Search Results (436)

This study proposes and validates a framework that integrates Grey Wolf Optimization (GWO) with Extreme Gradient Boosting (XGBoost) for estimating the Poisson’s ratio of auxetic structures. First, for 320 models derived from Computer-Aided Design-based (CAD-based) unit-cell designs, a systematic sweep of diameter and cellular dimensions was conducted to obtain porosity coverage in the 45–85% range. Subsequently, elastic modulus and Poisson’s ratio were computed via finite element analysis (FEA) at three mesh resolutions (0.20/0.25/0.30 mm), and relationships between design variables and outputs were examined using correlation heatmaps and Locally Weighted Scatterplot Smoothing (LOWESS) curves. GWO optimized the XGBoost hyperparameters through a multi-band narrowed search strategy; performance was evaluated using Mean Absolute Error (MAE), Root Mean Squared Error (RMSE), Mean Squared Error (MSE), and Coefficient of Determination ($R^2$) metrics, as well as residual diagnostics and Ground Truth–Prediction alignments for Poisson’s ratio. Across all configurations, $R^2\ge 0.994$ and absolute errors are on the order of ∼${10}^{-3}$; the 0.25 mm mesh stands out in terms of overall balance with the lowest squared-error profile and the highest $R^2$, the 0.30 mm mesh is practically equivalent in terms of MAE, and the 0.20 mm mesh is comparatively weaker. Residual diagnostics—comprising a pattern-free cloud around zero, slight right-skewness, and limited heteroskedasticity—indicate low bias and no substantive model-specification issues. The findings align with physical insight, confirming that Poisson’s ratio shifts toward more negative values as porosity increases and toward less negative values as diameter increases. The proposed GWO–XGBoost framework provides a reliable pre-screening tool for rapid design exploration and Poisson’s-ratio-targeted optimization, with the potential to reduce the need for additional FEA simulations and experimental iterations during early-stage design. Full article

Ultrasound coupling technology is pivotal to ensuring high-quality diagnostic imaging, yet conventional water-based gels face persistent challenges, including acoustic impedance mismatch, air-bubble formation, dehydration, messiness, and cross-contamination risks. This review presents a comprehensive analysis of the evolution, materials science, and clinical performance of ultrasound gel pads, an advanced alternative engineered for superior acoustic transmission, hygiene, and patient comfort. Historical progression from early coupling agents to modern polymeric and hydrogel-based pads is traced, highlighting breakthroughs such as bilayer hydrogels, nanocomposite reinforcements, metamaterial-inspired designs, and patient-specific 3D-printed pads. Comparative evaluations demonstrate that gel pads, particularly those integrating nanotechnology, rival but often outperform traditional gels in transmission efficiency, near-field resolution, and adaptability to complex anatomical surfaces, while offering reusability and reduced environmental impact. For instance, solid gel pads achieved 92.3% stone disintegration, compared with 45.5% for semi-liquid gel, in ESWL phantom studies (p < 0.001). Materials, including polyacrylamide, silicone, and advanced hydrogels, are analyzed for mechanical properties, biocompatibility, and sustainability, with emphasis on biodegradable and locally sourced alternatives. Manufacturing innovations ranging from continuous casting to additive manufacturing enable customization, functional integration, and scalable production, although cost, supply chain stability, and regulatory compliance remain critical barriers. By uniting advances in materials engineering, nanotechnology, and precision manufacturing, ultrasound gel pads have demonstrated strong potential to advance coupling media for diagnostic, therapeutic, and wearable ultrasound applications, enabling higher diagnostic accuracy, streamlined workflows, and patient-centered care across diverse clinical and resource-limited settings. Full article

Effective absorption in the S-band usually requires relatively thick absorbing materials. However, growing application demands necessitate the development of high-performance materials with subwavelength thickness. This study presents a broadband absorbing metamaterial for the S-band, based on a novel structural design featuring a nested hexagonal metal resonant layer integrated with a carbonyl iron powder (CIP)/charcoal (CH)/epoxy resin (ER) composite slab. This structural innovation enables exceptional S-band absorption within a subwavelength thickness, effectively overcoming the inherent physical limitations of traditional materials. By combining the arch measurement method and simulations over the 2–18 GHz, we demonstrate that the metal resonant layer of the metamaterial plays a key role in controlling the electromagnetic field vector distribution. This work investigates the mechanism for enhancing S-band absorption in metamaterials through the redistribution of electromagnetic field vectors. Additionally, magnetic loss from CIP/CH/ER and dielectric loss from the resonators further enhance absorption performance. The designed absorbing metamaterial exhibits effective absorption at a thickness of only 2.25 mm, with a reflection loss (RL) below −10 dB from 2.2 to 3.8 GHz. Simultaneously, it can maintain a radar cross-section (RCS) below −10 dBm² in a wide-angle range of ±160°. Furthermore, a superhydrophobic coating with a contact angle of 152° was prepared for absorbing metamaterial. This coating allowed the metamaterial to preserve its microwave absorption performance while imparting self-cleaning capability. This study proposes a multifunctional absorbing metamaterial for efficient absorption in the S-band. Full article

