Smart Materials and Structures

Energy harvesting technologies have been explored by researchers for more than two decades as an alternative to conventional power sources (e.g. batteries) for small-sized and low-power electronic devices. The limited life-time and necessity for periodic recharging or replacement of batteries has been a consistent issue in portable, remote, and implantable devices. Ambient energy can usually be found in the form of solar energy, thermal energy, and vibration energy. Amongst these energy sources, vibration energy presents a persistent presence in nature and manmade structures. Various materials and transduction mechanisms have the ability to convert vibratory energy to useful electrical energy, such as piezoelectric, electromagnetic, and electrostatic generators. Piezoelectric transducers, with their inherent electromechanical coupling and high power density compared to electromagnetic and electrostatic transducers, have been widely explored to generate power from vibration energy sources. A topical review of piezoelectric energy harvesting methods was carried out and published in this journal by the authors in 2007. Since 2007, countless researchers have introduced novel materials, transduction mechanisms, electrical circuits, and analytical models to improve various aspects of piezoelectric energy harvesting devices. Additionally, many researchers have also reported novel applications of piezoelectric energy harvesting technology in the past decade. While the body of literature in the field of piezoelectric energy harvesting has grown significantly since 2007, this paper presents an update to the authors' previous review paper by summarizing the notable developments in the field of piezoelectric energy harvesting through the past decade.

This review serves as a comprehensive design strategy for designing quasi-zero stiffness (QZS) mechanical metamaterials (MMs). It discusses their underlying deformation mechanisms that enable the attainment of QZS behavior under both compressive and tensile loadings. While the QZS characteristic of metamaterials has garnered considerable attention, further research is essential to unlock their potential fully. Numerous QZS metamaterials have been meticulously reviewed. They comprise various elements and mechanisms, including positive and negative stiffness elements (PS and NS), PS elements with variable stiffness, bending mechanisms employing stiff joints/areas, buckling, buckling-rotating, and bending/buckling deformation mechanisms leading to a QZS feature. Furthermore, the capability of multi-material, adaptive, smart metamaterials, origami (bending around the hinge of the folded joints), and kirigami lattices (out-of-plane buckling via cutting patterns) are weighted. These diverse mechanisms contribute to achieving QZS behavior in metamaterials under both compression and tension loads, which is paramount for various mechanical applications such as passive vibration isolation. This review effectively categorizes QZS metamaterials based on their underlying mechanisms, providing scholars with valuable insights to identify suitable mechanisms for the desired QZS feature.

Sports concussions are a public health concern. Improving helmet performance to reduce concussion risk is a key part of the research and development community response. Direct and oblique head impacts with compliant surfaces that cause long-duration moderate or high linear and rotational accelerations are associated with a high rate of clinical diagnoses of concussion. As engineered structures with unusual combinations of properties, mechanical metamaterials are being applied to sports helmets, with the goal of improving impact performance and reducing brain injury risk. Replacing established helmet material (i.e. foam) selection with a metamaterial design approach (structuring material to obtain desired properties) allows the development of near-optimal properties. Objective functions based on an up-to-date understanding of concussion, and helmet testing that is representative of actual sporting collisions and falls, could be applied to topology optimisation regimes, when designing mechanical metamaterials for helmets. Such regimes balance computational efficiency with predictive accuracy, both of which could be improved under high strains and strain rates to allow helmet modifications as knowledge of concussion develops. Researchers could also share mechanical metamaterial data, topologies, and computational models in open, homogenised repositories, to improve the efficiency of their development.

Miniaturized pneumatic artificial muscles (MPAMs), designed to replicate natural muscle actuation, offer unique attributes such as a high power-to-weight ratio, flexibility, easy integration, and compactness, making them favourable for many applications. The present paper aims at the development of an accurate semi-analytical force model considering the effect of the bladder material and friction terms to predict the nonlinear force-deformation response of MPAMs during contraction and expansion cycles. Existing force models for MPAMs exhibit limitations to accurately capturing the force-deformation behaviours due to several simplification factors. This study enhances these models by integrating correction terms to accurately address the nonlinearity and frictional effects exhibited by MPAMs. An analysis of the hysteresis loops resulting from the cyclic loading and unloading of MPAMs under specific pressures is undertaken to compare different methodologies in order to determine the most accurate correction terms. To investigate the nonlinear behaviour of MPAMs, the stress-strain relationship of the bladder material and results from force-deformation experimental tests on the entire actuator are considered and for the effect of friction term, theoretical and empirical approaches are investigated. Results suggest that the theoretical force model based on analytical and empirical friction forces, respectively, slightly overestimates and underestimates the force experienced by MPAMs during contraction while slightly underestimate and overestimates during expansion, respectively. A comparative analysis between MPAMs featuring Ecoflex-50 silicone and Ecoflex30 + PDMS mixture as bladder materials has also been conducted to further investigate the effect of bladder materials on their force and contraction outputs under inlet pressures ranging from 0 kPa to 300 kPa. It is shown that the MPAM feauting Ecoflex-50 bladder, exhibits lower dead-band pressure and an overall reduced blocked force in comparison to MPAM with bladder made of Ecoflex30 + PDMS while exhibiting a substantially enhanced free contraction capacity.

