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.
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ISSN: 1361-665X
Smart Materials and Structures is a multi-disciplinary journal dedicated to technical advances in (and applications of) smart materials, systems and structures; including intelligent systems, sensing and actuation, adaptive structures, and active control.
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Mohsen Safaei et al 2019 Smart Mater. Struct. 28 113001
Elinor Barnett et al 2024 Smart Mater. Struct. 33 065010
Despite bone screws being the most commonly inserted implant in orthopaedic surgery, 10% of fracture fixation failure is a result of screw migration or pullout. In this study, the effect of four auxetic structures on the pullout performance of a novel unthreaded bone fastener was investigated through experiments and numerical simulations. The auxetic fasteners included the re-entrant, rotating squares, missing rib, and tetrachiral structures. Parametric CAD models were developed for each, and polymer samples manufactured using a stereolithography process. Pullout testing using bone analogue material found the rotating squares fastener to achieve superior pullout resistance 2.5 times that of the non-auxetic control sample. With a pullout to push-in force ratio of 33.7, this fastener achieved high pullout resistance with a low insertion force improving ease of installation. The Poisson's ratio of the structure was determined using image analysis to be −1.31, similar to the missing rib and re-entrant types. The low axial stiffness of 12.1 N mm−1 for the rotating squares fastener was the reason for superior performance, allowing axial and resulting transverse strain to be initiated at relatively low load. The effect of increased diametral interference was investigated, and the re-entrant structure found to be superior with pullout resistance improved by 342%. This work provides a foundation for further development of unthreaded auxetic bone fasteners, which have the potential to replace screws for some orthopaedic applications and significantly reduce the prevalence of pullout as a failure mode.
Amir Pagoli et al 2022 Smart Mater. Struct. 31 013001
Soft actuators can be classified into five categories: tendon-driven actuators, electroactive polymers, shape-memory materials, soft fluidic actuators (SFAs), and hybrid actuators. The characteristics and potential challenges of each class are explained at the beginning of this review. Furthermore, recent advances especially focusing on SFAs are illustrated. There are already some impressive SFA designs to be found in the literature, constituting a fundamental basis for design and inspiration. The goal of this review is to address the latest innovative designs for SFAs and their challenges and improvements with respect to previous generations, and to help researchers to select appropriate materials for their application. We suggest seven influential designs: pneumatic artificial muscle, PneuNet, continuum arm, universal granular gripper, origami soft structure, vacuum-actuated muscle-inspired pneumatic, and hydraulically amplified self-healing electrostatic. The hybrid design of SFAs for improved functionality and shape controllability is also considered. Modeling SFAs, based on previous research, can be classified into three main groups: analytical methods, numerical methods, and model-free methods. We demonstrate the latest advances and potential challenges in each category. Regarding the fact that the performance of soft actuators is dependent on material selection, we then focus on the behaviors and mechanical properties of the various types of silicone that can be found in the SFA literature. For a better comparison of the different constitutive models of silicone materials proposed and tested in the literature, ABAQUS software is here employed to generate the engineering and true strain-stress data from the constitutive models, and compare them with standard uniaxial tensile test data based on ASTM412. Although the figures presented show that in a small range of stress–strain data, most of these models can predict the material model acceptably, few of them predict it accurately for large strain-stress values. Sensor technology integrated into SFAs is also being developed, and has the potential to increase controllability and observability by detecting a wide variety of data such as curvature, tactile contacts, produced force, and pressure values.
Micheal Sakr and Ayan Sadhu 2024 Smart Mater. Struct. 33 033001
Digital twins (DTs) have witnessed a paramount increase in applications in multidisciplinary engineering systems. With advancements in structural health monitoring (SHM) methods and implementations, DT-based maintenance and operation stages have been implemented significantly during the life cycle of civil infrastructure. Recent literature has started laying the building blocks for incorporating the concept of DTs with SHM of large-scale civil infrastructure. This paper undertakes a systematic literature review of studies on DT-related applications for SHM of civil structures. It classifies the articles based on thematic case studies: transportation infrastructure (i.e. bridges, tunnels, roads, and pavements), buildings, off-shore marine infrastructure and wind turbines, and other civil engineering systems. The proposed review is further uniquely sub-classified using diverse modeling approaches such as building information modeling, finite element modeling, 3D representation, and surrogate and hybrid modeling used in DT implementations. This paper is solely focused on applications relating DTs to SHM practices for various civil engineering infrastructures, hence highlighting its novelty over previous reviews. Gaps and limitations emerging from the systematic review are presented, followed by articulating future research directions and key conclusions.
Parham Mostofizadeh et al 2024 Smart Mater. Struct. 33 065001
In this paper, surface conductive heating was utilized to actively control the stiffness of lattice metamaterials manufactured employing multi-material 3D printing. To create an electrical surface conduction, additively manufactured samples in single and dual material configurations were dip coated in a solution of carbon black in water. Electro-thermo-mechanical tests conducted successfully demonstrated that the low-cost conductive coating can be used to actively alter the stiffness of the structure through surface joule heating. The process was found to result in repeatable and reproduceable stiffness tuning. Stiffness reductions of 56% and 94% were demonstrated for single and dual material configurations under the same electrical loading. The proposed methodology can be implemented to actively control the properties of polymeric lattice materials/structures where the change in the composition of polymers (introduce bulk electrical conductivity) is difficult and can have a wide range of applications in soft robotics, shape-changing, and deployable structures.
