Auxetic materials are unusual in that they expand sideways and get fatter when stretched, and get thinner when compressed. Such unusual behaviour is known as negative Poisson’s ratio. Negative Poisson’s ratio can enhance mechanical properties, like indentation resistance and energy absorption, giving potential improvements to protective equipment. Negative Poisson’s ratio also causes bi-axial expansion and domed curvature, which could improve the fit and comfort of shoes and clothing. Indeed, auxetic materials are gaining much interest in the sports and fitness equipment sector, with products such as shoes, helmets and clothing emerging. There are many types of auxetic materials that could bring benefits to such equipment, including fibre-reinforced composites, flexible 3D printed structures, fabrics/textiles and foams.
Auxetic open cell foam was first made in the 1980s, and much of our work exploring how negative Poisson’s ratio could improve sports equipment has used this material. Such open cell auxetic foam is, however, much less stiff than the closed cell foam typically used in sports equipment, as air can pass between the open cells. Recently, we made closed cell auxetic foam with similar stiffness to the foam used in sports and fitness equipment . This means we are now much closer to actually applying our auxetic foams to sports equipment. Our auxetic foam is made with a simple steaming process. We put the foam in boiling water for several hours and then allow the trapped steam to condense (or shrink), at room temperature, then naturally evaporate. The process leaves auxetic foam with a contorted cell structure of buckled and kinked cell walls. When the auxetic foam is stretched these contorted cells straighten, causing sideways expansion and negative Poisson’s ratio.
We see many opportunities for wearable technologies within the sports and fitness equipment sector, and would like to work with others within the E-Textiles network. Indeed, we are exploring how to combine stretchable sensors within novel apparel and protective equipment utilizing closed cell auxetic foams, so we can measure how they change shape and exert or transmit pressure. Such smart materials could be embedded within sports products like shoes, helmets and clothing, to measure and assess the benefits of negative Poisson’s ratio during use. If you are interested in our work, have relevant expertise and would like to work with us, then we would love to hear from you.
Drs Tom Allen, Olly Duncan and Charlotte Moroney and Professor Andy Alderson
Email: T.Allen@mmu.ac.uk, O.Duncan@mmu.ac.uk and A.Alderson@shu.ac.uk
 Manchester Metropolitan University
 Sheffield Hallam University