Auxetic structures are a subclass of mechanical metamaterials with the governing property of a negative Poisson's ratio. Therefore, such materials expand laterally to the tensile direction. [1] Other properties derived from this, are an increased thermal shock resistance, [2] a higher fracture toughness, [3,4] and indentation resistance. [5][6][7] Hence, auxetic materials are suitable for ballistic and blast protection [8][9][10][11] or as crash absorbers due to increased energy dissipation. [12,13] For those applications, the dynamic loading case is of special interest [14][15][16] as well as a lightweight approach through truss structures [17] or hybrid materials. Bronder et al. [18] electrochemically coated a printed polymeric auxetic structure with a thin layer of nickel and were able to improve properties such as plastic collapse stress (PCS) or plateau stress and thus also the energy absorption capacity.Additional applications of auxetics are press-fit fasteners, [19] stents, arterial prosthesis, [20] or sport-protective equipment. [21] Also, fiber-reinforced composites make use of the auxetic effect, either through a suitable stacking sequence of the fiber layers, [22,23] thereby increasing the lowvelocity impact resistance and showing a more localized initial damage, [24] or with auxetic fibers inside the polymer matrix, [25] which increases the fiber pull-out resistance. Both variants counteract the two common failure mechanisms of composite materials.Even though mechanical metamaterials are designed, artificial materials, the auxetic effect can also be found in nature, for example, in some cubic crystal lattices [26] or some forms of skin. [27][28][29] The first manufactured auxetic foam, however, was produced by Lakes et al. [6] Through triaxial compression in combination with heating, they were able to convert a conventional polyurethane foam to an auxetic one. Thereby, they marked a starting point for the investigation of auxetic materials to exploit their high tailoring potential. Today, auxetics have been developed across all material classes and sizes, even down to a molecular level. [30] How the microstructure for auxetic behavior works is easiest explained for a 2D case (Figure 1). Almgren [31] was the first to describe the re-entrant honeycomb structure as a possibility to display a negative Poisson's ratio. Also rotating rigid bodies [32] with connecting hinges at their corners, such as rectangles, triangles, and so on, are a possibility to achieve an auxetic effect. More recent microstructures with auxetic properties are chiral lattices [33][34][35][36] or double-arrowhead structures [37] (Figure 1b). To enhance the energy absorption capacity of the auxetics, an additional half strut can be added in the middle of the unit cell. [38] This increases the stability after initial deformation as well as preserves the auxetic effect and its benefits, thus creating a multifunctional auxetic structure applicable to many crash absorber and protection devices.With the improvement of additive manufacturi...