Fault diagnosis (FD), comprising fault detection, isolation, identification and accommodation, enables structural health monitoring (SHM) systems to operate reliably by allowing timely rectification of sensor faults that may cause data corruption or loss. Although sensor fault identification is scarce in FD of SHM systems, recent FD methods have included fault identification assuming one sensor fault at a time. However, real-world SHM systems may include combined faults that simultaneously affect individual sensors. This paper presents a methodology for identifying combined sensor faults occurring simultaneously in individual sensors. To improve the quality of FD and comprehend the causes leading to sensor faults, the identification of combined sensor faults (ICSF) methodology is based on a formal classification of the types of combined sensor faults. Specifically, the ICSF methodology builds upon long short-term memory (LSTM) networks, i.e. a type of recurrent neural networks, used for classifying 'sequences', such as sets of acceleration measurements. The ICSF methodology is validated using real-world acceleration measurements from an SHM system installed on a bridge, demonstrating the capability of the LSTM networks in identifying combined sensor faults, thus improving the quality of FD in SHM systems. Future research aims to decentralize the ICSF methodology and to reformulate the classification models in a mathematical form with an explanation interface, using explainable artificial intelligence, for increased transparency.

Polyvinyl alcohol fiber reinforced engineered cementitious composite (ECC) using piezoelectric polymer film has attracted significant interest due to its energy harvesting potential. This work provides a theoretical model for evaluating the energy harvesting of bendable ECC using surface-mounted polyvinylidene fluoride (PVDF). In the mechanical part, concrete damage plasticity model based on the explicit dynamic analysis was utilized to simulate the dynamic flexural behavior of ECC beam under different dynamic loading rates. The mechanism of force transfer through the bond layer between the PVDF film and ECC specimen was simulated by a surface-surface sliding friction model wherein the PVDF film was simplified as shell element to reduce computational cost. Then, the electromechanical behavior of the piezoelectric film was simulated by a piezoelectric finite element model. A simplified model was also given for a quick calculation. The theoretical model was verified with the experimentally measured mechanical and electrical results from the literature. Finally, a parametric analysis of the effects of electromechanical parameters on the efficiency of energy harvesting was performed. The verified theoretical model can provide a useful tool for design and optimization of cementitious composite systems for energy harvesting application.

Two-way shape memory polymers are stimulus-responsive materials capable of changing their shape between two configurations based on an on/off thermal stimulus. While the traditional effect has been studied under the application of an external mechanical load, it was demonstrated also in the absence of an external load. Such a response only relies on a carefully tailored macromolecular architecture of the polymer combined with a specific thermo-mechanical protocol. In particular, semicrystalline networks, either consisting of a multi-phase copolymer network or a homopolymer based network with broad phase transitions, have been proposed to this aim under ad hoc thermo-mechanical histories. In this work, the two-way shape memory behavior is studied on a poly(-caprolactone)-based network, crosslinked by means of a sol–gel approach and tailored on the selection of the molecular weight of the precursor polymer. Changing the prepolymer precursor allowed to tune the melting/crystallization regions of the networks, thus the thermal region of the reversible shape memory effect. The application of properly designed thermo-mechanical cycles allowed to study the two-way shape memory effect without the application of an external load under tensile conditions. Given a specific network, the stress-free actuation of the reversible elongation-contraction cycle under tensile conditions was induced across its specific melting/crystallization region. The extent of the effect was found to depend on the crystalline fraction remaining for the given actuation temperature and on the tensile stretched state imposed on the materials during the training step. The results were compared with the response achieved under the traditional two-way shape memory protocol under stress. The stress-free two-way shape memory effect was also successfully demonstrated and emphasized, under flexural conditions, which suggests the potential of these materials as intrinsically reversible actuators, promising for applications in the biomedical field and/or for soft robotics.

Functionally graded metamaterials represent a cutting-edge approach to designing structures with cellular materials. By manipulating parameters in specific regions, a customized mechanical response is achieved, optimizing material utilization. Despite various proposed methods to generate functionally graded structures, the challenge of high computational costs persists. This study introduces a novel and computationally efficient inverse design method for beams featuring segment-wise graded metamaterials. Using a semi-analytical approach based on Castigliano's second theorem, this method significantly reduces computational demands. The approach leverages prior experimental and computational characterizations of the transverse deflection in rectangular, reentrant, and hexagonal honeycombs. Validation through finite element models and experimental tests on additively manufactured beams confirms the adequate performance of the method. The proposed framework successfully generates beams with targeted deflections, demonstrating the method's capability for inverse design under specific loading and boundary conditions. This approach not only optimizes material utilization but also broadens the application scope of functionally graded metamaterials in structural design.