Daniel Zabek et al 2021 Smart Mater. Struct. 30 035002
Mechanical vibrations from heavy machines, building structures, or the human body can be harvested and directly converted into electrical energy. In this paper, the potential to effectively harvest mechanical vibrations and locally generate electrical energy using a novel piezoelectric-rubber composite structure is explored. Piezoelectric lead zirconate titanate is bonded to silicone rubber to form a cylindrical composite-like energy harvesting device which has the potential to structurally dampen high acceleration forces and generate electrical power. The device was experimentally load tested and an advanced dynamic model was verified against experimental data. While an experimental output power of 57 μW cm−3 was obtained, the advanced model further optimises the device geometry. The proposed energy harvesting device generates sufficient electrical power for structural health monitoring and remote sensing applications, while also providing structural damping for low frequency mechanical vibrations.
P Narayanan et al 2024 Smart Mater. Struct. 33 043001
Hard-magnetic soft materials (hMSMs) are smart composites that consist of a mechanically soft polymer matrix impregnated with mechanically hard magnetic filler particles. This dual-phase composition renders them with exceptional magneto-mechanical properties that allow them to undergo large reversible deformations under the influence of external magnetic fields. Over the last decade, hMSMs have found extensive applications in soft robotics, adaptive structures, and biomedical devices. However, despite their widespread utility, they pose considerable challenges in fabrication and magneto-mechanical characterization owing to their multi-phase nature, miniature length scales, and nonlinear material behavior. Although noteworthy attempts have been made to understand their coupled nature, the rudimentary concepts of inter-phase interactions that give rise to their mechanical nonlinearity remain insufficiently understood, and this impedes their further advancements. This holistic review addresses these standalone concepts and bridges the gaps by providing a thorough examination of their myriad fabrication techniques, applications, and experimental, and modeling approaches. Specifically, the review presents a wide spectrum of fabrication techniques, ranging from traditional molding to cutting-edge four-dimensional printing, and their unbounded prospects in diverse fields of research. The review covers various modeling approaches, including continuum mechanical frameworks encompassing phenomenological and homogenization models, as well as microstructural models. Additionally, it addresses emerging techniques like machine learning-based modeling in the context of hMSMs. Finally, the expansive landscape of these promising material systems is provided for a better understanding and prospective research.
Amanda White et al 2024 Smart Mater. Struct. 33 055053
Inflatable structures, promising for future deep space exploration missions, are vulnerable to damage from micrometeoroid and orbital debris impacts. Polyvinylidene fluoride-trifluoroethylene (PVDF-trFE) is a flexible, biocompatible, and chemical-resistant material capable of detecting impact forces due to its piezoelectric properties. This study used a state-of-the-art material extrusion system that has been validated for in-space manufacturing, to facilitate fast-prototyping of consistent and uniform PVDF-trFE films. By systematically investigating ink synthesis, printer settings, and post-processing conditions, this research established a comprehensive understanding of the process-structure-property relationship of printed PVDF-trFE. Consequently, this study consistently achieved the printing of PVDF-trFE films with a thickness of around 40 µm, accompanied by an impressive piezoelectric coefficient of up to 25 pC N−1. Additionally, an all-printed dynamic force sensor, featuring a sensitivity of 1.18 V N−1, was produced by mix printing commercial electrically-conductive silver inks with the customized PVDF-trFE inks. This pioneering on-demand fabrication technique for PVDF-trFE films empowers future astronauts to design and manufacture piezoelectric sensors while in space, thereby significantly enhancing the affordability and sustainability of deep space exploration missions.
Xin Ren et al 2018 Smart Mater. Struct. 27 023001
Materials and structures with negative Poisson's ratio exhibit a counter-intuitive behaviour. Under uniaxial compression (tension), these materials and structures contract (expand) transversely. The materials and structures that possess this feature are also termed as 'auxetics'. Many desirable properties resulting from this uncommon behaviour are reported. These superior properties offer auxetics broad potential applications in the fields of smart filters, sensors, medical devices and protective equipment. However, there are still challenging problems which impede a wider application of auxetic materials. This review paper mainly focuses on the relationships among structures, materials, properties and applications of auxetic metamaterials and structures. The previous works of auxetics are extensively reviewed, including different auxetic cellular models, naturally observed auxetic behaviour, different desirable properties of auxetics, and potential applications. In particular, metallic auxetic materials and a methodology for generating 3D metallic auxetic materials are reviewed in details. Although most of the literature mentions that auxetic materials possess superior properties, very few types of auxetic materials have been fabricated and implemented for practical applications. Here, the challenges and future work on the topic of auxetics are also presented to inspire prospective research work. This review article covers the most recent progress of auxetic metamaterials and auxetic structures. More importantly, several drawbacks of auxetics are also presented to caution researchers in the future study.
