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Multifunctional soft robots are emerging as a new-generation intelligent device for challenging environments. To meet the requirements of smart applications and soft robotics, developing a soft actuator capable of multiple functions and mechanical deformation is essential. In this study, we designed a free-standing magnetic soft actuator constructed from iron oxide (Fe2O3) nanoparticles and poly-(vinyl alcohol) (PVA), that responds to both moisture and magnetic fields. We used computational modeling (density functional theory and ab initio molecular dynamics) to explain the experimental findings demonstrating the deformation and high-bending angle (∼150°), which is about twice as large under combined moisture and magnetic field exposure compared to their individual effect. Additionally, a flower-shaped soft robot was designed by using the continuous bending deformation of the actuator in response to moisture changes, performing directional bending in an ambient environment. These findings demonstrate the material's sensitivity to moisture and magnetic fields, opening up new possibilities for designing responsive structures in the smart device industry. [ABSTRACT FROM AUTHOR]

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The magnetic soft materials that can actuate in response to the external magnetic stimulus are of a high potential for the soft robotics and biomedical applications. This work reports on the new magneto-responsive materials based on the bio-based poly(butylene succinate-co-butylene adipate) (PBSA). The synthesized magnetite (Fe3O4) nanoparticles were incorporated into the PBSA biopolymer matrix with the Fe3O4 content up to 3 wt%. The addition of Fe3O4 enhanced the magnetic and mechanical properties of the composites; all composites exhibited only the slight increases in the storage modulus in the presence of the external magnetic flux density of 10,000 G. The 3%v/v Fe3O4/PBSA composite exhibited only a small increase in the storage modulus from 7.23 × 105 Pa to 7.55 × 105 Pa without and with the magnetic flux density 10,000 G, respectively. The slight increases observed indicate the retained and favorable material flexibility and softness favoring the magnetic actuation. For the bending performance, the largest bending distance of 15.6 mm with the response time within 10 s and the maximum magnetophoretic force of 0.14 mN under the applied low magnetic flux density of 1300 G were obtained from the composite with the Fe3O4 content of 3.0%v/v with the composite magnetization of 1.719 emu g− 1. The fabricated soft magneto-responsive materials based on PBSA and Fe3O4 nanoparticle are shown here to be potential as soft magneto-responsive materials in the field of actuator such as soft robotic components. [ABSTRACT FROM AUTHOR]

To treat cardiovascular diseases (i.e., a major cause of mortality after cancers), endovascular-technique-based guidewire has been employed for intra-arterial navigation. To date, most commercially available guidewires (e.g., Terumo, Abbott, Cordis, etc.) are non-steerable, which is poorly suited to the human arterial system with numerous bifurcations and angulations. To reach a target artery, surgeons frequently opt for several tools (guidewires with different size integrated into angulated catheters) that might provoke arterial complications such as perforation or dissection. Steerable guidewires would, therefore, be of high interest to reduce surgical morbidity and mortality for patients as well as to simplify procedure for surgeons, thereby saving time and health costs. Regarding these reasons, our research involves the development of a smart steerable guidewire using electroactive polymer (EAP) capable of bending when subjected to an input voltage. The actuation performance of the developed device is assessed through the curvature behavior (i.e., the displacement and the angle of the bending) of a cantilever beam structure, consisting of single- or multi-stack EAP printed on a substrate. Compared to the single-stack architecture, the multi-stack gives rise to a significant increase in curvature, even when subjected to a moderate control voltage. As suggested by the design framework, the intrinsic physical properties (dielectric, electrical, and mechanical) of the EAP layer, together with the nature and thickness of all materials (EAP and substrate), do have strong effect on the bending response of the device. The analyses propose a comprehensive guideline to optimize the actuator performance based on an adequate selection of the relevant materials and geometric parameters. An analytical model together with a finite element model (FEM) are investigated to validate the experimental tests. Finally, the design guideline leads to an innovative structure (composed of a 10-stack active layer screen-printed on a thin substrate) capable of generating a large range of bending angle (up to 190°) under an acceptable input level of 550 V, which perfectly matches the standard of medical tools used for cardiovascular surgery. [ABSTRACT FROM AUTHOR]

