## 1 Introduction
Compliant conformable foam is typically closest to the head (which is currently believed not to contribute to helmet function [22]), with stiffer, denser foam lining a hard-plastic shell [23]. A slip plane between a helmet's shell and deformable layer aims to reduce rotational acceleration and is now included in many cycling and snowsports helmets [24]. Evidence supporting slip plane technology's ability to reduce the likelihood of concussion is based on computational models, which have been criticized for not including physical validation [25, 26, 27]. A similar concept, increasing rotational deformation by reducing shear modulus, has reduced the severity of oblique impacts [22]. Despite helmet developments, there is evidence to suggest that the number of sport induced head injuries has not decreased. Firstly, [PERSON] et al. [28] compared the number of concussed players in the National Football League (NFL) from the 1996-2001 and the 2002-2007 seasons. Suggested reasons for this lack of improvement in concussion rates are inadequate helmet design and certification. As recently as 2017, similar studies in snowsports saw helmet use rise dramatically after multiple high-profile deaths, serious injuries, and awareness campaigns, but with relatively little decrease in head injury rates [29,30]. Auxetic foams (with a negative Poisson's ratio) have shown higher energy absorption than their conventional counterparts [31, 32, 33], and can be tailored to test the effect of mechanical characteristics (i.e., Young's modulus and Poisson's ratio) in complex situations (i.e., impact tests) [34,35]. Lakes originally fabricated auxetic foams through a compression and heat treatment process [36]. Conventional open cell foams were compressed triaxially, to produce a re-entrant cellular structure, in molds with a volumetric compression ratio (VCR) between 1.4 amd 4 (VCR = initial volume/final volume). The molds were heated (163-171 \({}^{\circ}\)C) to soften the foam, and then cooled to room temperature to set the re-entrant cellular structure. Auxetic foams fabricated in this way typically have: (i) higher density due to volumetric compression; and (ii) quasi-linear compressive stress/strain relationship rather than cellular collapse beyond \(\sim\)5% compression typical in conventional open cell foams [37]. Auxetic conversion methods have developed, including suggestions of multiple heating cycles [38], mold lubrication [39], cooling the foam slowly within the mold [40], and passing pins through the foam to control compression [41; 42]. Pins allowed lateral stretching but through thickness compression, producing auxetic foams with comparable density and compressive stress strain relationships to their unconverted counterpart [34; 43]. Open cell auxetic foams exhibit peak forces \(\sim\)3 to \(\sim\)8 times lower than their unconverted counterparts when impacted in scenarios similar to sporting standards [31; 32; 38; 43; 44]. Previous work has impact tested foams in isolation [33; 44; 45] or covered with thin polypropylene shells [31; 32; 38; 43; 44]. Comparisons between auxetic and conventional foams have not been made in more complex multi-material systems, such as helmets. Closed cell foams with low magnitude, incremental negative Poisson's ratio have been fabricated [46], but cells are reported to rupture during heat and compression. Therefore, the stability and practical benefits of closed cell auxetic foam is still in question and at present there are no studies that include impact testing of this type of foam (either as part of a complex system or in isolation). This preliminary investigation aims to determine the effect on GSI when replacing an open cell conformable foam layer with an auxetic foam. Recent work assumes that the 'comfort layer' does not contribute to reduce impact severity [22]. Thermo-mechanically treating open cell foam alters its characteristics changing stress strain relationship, density, and Poisson's ratio.