A rotational multi-stable energy harvester has been presented in this paper for harnessing broadband ultra-low frequency vibrations. The novel design adopts a toroidal-shaped housing to contain a rolling sphere magnet which absorbs mechanical energy from bidirectional base excitations and performs continuous rotational movement to transfer the energy using electromagnetic transduction. Eight alternating tethering magnets are placed underneath its rolling path to induce multi-stable nonlinearity in the system, to capture low-frequency broadband vibrations. Electromagnetic transduction mechanism has been employed by mounting eight series connected coils aligned with the stable regions in the rolling path of the sphere magnet, aiming to achieve greater power generation due to optimized rate of change of magnetic flux. A theoretical model has been established to explore the multi-stable dynamics under varying low-frequency excitation up to 5 Hz and 3 g acceleration amplitudes. An experimental prototype has been fabricated and tested under low frequency excitation conditions. The harvester is capable of operating in intra-well, cross-well, and continuous rotation mode depending on the input excitation, and the validated physical device can generate a peak power of 5.78 mW with 1.4 Hz and 0.8 g sinusoidal base excitation when connected to a 405 Ω external load. The physical prototype is also employed as a part of a self-powered sensing node and it can power a temperature sensor to get readings every 13 s on average from human motion, successfully demonstrating its effectiveness in practical wireless sensing applications.

To pursue a variable-capacitance working principle, transducers based on soft electroactive polymers (EAPs) need deformable electrodes that match the compliance and stretchability of the EAP polymeric substrates. A variety of manufacturing procedures are available to create conductive materials that can achieve this, including solutions that can provide remarkably low resistivity. However, the simplest and most feasible options often involve the use of particle-filled (e.g. carbon-filled) polymer composites, which, while easy to produce, tend to exhibit relatively high resistivity. This high level of resistivity, combined with the inherent capacitance of EAP transducers, introduces dynamic effects in the devices electrical activation, which may affect performance. This paper investigates the impact of electrode resistivity on the electrical dynamics of EAP devices, combining continuum models and experimental validations. We use a continuum generalisation of known resistive-capacitive (RC) transmission line models to accurately predict voltage gradients on the surfaces of electrostatic transducers subject to rapidly varying voltages. We then present an experimental validation by measuring the spatial voltage distributions over carbon-based polymeric electrodes of dielectric elastomer (DE) transducers, and find a good agreement with our model predictions. We use our validated model to provide general estimates of the typical charging time and limit working frequency ranges of DE devices as a function of their dimensional scale and electrode sheet resistance. Our model provides useful indications for designing compliant electrodes in EAP transducers given target performance, or to understand the working limits of devices with given geometry and dielectric-electrode properties.

The integration of 3D printed constructions into civil projects has created new opportunities for economically efficient construction. However, preserving the long-term structural integrity of 3D-printed structures poses considerable challenges. This study covers the importance of structural health monitoring (SHM) and deployment of sensors for condition monitoring of 3D-printed civil infrastructure. It explores a wide range of sensors that might be used for continual evaluation and assessment of structural efficiency and the challenges related to SHM in these components. The report provides cost benefit analysis and case studies describing effective sensor installations in 3D-printed structures, demonstrating the ability of the technology to enhance the safety and integrity of infrastructure systems. It also identifies potential challenges and issues that must be resolved before sensor-based SHM can be successfully used in 3D-printed civil structures. The research emphasizes the potential of maintenance planning and decision support systems for optimizing maintenance schedules, reducing downtime, and increasing cost-effectiveness. This research is critical for academics, engineers, and professionals using sensors for 3D-printed structural systems.

Monitoring the vital signs of the injured in accidents is crucial in emergency rescue process. Fabric-based sensing devices show a vast range of potential applications in wearable healthcare monitoring, human motion and thermal management due to their wearable flexibility and high sensitivity. Nevertheless, flexible electronic devices for both precise monitoring of health under low strain and motion under large strain are still a challenge in extremely harsh environment. Therefore, development of sensors with both high sensitivity and wide strain range remains a formidable challenge. Herein, a wearable flexible strain sensor with a one-dimensional/two-dimensional (1D/2D) composite conductive network was developed for healthcare and motion monitoring and thermal management by coating 1D silver nanowires (AgNWs) and 2D Ti₃C₂T_x MXene composite films on nylon/spandex blended knitted fabric (MANS). The MANS strain sensor can simultaneously achieve high sensitivity (gauge factor for up to 267), a wide range of detection (1%–115%), excellent repeatability and cycling stability (1000 cycles). The sensor can be utilized for human health monitoring including heartbeat, pulse detection, breathing and various human motion. Moreover, the MANS sensor also has the electrical heating properties and voltage control temperature between 20 °C–110 °C can achieved at low voltage. In addition, the MANS shows hydrophobicity with water contact angle of 137.1°. The MXene/AgNWs composite conductive layer with high sensitivity under low and large strains, electrical thermal conversion, and hydrophobicity has great potential for precisely monitoring health and motion of the injured in emergency rescue in harsh environment.

With the continuous advancement of ultra-low-power electronic devices, capturing energy from the surrounding environment to power these smart devices has emerged as a new direction. However, most of the mechanical energy available for harvesting in the environment exhibits ultra-low frequencies. Therefore, the feasibility of self-powering low-power devices largely depends on the effective utilization of this ultra-low-frequency mechanical energy. Consequently, this work proposes an enhanced electromagnetic energy harvester based on a dual ratchet structure with secondary energy recovery. It converts ultra-low frequency vibrations into fast rotational movements by means of a rack and pinion mechanism, thus achieving high power output while maintaining a simple structure. Experimental tests demonstrate that the proposed harvester exhibits excellent power output under ultra-low-frequency external excitation. Under external excitation with a frequency of 1.5 Hz and an amplitude of 22 mm, with the optimal load matched at 20 Ω, the maximum power output reaches 598 mW, with a power density of 1572.65 μW cm⁻³. The secondary energy recovery power accounts for 34.4%, resulting in a 52.56% enhancement in the energy harvester's output performance. Additionally, hand-cranking tests indicate that the fabricated prototype of the electromagnetic energy harvester can power some common electronic devices, including smartphones, showcasing significant application potential.