Air cylinders are actuators that slide a piston inside cylinders by applying air pressure. We propose an air-cylinder-type artificial muscle that can be flexibly bent by using a flexible tube for the cylindrical part. The actuator output was a string connected to a piston. When the air-cylinder-type artificial muscle bends, the inner wall of the tube and the string come into contact, causing output fluctuations owing to friction. In this study, we investigated the output when an artificial muscle was bent. After describing the structure of the air-cylinder-type artificial muscle, the measurement results of the resistance force at each part of the actuator are presented. A theoretical output inspired by the capstan equation was derived, and its validity was verified by comparison with experimental results. [ABSTRACT FROM AUTHOR]

High-performance flexible actuators, integral components of soft robotics, hold promise for advancing applications in safe human–robot interactions, healthcare, and various other fields. Notable among these actuators are flexible electrochemical systems, recognized for their merits in low-voltage manipulation, rapid response speed, and cost-effectiveness. However, the optimization of output strain, response speed, and stability presents a significant challenge in this domain. Despite the application of diverse electrochemically active materials to enhance actuation performance, a critical need persists for corresponding electrical-mechanical models to comprehensively grasp actuation mechanisms. In this study, we introduce a novel electrochemical actuator that utilizes conductive polymer ionogel as active electrodes. This ionogel exhibits exceptional properties, including high conductivity, flexibility, and electrochemical activity. Our electrochemical actuators exhibit noteworthy bending strain capabilities and rapid response rates, achieving frequencies up to 10 Hz at a modest voltage of 1 V. An analytical model integrating ion migration and dynamic processes has been established to elucidate actuator behavior. Simulation results highlight that electrodes characterized by low resistance and high capacitance are optimal for simultaneous enhancement of bending strain and blocking force. However, the augmentation of Young's modulus, while increasing blocking force, compromises bending strain. Furthermore, a larger aspect ratio proves beneficial for unidirectional stress output, leading to increased bending strain, while actuator blocking force diminishes with greater length. These findings underscore the intricate interplay between material properties and dimensions in optimizing the performance of flexible electrochemical actuators. This work provides important practical and theoretical guidance for the manufacture of high-performance flexible actuators and the search for new smart materials. [ABSTRACT FROM AUTHOR]

Graphene possesses not only excellent optical, electrical and thermal properties but also outstanding thermoelectric and photothermal conversion capabilities. The application of graphene with functional responses plays a vital role in the development of intelligent materials intended for flexible actuators and intelligent robots. Cellulose paper is foldable and is a hydrophilic network comprised of porous cellulose fibres. In this paper, graphene composites as well as cellulose paper were taken as basic materials, which were further combined with other polymer materials (biaxially oriented polypropylene film (BOPP) and poly (3, 4-ethylenedioxythiophene/polystyrene sulfonate) (PEDOT: PSS). PEDOT: PSS solution can penetrate into the porous network of the paper to form a high-strength nanocomposite structure. A flexible thin film driver capable of multiple stimuli under light, heat and humidity was produced. Through characterization, it was found that the bending ability of the composite film was significantly improved in response to stimulation. [ABSTRACT FROM AUTHOR]

