Bioinspired metagel with broadband tunable impedance matching

Enhanced Ultrasound Transmission through Skull Using Flexible Matching Layer with Gradual Acoustic Impedance

Transcranial ultrasound imaging and therapy have gained significant attention due to their noninvasive nature, absence of ionizing radiation, and portability. However, the presence of the skull, which has a high acoustic impedance, presents a challenge for the penetration of ultrasound into intracranial tissue. This leads to a low transmission of ultrasound through the skull, hindering energy focusing and imaging quality. To address this challenge, we propose a novel approach that utilizes a flexible matching layer with gradual acoustic impedance to enhance ultrasound transmission through the skull. This matching layer is constructed using Poly(dimethylsiloxane) (PDMS)/tungsten powders as the structural component responsible for the gradual impedance, while agarose serves as the flexible matrix. Our simulation and experimental results demonstrate that the matching layer with an exponential gradual acoustic impedance significantly improves the ultrasound transmission coefficient across a wide frequency range compared to traditional quarter wavelength matching layers. Specifically, at 2 MHz, the maximum transmission coefficient reaches 49.5%, more than four times higher than that of the skull without a matching layer (only 11.7%). Additionally, the good flexibility of our matching layer ensures excellent adhesion to the curved surface of the skull, further enhancing its application potential in transcranial ultrasound imaging and therapy. The improved transmission performance allows for a lower ultrasound transmission power, effectively addressing overheating and safety issues.

Ultrasonic barrier-through imaging by Fabry-Perot resonance-tailoring panel

Imaging technologies that provide detailed information on intricate shapes and states of an object play critical roles in nanoscale dynamics, bio-organ and cell studies, medical diagnostics, and underwater detection. However, ultrasonic imaging of an object hidden by a nearly impenetrable metal barrier remains intractable. Here, we present the experimental results of ultrasonic imaging of an object in water behind a metal barrier of a high impedance mismatch. In comparison to direct ultrasonic images, our method yields sufficient object information on the shapes and locations with minimal errors. While our imaging principle is based on the Fabry-Perot (FP) resonance, our strategy for reducing attenuation in our experiments focuses on customising the resonance at any desired frequency. To tailor the resonance frequency, we placed an elaborately engineered panel of a specific material and thickness, called the FP resonance-tailoring panel (RTP), and installed the panel in front of a barrier at a controlled distance. Since our RTP-based imaging technique is readily compatible with conventional ultrasound devices, it can realise underwater barrier-through imaging and communication and enhance skull-through ultrasonic brain imaging.

Remote Water-to-Air Eavesdropping with a Phase-Engineered Impedance Matching Metasurface

Efficiently receiving underwater sound remotely from air is a long-standing challenge in acoustics hindered by the large impedance mismatch at the water-air interface. Here, a phase-engineered water-air impedance matching metasurface is proposed and experimentally demonstrated for remote and efficient water-to-air eavesdropping. The judiciously designed metasurface with near-unity transmission efficiency, long monitoring distance, and high mechanical stiffness is capable of making the water-air interface acoustically transparent and, at the same time, freewheelingly patterning the transmitted wavefront. This enables efficient control over the effective spatial location of a distant airborne sensor such that it can measure underwater signals with large signal-to-noise ratios as if placed close to the physical underwater source. Such airborne eavesdropping of underwater sound is experimentally demonstrated with a measured sensitivity enhancement of nearly 10⁴ at 8 kHz, far from achievable with the current state-of-the-art methods. Moreover, the opportunities of using the proposed metasurface for cross-media orbital-angular-momentum-multiplexed communication and underwater acoustic window are also demonstrated. This metasurface opens new avenues for communication and sensing in inhomogeneities with totally reflective interfaces, which may be translated to nano-optics and radio frequencies.

Electron Tomography and Machine Learning for Understanding the Highly Ordered Structure of Leafhopper Brochosomes

Insects known as leafhoppers (Hemiptera: Cicadellidae) produce hierarchically structured nanoparticles known as brochosomes that are exuded and applied to the insect cuticle, thereby providing camouflage and anti-wetting properties to aid insect survival. Although the physical properties of brochosomes are thought to depend on the leafhopper species, the structure-function relationships governing brochosome behavior are not fully understood. Brochosomes have complex hierarchical structures and morphological heterogeneity across species, due to which a multimodal characterization approach is required to effectively elucidate their nanoscale structure and properties. In this work, we study the structural and mechanical properties of brochosomes using a combination of atomic force microscopy (AFM), electron microscopy (EM), electron tomography, and machine learning (ML)-based quantification of large and complex scanning electron microscopy (SEM) image data sets. This suite of techniques allows for the characterization of internal and external brochosome structures, and ML-based image analysis methods of large data sets reveal correlations in the structure across several leafhopper species. Our results show that brochosomes are relatively rigid hollow spheres with characteristic dimensions and morphologies that depend on leafhopper species. Nanomechanical mapping AFM is used to determine a characteristic compression modulus for brochosomes on the order of 1-3 GPa, which is consistent with crystalline proteins. Overall, this work provides an improved understanding of the structural and mechanical properties of leafhopper brochosomes using a new set of ML-based image classification tools that can be broadly applied to nanostructured biological materials.