Daniel Haid et al 2023 Smart Mater. Struct. 32 113001
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.
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Zheng Tian et al 2024 Smart Mater. Struct. 33 065031
Electrical and mechanical energy converts around the nature, and electromechanical coupling effect is applied in various conditions such as mechanical sensing, electrical actuation, and self-powering. During the energy type conversion, electromechanical parameters are among the key issues, such as enlarging the sensitivity and range of mechanical sensing, and energy harvesting efficiency. In this work, a mechanical manipulated approach with stretchable electret is proposed to continuously manipulate the electromechanical parameters. An electromechanical coupling demonstration with pre-stretched electret films and non-contact electrodes are applied, verifying high and regulable electromechanical coupling parameters, and it is advantaged from large deformable and overload permissible capability. This mechanical manipulation approach proposes a new possibility on simplifying the structural and mechanical design of various electromechanical devices, and further enhancing the general applicability with certain geometry and material with ultra-high and tunable electromechanical coupling parameters.
Yi Sun et al 2024 Smart Mater. Struct. 33 065032
Soft grippers exhibit good adaptability, but their grasping performance is limited. Variable-stiffness technology has been applied to soft grippers to address this problem. Therefore, a variable bending stiffness module (VBSM) with electrostatic layer jamming based on a giant electrorheological fluid (ELJ-GERF) for soft robots is proposed in this study, which exhibits a faster response time and a wider range of stiffness variation. A VBSM prototype is fabricated, and a theoretical model is established. The stiffness is mainly affected by the electrode quantity, overlapping area of electrode plates, insulator and conductive layers' thickness, medium thickness and the exciting voltage. Direct current (DC) voltage experiments and alternating current (AC) voltage experiments were conducted on the test samples of filled with air (ELJ-AIR), silicone oil (ELJ-OIL), and ELJ-GERF. The experimental result show that stiffness-regulation of the VBSM can be achieved by adjusting the exciting voltage, and AC voltage being more suitable for regulating the stiffness of the VBSM than DC voltage. For AC voltage, the stiffness of ELJ-GERF increases to 53.5 times when a 4 kV voltage is applied. The stiffness variation range is about 2 to 3 times greater than that of ELJ-AIR or ELJ-OIL. Through the stiffness characterization experiment, the stiffness of the VBSM in this study is influenced by the viscosity of the GERF and the gap between the electrode plates. Through the capacitance test, the VBSM exhibits self-sensing ability. Finally, the VBSM is applied to a soft gripper, the vibration performance and variable stiffness performance in its application are verified.
Yangyi Shi et al 2024 Smart Mater. Struct. 33 067002
This paper proposes a multi-band composite wearable antenna for wireless communication, which uses a monopole structure as the radiating body and achieves multi-band characteristics through slit-loading and multi-branching methods. A polymer composite substrate with high dielectric constant and low dielectric loss was prepared using in situ polymerization, and the optimal dielectric constant and loss angle tangent were obtained by controlling the coating ratio of melamine formaldehyde resin to carbon nanotubes (MWCNTs) and the filler doping rate to achieve miniaturization of the antenna. Comparative experimental results show that the obtained composites have high flexibility and good dielectric properties. The antenna operates in the frequency bands of 2.21–2.52 GHz, 3.07–3.87 GHz, and 4.36–6.03 GHz, which cover the frequency bands of WLAN and WiMAX and 5G applications. The antenna was fabricated and tested, and its performance roughly matched the simulation results. Meanwhile, the antenna has passed the SAR safety test and maintained a stable performance under different curvatures, so it has potential applications in the wireless communication system.
Xujin Liu et al 2024 Smart Mater. Struct. 33 065030
To enhance the performance of the hydraulic electric energy harvesting suspension, several steps were taken. Firstly, a resistance feedback control strategy was proposed in this paper based on the controllable damping characteristics. Subsequently, a model of the suspension was established based on its structure and working principle. And a simulation model was created, the tracking effect of current as well as the enhancement of suspension performance, were investigated to preliminarily validate the effectiveness of the control strategy. Finally, a bench test was conducted to verify the proposed strategy. Experimental results demonstrated that the peak harvesting can reach about300 W, and the average is 190.3 W. The vertical acceleration of the body in the hydraulic electric energy harvesting suspension decreased by 18.01% when employing the control strategy compared to the passive suspension.
Jin Dai et al 2024 Smart Mater. Struct. 33 065029
The development digital hydraulics demands higher performance on high-speed on/off valves. In order to fully exploit the energy saving advantages of digital hydraulics, advanced high-speed valves are expected to possess a fast response and a large nominal flow rate simultaneously. Energy-coupled-actuator (ECA) utilizes the shear working mode of magnetic rheological fluid to achieve reciprocating motion of the valve spool through the coupling/decoupling of a pair of disks and a translational piece and its driving force is not affected by the valve spool's position. The reported advantages of ECA meets the design requirements of actuators for high-speed on/off valve. This study gives the detailed design proposal of high-speed valve based on ECA (ECAV). The work also established a multi-physics coupled model for ECAV, calculated the key parameters of the valve driving system, and predicted the switching performance of ECAV. Finally, a prototype of ECAV with updated sealing solution between the actuator and valve block was fabricated and experimental tested. The results indicate that for current ECAV prototype successfully established 40 l min−1@5 bar (1.5 mm stroke) using response time less than 7 ms. Moreover, the prototype only consumed 14 ms to reach a long stroke of 5 mm with a significantly increased ratio of stroke over response time.