The inspection, maintenance, and repair of complex pipelines have motivated the development of soft robots with highly flexible and good adaptability. In this study, inspired by the unique locomotion of earthworms, we developed a type of smart material–driven soft modular pipe robot capable of stable manipulation and performing in unstructured pipe environments, which easily assembles into more complex configurations with multiple modules for practical use. Our prototype robot consists of three soft telescopic modules connected in series with flexible bellows and a tail friction mechanism, where the modules adopt a high-energy density shape memory alloy spring as an actuator. Based on analyzing the peristaltic process of the module inside the pipe, it is ensured that the geometric constraint performance of the braided mesh pipe is reasonably matched with the thermomechanical performance of the SMA spring to realize the alternating conversion of anchoring and releasing. By optimizing the overall robotic structure, it is demonstrated that our robot achieves robust crawling in horizontal, vertical, variable-diameter, and curved pipes, wet pipes with the partial presence of water, and pipes with complex cavities through simple open-loop on/off control.

Wave energy is a widespread clean energy source, but harvesting low-frequency wave energy efficiently remains a challenge. In this paper, a frequency-increasing piezoelectric wave energy harvester (FPWEH) based on gear mechanism and magnetic rotor is proposed. The gear mechanism transforms the vertical motion of the wave into the higher-frequency rotational motion of the magnetic rotor. The magnetic rotor is equipped with several rotating magnets and one revolution of the magnetic rotor enables multiple excitations of the piezoelectric cantilevers. Therefore, the wave excitation frequency is increased, so that the FPWEH can obtain better output performance. The major factors influencing output performance are determined through theoretical and simulation analysis, and a test system to simulate the wave environment is established. According to experimental findings, the FPWEH can generate an output voltage of 69.82 V and a maximum power of 28.33 mW when the external resistance is 20 kΩ. It can also successfully power thermohygrometer and light-emitting diodes. These results validate the feasibility of the FPWEH for providing electricity to electronics with low power requirements. This research also offers a novel approach to harvesting low-frequency wave energy.

The integration of 3D printed constructions into civil projects has created new opportunities for economically efficient construction. However, preserving the long-term structural integrity of 3D-printed structures poses considerable challenges. This study covers the importance of structural health monitoring (SHM) and deployment of sensors for condition monitoring of 3D-printed civil infrastructure. It explores a wide range of sensors that might be used for continual evaluation and assessment of structural efficiency and the challenges related to SHM in these components. The report provides cost benefit analysis and case studies describing effective sensor installations in 3D-printed structures, demonstrating the ability of the technology to enhance the safety and integrity of infrastructure systems. It also identifies potential challenges and issues that must be resolved before sensor-based SHM can be successfully used in 3D-printed civil structures. The research emphasizes the potential of maintenance planning and decision support systems for optimizing maintenance schedules, reducing downtime, and increasing cost-effectiveness. This research is critical for academics, engineers, and professionals using sensors for 3D-printed structural systems.

Recent advancements in fabrication techniques, such as the development of powder metallurgy, have made it possible to tailor the mechanical properties of functionally gradient piezoelectric (FGP) micro/nanostructures. This class of structures can be used to improve the performance of many micro/nanoelectromechanical systems because of their spatially varying mechanical and electrical properties. The importance of FGP micro/nanoscale structures has been demonstrated by the growing number of published works on their size-dependent mechanical characteristics, including their static bending, buckling, vibration, energy harvesters and wave propagation using scale-dependent continuum-based models. Reviewing recent developments in the field of non-classical continuum mechanics, this paper examines the size-dependent mechanical analysis of porous FGP micro/ nanostructures. Five sophisticated theories of piezoelectricity—modified couple stress, strain gradient, surface effect, as well as nonlocal and nonlocal strain gradient theory, for example—are given special consideration in light of their potential to forecast unusual mechanical performance and wave characteristics in porous FGP micro/nanostructures and devices. In the future, porous FGP micro/nanostructures with multi-field couplings may be studied or designed, and this article may be a helpful resource.

Bistable structures have attracted attention due to their unique properties and potential applications in soft robotics, logic gates and energy harvesting devices. The bi-stability is always an inherent property if the bistable structures are pre-designed. A reprogrammable bistable structure that does not require re-designing and re-fabricating the prototype is highly desirable. Despite its vast potential and burgeoning interest, the field of reprogrammable bistable structures lacks a cohesive and comprehensive review. Therefore, this paper presents a state-of-the-art review of recent advances in the basic structural forms, key parameters determining bistable characteristics, active regulation mechanisms, and potential applications of reprogrammable bistable structures. It also presents the remaining challenges and suggests possible future research directions in the field of reprogrammable bistable structures. This review will provide valuable insights for researchers and engineers to explore the vast potential of reprogrammable bistable structures.

