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Topological physics has been driving exciting progress in the area of condensed matter physics, with findings that have recently spilled over into the field of metamaterials research inspiring the design of structured materials that can govern in new ways the flow of light and sound. While so far these advances have been driven by fundamental curiosity-driven explorations, without a focused interest on their technological implications, opportunities to translate these findings into applied research have started to emerge, in particular in the context of sound control. Our team has been leading a highly collaborative research effort on advancing the field of topological acoustics, dubbed ‘New Frontiers of Sound’ and connecting it to technological opportunities for computing, communications, energy and sensing. In this comment, we outline our vision towards the future of topological sound, and its translation towards industry-relevant functionalities and operations based on extreme control of acoustic and phononic waves. 

Acoustic metasurfaces are two-dimensional architected materials designed to enable non-trivial control of waves, with a thickness that is either thinner than or comparable to the wavelength. However, most metasurfaces today have a fixed geometry and lack the ability to tune acoustic waves on command. This limits their ability to perform multiple functions, such as beam steering and dynamic focusing. This study introduces inflatable acoustic metasurface (IAM) lenses that enable tunable focusing. The IAMs feature two-dimensional diffractive focusing patterns embedded in a membrane that can be inflated nonplanarly through hydraulic control. It is experimentally demonstrated that inflation allows continuous focal length adjustment from –2.49λ to +3.17λ. To characterize the lens performance, changes in focal characteristics, including peak pressure, full width at half-maximum, and full length at half-maximum, are tracked at different levels of inflation. Furthermore, it is shown that IAMs can correct aberrations that occur as the angle of incidence increases in conventional planar lenses. To validate this, IAMs were tested in a concave configuration at a 20° oblique incidence angle. The results of this study may be applicable to fields requiring continuous and real-time response in tunable focusing, including acoustic imaging and communication, ultrasound surgery, and neuromodulation. 

Abstract Extrusion 3D‐printing of biopolymers and natural fiber‐based biocomposites enables the fabrication of complex structures, ranging from implants' scaffolds to eco‐friendly structural materials. However, conventional polymer extrusion requires high energy consumption to reduce viscosity, and natural fiber reinforcement often requires harsh chemical treatments to improve adhesion. We address these challenges by introducing a sustainable framework to fabricate natural biocomposites usingChlorella vulgarismicroalgae as the matrix. Through bioink optimization and process refinement, we produced lightweight, multifunctional materials with hierarchical architectures. Infrared spectroscopy analysis reveals that hydrogen bonding plays a critical role in the binding and reinforcement ofChlorellacells by hydroxyethyl cellulose (HEC). As water content decreases, the hydrogen bonding network evolves from water‐mediated interactions to direct hydrogen bonds between HEC andChlorella, enhancing the mechanical properties. A controlled dehydration process maintains continuous microalgae morphology, preventing cracking. The resulting biocomposites exhibit a bending stiffness of 1.6 GPa and isotropic heat transfer and thermal conductivity of 0.10 W/mK at room temperature, demonstrating effective thermal insulation. These characteristics makeChlorellabiocomposites promising candidates for applications requiring both structural performance and thermal insulation, offering a sustainable alternative to conventional materials in response to growing environmental demands. 

In this Letter, we present the first experimental demonstration of the temporal refraction of acoustic waves in a phononic lattice. A step change in grounding stiffness results in a discontinuous change in group velocity across a so-called temporal boundary. This leads to frequency translation of incident signals, which maintain constant wavelength. We use the system to construct phononic analogs of the classical Snell and Fresnel relationships for temporal boundaries, providing evidence of temporal refraction. Last, we propose the ability to design systems to achieve tunable slow sound.