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Bouguermouh Karima et al 2024 Smart Mater. Struct. 33 063001
Four-dimensional (4D) printing has recently received much attention in the field of smart materials. It concerns using additive manufacturing to obtain geometries that can change shape under the effect of different stimuli. Such a technique enables the fabrication of 3D printed parts with the additional functionality of scalable, programmable, and controllable part shapes over time. This review provides a comprehensive examination of advances in the field of 4D printing, emphasizing the integration of fiber reinforcement and auxetic structures as crucial building blocks. The incorporation of fibers enhances structural integrity, while auxetic design principles contribute unique mechanical properties, such as negative Poisson's ratio and great potential for energy absorption due to their specific deformation mechanisms. Therefore, they present potential applications in aerospace, drones, and robotics. The objective of this review article is first to describe the distinctive properties of shape memory polymers, auxetic structures, and composite (fiber-reinforced) materials. A review of applications that use combinations of such materials is also presented when appropriate. The goal is to get a grip on the delicate balance between the different properties achievable in each case. The paper concludes by describing recent advances in 4D printing of fiber-reinforced auxetic structures.
Xuan Phu Do and Seung Bok Choi 2024 Smart Mater. Struct. 33 053001
In this review article, different structural types of the magnetic core required for activation of magnetorheological elastomer (MRE) and magnetorheological fluid (MRF) are introduced in terms of design feature, magnetic flux analysis and performance, installation with primary structure and close relationship to material types. As a first step, dynamic functions related to the chosen models are summarized and discussed according to the magnetic field variations including the field-dependent damping force and torque of the application systems. To address on the practical feasibility, main issues of design process are also pointed out and are discussed stating the manufacturing feasibility and the scaled factors of dynamic variables. Then, after analysing the featured models and dynamic functions, the derivation approaches to establish mathematical models of the magnetic circuit core (MCC) are provided and compared as a valuable reference for checking both simplicity and accuracy. In this stage, the chosen symbolized magnetic circuit models are clearly described about linear or/and nonlinear behaviours of the input (current) and output (magnetic field). In addition, a couple of commercial software to design the magnetic circuit model is introduced since they can be effectively adopted to analyse the MCCs of many application systems utilizing MRE and MRF without any difficulty.
Ravindra Masana et al 2024 Smart Mater. Struct. 33 043002
Structures inspired by the Kresling origami pattern have recently emerged as a foundation for building functional engineering systems with versatile characteristics that target niche applications spanning different technological fields. Their light weight, deployability, modularity, and customizability are a few of the key characteristics that continue to drive their implementation in robotics, aerospace structures, metamaterial and sensor design, switching, actuation, energy harvesting and absorption, and wireless communications, among many other examples. This work aims to perform a systematic review of the literature to assess the potential of the Kresling origami springs as a structural component for engineering design keeping three objectives in mind: (i) facilitating future research by summarizing and categorizing the current literature, (ii) identifying the current shortcomings and voids, and (iii) proposing directions for future research to fill those voids.
P Narayanan et al 2024 Smart Mater. Struct. 33 043001
Hard-magnetic soft materials (hMSMs) are smart composites that consist of a mechanically soft polymer matrix impregnated with mechanically hard magnetic filler particles. This dual-phase composition renders them with exceptional magneto-mechanical properties that allow them to undergo large reversible deformations under the influence of external magnetic fields. Over the last decade, hMSMs have found extensive applications in soft robotics, adaptive structures, and biomedical devices. However, despite their widespread utility, they pose considerable challenges in fabrication and magneto-mechanical characterization owing to their multi-phase nature, miniature length scales, and nonlinear material behavior. Although noteworthy attempts have been made to understand their coupled nature, the rudimentary concepts of inter-phase interactions that give rise to their mechanical nonlinearity remain insufficiently understood, and this impedes their further advancements. This holistic review addresses these standalone concepts and bridges the gaps by providing a thorough examination of their myriad fabrication techniques, applications, and experimental, and modeling approaches. Specifically, the review presents a wide spectrum of fabrication techniques, ranging from traditional molding to cutting-edge four-dimensional printing, and their unbounded prospects in diverse fields of research. The review covers various modeling approaches, including continuum mechanical frameworks encompassing phenomenological and homogenization models, as well as microstructural models. Additionally, it addresses emerging techniques like machine learning-based modeling in the context of hMSMs. Finally, the expansive landscape of these promising material systems is provided for a better understanding and prospective research.
