Many sports participants, particularly engineers, are already well aware of the limitations of their protective equipment. Un-representative certifications and standards can lead to a dangerous mismatch between user expectations and equipment function. Deficiencies in protective equipment can contribute to life changing injuries of professional, celebrity and recreational sports people. The challenge is huge; protect users from both small, repeated impacts and large, high speed collisions and do this without hindering movement or performance. Smart materials could contribute to a solution: shear thickening fluids (STFs, D3O Video) are widely used as energy absorbing and dissipating layers that remain flexible during normal use. However dramatic changes in strain rate (impact speed) and temperature mean that they are not always able to offer protection, even if they pass certification tests. Initially, searching for a material that could reduce the likelihood of sporting injuries without restricting movement; I joined the Materials and Engineering Research Institute at Sheffield Hallam University and now work closely with an interdisciplinary research group in the Centre for Sports Engineering Research and Manchester Metropolitan University (MMU).
Auxetic materials have folding and/or rotating structures giving them a negative Poisson’s ratio (Auxetic Video) which is the ratio of lateral (width-ways) to axial (length-ways) deformation. While most material get thinner when we pull on them (think of stretching an elastic band) auxetic materials get fatter. In the other direction, auxetics get thinner (and denser) when compressed. My director of studies, Professor Andrew Alderson, moved to Sheffield Hallam University in 2014, bringing over twenty years of research in auxetic materials. Dr Tom Allen and Dr Leon Foster were quick to start working with him to improve pads [1–4], helmets  and equipment comfort , and to re-vamp auxetic foam fabrications [7–11].
Perhaps the most easily visualised benefit of auxetics is their contraction during indentation. Increasing a material’s density generally increases its stiffness. An auxetic pad could resist a penetrating object such as a stud , while remaining compliant in impacts over a larger area. Possible improvements go beyond density increases, auxetics can theoretically dissipate more energy than a conventional material. Additionally, as Poisson’s ratio decreases from 0.5 to -1 (becoming more auxetic), shear modulus increases. Shear modulus  defines a materials resistance to shape change. Rubber (with a Poisson’s ratio of 0.5) will readily change shape while auxetic materials won’t. They will deform as a flatter unit during indentation, giving a larger compressed area and meaning more material is compressed — this should increase the energy absorbed. Auxetic’s versatility in reacting to the shape of impacting bodies is three fold; higher density, more lateral deformation and more compressed material. I have recently shown lateral material flow and changes to the shape of the upper surface during indentation; I presented the results at the 14th conference for Auxetics and related systems and am currently preparing a manuscript to submit to a journal to be considered for publication.
The unique properties of auxetic materials could also improve helmet design. Helmet standards are regularly criticised for only specifying direct impacts and linear accelerations. Rotational acceleration during oblique impacts causes a wider range of life threatening injuries than linear acceleration, and can be reduced by designing helmets that have layers that can rotate (MIPS Video). It is still unclear whether a helmet’s open cell foam layer, often included to make a helmet more comfortable, is too compliant, too rigid or too thin to reduce rotational acceleration. Following an early pilot study clarifying that changing the open cell layer can affect the attenuation of linear acceleration , work is now underway measuring the shear modulus and rotational acceleration of helmets with conventional (low shear modulus) and auxetic (high shear modulus) open cell layers.
Comfort, due to auxetics’ unique shape change, is a benefit claimed in commercial products. Nike’s Flyknit running shoe expands with a runner’s foot during impact, reducing uncomfortable pressure points. Underarmour’s clutch fit range is claimed to dome during bending and conform to anatomical features in the wearer’s foot, a unique feature of auxetics. D3O now market their ‘Trust’ helmet pad system, featuring the established bow-tie shaped auxetic geometry. Future developments could include gradient foam sheets, with a range of Poisson ratios that can tailor the impact performance and fit of a single pad [8,10]. Smart garments incorporating auxetic and gradient foams could include rugby tops  and protection for snow-sports .
To test auxetics’ claimed benefits, we have created  and explained  a fabrication method that eliminates density and stiffness mismatches to produce a range of Poisson’s ratios during fabrication. For more information on any of the above, see our comprehensive review , our group’s BBC appearance (Auxetics on the BBC) or Professor Alderson’s previous blog (Auxetics for safety blog). Watch this space for a detailed look at indentation resistance, our take on auxetic closed cell foams that could replace materials traditionally found in sporting protective equipment  and additively manufactured auxetic structures.
Author: Olly Duncan, Materials and Engineering Research Institute, Sheffield Hallam University
- Allen T, Duncan O, Foster L, Senior T, Zampieri D, Edeh V, et al. Auxetic foam for snow-sport safety devices. Snow Sport Trauma Saf Proc Int Soc Ski Saf. 2016.21.
- Duncan O, Foster L, Senior T, Alderson A, Allen T. Quasi-static characterisation and impact testing of auxetic foam for sports safety applications. Smart Mater Struct. 2016.25(5).
- Allen T, Martinello N, Zampieri D, Hewage T, Senior T, Foster L, et al. Auxetic foams for sport safety applications. Procedia Eng. 2015.112(0). 104–9.
- Allen T, Shepherd J, Hewage TAM, Senior T, Foster L, Alderson A. Low-kinetic energy impact response of auxetic and conventional open-cell polyurethane foams. Phys Status Solidi Basic Res. 2015.9. 1–9.
- Foster L, Peketi P, Allen T, Senior T, Duncan O, Alderson A. Application of Auxetic Foam in Sports Helmets. Appl Sci. 2018.8(3). 354.
- Moroney C, Alderson A, Allen T, Sanami M, Venkatraman P. The Application of Auxetic Material for Protective Sports Apparel. Proceedings. 2018.2(6). 251.
- Duncan O, Foster L, Senior T, Allen T, Alderson A. A Comparison of Novel and Conventional Fabrication Methods for Auxetic Foams for Sports Safety Applications. Procedia Eng. 2016.147(0). 384–9.
- Allen T, Hewage T, Newton-Mann C, Wang W, Duncan O, Alderson A. Fabrication of Auxetic Foam Sheets for Sports Applications. Phys Status Solidi Basic Res. 2017.254(12).
- Duncan O, Allen T, Foster L, Gatt R, Grima JN, Alderson A. Controlling Density and Modulus in Auxetic Foam Fabrications—Implications for Impact and Indentation Testing. Proceedings. 2018.2(6). 250.
- Duncan O, Allen T, Foster L, Senior T, Alderson A. Fabrication, characterisation and modelling of uniform and gradient auxetic foam sheets. Acta Mater. 2017.126. 426–37.
- Duncan O, Clegg F, Essa A, Bell AMT, Foster L, Allen T, et al. Effects of Heat Exposure and Volumetric Compression on Poisson’s Ratios, Young’s Moduli, and Polymeric Composition During Thermo-Mechanical Conversion of Auxetic Open Cell Polyurethane Foam. Phys Status Solidi. 2018.
- Duncan O, Shepherd T, Moroney C, Foster L, Venkatraman PD, Winwood K, et al. Review of auxetic materials for sports applications: Expanding options in comfort and protection. Appl Sci. 2018.8(6). 941.
- Foster L, Peketi P, Allen T, Senior T, Duncan O, Alderson A. Application of auxetic foam in sports helmets. Appl Sci. 2018.8(3).
- Fan D, Li M, Qiu J, Xing H, Jiang Z, Tang T. A Novel Method for Preparing Auxetic Foam from Closed-cell Polymer Foam Based on Steam Penetration and Condensation ( SPC ) Process. ACS Appl Mater Interfaces. 2018.