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.
Purpose-led Publishing is a coalition of three not-for-profit publishers in the field of physical sciences: AIP Publishing, the American Physical Society and IOP Publishing.
Together, as publishers that will always put purpose above profit, we have defined a set of industry standards that underpin high-quality, ethical scholarly communications.
We are proudly declaring that science is our only shareholder.
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.
Open all abstracts, in this tab
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.
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.
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.
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.
Iman Valizadeh and Oliver Weeger 2024 Smart Mater. Struct. 33 065006
A major benefit of additive manufacturing technologies is precise control over structural topologies and material properties, which allows to tailor, for instance, energy absorption and dissipation. While vat photopolymerization is generally restricted to a single material, grayscale masked stereolithography (gMSLA) allows to customize material behavior by grading the light intensity within a structure. This study investigates the impact and opportunities of grayscale grading strategies on the rate-dependent mechanical behavior of structures fabricated by gMSLA. Considering the viscoelastic nature of polymers, rate-dependent energy dissipation is explored, introducing a parametric linear viscoelastic constitutive model for varying grayscales. The investigation includes the comprehensive characterization of mechanical properties, numerical finite element simulation, validation through experimental procedures, and exploration of dissipation energy under different strain rates. In this way, a rational function successfully determines the critical strain rate at which the maximum dissipation occurs. Overall, the research offers a comprehensive investigation of the mechanical dissipation behavior of graded 3D printed structures, laying the foundation for further studies and advancements aimed at optimizing these structures for enhanced energy absorption capabilities.
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.
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.
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.
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.
Open all abstracts, in this tab
Avadesh Yadav et al 2024 Smart Mater. Struct. 33 065044
Shape memory polymers are utilized in diverse fields, including deployable structures and components, owing to their advantageous properties like low density, cost-effectiveness, and ease of processing. The current study investigates the effect of dual dispersion of carbon fibers (CF) and silica nanoparticles (SN) on the recovery performance of shape memory epoxy composites. CF (0.25 wt.%) reinforced epoxy composite, SN (0.25 wt.%) reinforced epoxy composite, and dual (CF and SN 0.25 wt.% each) dispersed epoxy composite were developed using magnetic stirring and ultrasonic mixing to study the mechanical properties and shape memory behaviour. Flexural strength, tensile strength and fracture toughness of pristine epoxy was found to be 134.38, 52.15 MPa and 1.91 MPam1/2, respectively. The addition of CF resulted in a flexural strength of 135.70 MPa and a tensile strength of 53.01 MPa, while the incorporation of SN led to a flexural and tensile strength of 138.40 and 53.69 MPa, respectively. The fracture toughness of composites after adding CF and SN was found to be 2.22 and 2.39 MPam1/2, respectively. With dual dispersion, the flexural strength of 139.82 MPa, tensile strength of 54.64 MPa, and fracture toughness of 2.75 MPam1/2 were achieved. Dual dispersion has shown improved mechanical properties compared to single dispersion. The introduction of CF slightly decreased the shape fixity ratio (Rf) and shape recovery ratio (Rr) of the pristine epoxy from 98.67% and 96.62% to 96.67% and 95.15%, respectively. Similarly, the addition of SN further reduced these ratios to 98.00% and 91.83%, respectively. With a dual dispersion approach Rf and Rr were observed to be about 97.33% and 93.83%, respectively. The addition of fillers led to a reduction in Rf and Rr due to inhibition of polymeric chains, resulting in partial shape recovery. However, recovery time improved from 28 to 23 and 26 s with addition of CF and SN, respectively, in epoxy. With dual dispersion, a speedy recovery was achieved with a recovery time of 21 s. The findings of this study demonstrate the potential of dual dispersed fillers to improve the mechanical and shape memory properties of epoxy, which could find applications in the smart materials and structural engineering.
Lan Jiang et al 2024 Smart Mater. Struct. 33 065043
As urban traffic environments continue to grow in complexity, there is an urgent need for a versatile mode of transportation that seamlessly transitions between terrestrial and aerial mobility. In conventional magnetorheological damper (CMRD), the magnetorheological fluid flowing through the narrow annular gap between the piston and cylinder in CMRD results in a damping force directly proportional to velocity. As velocity increases, the damping force rises sharply, posing a significant risk to the vehicle's mechanical structure and passenger safety. This velocity sensitivity restricts their applications primarily to standard commercial vehicle suspension systems. They face significant challenges when it comes to high-speed impact scenarios. To overcome this limitation, enhance the shock-absorbing capacity of flying cars, ensure passenger safety, and improve passenger comfort during the landing phase, this study introduces a novel magnetorheological damper (NMRD) with unique internal channel structure embedded in a circular permanent magnet. In road travel mode, NMRD maintains a wide dynamic range. During high-speed impact landing, when the impact force exceeds the threshold, the pressure relief channel opens, effectively reducing the peak impact force. This feature greatly expands the application range of magnetorheological dampers. The researches included simulations of the electromagnetic induction phenomenon within the piston, The pressure relief damping force inside the NMRD valve was accurately measured by using material testing system, the peak force and peak acceleration experienced by the two dampers during impact were tested using a dedicated drop hammer apparatus. These tests demonstrate that the NMRD exhibits superior impact resistance performance compared to CMRD. This highlights the promising potential for the NMRD's application within the suspension systems of flying cars.
