Following high profile, life changing long term mental illnesses and fatalities in sports such as skiing, cricket and American football-sports injuries feature regularly in national and international news. A mismatch between equipment certification tests, user expectations and infield falls and collisions is thought to affect risk perception, increasing the prevalence and severity of injuries. Auxetic foams, structures and textiles have been suggested for application to sporting goods, particularly protective equipment, due to their unique form-fitting deformation and curvature, high energy absorption and high indentation resistance. The purpose of this critical review is to communicate how auxetics could be useful to sports equipment (with a focus on injury prevention), and clearly lay out the steps required to realise their expected benefits. Initial overviews of auxetic materials and sporting protective equipment are followed by a description of common auxetic materials and structures, and how to produce them in foams, textiles and Additively Manufactured structures. Beneficial characteristics, limitations and commercial prospects are discussed, leading to a consideration of possible further work required to realise potential uses (such as in personal protective equipment and highly conformable garments).

Large sheets of polyurethane open-cell foam were compressed (or stretched) using pins and a conversion mould whilst undergoing thermal softening and controlled cooling. Sheets (final dimensions 355 x 344 x 20 mm) were fabricated with uniform triaxial compression, with and without through-thickness pins, and also with different compression regimes (uniform triaxial compression or through-thickness compression and biaxial planar tension) in opposing quadrants. The samples fabricated under uniform triaxial compression with and without pins exhibited similar cell structure and mechanical properties. The sheets fabricated with graded compression levels displayed clearly defined quadrants of differing cell structure and mechanical properties. The graded foam quadrants subject to triaxial compression displayed similar cell structure, tangent moduli and negative Poisson's ratio responses to the uniform foams converted with a similar level of triaxial compression. The graded foam quadrants subject to through-thickness compression and biaxial planar tension displayed a slightly re-entrant through-thickness cell structure contrasting with an in-plane structure resembling the fully reticulated cell structure of the unconverted parent foam. This quadrant of graded foam displayed positive and negative Poisson's ratios in tension and compression, respectively, accompanied by high and low in-plane tangent modulus, respectively. The strain-dependent mechanical properties are shown to be fully consistent with expectations from honeycomb theory. The triaxially compressed quadrants of the graded sheet exhibited ~4 times lower peak acceleration than quadrants with through-thickness 2 compression and biaxial planar tension in 6 J impact tests using a steel hemispherical drop mass.

This study compared low strain rate material properties and impact force attenuation of auxetic foam and the conventional open-cell polyurethane counterpart. This furthers our knowledge with regards to how best to apply these highly conformable and breathable auxetic foams to protective sports equipment. Cubes of auxetic foam measuring 150 x 150 x 150 mm were fabricated using a thermo-mechanical conversion process. Quasi-static compression confirmed the converted foam to be auxetic, prior to being sliced into 20 mm thick cuboid samples for further testing. Density, Poisson's ratio and the stress-strain curve were all found to be dependent on the position of each cuboid from within the cube. Impact tests with a hemispherical drop hammer were performed for energies up to 6 J, on foams covered with a polypropylene sheet between 1 and 2 mm thick. Auxetic samples reduced peak force by ~10 times in comparison to the conventional foam. This work has shown further potential for auxetic foam to be applied to protective equipment, while identifying that improved fabrication methods are required.

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Open cell auxetic foams have shown potential for use in sporting and other protective equipment.Previous conversion methods for auxetic closed cell foam used a bespoke pressure vessel. This work expands upon a recent method, using steam absorption followed by cooling and condensation, to change the cell shape of closed cell foam to impart a negative Poisson's ratio.Aiming to present a simple, repeatable conversion method, foam samples (20 ⅹ 10 ⅹ 100 mm) floating in water filled (~20°C), covered ceramic containers (300 ⅹ 200 ⅹ 100 mm) were heated in an oven at 105°C for between 1 and 6.5 hours. Based on the results for the smaller samples, a larger sample of foam (30 ⅹ 100 ⅹ 100 mm) was also converted, with a steaming time of 8 hours.Final volume ratio (original/final volume) increased with heating time, up to a maximum of 4.6.The amount and angle of re-entrant (inward folding) cell walls increased with final volume ratio as cells contracted further, evidenced by micro computed tomography. Poisson's ratios, measured using digital image correlation, were as low as -0.3 in tension and -1.1 in compression. Tensile Young's modulus increased from ~2 MPa up to ~6 MPa with final volume ratio and compressive Young's moduli reduced from ~1.5 MPa for low values of final volume ratio between one and three, then remained close to 1.5 MPa.