This review serves as a comprehensive design strategy for designing quasi-zero stiffness (QZS) mechanical metamaterials (MMs). It discusses their underlying deformation mechanisms that enable the attainment of QZS behavior under both compressive and tensile loadings. While the QZS characteristic of metamaterials has garnered considerable attention, further research is essential to unlock their potential fully. Numerous QZS metamaterials have been meticulously reviewed. They comprise various elements and mechanisms, including positive and negative stiffness elements (PS and NS), PS elements with variable stiffness, bending mechanisms employing stiff joints/areas, buckling, buckling-rotating, and bending/buckling deformation mechanisms leading to a QZS feature. Furthermore, the capability of multi-material, adaptive, smart metamaterials, origami (bending around the hinge of the folded joints), and kirigami lattices (out-of-plane buckling via cutting patterns) are weighted. These diverse mechanisms contribute to achieving QZS behavior in metamaterials under both compression and tension loads, which is paramount for various mechanical applications such as passive vibration isolation. This review effectively categorizes QZS metamaterials based on their underlying mechanisms, providing scholars with valuable insights to identify suitable mechanisms for the desired QZS feature.

The photomechanical effect (PME), characterized by light-induced mechanical deformation in materials, has gained significant attention across various domains. Photomechanical modeling, integrating photochemistry and mechanical behavior in photoactive materials, is a crucial tool for understanding and optimizing functionality. In this review, we provide an overview of recent developments in mechanical modeling and numerical simulations, focusing on finite element simulations in organic photoactuators. We conducted a systematic literature search from the discovery of the PME, examining progress in modeling diverse organic photoactuators, including polymer-based and liquid crystal elastomer. Integrating light and mechanical constitutive models has enabled the accurate representation of the photomechanical responses of these materials. This review summarizes methods for simulating light-induced deformation, factors influencing photomechanical responses, and current field limitations. Additionally, this review introduces mechanical models as indispensable tools for describing the mechanical behavior of organic photoactuators. In conclusion, developing novel organic photoactuators requires establishing generalized photomechanical couplings to optimize design, enhance light-induced responses, and facilitate cost-effective commercialization. This review serves as a valuable resource for researchers interested in this field, stimulating further exploration of organic photoactuator applications.

Transparent thermoelectric materials are a special kind of material that converts thermal energy into electrical power and possesses unique properties for transparent electronics and future energy applications. These materials are being studied for specific applications such as windowpanes, photovoltaic panels, sensor displays, smart electronic devices, and more. For such applications, it is desirable that the thermoelectric materials be in the form of thin films or coatings, be optically transparent, and exhibit excellent thermoelectric performance. Understanding the electrical, thermal, and optical properties of materials is crucial for the development of transparent thermoelectric devices. This paper discusses the current progress in the development of transparent thermoelectric materials.

As an excellent vibration energy harvesting material, iron-cobalt-vanadium alloy can be applied in seismic vibration monitoring. In this paper, a self-powered stepped composite structure based on iron-cobalt-vanadium alloy for long-term seismic monitoring is proposed, which can convert the mechanical energy generated by low-frequency transient seismic vibration into a voltage signal for self-powered monitoring. On the basis of its mechanical analysis, a mechano-magneto-electric coupling model is established. The relation between the performance of the voltage and the performance of the material is derived, a variety of magnetostrictive composite structures are produced, the properties of the materials used and the voltage performance generated by the structures are compared and analysed, and a simulated earthquakes platform is constructed for experimenting, and the maximum voltage is 620 mV under a transient force of 1 N, which proves that the composite structure of iron-cobalt-vanadium alloy is excellent in terms of voltage output. Finite element simulation is also used to analyse the role of generated magnetic field on the voltage output of the structure under different bias magnet arrangements, and the sensor is further optimised. Simulated seismic experiments were then carried out to analyse the voltage characteristics and energy harvesting capability. Experimentally, it was confirmed that the generated voltage and deflection were linear with R2 = 0.9966, and the fitting results are accurate. The structure produces a voltage of 1280 mV, an output power of 14.13 mW and a maximum power density of 139.55 mW/cm3 under a transient force of 2 N. The sensor has the advantages of simple structure, large output signal, easy fabrication and long term operation, therefore, this work highlights the feasibility of harvesting energy from seismic vibration for long term monitoring. It can have good prospective applications in the domain of developing self-powered seismic monitoring and transient vibration energy harvesting.

Autonomous underwater vehicles (AUVs) with greater maneuverability, efficiency, and resiliency are needed to meet the challenges of exploring and monitoring the underwater world, so we look to underwater creatures to uncover what makes them such excellent swimmers. Bio- inspired, soft robots can combine the performance of biological swimming with the robustness of soft construction, where the ideal robot has a jointless, flexible body with embedded muscles just like real fish. In this paper, we present a continuously deformable robotic trout with embedded electro-hydraulic HASEL artificial muscles, experimentally characterize its swimming kinematics, and report a reduced order numerical model which predicts the robot fish's natural frequencies and mode shapes. We characterized the robot's 3D full body swimming kinematics while submerged in water with digital image correlation. The soft robot undergoes whole body bending in response to internal muscle actuation and yields kinematics comparable to biological trout. Tail beat velocity was measured at the first three observed natural frequencies with a maximum of 69 mm s^-1 corresponding to a caudal fin trailing edge displacement of ±10 mm. We derive a beam-based fluid structure interaction (FSI) model which predicts swimming kinematics in response to embedded muscle forces and includes the effects of nonlinear vortex and convective forces on the robot's body. The nonlinear FSI model predicted the first three damped natural frequencies within 5% error and mode shapes which correlated with the experimental data. This paper contributes the design, fabrication, and characterization of a solid-state robotic trout featuring whole-body flexibility and embedded actuation through numerical modeling and experimental analysis.

