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Here, the authors report an effect analogous to optical activity in a 3D mechanical micro-lattice composed of chiral unit cells. They spatiotemporally resolve the motion of metamaterial beams at ultrasonic frequencies with nanometric precision and show up to 22° polarization rotation per unit cell.

That the unit cell of a metamaterial can't be considered vanishingly small like in ordinary crystals has long been deemed more burden than opportunity. The emergence of a characteristic length scale in metamaterial chains may change that trend.

In this work, authors demonstrate a model for non-reciprocal and non-Newtonian mechanical metamaterials by combining the concept of local resonances and fixing boundaries. Via computational models and impact experiments they show that stiffness substantially changes as a function of the loading velocity.

Shape-morphing metamaterials use geometric design to achieve advantageous properties, enabling innovations in fields from robotics to wearable devices. This Review proposes a unified classification of the design principles underlying shape-morphing behaviour, discusses available design tools and highlights emerging challenges in the development of shape-morphing metamaterials.

4D metamaterials offer the additional functionality of being responsive to external stimuli. Here, a metamaterial-based soft robot is composed of bilayer plates that can rotate and translate in response to thermal stimuli, allowing controlled motion.

The authors introduce frozen evanescent waves as the Bloch eigenmodes of periodic problems at zero frequency. Elastic waves serve as an example. Under special conditions, the decay length of the frozen evanescent phonons can become anomalously large.

Optically actuated microrobots enable precise microscale manipulation, yet achieving multiple degrees of freedom remains challenging. This work presents a chiral microrobot driven by optical tweezers that achieves controlled out-of-plane rotation with high torque and rotational velocity. In addition, it exhibits translation, rotation, and their controlled combination, enabling versatile actuation modes.

Metamaterials are rationally designed composites made of tailored building blocks, leading to effective medium properties beyond their ingredients. This Review surveys 3D metamaterials with unprecedented physical properties in electromagnetism and optics, acoustics and mechanics and transport, made possible by advances in design and manufacturing.

Here, the authors introduce beyond-nearest-neighbour interactions as a mechanism for molding the flow of waves in acoustic metamaterials. They find that for strong third-nearest-neighbour interactions, this mechanism allows for engineering roton-like acoustical dispersion relations under ambient conditions.

Long-range interactions have often been considered as a nuisance or correction to the desired features of metamaterials. Here, nonlocal interactions in 2D acoustic metamaterials are instead exploited as a tool to engineer peculiar wave dispersions, such as multiple roton-like minima, leading to negative and triple refraction.

Topological metamaterials are becoming increasingly interesting for their wave-confining capabilities, providing topologically robust guiding of light, sound and vibrations. Here, topological edge and disclination states in valley Hall sonic lattices are investigated via a non-commercial analytical approach combining the null-field method with multiple scattering techniques.

Here, the authors report phonon transmission through a nonlocal material slab embedded in a local material and perform an analysis based on a one-dimensional mass-and-spring model. Evidence of rich phonon behaviours, including sharp resonances related to bound-states-in-the-continuum, are found and reproduced by a developed higher-order-gradient effective medium theory for nonlocal materials.