Supershear surface waves reveal prestress and anisotropy of soft materials

Surface waves play important roles in many fundamental and applied areas from seismic detection to material characterizations. Supershear surface waves with propagation speeds greater than bulk shear waves have recently been reported, but their properties are not well understood. Here we describe theoretical and experimental results on supershear surface waves in rubbery materials. We find that supershear surface waves can be supported in viscoelastic materials with no restriction on the shear quality factor. Interestingly, the effect of prestress on the speed of the supershear surface wave is opposite to that of the Rayleigh surface wave. Furthermore, anisotropy of material affects the supershear wave much more strongly than the Rayleigh surface wave. We offer heuristic interpretation as well as theoretical verification of our experimental observations. Our work points to the potential applications of supershear waves for characterizing the bulk mechanical properties of soft solid from the free surface.

Manta Ray Inspired Soft Robot Fish with Tough Hydrogels as Structural Elements

The design of soft robots capable of navigation underwater has received tremendous research interest due to the robots' versatile applications in marine explorations. Inspired by marine animals such as jellyfish, scientists have developed various soft robotic fishes by using elastomers as the major material. However, elastomers have a hydrophobic network without embedded water, which is different from the gel-state body of the prototypes and results in high contrast to the surrounding environment and thus poor acoustic stealth. Here, we demonstrate a manta ray-inspired soft robot fish with tailored swimming motions by using tough and stiff hydrogels as the structural elements, as well as a dielectric elastomer as the actuating unit. The switching between actuated and relaxed states of this unit under wired power leads to the flapping of the pectoral fins and swimming of the gel fish. This robot fish has good stability and swims with a fast speed (∼10 cm/s) in freshwater and seawater over a wide temperature range (4-50 °C). The high water content (i.e., ∼70 wt %) of the robot fish affords good optical and acoustic stealth properties under water. The excellent mechanical properties of the gels also enable easy integration of other functional units/systems with the robot fish. As proof-of-concept examples, a temperature sensing system and a soft gripper are assembled, allowing the robot fish to monitor the local temperature, raise warning signals by lighting, and grab and transport an object on demand. Such a robot fish should find applications in environmental detection and execution tasks under water. This work should also be informative for the design of other soft actuators and robots with tough hydrogels as the building blocks.

Redefinable planar microwave passive electronics enabled by thermal controlled VO2/Cu hybrid matrix

A planar microwave array device with complex electromagnetic functional reconfigurability is demonstrated by means of phase transition film VO₂ to manipulate the electromagnetic distribution. Based on planar patch architecture, the microwave device can switch between antenna array and cascaded filter functions. Furthermore, hybrid EM functions such as cascaded antenna arrays and filters are enabled, themselves with further reconfigurability. Therefore, a single design realizes many mono and hybrid antenna and filter functions, which are determined by the order of the array. For simplicity of demonstration, a 2 × 2 array device working at three reconfigurable center frequency points of 3.1, 3.7, and 4.4 GHz, fully compatible with standard planar CMOS processing. A comprehensive design method is proposed to meet the design requirements of a patch-based antenna array and cascaded filter. Based on the functionally reconfigurable microwave device, the front-end circuit could be recombined to suitable for multifunctional microwave systems.

Photo-responsive hydrogel-based re-programmable metamaterials

This paper explores a novel programmable metamaterial using stimuli-responsive hydrogels with a demonstration of bandgap formation and tuning. Specifically, a photo-responsive hydrogel beam that can achieve re-programmable periodicity in geometric and material properties through patterned light irradiation is designed. Hydrogels consist of polymeric networks and water molecules. Many unique properties of hydrogels, including bio-compatibility, stimuli-responsiveness, and low dissipation make them ideal for enabling re-programmable metamaterials for manipulating structural dynamic response and wave propagation characteristics. Bandgap generation and tunability in photo-responsive hydrogel-based metamaterial (in the form of a diatomic phononic chain) as well as the effects of system parameters such as light exposure pattern and photo-sensitive group concentration on the bandgap width and center frequency are systematically studied. In agreement with finite-element model simulations, it is observed that an increase in light exposure region size reduces both the bandgap width and center frequency, while an increase in the concentration of photo-sensitive group increases bandgap width, attenuation and reduces its center frequency. This work unveils the potential of stimuli-response hydrogels as a new class of low-loss soft metamaterials, unlike most other soft materials that are too lossy to sustain and exploit wave phenomena.