Designing wideband energy harvesters using beam structures typically involves complexities, particularly in low-frequency and low-energy environments where the limitations of beam structures become more evident. To address these challenges, this study proposed a strategy for energy harvesting using a loaded-string system and established a theoretical model to investigate its performance. A parametric study was conducted for the string system, examining the effects of initial tension, mass location, material stiffness and excitation amplitude. The accuracy of the proposed model was verified through experimental validation. Both theoretical and experimental analyses observed a frequency shifting phenomenon, demonstrating the wideband characteristics of the system. Furthermore, the proposed string structure allows for convenient parameter adjustments, enabling the tuning of its natural frequency and operating bandwidth to meet more stringent practical requirements. The string system provides a new direction for designing energy harvesters to harness low-frequency energy from the ambient environment.

In the pursuit of sustainable solutions to the ever-increasing demand for renewable energy, mechanically compliant Thermoelectric Generators (TEGs) have garnered significant attention owing to the promise they present for application in generating power from waste heat in mechanically challenging scenarios. This review paper examines the ongoing advancements in the efficiency and applicability of TEGs through novel material engineering and design innovations. It delves into the improvement of their thermoelectric properties via micro- and nanostructural modifications and explores architectural advancements aimed at enhancing functionality and power output. Notably, the integration of TEGs into flexible, stretchable, and wearable electronics has been a significant development, expanding their applications in various domains such as healthcare monitoring, remote sensing, and consumer electronics. The review emphasizes the critical interplay between electronic, thermal, and mechanical aspects in optimizing TEG performance. By providing an in-depth exploration of these multifaceted interactions and highlighting the significant advancements in materials and design, this review aims to underscore the importance of TEGs in a cleaner and more efficient era of energy generation, with a particular focus on their emerging applications across diverse fields.

Autonomous underwater vehicles (AUVs) with greater maneuverability, efficiency, and resiliency are needed to meet the challenges of exploring and monitoring the underwater world, so we look to underwater creatures to uncover what makes them such excellent swimmers. Bio- inspired, soft robots can combine the performance of biological swimming with the robustness of soft construction, where the ideal robot has a jointless, flexible body with embedded muscles just like real fish. In this paper, we present a continuously deformable robotic trout with embedded electro-hydraulic HASEL artificial muscles, experimentally characterize its swimming kinematics, and report a reduced order numerical model which predicts the robot fish's natural frequencies and mode shapes. We characterized the robot's 3D full body swimming kinematics while submerged in water with digital image correlation. The soft robot undergoes whole body bending in response to internal muscle actuation and yields kinematics comparable to biological trout. Tail beat velocity was measured at the first three observed natural frequencies with a maximum of 69 mm s^-1 corresponding to a caudal fin trailing edge displacement of ±10 mm. We derive a beam-based fluid structure interaction (FSI) model which predicts swimming kinematics in response to embedded muscle forces and includes the effects of nonlinear vortex and convective forces on the robot's body. The nonlinear FSI model predicted the first three damped natural frequencies within 5% error and mode shapes which correlated with the experimental data. This paper contributes the design, fabrication, and characterization of a solid-state robotic trout featuring whole-body flexibility and embedded actuation through numerical modeling and experimental analysis.

Self-healing materials possess the capability to promptly repair minor damages occurring during service, thereby effectively preventing safety accidents. This paper investigates a multi-objective topology optimization method for the macro structure and microtubule network of self-healing materials around pure epoxy resin materials, aiming to enhance the damage healing capability of the microtubule network while meeting the mechanical performance requirements of the macro structure. By introducing the design variables of macro structure and microtubule network, the corresponding topological description functions are established respectively. And study applies logical operations and post-processing techniques to generate an embedded microtubule network structure description. The objective functions include the flexibility of the macro structure, the along-travel head loss, and the total length of the microtubule network, with material volume serving as a constraint. In order to determine the head loss of the three-dimensional microtubule network structure, a Hardy-Cross method based on flow initialization and loop search is proposed. Multi-objective topology optimization is designed based on moving morphable components algorithm, enumeration method and Pareto principle. Develop iterative termination conditions by assessing the disparity between Pareto solution sets in each generation, thereby ensuring algorithm convergence. The numerical example of the Messerschmitt–Bölkow–Blohm (MBB) beamyields a flexibility of 0.059 without a carrier and 0.0728 with a carrier the macrostructural flexibility without a carrier is 81.0% compared to with a carrier, and the macrostructural profiles and the overall flexibility of the MBB beams with/without a carrier are close to each other. This method serves as a reference for optimizing large-scale self-healing structures.

