Hierarchical Auxetic Mechanical Metamaterials

Abstract

Auxetic mechanical metamaterials are engineered systems that exhibit the unusual macroscopic property of a negative Poisson's ratio due to sub-unit structure rather than chemical composition. Although their unique behaviour makes them superior to conventional materials in many practical applications, they are limited in availability. Here, we propose a new class of hierarchical auxetics based on the rotating rigid units mechanism. These systems retain the enhanced properties from having a negative Poisson's ratio with the added benefits of being a hierarchical system. Using simulations on typical hierarchical multi-level rotating squares, we show that, through design, one can control the extent of auxeticity, degree of aperture and size of the different pores in the system. This makes the system more versatile than similar non-hierarchical ones, making them promising candidates for industrial and biomedical applications, such as stents and skin grafts.

Three Decades of Auxetics Research − Materials with Negative Poisson's Ratio: A Review

“…Another important landmark in rotating unit auxetic structures was development of hierarchical auxetic structures. [39] This structure relied on the auxetic behavior from sub unit structure and also extracted benefits from hierarchy. This hierarchical structure gives a wide range of negative Poisson's ratio in contrast to the non-hierarchical rotating units which exhibited Poisson's ratio of about À1.…”

“…Figure 21 shows a designed stent based on rotating squares mechanism. [121] Based on the hierarchical design of auxetic metamaterials, [39] a stent design was proposed which can reduce the side effects of conventional stents such as inflammation due to contact with foreign object in the body. Auxetic metamaterials can be used in skin grafts due to their adjustable porosity.…”

“…Auxetic metamaterials can be used in skin grafts due to their adjustable porosity. [39] The concept of auxetic smart bandage [123] is also promising as the release of medicine through pores can be controlled by exploiting the auxetic behavior from textiles. The auxetic materials have the opportunities in tissue engineering and biomedical applications.…”

Design of planar isotropic negative Poisson’s ratio structures

“…Moreover, rationally designed geometries and structural mechanisms have also been reported to achieve negative Poisson's ratio. Examples include foams with re-entrant ribs [5][6][7], chiral microstructures [8,9], rotating units [10][11][12][13][14] and origami structures [15][16][17]. In all these demonstrations, careful design of the microstructure has led to effective negative macroscopic Poisson's ratios (ν < 0), even though the bulk materials have a positive Poisson's ratio.…”

Auxetic Perforated Mechanical Metamaterials with Randomly Oriented Cuts

“…Normally, in a skin grafting procedure, a piece of skin is cut from a healthy region, perforated, stretched, and placed onto the target area, which is usually much larger than the graft. Previous studies by the same authors have already advocated the merits of using perforation patterns which results in auxetic behavior for this purpose, [ 12,15 ] since auxetic skin grafts are expected to cover a much larger surface area in comparison to designs which result in positive Poisson's ratio response. This study, however, takes this application for these auxetic metamaterials one step further.…”