show enhanced mechanical properties such as increased indentation, hardness and shear modulus. These properties have made it possible for auxetics to be studied for use in applications such as personal protective equipment (helmets, bulletproof vests and knee pads).

Interestingly,...