Active metamaterials address fundamental limitations of passive media and have widely been recognized as necessary in numerous compelling applications such as cloaking and extreme noise absorption. However, most practical devices of interest have yet to be realized due to the lack of a suitable strategy for implementing bulk active metamaterials—those that involve interacting cells and functionality beyond one dimension. Here, we present such an active acoustic metamaterial design with bulk modulus and anisotropic mass density that can be independently programmed over wide value ranges. We demonstrate this ability experimentally in several examples, targeting acoustic properties that are hard to access otherwise, such as a bulk modulus significantly smaller than air, strong mass density anisotropy, and complex bulk modulus and mass density for high reflectionless sound absorption. This work enables the transition of active acoustic metamaterials from isolated proof-of-concept demonstrations to versatile bulk materials.

4D textiles are a specific class of 4D printed materials obtained by printing flat patterns on elastically pre-tensioned textiles and being able to switch from planar systems to complex 3D objects after the textile pre-stretch is released. The mechanical balance between textile recovering strain and printed structure stiffness determines the final shape. This study is carried out by coupling pre-stretched Lycra to PLA and explores ways to control 4D textile shape transformations by varying pre-stretch (10% ÷ 60%), printed structure geometry (bar-shaped and star-shaped elements; star-shaped patterns), and printed element thickness (0.3 ÷ 3 mm) and mutual distance (2 ÷ 15 mm). By adjusting these parameters, a wide set of out-of-plane curvatures are obtained, ranging from flat, to dome-like and highly curved, wrapped or coiled shapes. Digital optical methods, including digital image analysis, 3D scanning, and digital image correlation, are used to evaluate the complexity of the final shape and strain state evolution during shape transformation. The geometry variation is measured in terms of height increase (maximum 45 mm for a star-shaped system, 30 mm for a multiple star pattern) and of area decrease (maximum 80% for a star-shaped system, 60% for a multiple star pattern). While most shape transformations occur immediately after printing ("direct 4D printing"), further shape evolutions may be triggered by heating above the PLA glass transition, allowing for the creation of dynamic structures whose shape changes upon external stimuli. The adhesion between the 3D printed element and the stretched textile is also examined, with a focus on determining the role of interfacial strength and the conditions that could enhance it. This study provides an overview of the primary design variables and valuable maps of their impacts on shape transformations in this broad scenario of influencing parameters.

A current research topic for dielectric elastomer (DE) materials is the reduction of the thickness of the DE layer in order to achieve a lower operating voltage with the same electric field strength. As the ratio of the layer thicknesses of the electrode to the elastomer is therefore more important, the mechanical properties of the electrode layers are of greater significance. Several research articles deal with investigations, exploring the influence of electrode materials on the behavior of the DE transducer and emphasizing its importance. In analytical models, however, the electrodes are not usually considered separately, but the parameters are identified for the entire DE composite, consisting of elastomer and electrode layers. In contrast, in this article the material characterization is carried out separately for the two materials in a first step. In a further step, a holistic model for multilayer DE transducers is derived on the basis of this material-specific characterization and subsequently validated with measurements. For the DE layers, ELASTOSIL ® 2030 (EL 2030), and for the electrode layers, ELASTOSIL ® LR 3162 (EL 3162) are investigated, frequently used materials for DE transducers that offer reproducible properties for the investigation. EL 3162 is a carbon black filled elastomer material that exhibits higher elastic and viscose stresses as well as a significant rate-independent hysteresis compared to EL 2030. Experimental investigations of DE transducers with different electrode thicknesses are examined to validate the model and to demonstrate the significance and influence of the electrode layers on the transducer's performance. Furthermore, the influence of the electrode properties on the actuator, generator and sensor behavior of the DE transducer is analyzed based on the developed model. Depending on the thickness and number of layers, this underlines the relevance of the electrode properties and provides information on the optimized design of the DE transducer.

Electric vehicle (EV) drivetrains have witnessed remarkable progress, prompting intensified research into advanced transmission systems. Magnetorheological fluids (MRF) clutches offer precise modulation of input currents, enabling swift and seamless torque delivery for EV transmission systems, owing to their exceptional performance. The transmission of an EV requires MRF-based clutches to deliver a precise and rapid torque transfer during gear shifting. In these scenarios, the inherent current rate-dependent hysteresis of the MRF-based clutches between the output torque and input current poses a significant challenge in accurately regulating output torque. Therefore, an accurate clutch model of the MRF-based clutches that can describe the rate-dependent hysteresis is crucial to achieve precise control of the output torque. This study investigates the nonlinear hysteresis phenomena using a prototyped MRF dual-clutch (MRFDC) for the transmission system of EVs, followed by a comprehensive analysis of three widely used hysteresis models: two parametric models, including the Bouc-Wen (BW) model and algebraic model (AM), and a non-parametric model, the NARX model. Accuracy, fitting time, and stack size are selected as the main indicators to evaluate the three models comprehensively. Results indicate that the NARX model has exceptional accuracy compared to the others, while it has a much higher memory requirement. The algebraic model shows a great advantage in computational efficiency because it has a straightforward expression. The BW model is in the middle position for all three indicators. To optimize the classic BW model (CBW), a fractional-order modified BW model (FOMBW) is proposed based on the polynomial input function and fractional-order derivatives. The proposed FOMBW model demonstrates superior capability in capturing asymmetric and rate-dependent characteristics compared to the CBW model. These findings provide the basis for choosing an appropriate model to effectively capture nonlinear current hysteresis phenomena within MRFDC with the requirement for precise torque control during gear shifting.