High-performance dual-responsive flexible actuators are crucial for soft robots, artificial muscles, and wearable devices. However, a simple and fast preparation of dual-responsive flexible actuators with excellent actuation performance remains a challenge. Herein, an electrohydrodynamic (EHD) printing method is proposed to prepare eutectic gallium-indium/cellulose nanofiber (EGaIn/CNF) film with asymmetric structure. EGaIn/CNF film is assembled with polyethylene (PE) with high anisotropic thermal expansion coefficient to fabricate an EGaIn/CNF-PE bilayer flexible actuator with gradient structure. Through the collaborative design of materials and structures, combined with the high conductivity of gradient-distributed EGaIn, excellent hygrosensitivity of gradient-distributed CNF, and high thermal expansion coefficient of PE, the EGaIn/CNF-PE flexible actuator exhibits excellent actuation performance under electrothermal and humidity response. The flexible actuator achieves a bending curvature of 3.3 cm−1 within 12 s under a low voltage of 4 V, and bending curvature of 3.06 cm−1 under a 30 % change in environmental humidity. Moreover, the fabricated flexible actuator is applied in the fields of flexible gripper, smart flexible switch, and biomimetic flower, which shows its commendable practical application potential. [Display omitted] • EGaIn/CNF composite film with asymmetric structure preparation using the EHD printing method. • High-performance flexible actuator with gradient-distributed structure for electrothermal and humidity response. • Flexible actuator with bending curvature of 3.3 cm−1 (4 V) and 3.06 cm−1 at a 30 % change in environmental humidity. • Designed flexible actuator for flexible gripper, the smart flexible switch, and biomimetic flower. [ABSTRACT FROM AUTHOR]

Soft actuators have a high potential for the creative design of flexible robots and safe human–robot interaction. So far, significant progress has been made in soft actuators' flexibility, deformation amplitude, and variable stiffness. However, there are still deficiencies in output force and force retention. This paper presents a new negative pressure-driven folding flexible actuator inspired by origami. First, we establish a theoretical model to predict such an actuator's output force and displacement under given pressures. Next, five actuators are fabricated using three different materials and evaluated on a test platform. The test results reveal that one actuator generates a maximum pull force of 1125.9 N and the maximum push force of 818.2 N, and another outputs a full force reaching 600 times its weight. Finally, demonstrative experiments are conducted extensively, including stretching, contracting, clamping, single-arm power assistance, and underwater movement. They show our actuators' performance and feature coupling hardness with softness, e.g., large force output, strong force retention, two-way working, and even muscle-like explosive strength gaining. The existing soft actuators desire these valuable properties. [ABSTRACT FROM AUTHOR]

Conductive polyacrylamide (PAM) hydrogels with salts that act as electrolytes have been used as transparent electrodes with high elasticity in flexible electronic devices. Different types and contents of raw materials will affect their performance in all aspects. We tried to introduce highly hydratable salts into PAM hydrogels to improve their water retention capacity. Different salts can improve the water retention capacity of PAM hydrogels to a certain extent. In particular, PAM hydrogels containing higher concentrations of lithium chloride (LiCl) and calcium chloride (CaCl2) showed an extremely strong water retention capacity and could retain about 90% and more than 98% of the initial water in the experimental environment at a temperature of 25 °C and a relative humidity of 60% RH, respectively. In addition, we conducted electrical conductivity tests on these PAM hydrogels with different salts. The PAM hydrogels containing LiCl also show outstanding conductivity, and the highest conductivity value can reach up to about 8 S/m. However, the PAM hydrogels containing CaCl2, which also performed well in terms of their water retention capacity, were relatively common in terms of their electrical conductivity. On this basis, we attempted to introduce single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), and graphene (GO) electronic conductors to enhance the electrical conductivity of the PAM hydrogels containing LiCl. The conductivity of the PAM hydrogels containing LiCl was improved to a certain extent after the addition of these electronic conductors. The highest electrical conductivity was about 10 S/m after we added the SWCNTs. This experimental result indicates that these electronic conductors can indeed enhance the electrical conductivity of PAM hydrogels to a certain extent. After a maximum of 5000 repeated tensile tests, the conductive hydrogel samples could still maintain their original morphological characteristics and conductivity. This means that these conductive hydrogel samples have a certain degree of system reliability. We made the PAM conductive hydrogels with high water retention and good conductivity properties into thin electrodes and applied them to an electric response flexible actuator with dielectric elastomer as the functional material. This flexible actuator can achieve a maximum area strain of 18% under an external voltage of 10 kV. This new composite hydrogels with high water retention and excellent conductivity properties will enable more possibilities for the application of hydrogels. [ABSTRACT FROM AUTHOR]