Xianxu 'Frank' Bai et al 2024 Smart Mater. Struct. 33 033002
In the last two decades, magnetorheological (MR) fluids have attracted extensive attention since they can rapidly and continuously control their rheological characteristics by adjusting an external magnetic field. Because of this feature, MR fluids have been applied to various engineering systems. This paper specifically investigates the application of MR fluids in shock mitigation control systems from the aspects of three key technical components: the basic structural design of MR fluid-based energy absorbers (MREAs), the analytical and dynamical model of MREAs, and the control method of adaptive MR shock mitigation control systems. The current status of MR technology in shock mitigation control is presented and analyzed. Firstly, the fundamental mechanical analysis of MREAs is carried out, followed by the introduction of typical MREA configurations. Based on mechanical analysis of MREAs, the structural optimization of MREAs used in shock mitigation control is discussed. The optimization methods are given from perspectives of the design of piston structures, the layout of electromagnetic coil, and the MR fluid gap. Secondly, the methods of damper modeling for MREAs are presented with and without consideration of the inertia effect. Then both the modeling methods and their characteristics are introduced for representative parametric dynamic models, semi-empirical dynamic models, and non-parametric dynamic models. Finally, the control objectives and requirements of the shock mitigation control systems are analyzed, and the current competitive methods for the ideal 'soft-landing' control objectives are reviewed. The typical control methods of MR shock mitigation control systems are discussed, and based on this the evaluation indicators of the control performance are summarized.
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Gonzalez et al
Rehabilitation is crucial for children with physical disabilities arising from various conditions. Traditional exoskeletons, reliant on electric motors and rigid components, have limitations. To overcome these, researchers are turning to soft wearable rehabilitation robots (SWRRs) with artificial muscles based on smart materials like twisted and coiled polymer actuators (TCPs). TCPs offer enhanced compliance, adaptability, comfort, safety, and reduced weight—critical for paediatric use. Despite facing challenges like low operating frequencies and high temperatures, TCPs are explored as potential artificial muscles for SWRRs, due to their advantages on the force they can generate, the strain and a linear behaviour. This study details a proof of concept for a paediatric rehabilitation system for ankles based on TCPs, including the actuator characterization, mechanical design, control strategy, and human-computer-interface (HCI). The resulting device achieved a 14 Nm torque, a 10° range of motion in dorsiflexion within 5 seconds, and integrated electromyographic HCI. This research marks a promising step towards innovative, soft wearable rehabilitation solutions for children with physical disabilities.
White et al
This work presents an investigation into the energy harvesting performance of a combination of PTFE and PVDF materials prepared using a one-step electrospinning technique. Before electrospinning, different percentages of the 1-micron PTFE powder were added to a PVDF precursor. The surface morphology of the electrospun PTFE/PVDF fibre was investigated using a scanning electron microscope (SEM) and tunnelling electron microscope (TEM). The structure was investigated using Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction analysis (XRD). A highly porous structure was observed with a mix of the α- and β-phase PVDF. The amount of β-phase was found to reduce when increasing the percentage of PTFE. The maximum amount of PTFE that could be added and still be successfully electrospun was 20%. This percentage showed the highest energy harvesting performance of the different PTFE/PVDF combinations. Electrospun fibres with different percentages of PTFE were deployed in a triboelectric energy harvester operating in the contact separation mode and the open circuit voltage and short circuit current were obtained at frequencies of 4 to 9 Hz. The 20% PTFE fibre showed 4 (51 to 202 V) and 7 times (1.3 to 9.04 µA) the voltage and current output respectively when compared with the 100% PVDF fibre. The Voc and Isc were measured for different load resistances from 1kΩ to 6GΩ and achieved a maximum power density of 348.5 mW/m2 with a 10 MΩ resistance. The energy stored in capacitors 0.1, 0.47, 1, and 10 µF from a book shaped PTFE/PVDF energy harvester were 1.0, 16.7, 41.2 and 136.8 µJ, respectively. The electrospun fibre is compatible with wearable and e-textile applications as it is breathable and flexible. The electrospun PTFE/PVDF was assembled into shoe insoles to demonstrate energy harvesting performance in a practical application.
Liao et al
Long Short-Time Memory (LSTM) deep neural networks are capable of learning order dependence in sequence problems and capturing long-term, non-linear temporal dependencies between the input and out of a system. With the long-term vision to model dynamical systems to which analytical or numerical methods are impossible or difficult to apply, this paper presents a study of modeling system dynamics and predicting responses using the LSTM networks, which have demonstrated excellent capability in predicting single-mode responses in a prior study. However, the LSTM network exhibits difficulties in modeling and predicting multi-mode responses accurately. To resolve the multi-mode issue, this paper presents an approach that obtains an equivalent network consisting of a set of sub-networks learned on isolated modes, and demonstrates its effectiveness on a simulated 2-degree-of-freedom (2DOF) mass-spring-damper system of nonlinear Duffing springs. The second part of the paper is focused on the application of the proposed approach in piezoelectric energy harvesting. Experiments are conducted on a harvester subjected to random base-motion excitation and exhibiting nonlinearity in its multi-mode response. Both the direct and mode-separation LSTM modeling approaches are applied to predict the output voltage given a random base-motion excitation. The mode-separation approach outperforms the direct approach significantly, and yields an excellent match between the actual and predicted responses. Specifically, for a test electrical voltage response of RMS value 0.2241 V, the difference between the actual test and predicted responses by using the mode-separation approach has an RMS value of 0.0504 V, compared to 0.1645 V obtained by using the direct LSTM approach. It is also much lower than the RMS value of 0.1835 V obtained by using the attention-based LSTM network, another comparison method. Leveraging a deep learning strategy, the validated approach opens up opportunities for accurately modeling energy harvesting systems of high complexities and/or strong nonlinearities.