A two degree-of-freedom (2-DOF) galloping piezoelectric energy harvesting system where both oscillators are excited by the incoming flow is studied in this paper. A coupled lumped-parameter model is developed to simulate the electro-fluid-structural coupling behaviour assuming a quasi-steady aerodynamic flow field. The differential equations governing the dynamics of the lumped system are converted into nonlinear algebraic equations employing the harmonic balance method (HBM) and then solved using Newton's method. The approximate analytical solutions are compared to the solutions obtained by numerical integration, and the results agree, both qualitatively and quantitatively. The solutions were subsequently used to investigate the effects of the bluff body dimension, mechanical, electrical, aerodynamic, and electromechanical parameters on the performance of the harvester and to illustrate how to optimise some of these parameters to reduce the cut-in wind speed and maximise the power output of the energy harvester. It will be shown that the energy harvesting performance could be significantly improved by introducing piezoelectric transducers on both oscillators and including both masses in the incoming flow. A comparative study between the proposed 2-DOF flow energy harvesting system, a single-degree-of-freedom (SDOF) galloping piezoelectric energy harvester (GPEH), and other configurations of 2-DOF GPEH shows that the present 2-DOF GPEH generates the largest power peak.

Harvesting energy from mechanical vibration using piezoelectric material is a common method to power small electronics and batteries. Implementation of multiple piezo-patches to plate-like structures increases the power capacity and frequency bandwidth of the energy harvester since multiple patches can capture more vibrational modes of the dense plate dynamics. To optimize the harvested electrical output using advanced harvesting circuits, equivalent circuit modeling (ECM) is a useful technique for representing the entire electromechanical system in circuit- simulator-software (LTspice). The ECM technique based on finite-element (FE) simulation has been used for plate-like structures with only one single-piezo-patch-harvester in the literature. However, when multiple patches are integrated into a plate-like structure, their coupling can cause charge cancelation, resulting in complex dynamics that cannot be handled by the existing ECM methods. This paper presents three new methodologies for extracting a multi-mode ECM of the harvesters (e.g.: multi-patch piezo-harvesters integrated into a plate), using an admittance-based system identification approach utilizing FE results. The first method incorporates the ECM of multi-patch harvester into a single ECM, so called OGC. It includes a circuit branch where multiple patches have only one ECM branch for each vibration mode, which requires less equivalent circuit elements and hence less computational cost. The second and third so-called respective ground uncoupled (RGU) and respective ground coupled (RGC) ECM methods generate circuit branches for each piezo-patch and for each vibration mode, whereby their electrical connection (e.g. parallel or series) is later configured in LTspice. The RGU method provides a general multi-patch modeling approach while RGC method provides ECM parameters for each patch simultaneously when they are all connected which is the case in network harvester analysis. The ECMs of parallel multi-patch harvesters are validated by system-level FE simulations. The proposed admittance-based ECM methods are reliable for deriving the system parameters of multi-patch harvesters.

Functionally graded metamaterials represent a cutting-edge approach to designing structures with cellular materials. By manipulating parameters in specific regions, a customized mechanical response is achieved, optimizing material utilization. Despite various proposed methods to generate functionally graded structures, the challenge of high computational costs persists. This study introduces a novel and computationally efficient inverse design method for beams featuring segment-wise graded metamaterials. Using a semi-analytical approach based on Castigliano's second theorem, this method significantly reduces computational demands. The approach leverages prior experimental and computational characterizations of the transverse deflection in rectangular, reentrant, and hexagonal honeycombs. Validation through finite element models and experimental tests on additively manufactured beams confirms the adequate performance of the method. The proposed framework successfully generates beams with targeted deflections, demonstrating the method's capability for inverse design under specific loading and boundary conditions. This approach not only optimizes material utilization but also broadens the application scope of functionally graded metamaterials in structural design.

To pursue a variable-capacitance working principle, transducers based on soft electroactive polymers (EAPs) need deformable electrodes that match the compliance and stretchability of the EAP polymeric substrates. A variety of manufacturing procedures are available to create conductive materials that can achieve this, including solutions that can provide remarkably low resistivity. However, the simplest and most feasible options often involve the use of particle-filled (e.g. carbon-filled) polymer composites, which, while easy to produce, tend to exhibit relatively high resistivity. This high level of resistivity, combined with the inherent capacitance of EAP transducers, introduces dynamic effects in the devices electrical activation, which may affect performance. This paper investigates the impact of electrode resistivity on the electrical dynamics of EAP devices, combining continuum models and experimental validations. We use a continuum generalisation of known resistive-capacitive (RC) transmission line models to accurately predict voltage gradients on the surfaces of electrostatic transducers subject to rapidly varying voltages. We then present an experimental validation by measuring the spatial voltage distributions over carbon-based polymeric electrodes of dielectric elastomer (DE) transducers, and find a good agreement with our model predictions. We use our validated model to provide general estimates of the typical charging time and limit working frequency ranges of DE devices as a function of their dimensional scale and electrode sheet resistance. Our model provides useful indications for designing compliant electrodes in EAP transducers given target performance, or to understand the working limits of devices with given geometry and dielectric-electrode properties.