We present a self-deformable flexible tweezer capable of simultaneous mechanical handling and electrical measurements. The tweezer has a soft cantilever with the dimensions 2 mm $\times8$ mm $\times $ 75- $100~\mu \text{m}$ , and undergoes self-deformation. The device is shown to be successfully capable of detecting electrical signals by gently touching the surface and grasping a spherical bead. The device demonstrated the lowest working voltage (1.5 VDC), force suitable for soft gripping, and curvature radius of 2 mm, that was one of the smallest values compared to that of similar state-of-the-art devices. The device was fabricated using a unique and highly reliable process that was specifically developed to produce flexible cantilevers with novel ionic polymer-metal composites (IPMCs). The materials used were poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) and an ionic liquid (IL). The PVDF-TrFE/IL gel was prepared using acetone as the solvent and the gel was coated with silver nanowires as the electrodes. The actuator with a length of 8 mm and containing 50 wt% IL yielded the largest bending displacement of 7 mm and minimum curvature radius of 2 mm at 1.5 VDC. [2022-0026] [ABSTRACT FROM AUTHOR]

The locomotor behavior of creatures in nature can bring a lot of inspiration for the fabrication of soft actuators. In this paper, we fabricated a bionic light-driven swimming soft robot that can perform grasping of tiny objects and achieve the task of object transfer. By adding carbon nanotubes (CNTs), the temperature-sensitive hydrogels can be endowed with light-responsive properties. The fabricated composite hydrogel structure can control the contraction and expansion of volume by light, which is similar to the contraction and diastole behavior of muscles. The oscillation of the fish tail and the grasping action of the normally closed micromanipulator can be achieved by the control of the irradiation of the xenon light source. The bending of the bionic arm can be controlled by the irradiation of a near-infrared (NIR) laser, which transforms the spatial position and posture of the micromanipulator. The proposed scheme is feasible for miniaturized fabrication and application of flexible actuators. This work provides some important insights for the study of light-driven microrobots and light-driven flexible actuators. [ABSTRACT FROM AUTHOR]

In this paper, a novel variable stiffness mechanism (VSM) based on specially designed S-springs made from shape memory alloy (SMA) is developed. Based on the stiffness model, by changing the state combination of the SMA S-springs with different thicknesses, the actuator's stiffness can be discretely adjusted online. The stiffness adjustment range of the actuator can be conveniently extended almost from 0 N/m to infinity by changing the preset spring angle manually offline without a complex mechanical mechanism or replacing any parts. A linear digital variable stiffness actuator (LDVSA) is also designed with the VSM. Some tests of the SMA S-springs, as well as the actuator's stiffness under different configurations, comparisons with other actuators and its trajectory tracking capacity are conducted to verify its design. Then, a dynamic model of the actuator is established and its bandwidth is analysed. Based on this, explosive experiments are designed and performed to explore the application potential of the actuator. Experimental results are provided to illustrate the explosive capacity and energy efficiency of the proposed design.

Wings are the main source of lift for Underwater Gliders (UGs) and play a decisive role in their motion performance. The wings of UGs are generally rectangular and symmetrical airfoils. In this study, a mini-UG with flexible camber trailing-edge wings was proposed and developed to investigate the influence of the trailing edge of the wings on the motion performance of UGs. Flexible deformation of the trailing edge was realized using a steer-by-wire actuator. The test results showed that the trailing edge of the wing could realize maximum upward/downward sloping angles of +16°/−16°. By combining computational fluid dynamics simulations and tank experiments, this study obtained the glide efficiency (in lift-drag ratio) and stable margins of a mini-UG with variable camber trailing-edge wings. The results showed that the angles of attack corresponding to the minimum lift-to-drag ratio were all negative in cases of downward sloping and positive in cases of upward sloping. Moreover, the suitable camber of the trailing edge, that is, downward-sloping trailing edge on descending glides and upward-sloping trailing edge on ascending glides, can not only greatly improve glide efficiency but also benefit flight stability. Tank experiments showed that the lift-to-drag ratio of the mini-UG with appropriate variable camber trailing-edge wings can be increased by at least 25% compared to those with a symmetrical airfoil.

