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Shape-changing substrates are pivotal for advancing minimally invasive, conformable bioelectronics, yet their implantation is hindered by inaccessible triggering mechanisms and complex geometric fabrication. Here, the authors report a digital 4D printing technique for fabricating customized shape memory hydrogel substrates enabling trigger-free shapeshifting with predefined onset periods.

A metamaterial-based acoustic sink has been designed to serve the purpose of absorbing the diverging waves and demonstrating three-dimensional sub-diffraction spherical sound wave focusing.

Metamaterials can be used to alter the transmission of light or sound, with their tailored structures permitting control over their optical or acoustic properties. Meiet al. present a thin-film acoustic metamaterial that provides efficient absorption of sound over a broad range of wavelengths.

Non-Abelian braiding—a candidate for realizing quantum logics—is demonstrated by controlling the geometric-phase matrix in a photonic chip, and its key characteristics are observed.

Acoustic impedance-matched surfaces do not reflect incident waves. Traditional means of acoustic absorption have so far resulted in imperfect impedance matching and bulky structures, or require costly and sophisticated electrical design. Inspired by electromagnetic metamaterials, a subwavelength acoustically reflecting surface with hybrid resonances and impedance-matched to airborne sound at tunable frequencies is now demonstrated.

Liquid crystal elastomers are used in actuators, though methods to pre-program actuation limits the geometric design. Here the authors show a method of treating the elastomers in free-radical initiator to program actuation.

Controlling elastic waves in medium is essential to many applications in mechanical to earthquake engineering. Ma et al. demonstrate selective suppression of different vibrational modes in a three-dimensional rod-shape structure, which shows fluid-like elasticity with only longitudinal waves propagating.

This work realizes high-dimensional SO(m) non-Abelian holonomy in integrated photonics for both classical and quantum optics realms and demonstrates the application of geometric phase-based linear optical computation.

Tuning actuation temperatures of liquid crystalline elastomers can help control actuation behaviour, but normally cannot be altered following the synthesis of the material. Here, the authors report the development of an elastomer with varying actuation temperature depending on the conditions.

The behaviour of sound waves in phononic crystals—metamaterials with spatially varying acoustic characteristics—is similar to that of electrons in solids. Now, phononic band inversion and Zak phases have been measured for a 1D phononic system.

Characterizing and classifying dynamic non-Hermitian skin effect is a key challenge in nonHermitian physics. Here, authors illustrated rich non-Hermitian skin dynamics and dynamic phases in one-dimensional systems with glide-time reversal symmetry.

Genome evolution of Allium genus and genomic basis underlying the pungency flavor formation remain poorly understood. Here, the authors assemble the genome of bunching onion and conduct population genetics analyses to reveal Allium genome evolution, flavor formation and domestication history.

Indoor sound fields are disordered due to reflections and multiple scattering, which scrambles acoustic communication. Here, based on tunable metasurfaces and communication theories, the authors demonstrate multi-channel, multi-spectral, cross-talk-free acoustic communication in a room.

The authors report an experimental realization of order-3 exceptional lines (EL3) in a stack of three coupled acoustic cavities. The EL3 are embedded in an order-2 exceptional surface. The authors use the winding number of resultants of the Hamiltonian matrix to diagnose the topology of EL3.

It is experimentally demonstrated that the non-Hermitian skin effect can convert localized topological modes into extended modes of unconventional shapes while preserving the topological characteristics, which presents opportunities for topological manipulations of waves and light.

Spin-orbit acoustics is determinant to provide new perspectives and functionalities for sound manipulations. Here the authors theoretically and experimentally demonstrate acoustic spin-orbit interaction enabling chiral sound-matter interactions with unprecedented applications.

Although it shows promise for applications, non-Abelian braiding is difficult to realize in electronic systems. Its demonstration using acoustic waveguides may provide a useful platform to study non-Abelian physics.

This Review describes topological phenomena that can be realized in acoustic and mechanical systems. Methods of symmetry breaking are described, along with the consequences and rich phenomena that emerge.

Non-Hermitian theory consists of mathematical structures that are used to describe open systems, which can give rise to non-Hermitian topology not found in Hermitian systems. This Review provides an overview of non-Hermitian band topology and discusses recent developments, such as the non-Hermitian skin effect and non-Hermitian topological classifications.