## 1 Introduction
[2] replaced part of the helmet's liner by aluminum honeycombs, using these as reinforcement material. Although this configuration provided better protection than the original EPS liner in impacts against a curbstone anvil, the performance in impacts against flat surfaces was worse than the original solution. In addition, the thickness necessary to accommodate the honeycomb layers is extremely limited. Thus, in a real accident scenario, an excessively thin layer of EPS foam could be easily penetrated, reaching the head. More recently, [PERSON] et al. [27] proposed a novel Angular Impact Mitigation (AIM) system for bicycle helmets, employing an elastically suspended aluminum honeycomb liner. The impact performance under normal and oblique impacts was compared to a standard EPS helmet. The results demonstrated that this new helmet could effectively improve impact mitigation, performing better than the EPS helmet for both normal and oblique impacts. However, penetration of the comfort liner was not addressed. [PERSON] et al. [1] proposed a new ski helmet liner that consists of an Acrylonitrile Butadiene Styrene (ABS) lamina with deformable cones. Energy is absorbed via a combination of folding and collapsing of the cones. According to the authors, the main advantage that such liner may introduce over common EPS pads is that it allows a better optimization of energy absorption for different impact sites. Recently, [PERSON] et al. [28] studied the feasibility of hierarchical lattice liners in comparison to currently employed solutions. The hierarchical lattice liner revealed to be a promising solution, reducing the head injury probability in comparison to an EPS liner. Another concept is the conehead(tm) shock absorbing liner, developed to absorb impact force more effectively. This concept proposed by [PERSON] [8] consists in a helmet foam liner made of two density layers, initially designed for motorcycle helmets. The outer layer is made of high-density foam and has truncated cones facing inwards. The inner layer is made of a softer, low density foam and has cones facing outwards. With this dual-density liner, the impact performance is improved for both low and high energy impacts. The collapsing of the cones causes the energy to spread sideways within the thickness of the foam liner instead of towards the head [8]. In addition, the head will experience a gradual deceleration due to the crushing of the cones, minimizing the energy induced to the head. Hence, there is a reduction in the forces translated across the thickness of the liner. This concept is among the most promising ones, being already used in a few commercial helmets. POC also developed a dual-density liner, similar to the conehead(tm), designed for sports [29]. Some standards are starting to promote helmet testing under multi-impacts such as SNELL and NOCSAE [20]. Other oil-derived plastics have been proposed to deal with multiple crashes. For instance, EPP is somewhat similar to EPS, presenting similar peak accelerations and impact duration for the same helmet, as verified by [PERSON] et al. [12]. In addition, it has the capacity to recover some of its dimensions depending on the impact severity [4]. A closed cell foam based on vinyl nitrile polymer (VNP) was also indicated as a good energy absorber that could be used as helmet liner [6]. In fact, stiff VNP is already used at the top of some American football helmets, together with EPP to provide extra protection against tackles and blocks that usually impact this area. Recently, KALI developed a new concept called nano core, which is an acrylic self-healing foam infused with carbon nanotubes. According to the manufacturer, this new liner dissipates impact energy more efficiently with a smaller volume [7]. The increase of CPU power led to the development of advanced FE models, with accurate modelling of helmet components (geometry, material properties, contact/interactions, etc.). Some of these models were developed to optimize helmets according to specific criteria [22; 30; 31; 32; 33; 34; 35].