Soft actuators have a high potential for the creative design of flexible robots and safe human–robot interaction. So far, significant progress has been made in soft actuators’ flexibility, deformation amplitude, and variable stiffness. However, there are still deficiencies in output force and force retention. This paper presents a new negative pressure-driven folding flexible actuator inspired by origami. First, we establish a theoretical model to predict such an actuator’s output force and displacement under given pressures. Next, five actuators are fabricated using three different materials and evaluated on a test platform. The test results reveal that one actuator generates a maximum pull force of 1125.9 N and the maximum push force of 818.2 N, and another outputs a full force reaching 600 times its weight. Finally, demonstrative experiments are conducted extensively, including stretching, contracting, clamping, single-arm power assistance, and underwater movement. They show our actuators’ performance and feature coupling hardness with softness, e.g., large force output, strong force retention, two-way working, and even muscle-like explosive strength gaining. The existing soft actuators desire these valuable properties.

An actuator built with flexible material has the advantage of smaller size and can withstand certain collisions better than actuators with rigid material. This paper proposes a crawling actuator model driven by dielectric elastomer (DE), which uses the electrically induced deformation of the DE membrane to drive the motion of the actuator. When the dielectric elastomer in the actuator is at higher voltage, the DE material produces higher deformation, and the deformation is transmitted to the ground through the friction foot thus driving the motion of the actuator. An interpolation fitting estimation algorithm (IFEA) was constructed based on the relevant material properties and principles. The pre-stretch length of the DE membrane was determined and verified through experiment; the verified results showed that the actuator has better driving performance when the membrane pre-stretching ratio is equal to 3. The crawling actuator can achieve a speed of about 50 mm/s at 4 kv and can reach 11 mm/s when loaded with four times its weight. The new crawling actuator achieved an excellent turning ability of 8.2°/s at 60% duty cycle and 32 Hz frequency. Compared with other types of crawling actuators, the actuator presented in this work has better load capacity and crawling performance.

Conductive polyacrylamide (PAM) hydrogels with salts that act as electrolytes have been used as transparent electrodes with high elasticity in flexible electronic devices. Different types and contents of raw materials will affect their performance in all aspects. We tried to introduce highly hydratable salts into PAM hydrogels to improve their water retention capacity. Different salts can improve the water retention capacity of PAM hydrogels to a certain extent. In particular, PAM hydrogels containing higher concentrations of lithium chloride (LiCl) and calcium chloride (CaCl2) showed an extremely strong water retention capacity and could retain about 90% and more than 98% of the initial water in the experimental environment at a temperature of 25 °C and a relative humidity of 60% RH, respectively. In addition, we conducted electrical conductivity tests on these PAM hydrogels with different salts. The PAM hydrogels containing LiCl also show outstanding conductivity, and the highest conductivity value can reach up to about 8 S/m. However, the PAM hydrogels containing CaCl2, which also performed well in terms of their water retention capacity, were relatively common in terms of their electrical conductivity. On this basis, we attempted to introduce single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), and graphene (GO) electronic conductors to enhance the electrical conductivity of the PAM hydrogels containing LiCl. The conductivity of the PAM hydrogels containing LiCl was improved to a certain extent after the addition of these electronic conductors. The highest electrical conductivity was about 10 S/m after we added the SWCNTs. This experimental result indicates that these electronic conductors can indeed enhance the electrical conductivity of PAM hydrogels to a certain extent. After a maximum of 5000 repeated tensile tests, the conductive hydrogel samples could still maintain their original morphological characteristics and conductivity. This means that these conductive hydrogel samples have a certain degree of system reliability. We made the PAM conductive hydrogels with high water retention and good conductivity properties into thin electrodes and applied them to an electric response flexible actuator with dielectric elastomer as the functional material. This flexible actuator can achieve a maximum area strain of 18% under an external voltage of 10 kV. This new composite hydrogels with high water retention and excellent conductivity properties will enable more possibilities for the application of hydrogels.