This work introduces a compact multi-resonant metamaterial absorber designed to achieve efficient electromagnetic absorption over several microwave frequency bands. The proposed configuration is based on a hybrid resonator arrangement that promotes strong electromagnetic interaction and enables multiple resonant modes within a single unit cell. Consequently, six distinct absorption peaks are obtained at 2.4, 5.21, 6.88, 9.77, 12.61, and 14.99 GHz, covering S-, C-, X-, and Ku-band applications. The absorber exhibits high absorption performance, exceeding 97% across most operating frequencies and slightly lower value is observed of 91.13% at 12.61 GHz, which indicates effective impedance matching with free space and efficient energy dissipation mechanisms. The absorption characteristics are further examined through surface current distributions, electric field confinement, and effective medium analysis, demonstrating that the multi-band response originates from the interaction of multiple resonant elements and intrinsic material losses. Moreover, the proposed structure maintains stable performance for different polarization angles and oblique wave incidence, confirming its polarization-insensitive and angularly stable behavior. To validate the design, a prototype is fabricated and experimentally characterized using a free-space measurement setup, showing close agreement with the simulated results. The compact geometry, low fabrication cost, and scalability of the proposed absorber make it a promising candidate for applications such as electromagnetic interference mitigation, radar cross-section reduction, and modern wireless communication systems. Full article

Auxetic metamaterials have outstanding negative Poisson’s ratio characteristics which can be beneficial in different industrial applications. The main aim of the present study is to investigate the effect of thickness-dependent functional grading (FG) on the mechanical response of two widely known auxetic geometries, namely re-entrant and anti-tetrachiral. Three different thickness-dependent FG versions of these geometries were compared against their counterparts without FG by using numerical simulations. The effect of thickness-dependent FG was also compared against non-auxetic geometry (honeycomb) to understand the effect of auxeticity. The validation experiments were performed by the production of sample geometries by laser powder bed fusion technology from Inconel 718 material and quasi-static compression testing. The results revealed that the grading direction is a key variable in design that significantly influences the deformation stability and stress distribution, and it was shown that thickness-dependent FG is a promising way to decrease the weight of auxetic structures without sacrificing SEA considerably. Full article

Bone plates are widely used in orthopaedic surgery to stabilise fractured bones and support healing following traumatic injuries or osteotomies. However, conventional metallic bone plates suffer from stress shielding and stiffness mismatch with bone, which can hinder optimal healing. Additive manufacturing enables the incorporation of novel metamaterial architectures into polymer-based implants to enhance mechanical properties. The fatigue behaviour of these implants during the healing period is critical to ensuring their structural integrity and long-term performance. In this study, the compressive fatigue performance of fused deposition modelling (FDM)-printed carbon fibre-reinforced polylactic acid (CF-PLA) bone plates were investigated. Four metamaterial structures—tetrachiral, re-entrant, rotating square, and hexagonal—were evaluated under strain-controlled cyclic loading at 20%, 40%, 60%, and 80% of their respective yield strains. The results showed a strong dependence of fatigue behaviour on lattice geometry. Among the tested configurations, the re-entrant structured bone plate exhibited the best overall fatigue performance, sustaining up to 100,000 cycles at moderate strain levels and showing delayed stiffness degradation under high strain conditions. In contrast, rotating square and hexagonal structures showed early stiffness loss and failure at higher strain levels. These findings highlight the importance of lattice design in fatigue performance, although FDM-induced printing defects significantly influence overall fatigue behaviour. Full article

Optical bistability is a nonlinear phenomenon enabling stable switching between two optical states and has important applications in optical communication and photonic neural networks (PNNs). However, conventional bistable devices often suffer from fabrication imperfections and scattering losses, which limit their robustness and dispersionless performance. In this study, we numerically investigate optical bistability from a one-dimensional photonic topological hypercrystal (PhH) composed of alternating hyperbolic metamaterials (HMMs) and dielectric layers. By designing a center-inversed symmetric layered PhH structure and introducing Kerr nonlinearity into the localized dielectric region of maximum electric field intensity at the inversion center, we achieve a robust, angle-insensitive optical bistability for TM polarization through phase variation compensation mechanism. When applied as a nonlinear activation function in PNNs, the bistable PhH exhibits performance comparable to conventional digital activation functions such as ReLU and Sigmoid in image-recognition tasks. Our work paves the way for integrating topological bistable devices into next-generation PNNs. Full article