Chen et al
Auxetic metamaterials have shown good stability and uniform deformation capabilities against influences (vibration, temperature change, load change), making them significant in maintaining and adjusting the bandgap of phonon crystals. The low-frequency and broadband are the important goals of phonon crystals. For most traditional re-entrant honeycomb structures (T-RHS), the bandgap range is narrow and tunability is poor. Here, an auxetic hybrid structure with tunable acoustic bandgap (AHS-T) consisting of periodic mass inclusions integrated with traditional re-entrant honeycomb and chiral hybrid is proposed. Aiming at investigating the tunability of the bandgap in the low-frequency range. Compared with T-RHS, the bandgap real-time adjustment and wider bandwidth of the AHS-T be realized during compression and tension. The numerical results show that the bandgap of the AHS-T can be flexibly tailored by reasonably adjusting the strain and geometrical configurations of AHS-T. The bandwidth of AHS-T can be increased to 87.1% when the bottom diameter and column height H of the scatterer are changed reasonably. Moreover, the deformation behavior of auxetic material has an auxiliary effect on expanding the bandwidth. Compared with the structure which is not subjected to load, the adjustable amplitude of the bandgap is 41%. The findings of this work provide a design idea for manipulating elastic waves in dynamic environment.
Liu et al
As the integral constituent of atomic force microscope (AFM), Piezoelectric micro-positioning platform (PMP) plays an pivotal role in AFM working accuracy. However, the PMP platform has hysteretic nonlinear characteristics, which bring challenges to high-precision positioning applications, especially in large travel applications. In this paper, the nonlinear P-I model and the linear ARX dynamic model are cascaded to form the
Hammerstein model to characterize the dynamic characteristics of PMP, and the mixed algorithm of the beetle antennae search-differential evolution (BAS-DE) is designed to identify the parameters of the established model. In order to eliminate the hysteresis effect, a compound controller based on adaptive inverse compensation is proposed, which is composed of feedforward controller of P-I inverse model and MPC feedback controller. As the compound controller depends on modeling accuracy, the tracking error caused by model mismatch is improved by adaptive mechanism. The experimental tracking results of sinusoidal signals and triangular signals of different frequencies show that the proposed method can improve the tracking performance of PMP and verify its effectiveness
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Alberto Gonzalez et al 2024 Smart Mater. Struct.
Rehabilitation is crucial for children with physical disabilities arising from various conditions. Traditional exoskeletons, reliant on electric motors and rigid components, have limitations. To overcome these, researchers are turning to soft wearable rehabilitation robots (SWRRs) with artificial muscles based on smart materials like twisted and coiled polymer actuators (TCPs). TCPs offer enhanced compliance, adaptability, comfort, safety, and reduced weight—critical for paediatric use. Despite facing challenges like low operating frequencies and high temperatures, TCPs are explored as potential artificial muscles for SWRRs, due to their advantages on the force they can generate, the strain and a linear behaviour. This study details a proof of concept for a paediatric rehabilitation system for ankles based on TCPs, including the actuator characterization, mechanical design, control strategy, and human-computer-interface (HCI). The resulting device achieved a 14 Nm torque, a 10° range of motion in dorsiflexion within 5 seconds, and integrated electromyographic HCI. This research marks a promising step towards innovative, soft wearable rehabilitation solutions for children with physical disabilities.
Pattarinee White et al 2024 Smart Mater. Struct.
This work presents an investigation into the energy harvesting performance of a combination of PTFE and PVDF materials prepared using a one-step electrospinning technique. Before electrospinning, different percentages of the 1-micron PTFE powder were added to a PVDF precursor. The surface morphology of the electrospun PTFE/PVDF fibre was investigated using a scanning electron microscope (SEM) and tunnelling electron microscope (TEM). The structure was investigated using Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction analysis (XRD). A highly porous structure was observed with a mix of the α- and β-phase PVDF. The amount of β-phase was found to reduce when increasing the percentage of PTFE. The maximum amount of PTFE that could be added and still be successfully electrospun was 20%. This percentage showed the highest energy harvesting performance of the different PTFE/PVDF combinations. Electrospun fibres with different percentages of PTFE were deployed in a triboelectric energy harvester operating in the contact separation mode and the open circuit voltage and short circuit current were obtained at frequencies of 4 to 9 Hz. The 20% PTFE fibre showed 4 (51 to 202 V) and 7 times (1.3 to 9.04 µA) the voltage and current output respectively when compared with the 100% PVDF fibre. The Voc and Isc were measured for different load resistances from 1kΩ to 6GΩ and achieved a maximum power density of 348.5 mW/m2 with a 10 MΩ resistance. The energy stored in capacitors 0.1, 0.47, 1, and 10 µF from a book shaped PTFE/PVDF energy harvester were 1.0, 16.7, 41.2 and 136.8 µJ, respectively. The electrospun fibre is compatible with wearable and e-textile applications as it is breathable and flexible. The electrospun PTFE/PVDF was assembled into shoe insoles to demonstrate energy harvesting performance in a practical application.