Electroresponsive devices have attended significant interest for actuators and artificial muscles in recent days but are susceptible to low deformation at applied voltages. To address this problem, herein, we have prepared novel flexible composite gel based on polyvinyl chloride (PVC), dibutyl adipate (DBA), and functionalized reduced graphene oxide (f‐rGO) via solution casting technique. The structural, morphological, optical, mechanical, and thermal properties of the composite PVC/f‐rGO gel were characterized by using various techniques. In order to test the usefulness of PVC/f‐rGO, we fabricated the new planar PVC/f‐rGO‐based gel actuator, by holding the gel between two electrodes. Stimuli‐responsive deformation of ~2.7 mm was measured at a voltage of 1200 V for an optimized gel in the designed device. The loading of silane‐f‐rGO greatly enhanced the stimulation, response time and flexibility of the PVC gel. The highest elongation at break of about 433% was achieved for f‐rGO‐loaded PVC gel owing to strong dipole–diploe interactions between alpha hydrogen of PVC and ‐NH2 of f‐rGO. We believe that highly electro‐stimulated, soft and, flexible PVC/f‐rGO gels could open a new avenue for the fabrication of advanced artificial muscles and tunable soft actuators for number of applications. [ABSTRACT FROM AUTHOR]

Stimulated devices are highly demanded for actuators and artificial muscles in the recent era but susceptible to low deformation at an applied voltage. In the present work, ionic liquids (ILs) based gel films were prepared from the polyvinyl chloride (PVC), dibutyl adipate (DBA), 1‐butyl, 3‐methimidazolium chloride, and 1‐pentyl‐3‐methylimidazolium hexafluorophosphate by a simple solvent evaporation method. The structural, morphological, optical, and mechanical properties of the composite PVC/ILs gel were characterized by Fourier‐transform infrared spectroscopy (FTIR), Large Angle X‐ray scattering (LAXS), UV–visible (UV–vis) absorption spectroscopy, scanning electron micrpscopy (SEM) and elemental mapping. We found that the displacement of plasticized PVC gels‐based actuator was 0.1 mm with the response time of 0.33 s at an induced voltage of 1000 V. The loading of 0.02% of IL (fluorides) with PVC gel showed maximum deformation of 0.16 mm with a relatively rapid response time of 0.2 s. These high deformation and response time values of IL fluoride‐based gels are dramatically higher than reported PVC gels. Likewise, the loading of IL fluorides in the PVC gel showed a high elongation value at the break of about 377%. This work suggests that the flexible gel based on IL fluorides and PVC could be a potential candidate for the fabrication of high‐performance artificial muscles and tunable soft actuators. [ABSTRACT FROM AUTHOR]

A water hydraulic flexible gripper with three-fingered structure is developed to deal with the problem of poor adaptability for the existing underwater gripper. This gripper driven by water hydraulics can realize flexible grasping and possess simple structure, high pressure-bearing, strong adaptability and capability of anti-jamming to the water environment and easy to control. In particular, the water hydraulic flexible gripper system is an open system in relation to the underwater environment, with the water source being supplied directly by the underwater environment, eliminating the effects of back pressure generated by the underwater environment in comparison with the closed system of other grippers. It is a good solution to solve the problem of poor adaptability for the existing underwater gripper in underwater environment. The flexible actuator model is established to explore the key parameters influencing the deformation characteristics. The effects of different inlet pressure, knuckle length, wall thickness and material of the inner skeleton and external surface on the deformation characteristics of the flexible actuator are investigated through simulation. It is found that, for the flexible actuator, the wall thickness of the inner skeleton is selected as 1 mm and the inner skeleton length is designed with the first knuckle of 30 mm and the second knuckle of 80 mm. Based on the optimal parameters obtained through simulation, the prototype of flexible actuator and flexible gripper are fabricated. Experiment is performed to test the deformation of the flexible actuator under different inlet pressure. It is found that the experiment results are consistent with the simulation results. Both of the experiment and simulation results exhibit that the total deformation of the flexible actuator is proportional to the inlet pressure. The research will lay foundation for the optimal design of flexible actuator used for the underwater gripper driven by water hydraulics.