The tunable coefficient of thermal expansion (CTE) and Poisson’s ratio (PR) properties of metamaterials help address issues caused by drastic temperature variations and external loads. In this work, we propose a novel bimaterial thermal expansion and PR integrated tunable 2D metamaterial structure. Under certain parameter constraints, the structure based on an Al alloy/low carbon steel (LCS) combination demonstrates a wide tunability, with the CTE ranging from −47 to 28 ppm/°C and the PR varying from −14.8 to 7.3. A general thermoelastic equation is adopted to establish the relationship between temperature, external force, and displacement, which is then assembled into a theoretical model. Through theoretical analysis and numerical simulations, the underlying mechanisms of the proposed 2D metamaterial structure’s CTE, PR, and their relationship with geometric parameters and elastic modulus ratios are revealed. CTE and PR experiments are conducted to validate the theoretical modeling. Finally, the coupling relationship between CTE and PR is revealed. Full article

Additive manufacturing has emerged as a powerful approach for producing architected materials with tailored mechanical properties and enhanced energy absorption capabilities. By enabling precise control over geometry, relative density, and hierarchical topology, additive manufacturing facilitates the design of lightweight cellular structures with superior crashworthiness compared to conventional energy-absorbing materials. This review provides a comprehensive overview of recent advances in additively manufactured energy-absorbing structures, with particular emphasis on the interplay between structural architecture, fabrication technologies, and mechanical performance. Key additive manufacturing processes, including fused deposition modeling, stereolithography, selective laser sintering, and multi-jet fusion, are evaluated in terms of their fabrication capabilities, material compatibility, and inherent limitations. Special attention is given to the mechanical behavior of representative architectures, including two-dimensional cellular structures, three-dimensional lattice geometries, sandwich systems, and emerging four-dimensional programmable materials. Depending on topology and material system, additively manufactured lattices can achieve specific energy absorption values exceeding 20–40 J g⁻¹, significantly outperforming many conventional foams. Finally, current challenges, such as process-induced defects, anisotropic mechanical behavior, and the lack of standardized testing methodologies, are discussed, along with future research directions, including multi-material printing, functionally graded architectures, and adaptive metamaterials for next-generation impact mitigation systems. Full article

This study introduces a switchable and tunable multimodal, multi-peak, perfect terahertz absorber, utilizing a composite structure of graphene and double concentric metal rings. From bottom to top, the absorber consists of a gold substrate, a SiO₂ dielectric layer, a patterned graphene layer, another SiO₂ dielectric layer, and double concentric metal rings on the top. The structure achieves three high-absorption resonance peaks in the far-infrared band: a relatively broad peak with 99.05% absorptance at 38.128 THz, and two extremely narrow peaks with 99.56% and 97.23% absorptance at 47.909 THz and 49.873 THz, respectively. Analysis of the absorption spectra and electric field distributions reveals that the generation mechanism of Peak I is Fabry–Pérot cavity resonance, while Peaks II and III result from the coupling between the high-order localized surface plasmons in the outer ring and the graphene surface plasmon polaritons. Benefiting from graphene’s excellent electrical tunability, the absorption peaks’ positions and intensities can be dynamically tuned by varying the Fermi level. The core innovation of this work lies in the high-level integration of multiple functionalities. By leveraging the sensitive response of Peak III to variations in the Fermi level, a four-input AND logic gate is embedded within the metamaterial absorber in this frequency band. The Fermi levels of four independent graphene regions serve as the binary inputs, while the absorption state of Peak III is defined as the logical output. Additionally, the two narrow peaks display high sensitivity to the surrounding refractive index, with sensitivities of 30.1 THz/RIU and 62.5 THz/RIU, demonstrating significant potential for sensing. This multifunctional integrated device combines tunable absorption, a logic gate, and sensing capabilities, making it promising for terahertz communication systems, intelligent sensing networks, and reconfigurable platforms. Full article

Planar microwave (MW) sensors offer high-resolution, non-invasive technology for monitoring critical soil properties, serving as a support for modern precision agriculture. While laboratory studies confirm their exceptional sensitivity, the widespread adoption of these sensors is severely impeded by critical translational challenges that constitute a defining “lab-to-field gap”. These barriers include high sensor-to-sensor variability, debilitating thermal cross-sensitivity, soil heterogeneity necessitating unique site-specific calibration, and the enduring tension between high-performance and cost-effective scaling. This review systematically synthesizes the current state of planar permittivity MW technology, moving beyond technical mechanisms to critically assess these operational limitations. We detail advanced architectural strategies designed to bridge this gap, focusing particularly on the transition toward more robust solutions. The key strategies analyzed include the adoption of differential sensor designs using microstrip patch antennas to mitigate common-mode environmental errors, the integration of ultra-compact metamaterial structures such as split-ring resonators (SRRs) and complementary split-ring resonators (CSRRs) for enhanced field robustness and deep soil sensing, and the necessity of multi-parameter sensing capabilities (moisture, pH, and salinity). By establishing a comprehensive roadmap that prioritizes field stability, cost efficiency, and seamless IoT integration, this review demonstrates that planar MW sensors are poised to become reliable and scalable tools. Addressing these critical translational hurdles will ensure optimal resource management, significantly enhance crop productivity, and enable sustainable practices within smart farming ecosystems. Full article