Luke B Demo et al 2024 Smart Mater. Struct. 33 065042
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.
Jiaxi Jin et al 2024 Smart Mater. Struct. 33 065045
This paper addresses the modeling of dynamic hysteresis and creep in piezoelectric actuators, and employs feedforward open-loop control based on inverse models to compensate for hysteresis and creep phenomena. The comprehensive model consists of quasi-dynamic and dynamic components. The quasi-dynamic model combines the quasi-dynamic Prandtl–Ishlinskii (PI) model with an PI-based linear time-invariant model, while the dynamic part utilizes the auto-regressive exogenous model. The model accurately describes creep and dynamic hysteresis with modeling errors of less than 0.01 μm and 0.14 μm, respectively. The inversion of the comprehensive model has been proven to exhibit unique convergence. Under inverse feedforward control, the improvement in dynamic hysteresis and hysteresis with creep can be achieved at 94% and 83%, respectively. The comprehensive model proposed in this paper accurately describes the dynamic hysteresis and creep phenomena in piezoelectric actuators and realizes open-loop compensation control, achieving precise actuation of piezoelectric actuators.
Zhang Li et al 2024 Smart Mater. Struct. 33 063002
Traditional robots with constant stiffness demonstrate reliable output power and positioning precision, which may conversely reduce their flexibility and adaptability or even incur greater damage for accidental collisions with the environment or humans. Here, we review state-of-the-art robots with a variable stiffness mechanism, which is a key design concept that is widely used to improve robot reliability and impart new functionalities. To determine the similarities and differences between variable stiffness methods, we discuss the existing principles for variable stiffness of both rigid and soft robots, such as coupled and uncouple structures, thermal stimuli and magneto-rheological approaches. We hope this paper can help readers better understand these methods with regard to interesting applications. In addition, we also outline challenges and perspectives, where a simpler structure, larger band and faster response of stiffness modulation are required for robots in the future.
Open all abstracts, in this tab
Zhang Li et al 2024 Smart Mater. Struct. 33 063002
Traditional robots with constant stiffness demonstrate reliable output power and positioning precision, which may conversely reduce their flexibility and adaptability or even incur greater damage for accidental collisions with the environment or humans. Here, we review state-of-the-art robots with a variable stiffness mechanism, which is a key design concept that is widely used to improve robot reliability and impart new functionalities. To determine the similarities and differences between variable stiffness methods, we discuss the existing principles for variable stiffness of both rigid and soft robots, such as coupled and uncouple structures, thermal stimuli and magneto-rheological approaches. We hope this paper can help readers better understand these methods with regard to interesting applications. In addition, we also outline challenges and perspectives, where a simpler structure, larger band and faster response of stiffness modulation are required for robots in the future.
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.
Open all abstracts, in this tab
Hua et al
As a new type of medical equipment, capsule robots are actuated wirelessly by space magnetic field, which have important application advantages in the diagnosis and treatment of gastrointestinal diseases. Active locomotion is the basis of medical operation for capsule robots, as well as an important guarantee to avoid misdetection and retention in the body. Furthermore, the pose estimation of the capsule robots in the gastrointestinal tract can provide accurate information for medical operation and improve work efficiency. Specific medical operation is one of the ultimate goals of capsule robots, and it is the key to realize the non-invasive diagnosis and treatment technology. Moreover, replacing traditional chemical batteries with wireless power transfer technology not only reduces the dimensions of the capsule robots, but also provides unlimited possibilities for the development of medical operations. In this work, the state-of-the-art capsule robots are reviewed according to the research directions of the locomotion, pose, medical operation and wireless power transmission reported from 2018 to 2023. In light of the four main directions of the capsule robots, some important research achievements and approaches are summarized. In particular, some outstanding advances on innovative structure, efficient methodology and appropriate application of the capsule robots are introduced in details. Finally, an overview of the significant issues occurred in the capsule robots is reported, and the developing trends are discussed.
Rodriguez-Morales et al
Four-dimensional (4D) printing has emerged as a branch of Additive Manufacturing (AM) that utilizes stimuli-responsive materials to generate three-dimensional (3D) structures with functional features. In this context, constitutive models play a paramount role in designing engineering structures and devices using 4D printing, as they help understand mechanical behavior and material responses to external stimuli, providing a theoretical framework for predicting and analyzing their deformation and shape-shifting capabilities. This article thoroughly discusses available constitutive models for single-printed and multi-printed materials. Later, we explore the role of machine learning algorithms in inferring constitutive relations, particularly in viscoelastic problems and, more recently, in shape memory polymers. Moreover, challenges and opportunities presented by both approaches for predicting the mechanical behavior of 4D printed polymer materials are examined. Finally, we concluded our discussion with a summary and some future perspectives expected in this field. This review aims to open a dialogue among the mechanics community to assess the limitations of analytical models and encourage the responsible use of emerging techniques, such as machine learning. By clarifying these aspects, we intend to advance the understanding and application of constitutive models in the rapidly growing field of 4D printing.
