Micro‐Scale Auxetic Hierarchical Mechanical Metamaterials for Shape Morphing

Abstract

years, it has been possible to note an emergence of promising directions of studies that address the possibility of observing these effects. In fact, one of the most interesting directions of studies related to this topic are mechanical metamaterials, [3,[28][29][30][31][32] that is, structures that can exhibit counterintuitive mechanical behavior based primarily on their design. Mechanical metamaterials are known for their ability to exhibit counterintuitive mechanical properties such as auxetic behavior, [33][34][35][36][37][38][39][40] negative stiffness [2,41] and negative compressibility [42,43] that are essential in many industries. However, in a vast majority of cases, standard mechanical metamaterial proved insufficient while searching for structures capable of exhibiting versatile deformation patterns. This in turn led to intensive studies devoted to hierarchical mechanical metamaterials, [44][45][46] that is, structures composed of elements having their own geometry that normally can deform irrespective of the rest of the system.Despite the large popularity of hierarchical mechanical metamaterials, it is a relatively new thread in the field of mechanical metamaterials. It seems that some of the first studies devoted to this topic were published by Gatt et al. [45] and Cho et al. [46] where the famous hierarchical rotating square system was proposed. In the following years, it was demonstrated [45][46][47][48][49] that this structure can exhibit tunable auxetic behavior where the extent of auxeticity depends on the relative rate of the deformation of hierarchical levels constituting the system. Recently, it was also reported that the design of the hierarchical square system can be modified in order to change its characteristic. [4,[50][51][52][53] Similar studies based primarily on the extent of the observed auxeticity were also conducted for structures based on rotating rectangles [54] as well as re-entrant and other honeycombs. [54][55][56][57][58] Even though the concept of hierarchical mechanical metamaterials proved to be very popular and resulted in numerous publications, it is possible to note that the hierarchical structures proposed up to date normally share several limitations. First, most of the known hierarchical mechanical metamaterials are designed in a way where their different hierarchical levels correspond either to the same deformation mechanism or to structures having very similar Poisson's ratio. Hence, the possible range of the resultant auxetic behavior is relatively small and may prove to be insufficient in the case of applications where functional materials must significantly change their mechanical response. Another limitation of the already reported hierarchical metamaterials is the fact that in a vast majority of cases, their ability to exhibit the tunable mechanical behavior Shape morphing and the possibility of having control over mechanical properties via designed deformations have attracted a lot of attention in the materials community and led to a variety of applications w...

Variable Dual Auxeticity of the Hierarchical Mechanical Metamaterial Composed of Re‐Entrant Structural Motifs

“…More specifically, if one was to construct the structure composed of multiple unit-cells corresponding to a different deformation pattern, then in general, it would be possible to arbitrarily modify the shape of the entire structure. If the mismatch in the Poisson's ratio between different parts of the system would be significant, it would be possible to observe considerable shape-morphing similarly to the concept proposed in [66] (in the aforementioned study, it was demonstrated that an initial shape of the metamaterial subjected to the mechanical deformation can be significantly modified depending on the ratio in the Poisson's ratio associated with different parts of the structure). The possibility of exhibiting controllable shape-morphing could be used in the design of stents in order to better support specific parts of the blood vessel that are particularly weak.…”

“…Hierarchical mechanical metamaterials [59,60,61,62,63,64,65,66] are a class of structures composed of multiple structural levels having their own geometry that typically can deform irrespective of each other. This, in some cases, allows the system to exhibit quantitatively very different mechanical properties without the need of being reconstructed.…”

Mechanical behaviors of a novel auxetic honeycomb characterized by re-entrant combined-wall hierarchical substructures

“…Besides, with the development of micromachining, constructing hierarchical auxetic honeycombs at the micro-scale has received extensive attention [55,56]. Although the concept of the RCW honeycomb proposed in this manuscript is confirmed by the conventional 3D printing process, its further manufacturing and research at the microscale, such as by the microprinting technology, is still of significance.…”

“…Considering the huge prospect in many applications such as medical equipment and flexible electronics, it is considered to be one of the most promising directions in this field [51][52][53][54]. Dudek et al [55] developed and fabricated 2D and 3D hierarchical mechanical metamaterials at the microscale by the 3D microprinted polymers, and found that the proposed designs have great capability of shape morphing. Cho et al [56] proposed a novel strategy to construct multilevel hierarchical metamaterials with programmable geometric shapes by the fractal cutting strategy, and confirmed it at both the macroscale and microscale.…”

Brillouin Light Scattering Characterisation of Gray Tone 3D Printed Isotropic Materials

“…A few extensive studies have been performed on different polymers to obtain the printable properties as a function of printing parameters, via Raman scattering or other characterisation techniques [14,15]. However, from one setup to another and for different reasons, characterisation must be repeated and adapted each time.…”