Christian Heinrich et al 2024 Smart Mater. Struct.
The transformation of metastable austenite to martensite under mechanical loading can be harnessed to create a material sensor which records a measure of the load history without the need for electrical energy and can be read out at arbitrary intervals via eddy current probing, thus leading to an ultra-low-power sensing solution.
This paper presents possibilities of processing this load amplitude-dependent evolution of martensite content loading for component fatigue analysis. The general method is based on using a theoretical material model typically used in FEM analyses which includes hardening plasticity and phase transformation to precompute tables of stress amplitude or cumulative damage corresponding to different sensor readings which can be stored on a low power processing system onboard the component for energy-efficient lookup. 
At nominal single amplitude loading, the sensor can be used as a load cycle counter for known loads or as an overload detection device upon divergent martensite content rise. Interpretation of block program loading is less practical due to resolution issues. Under random loading, sequence effects get averaged out; interpretation is easiest with narrow load spectra, but information can be gained from very wide spectra as well. Multiple sensors at different locations can aid interpretation. Uncertainty due to necessary assumptions and untreated influences of temperature and loading rate is discussed.
Luke Benjamin Demo et al 2024 Smart Mater. Struct.
In recent years, there has been growing interest in self-sensing structural materials across research and industry sectors. Detecting and locating structural damage typically requires numerous sensors wired to a data acquisition (DAQ) circuit, rendering implementation impractical in real structures. This paper proposes an innovative, cost-effective sensor network for damage detection and localization in fiber-reinforced polymer composites. The innovation encompasses three key elements: (1) utilizing carbon fiber tows within the composite as piezoresistive sensors, eliminating the need for additional foreign sensor devices; (2) introducing a novel sensor layout wherein sensor tow branches with varied resistance values are connected in parallel, reducing the number of connections to the DAQ circuit and cutting manufacturing costs significantly; (3) developing a practical sensor terminal fabrication technique to minimize manufacturing expenses. The proposed design methodology for the branch resistance values is first validated using a demonstration panel. Subsequently, the overall strategy is assessed by conducting impact tests on carbon and glass fiber-reinforced composite specimens. Results validate the sensor's ability to accurately detect and locate structural damage.
Diego Di Brizzi et al 2024 Smart Mater. Struct.
Auxetics are a class of materials and metamaterials with a negative Poisson's ratio (ν) and have gained tremendous popularity over the last three decades. Many studies have focused on characterizing designs that allow obtaining a negative ν. However, some open issues remain concerning understanding the auxetic behavior in operational conditions. Studies have been centered on analyzing the response of specific auxetic topologies instead of treating auxeticity as a property to be analyzed in a well-defined structural context. This study aims to contribute to the investigation of auxetic materials with a structural application, focusing on maximizing performance. The field of application of auxetics for designing inserts was selected and a model of a nail-cavity system was created to determine the effects of different design choices on the system behavior by exploring relationships between selected parameters and the auxetic insert behavior. The exploration combines finite element modeling analyses with their surrogate models generated by supervised learning algorithms. This approach allows for exploring the system's behavior in detail, thus demonstrating the potential effectiveness of auxetics when used for such applications. A list of design guidelines is elaborated to support the exploitation of auxetics in nail-cavity systems.
Huanpeng Hong et al 2024 Smart Mater. Struct.
Iron-based shape memory alloys (FeSMA) are emerging as promising materials for use in post-tensioning concrete structures to provide self-centering capabilities during a seismic event. Past experimental studies on FeSMA focused on strengthening or repairing existing structural components. In addition to the structural rehabilitation for gravity loading, FeSMA also have potential for use in self-centering columns subjected to seismic loads. However, the basic material properties, such as strength, ductility, recovery strain, actuation stress (i.e., prestress) stability, low-cycle fatigue resistance, and temperature dependence of FeSMA related to self-centering column applications have not been studied so far. To fill this knowledge gap and determine the feasibility of using FeSMA in self-centering columns, this study performed a comprehensive characterization and analysis of FeSMA both before and after actuation (i.e., thermal stimulation). The strength, ductility, and recovery strain of FeSMA before actuation were tested at different temperatures from -40℃ to 50℃. After actuation, the actuation stress, low-cycle fatigue resistance, and strain capacity of FeSMA were tested at different temperatures from -40℃ to 50℃, prestrain levels from 4% to 30%, and under low-cycle fatigue loading with strain amplitudes from 0.5% to 1.0%. The results from this study demonstrated that FeSMA exhibit high ductility, cyclic actuation stress stability, and low-cycle fatigue resistance at temperatures from -40℃ to 50℃. Furthermore, it was found that increasing the prestrain level can effectively increase the post-actuation strain amplitude at which the actuation stress reduces to zero. A prestrain level between 15% and 20% is recommended for application of FeSMA in self-centering columns. The research findings from this study demonstrated the feasibility of using FeSMA in self-centering columns subject to seismic loading.