In view of the shortcomings of the traditional high-rigidity robots that can't ensure safety of human-robot interaction and the adaptability of complex environment, a variable stiffness flexible actuator is proposed for the robot joint based on the magnetorheological (MR) effect of MR Fluids. In this paper, the principle of flexible joint drive is detailed, and design the structure of the drive and perform mechanical analysis is designed. At the same time, the magnetic field analysis of the actuator is carried out by using the magnetic field simulation software Maxwell. Finally, the actuator is simulated and verified. Comparing the analysis of the results, the actuator has the characteristics of simple structure, easy control and wider active variable stiffness adjustment range, and can absorb the vibration or shock energy, and improve the robot joint output ability.

A water hydraulic flexible gripper with three flexible actuators is proposed, which integrates a soft external skin and a flexible inner skeleton to enhance resistance against water pressure. To investigate the deformation characteristics of the water hydraulic flexible actuator, a mathematical model is established to describe the relationship between water pressure and actuator deformation. The effects of different materials, structural sizes, and inlet pressure are analyzed through simulation. A Kriging model is utilized to fit the simulation results, while a multi-island genetic algorithm is employed to obtain an optimal solution for the structure of the flexible actuator. Finally, the deformation of the flexible actuator under various inlet pressures is quantified through experimental measurements, and the obtained results exhibit degree of consistency with the simulation outcomes. After completing static experiments at a simulated sea depth of 1000 m, the tip forces of flexible actuators were tested, and then land and underwater grasping experiments were conducted. The water-hydraulic flexible actuator developed in this study has great potential for handling fragile or deformable objects during deep-sea. This promotes the application of water-hydraulic flexible manipulators in the field of underwater robotics, providing higher flexibility and adaptability compared to traditional rigid manipulators. [Display omitted] • A water hydraulic flexible gripper with three flexible actuators is proposed. • Deformation characteristics of the flexible actuator is established. • Kriging model integrated with MIGA is used to obtain the optimal solution for flexible actuator. • Deformation of flexible actuator under different inlet pressure is validated by experiments. [ABSTRACT FROM AUTHOR]

One of the limitations in the development of really soft robotic devices is the development of soft actuators. In recent years, our research group has developed a new flexible shape memory alloy actuator that provides more freedom of movements and a better integration in wearable robots, especially in soft wearable robots. Shape memory alloy wires present characteristics such as force/weight ratio, low weight, and noiseless actuation, which make them an ideal choice in these types of applications. However, the control strategy must take into account its complex dynamics due to thermal phase transformation. Different control approaches based on complex non-linear models and other model-free control methods have been tested on real systems. Some exoskeleton prototypes have been developed, which demonstrate the utility of this actuator and the advantages offered by these flexible actuators to improve the comfort and adaptability of exoskeletons. [ABSTRACT FROM AUTHOR]

