Take a thin cylindrical shell and twist it; it will buckle immediately. Such unavoidable torsional buckling can lead to systemic failure, for example by disrupting the blood flow through arteries. In this study, we prevent this torsional buckling instability using a combination of auxeticity and orthotropy in cylindrical metamaterial shells with a holey pattern. When the principal axes of the orthotropic meta-shell are relatively aligned with that of the compressive component of the applied stress during twisting, the meta-shell uniformly shrinks in the radial direction as a result of a local buckling instability. This shrinkage coincides with a softening-stiffening transition that leads to ordered stacking of unit cells along the compressive component of the applied stress. These transitions due to local instabilities circumvent the usual torsional instability even under a large twist angle. This study highlights the potential of tailoring anisotropy and programming instabilities in metamaterials, with potential applications in designing mechanical elements for soft robotics, biomechanics or fluidics. As an example of such applications, we demonstrate soft torsional compressor for generating pulsatile flows through a torsion release mechanism.

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Metamaterials are artificial structures with properties that are rare or non-existent in nature. These properties are created by the geometry and interconnection of the metamaterial unit cells. In active metamaterials, sensors and actuators are embedded in each unit cell to achieve greater design freedom and tunability of properties after the fabrication. While active metamaterials have been used in vibration control applications, the influence of applied control architectures on damping performance has not been thoroughly studied yet. This paper discusses the relationship between suitable control architectures for increased damping in finite active metamaterials and the number of damped modes. A metamaterial beam consisting of links with measured and actuated joints is considered. Optimal controllers for each of the considered scenarios are designed in the modal domain using linear-quadratic regulator (LQR). We show that, when all modes of a structure should be damped, the optimal solution can be reduced to a decentralised controller. When modes in a smaller range of frequencies are targeted, distributed controllers show better performance. The results are confirmed with experiments.

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Metamaterials can display unusual and superior properties that come from their carefully designed structure rather than their composition. Metamaterials have permeated large swatches of science, including electromagnetics and mechanics. Although metamaterials hold the promise for realizing technological advances, their potential to enhance interactions between humans and materials has largely remained unexplored. Here, we devise a class edible mechanical metamaterials with tailored fracture properties to control mouthfeel sensory experience. Using chocolate as a model material, we first demonstrate how to create and control the fracture anisotropy, and the number of cracks, and demonstrate that these properties are captured in mouthfeel experience. We further use topology optimization to rationally design edible metamaterials with maximally anisotropic fracture strength. Our work opens avenues for the use of metamaterials to control fracture and to enhance human-matter interactions.@en