Joshua Knospler et al 2024 Smart Mater. Struct.
Soft robots have revolutionized machine interactions with humans and the environment to enable safe operations. The fixed morphology of these soft robots dictates their mechanical performance, including strength and stiffness, which limits their task range and applications. Proposed here are modular, reconfigurable soft robots with the capabilities of changing their morphology and adjusting their stiffness to perform versatile object handling and planar or spatial operational tasks. The reconfiguration and tunable interconnectivity between the elemental soft, pneumatically driven actuation units is made possible through integrated permanent magnets with coils (PMC). The proposed concept of attaching/detaching actuators enables these robots to be easily rearranged in various configurations to change the morphology of the system. While the potential for these actuators allows for arbitrary reconfiguration through parallel or serial connection on their four sides, we demonstrate here a configuration called ManusBot. ManusBot is a hand-like structure with digits and palm capable of individual actuation. The capabilities of this system are demonstrated through specific examples of stiffness modulation, variable payload capacity, and structure forming for enhanced and versatile object manipulation and operations. The proposed modular, soft robotic system with interconnecting capabilities significantly expands the versatility of operational tasks as well as the adaptability of handling objects of various shapes, sizes, and weights using a single system.
Pedro M. Ferreira et al 2024 Smart Mater. Struct.
In the field of structural engineering, the integration of smart materials and structural health monitoring (SHM) has given rise to self-sensing materials (SSM), leading to a paradigm shift in SHM. This paper focuses on the interplay between self-sensing capabilities and the piezoelectric properties of lead zirconate titanate (PZT) and barium titanate (BT) in aluminium components. Leveraging Friction Stir Processing (FSP), the study explores the synthesis and performance of self-sensing materials with embedded piezoelectric particles, potentially transforming structural engineering. The paper highlights FSP as a key methodology for incorporating piezoelectric particles into structural materials, showcasing its potential in developing self-sensing materials with enhanced functionalities. A specific focus is placed on integrating PZT and BT particles into AA2017-T451 aluminium parts using FSP, with metallographic assessments and mechanical property evaluations conducted to analyse particle distribution and concentration. This study shows how BT and PZT particles are incorporated into AA2017-T451 aluminium to create a self-sensing material that responds to external stimuli. Under cyclic loading, the self-sensing materials exhibit a linear load-electrical response correlation, with sensibility increasing at lower frequencies. Metallographic analysis shows homogeneous particle distribution, while PZT induces increased brittleness and brittle fractures. Yield strength remains relatively stable, but ultimate strength decreases post-FSP. Hardness variations indicate weaker bonding with PZT particles. Eddy's current testing aligns with hardness profiles, and sensorial characterization reveals a non-linear frequency-sensibility relationship, showcasing the SSMs' suitability for low-frequency applications, particularly with PZT embedment.
Mahdi Alaei Varnosfaderani et al 2024 Smart Mater. Struct. 33 065019
Inspired by the bending vibration observed in the biological locomotions such as those found in snakes, horned lizards, and sandfish, we have developed a novel vibro probe utilizing bending resonance modes to study the bending vibration effects in assisting penetration into granular materials. This approach contrasts with traditional probes that rely on longitudinal vibrations for penetration. This newly developed probe was used to experimentally investigate the impact of bending vibration in reducing the required penetration force and enhancing the penetration process within granular materials such as lunar or Martian regolith. The bending vibrations were excited by thin piezo patches attached to the probe's machined surface without increasing the probe's outside diameter. This simple mechanism enables pushing the whole probe inside the granular materials. Experimental modal analysis was employed to determine the resonance frequencies of the probe. Subsequently, the probe was pushed into granular materials, both with and without the bending vibrations, by a linear actuator. Experimental results indicated that employing bending vibration in one direction led to a reduction in penetration force by up to 27% while utilizing two directions resulted in a reduction of up to 42%. Additionally, when the probe stopped penetrating the soil due to insufficient axial force, bi-directional bending vibration proved more effective in swiftly fluidizing the surrounding soil. These findings highlight the efficacy of bending vibrations in compact subsurface drilling tools.
Seiki Chiba et al 2024 Smart Mater. Struct. 33 065016
Actuators, sensors, and generators using dielectric elastomers (DEs) are inexpensive and light, and can be easily to structured, multilayer-able, and very efficient. They are ideal for an eco-energy society. In the latest technology, an only 0.15 g DE can lift an 8 kg weight by 1 mm or more in just 88 ms. The near future, it can be applied to efficient drive systems of humanoid robots, systems that assist in driving the motors of electric vehicles, and various industrial machinery. It is highly likely that very thin and miniaturized DE sensors would also support the driving of motors. In addition, DE generators, which can be applied to various external forces, have attracted significant attention as a renewable energy source. In this paper, we discuss the R&D status of DEs using mainly commercially available elastomer materials, give examples of issues, and discuss and their potential applications, and usefulness. The excellent performance of the DEs mentioned above is largely due to their carbon-based electrodes. In this study, various carbon materials (including carbon grease, carbon black, MWCNT, and SWCNT) and their DE performances were compared.