Chen et al
This paper proposes an inertia-driven resonant piezoelectric stack pump based on a flexible support structure to solve the problem that the piezoelectric stack cannot effectively drive the diaphragm pump to transport liquid due to too small output displacement and too high resonant frequency when one end is fixed. Under the inertial force generated by the vibration of the piezoelectric stack's mass center during its deformation, the whole piezoelectric stack will vibrate with the flexible support structure; and a large displacement and inertial force can be achieved to drive the pump at the resonant frequency. Piezoelectric pumps are designed with a diaphragm pump and a piezoelectric stack based on the flexible support structure. The piezoelectric vibrator includes a piezoelectric stack, a preloading component and a flexible support plate. A fixed support plate and three flexible support plates with different stiffnesses were fabricated and assembled with the same piezoelectric stack and diaphragm pump respectively to construct four piezoelectric pump prototypes with different resonant frequencies. The temperature rise characteristics of the piezoelectric stack were experimentally studied to determine the safe range of the driving voltage and frequency. Then the output performances of the piezoelectric pumps were tested. Under a sinusoidal driving voltage of 100 Vpp, the piezoelectric pump based on the fixed support structure cannot pump water, while the piezoelectric pumps based on the flexible support structure achieved the maximum flow rates of 89.0 ml/min, 123.4 ml/min and 197.4 ml/min at the resonant frequencies of 262 Hz, 297 Hz and 354 Hz, and the maximum backpressures of 4.4 kPa, 7.5 kPa and 11.0 kPa at 266 Hz, 309 Hz and 365 Hz.
Yang et al
2D braided shape memory composite (SMPC) tubes, with near-net shape manufacturing and programmable, are widely utilized in smart structures. Here we have developed braided tubes of continuous carbon fiber reinforced shape memory polyurethane (SMPU) composites. This innovative design yields a synergistic boost in both mechanical strength, shape memory functionality, and dual-trigger responsiveness. The mechanical properties, electrical/thermal shape memory performance, and recovery force of the SMPC tubes with various braiding angles have been investigated. The effects of braiding angle, temperature dependence, and applied current on the mechanical properties and shape memory properties were revealed. We found a substantial increase in compression load and ring stiffness as the braiding angle increased and the temperature decreased. The SMPC tubes exhibited a recovery ratio of 99% under electrical and thermal triggering, demonstrating a more rapid shape recovery compared to the SMPU tubes solely under thermal triggering. The large-angle specimens exhibited shorter recovery times, higher recovery forces (up to 11.40 N), and faster responses upon electrical stimulation. The ability of SMPC tubes to generate a recovery force several times greater than their weight holds great potential for expanding the applications of smart actuators.
Chen et al
This comprehensive review explores the design and applications of machine learning techniques to acoustic metamaterials (AMs) and phononic crystals (PnCs), with a particular focus on deep learning. AMs and PnCs, characterized by artificially designed microstructures and geometries, offer unique acoustic properties for precise control and manipulation of sound waves. Machine learning, including deep learning, in combination with traditional artificial design have promoted the design process, enabling data-driven approaches for feature identification, design optimization, and intelligent parameter search. Machine learning algorithms process extensive acoustic metamaterial data to discover novel structures and properties, enhancing overall acoustic performance. This review presents an in-depth exploration of applications associated with machine learning techniques in AMs and PnCs, highlighting specific advantages, challenges and potential solutions of applying of using machine learning algorithms associated with machine learning techniques. By bridging acoustic engineering and machine learning, this review paves the way for future breakthroughs in acoustic research and engineering.
Trending on Altmetric
Open all abstracts, in this tab
Luke B Demo et al 2024 Smart Mater. Struct. 33 065042
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.
Joshua Knospler et al 2024 Smart Mater. Struct. 33 065040
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. 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. 33 065037
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 SSMs 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 SSMs 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 SSM that responds to external stimuli. Under cyclic loading, the SSMs 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'scurrent 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.
Takashi Ozaki et al 2024 Smart Mater. Struct.
Technologies aimed at digitizing worker actions to enhance human labor tasks and mitigate accidents and disabilities have garnered significant attention. This study focuses on monitoring the force exerted by the fingers and developing a wearable fingertip force sensor based on a simple elliptical ring structure in conjunction with a commercially available resistive bend sensor. Resembling a ring accessory, the sensor is easy to attach and detach, and exhibits high sensitivity, with a resistance change of approximately 9% for a fingertip load of 1N. Furthermore, to mitigate crosstalk during finger flexion, we propose a combined configuration employing this ring-shaped sensor alongside another sensor designed for measuring and rectifying finger flexion angles. Additionally, we introduce an empirically derived fitting function and a straightforward calibration procedure to extract the function's parameters. The results indicate that the proposed system achieves an average RMS error of 0.53 N for force estimations of approximately 5 N, even amidst finger bending and postural changes.
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.
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.
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.