# Review of Auxetic Materials for Sports Applications: Expanding Options in Comfort and Protection
## 3 Common Auxetic Materials and Structures
### Fabricating and Characterising Auxetic Foam  
Auxetic foam is often fabricated to experimentally show expected enhancements (i.e., to impact force attenuation or vibration damping) due to NPR [2; 33; 183; 32]. Auxetic foam fabrications typically change the cell structure of open cell foam (referred to herein as the parent foam) to give it an NPR. [PERSON] thermo-mechanical fabrication method [9] first applies a volumetric compression ratio (VCR, normally defined as original/final volume and typically between 2 and 5 [184]) to a parent foam in all three orthogonal axes to buckle cell ribs [118]. The applied compression changes the cell shape and causes the re-entrant structure, as cell ribs buckle beyond \(\sim\)5% compression [118]. The compressed foam is then heated to a set temperature to encourage permanent plastic deformation. The temperature is typically between 130 and 220 \({}^{\circ}\)C [13], often referred to as the'softening temperature' [20; 185]. Finally, the foam is cooled to fix the imposed structure [9]. Buckling the originally straight ribs (Figure 3e) gives the polyhedral cells a re-entrant, contorted cell structure [9] (Figure 3a,d). Typical sizes were initially small, in the order of 20 \(\times\) 20 \(\times\) 60 mm (following fabrication), although larger'scaled up' samples have subsequently been fabricated (e.g., [185; 186; 187]). Fabrication methods for auxetic foams have developed and diversified since [PERSON] initial study [9]. Stretching samples after cooling was introduced to reduce adhesion between cell ribs and residual stresses [189]. More recently, fabrication processes have been split into stages including; multiple heating cycles (with foam removed from the mould and stretched by hand in between to reduce residual stresses, flaws and creases [190]) and the addition of an annealing stage--heating below the softening temperature. Foams are typically annealed at 100 \({}^{\circ}\)C [37; 185; 190], but alternatives to annealing include slow cooling in the mould in air [20; 191] or cooling outside the mould in air [192]. In an attempt to reduce creases and flaws when inserting foam into the mould, olive or vegetable oil and WD-40 have been used to lubricate moulds [185; 189; 193]. Another line of investigation has attempted to increase the range of Poisson's ratios and Young's moduli achievable following thermo-mechanical fabrications. The heat, time and compression applied during fabrication can be adjusted to give higher magnitude NPR and increased stiffness [129]. Heating for longer or at a higher temperature ('over-heating' while using typical compression levels) gives a positive or near zero Poisson's ratio re-entrant foam [30; 194]. Over-heated re-entrant samples have comparable density to typical auxetic foams and a linear stress strain curve without the presence of a plateau region [30; 194]. Comparing auxetic and over-heated foam with a positive Poisson's ratio [194] could demonstrate the effect of Poisson's ratio on characteristics such as vibration damping or impact force attenuation. In depth analysis shows the complexity and diversity of chemical constitution in polymers (including those found in foams) [20]. Polymeric microstructure [20], and microstructural changes caused by heating [195] have only recently been investigated in relation to auxetic foam fabrication. [PERSON] & Zeng targeted styrene acrylonitrile copolymer (SAN) particle bonding, which has a glass transition temperature of around 110 \({}^{\circ}\)C. Such, or other, electron donating groups [15] can provide fixing mechanisms for imposed structures. Sporting products often include foam sections which have a larger planar area than typical auxetic foam samples (-20 \(\times\) 20 \(\times\) 60 mm) [9; 185; 196], and numerous scaled up fabrications have been attempted [185; 186; 4; 7; 23; 187; 188; 185; 187]. Large fabricated samples often exhibit random and ordered inhomogeneity.