Metamaterials possess high freedom on structural design, yet their ability to modulate electromagnetic waves is subject to intrinsic constraints that are independent of specific meta-atom geometries. The constraints are revealed by analyzing the statistical amplitudes and phases of transmission and reflection wave in some representative metamaterials. Based on scattering theory, a reconstructed and more general description of the electromagnetic modulation process in metamaterials is established. Two explicit and geometry-independent corollaries concerning the coupling between transmission and reflection waves are further obtained and verified. The results provide a new perspective on the fundamental modulation mechanism of metamaterials on electromagnetic waves. Full article

Obvious size-dependent bending responses are observed in porous polymer beams, particularly as their thickness approaches the scale of the lattice constant. However, the relationship between the size dependency and the microstructure remains unclear. Direct numerical simulations are computationally expensive due to the complexity of the microstructures, while classical multiscale methods, which neglect the surface effect, yield results that deviate significantly from actual behavior. In this study, an equivalent model for porous polymer beams incorporating surface-driven moment of inertia is developed to capture the size-dependent Young’s modulus by introducing a surface strength factor and surface thickness. Then, an online prediction framework based on the offline dataset generated by the moment-of-inertia-dependent surface homogenization method was established for size-dependent bending response. The proposed framework is evaluated in terms of accuracy and computational efficiency. Results show that the classical multiscale homogenization method can produce relative errors as high as 1108%, whereas the surface homogenization method maintains relative errors below 4%. Moreover, the computational cost is substantially reduced compared to direct numerical simulations. This work not only uncovers the underlying moment-of-inertia-dependent surface mechanism of the size-dependent behavior in metamaterial beams but also delivers an accurate and efficient tool for their structural design and performance prediction. Full article

To address the limitations of conventional metamaterials in thermo-mechanical coupling environments, this study proposes a multifunctional metamaterial structure through material selection and structural optimization, demonstrating stable vibration isolation performance under thermal fluctuations. The thermal deformation mechanisms and zero thermal expansion (ZTE) behavior of curved-beam unit cell are systematically examined through the chained beam constraint model (CBCM). A novel dual-zero metamaterial featuring both quasi-zero-stiffness (QZS) and ZTE characteristics is developed using curved-beam unit cell design. A parametric analysis, through finite element modeling, systematically investigated the effects of geometric parameters and material properties on the thermal expansion deformation and mechanical responses in the curved-beam unit cell structure. Furthermore, cylindrical metamaterials featuring dual-zero properties were engineered, and their deformation control mechanisms and vibration characteristic evolution across a broad temperature range were systematically studied. The simulation results indicate that while the Al–Al structure exhibits a significant resonance peak shift of up to 64.32% at 200 °C, the Al–Steel zero-stiffness design restricts this shift to only 7.72%. Furthermore, the Steel–Invar configuration demonstrates exceptional vibrational stability, with its center frequency shifting marginally from 5.58 Hz to 5.61 Hz at 200 °C. This methodology presents a viable solution for engineering metamaterials in extreme-temperature environments. Full article

This study presents a predictive manufacturing framework for customizing the biomechanical response of a 3D printed ergonomic armrest based on relaxed Voronoi metamaterials. A double curved armrest geometry was combined with parametric lattice generation, stereolithography printing in BioMed Elastic 50A resin, uniaxial compression testing of cylindrical lattice specimens, and homogenized finite element simulations using a CT derived forearm model under 15, 30, and 45 N loading. The results showed that both cell size and ligament thickness strongly affected compressive behavior, with smaller cells and thicker ligaments producing higher stiffness and earlier densification. Among the uniform configurations selected for simulation, the E-9-1.5 lattice provided the most balanced response, maintaining contact pressure below about 70 kPa up to 45 N, whereas the stiffer E-7-1.5 configuration exceeded 160 kPa and the E-7-1 configuration surpassed 100 kPa at higher load. Based on these findings, a functionally graded Voronoi concept was developed to combine a more compliant central zone with a stiffer peripheral support region while preserving conformity to the complex armrest boundary. Overall, the results show that relaxed Voronoi lattices offer a computationally efficient route toward anatomically conforming and mechanically tunable cushioning interfaces. Full article