Underwater metamaterial absorber with impedance-matched composite

By using a structured tungsten-polyurethane composite that is impedance matched to water while simultaneously having a much slower longitudinal sound speed, we have theoretically designed and experimentally realized an underwater acoustic absorber exhibiting high absorption from 4 to 20 kHz, measured in a 5.6 m by 3.6 m water pool with the time-domain approach. The broadband functionality is achieved by optimally engineering the distribution of the Fabry-Perot resonances, based on an integration scheme, to attain impedance matching over a broad frequency range. The average thickness of the integrated absorber, 8.9 mm, is in the deep subwavelength regime (~λ/42 at 4 kHz) and close to the causal minimum thickness of 8.2 mm that is evaluated from the simulated absorption spectrum. The structured composite represents a new type of acoustic metamaterials that has high acoustic energy density and promises broad underwater applications.

Lotus Metasurface for Wide-Angle Intermediate-Frequency Water-Air Acoustic Transmission

Only 0.1% of the acoustic energy can transmit across the water-air interface because of the huge acoustic impedance mismatch. Enhancing acoustic transmission across the water-air interface is of great significance for sonar communications and sensing. However, due to the interface instability and subwavelength characteristics of acoustic metamaterials, wide-angle intermediate-frequency (10 kHz-100 kHz) water-air acoustic transmission remains a great challenge. Here, we demonstrate that the lotus leaf is a natural low-cost acoustic transmission metasurface, namely, the lotus acoustic metasurface (LAM). Experiments demonstrate the LAM can enhance the acoustic transmission across the water-air interface, with an energy transmission coefficient of about 40% at 28 kHz. Furthermore, by fabricating artificial LAMs, the operating frequencies can be flexibly adjusted. Also, the LAM allows a wide-angle water-to-air acoustic transmission. It will enable various promising applications, such as detecting and imaging underwater objects from the air, communicating between ocean and atmosphere, reducing ocean noises, etc.

Tunable Fluid-Type Metasurface for Wide-Angle and Multifrequency Water-Air Acoustic Transmission

Efficient acoustic communication across the water-air interface remains a great challenge owing to the extreme acoustic impedance mismatch. Few present acoustic metamaterials can be constructed on the free air-water interface for enhancing the acoustic transmission because of the interface instability. Previous strategies overcoming this difficulty were limited in practical usage, as well as the wide-angle and multifrequency acoustic transmission. Here, we report a simple and practical way to obtain the wide-angle and multifrequency water-air acoustic transmission with a tunable fluid-type acoustic metasurface (FAM). The FAM has a transmission enhancement of acoustic energy over 200 times, with a thickness less than the wavelength in water by three orders of magnitude. The FAM can work at an almost arbitrary water-to-air incident angle, and the operating frequencies can be flexibly adjusted. Multifrequency transmissions can be obtained with multilayer FAMs. In experiments, the FAM is demonstrated to be stable enough for practical applications and has the transmission enhancement of over 20 dB for wide frequencies. The transmission enhancement of music signal across the water-air interface was performed to demonstrate the applications in acoustic communications. The FAM will benefit various applications in hydroacoustics and oceanography.

Soft Materials by Design: Unconventional Polymer Networks Give Extreme Properties

Hydrogels are polymer networks infiltrated with water. Many biological hydrogels in animal bodies such as muscles, heart valves, cartilages, and tendons possess extreme mechanical properties including being extremely tough, strong, resilient, adhesive, and fatigue-resistant. These mechanical properties are also critical for hydrogels' diverse applications ranging from drug delivery, tissue engineering, medical implants, wound dressings, and contact lenses to sensors, actuators, electronic devices, optical devices, batteries, water harvesters, and soft robots. Whereas numerous hydrogels have been developed over the last few decades, a set of general principles that can rationally guide the design of hydrogels using different materials and fabrication methods for various applications remain a central need in the field of soft materials. This review is aimed at synergistically reporting: (i) general design principles for hydrogels to achieve extreme mechanical and physical properties, (ii) implementation strategies for the design principles using unconventional polymer networks, and (iii) future directions for the orthogonal design of hydrogels to achieve multiple combined mechanical, physical, chemical, and biological properties. Because these design principles and implementation strategies are based on generic polymer networks, they are also applicable to other soft materials including elastomers and organogels. Overall, the review will not only provide comprehensive and systematic guidelines on the rational design of soft materials, but also provoke interdisciplinary discussions on a fundamental question: why does nature select soft materials with unconventional polymer networks to constitute the major parts of animal bodies?