New directions in metamaterials

Multi-material 3D printing techniques are now enabling the rational design of metamaterials with both complex geometries and multiple materials compositions. Here, deep-learning methods are used to identify, among planar network structures, the rare designs that yield very unusual and desirable combinations of materials properties.

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.

Origami is a promising source of inspiration in designing foldable structures and reconfigurable metamaterials. Here, building on exact folding kinematic conditions, an algorithmic design of rigidly-foldable origami structures is presented, allowing the engineering of metamaterials with arbitrary complex crease patterns.

The 3D stiffness of a self-folded metamaterial structure is limited by the low stiffness required by the folding process. Here, the stiffness limits of self-folding bilayers are theoretically established by a nonlinear model and experimentally validated on polymer-metal composites, providing the optimal combinations of geometrical and mechanical properties of folded constructs.

Origami-inspired metamaterials are attractive for their programmable shape-shifting properties but are typically characterized by low structural rigidity. Here, 3D heterogeneous origami structures display highly reconfigurable mechanical properties, including finely controllable and reversible stiffness variation.

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.

Shape-shifting structures are important building blocks in the design of reconfigurable materials and devices with advanced functionalities. Here, versatile metamaterials with 3D-to-3D shape-shifting behavior upon thermal activation are fabricated by adapting a 3D printer to print on curved surfaces.

The intriguing physics of correlated flat bands in moiré superlattices can be mimicked, in classical physics, by twisted acoustic plates with periodic holes. Here, the authors derive a combined analytical and numerical approach that provides computational advantage in band engineering of holey bilayer plates.

Active metamaterials can host non-Hermitian interactions that defy the conservation laws of linear elasticity, leading to unusual phenomena such as one-way energy transmission and odd-elastic moduli. Here, robust unidirectional Rayleigh surface waves are found in active media comprising both gyroscopic and odd-elastic effects.

Topological mechanical metamaterials have been considered effective for energy manipulation via edge states, but manipulating these states remains challenging. Here, a Kresling origami mechanical metamaterial hosts multiple topological edge states at finite frequencies, which can be manipulated and transferred across the boundaries of the system by adjusting the lattice torsion.

Surface acoustic waves are important in a wide range of applications such as telecommunication filters, sensors, and microfluidic devices. Here, patterning of a phononic metamaterial formed of annular hole resonators enables frequency control of the surface wave phase velocities.

Chiral mechanical metamaterials enable unusual effects, such as coupling between strain and twist. Here, manufactured microstructured samples with >10⁵ chiral unit cells exhibit large characteristic lengths, in agreement with analytical and numerical modelling and micropolar continuum elasticity.

Modeling artificial nanostructures in terms of effective materials parameters is important for gaining physical insight into their behavior and facilitating their optimization. Here, an analytical effective medium theory for the heat capacity of holey phononic crystals is derived, revealing the effect of the emergent anisotropic elastic response of the metasolid.

Scattered elastic waves provide non-invasive diagnostics and dynamic characterization of metamaterials, but extracting information from small-size samples is challenging. Here, convolutional neural networks are used to interpret diffracted waves, revealing how sample-edge scattering provides the most significant information on macroscopic metamaterial properties.