A flexible actuator finite element is developed and applied for the modelling of spatial mechanisms present in several industrial applications. A total Lagrangian framework is employed for the development of the finite deformation dynamic equilibrium using solid-like shell and 3D frame finite elements. Exploiting the total Lagrangian aspect of the formulation, the actuator motion is imposed by controlling the element reference configuration. It has the advantage of retaining the actuated bar flexibility, an important factor when simulating flexible mechanisms, and not requiring special treatments as constraint enforcement impositions. As the employed elements use alternative nodal parameters such as positions and generalized vectors to describe their kinematics, a treatment on the introduction of rotational connections—spherical, revolute and pinned joints—largely present in actuated mechanisms, is developed. The nonlinear equations of motion are solved by the Newton–Raphson method. Examples are presented to evaluate the proposed flexible actuator finite element regarding its dynamical behaviour in mechanisms where its use is of importance. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • A new classification of actuation systems is proposed according to the stable dimension of actuator output as acceleration, velocity or position. • Actuators of acceleration or velocity-type can be transformed into velocity or position-type by means of some additive structure becoming part of the actuation system. • Rubber-made actuators constitute a large new class of position-type actuators. • Antagonism can be applied especially to SISO position-type actuators for realizing MIMO position-type actuators. Abstract A general actuator definition is proposed, which characterizes the actuator by a force function with two positive components: a motor-component and a resistive-component. For a constant input value, a stable equilibrium can result under the form of an output which can have the dimension of an acceleration, a velocity or a position, thus giving the actuator its corresponding type. We also introduce a concept of type transformation from acceleration or velocity-type to velocity or position-type. Such possibility of actuator transformation leads us, on the one hand, to propose original interpretations of different technical devices, like a weight-driven clock or a lock, understood as position-type actuators, or an hourglass, understood as velocity-type actuator; and, on the other hand, to distinguish, among the relatively recent class of position-type actuators, these resulting from a transformation of type, like the so-called elastic actuators, from those for which stable open-loop positioning results from opposing force tendencies inside the materials constituting the actuator. We also emphasize the role of a biomimetic antagonism principle for defining multiple inputs-multiple outputs position-type actuators. [ABSTRACT FROM AUTHOR]

Abstract The performance of micro devices actuated using electroactive polymers is often limited by the high stiffness of metallic electrodes, which prevents the polymeric components extension. In this work we report on an approach allowing to reduce the use of metals to a minimum by incorporating softer conductive polymer electrodes. A multilayered actuator incorporating polyimide (PI) substrate and a thin electroactive polymer (EAP) layer of P(VDF-TrFE-CFE) squeezed between polyaniline (PANI) electrodes was fabricated and characterized. The PANI layer was electrodeposited on the electroactive structures using cyclic voltammetry. Our results show that while the use of conjugate conductive polymer electrodes is feasible, the influence of the thin metallic seed layers required for electro-deposition is significant. The proposed electro-deposition based polymeric process is highlighted by its simplicity and relatively low cost, compared to common Physical Vapor Deposition processes, and can be extended to other polymers. Graphical Abstract Highlights • Use of polyaniline electrodes instead of routinely used metal electrodes to increase actuator's flexibility • Electropolymerization (cyclic voltammetry) of polyaniline integrated with electroactive P(VDF-TrFE-CFE) layer microfabrication methods • Fabrication, actuation and characterization of functional thin film bimorph actuator that is mainly made of polymers [ABSTRACT FROM AUTHOR]

Hand recovery process is a major issue in the rehabilitation field as hands are vital to perform most daily activities. This paper presents the design and evaluation of a one actuated degree-of-freedom exoskeleton finger using flexible actuator to assist patients in rehabilitation process and more specifically in tendon recovery protocols. The control strategy based on the motion-copying system is used to be able to take advantages of the sensation of the patient. The wearability and adaptability of the device are improved, compared to other specific devices, mainly by the use of remote actuation and light-weight materials. Evaluation of the device in terms of wearability, adaptability, repeatability and accuracy of the position estimation are conducted. All these criteria are confirmed. The next step is to test the device in actual recovery protocols to evaluate its efficiency. [ABSTRACT FROM AUTHOR]

ABSTRACT: Electro‐active polymers (EAPs) such as P(VDF‐TrFE‐CTFE) are greatly promising in the field of flexible sensors and actuators, but their low dielectric strength driven by ionic conductivity is a main concern for achieving high electrostrictive performance. It is well known that there is a quadratic dependence of the strain response and mechanical energy density on the applied electric field. This dependence highlights the importance of improving the electrical breakdown EAPs while reducing the dielectric losses. This article demonstrates that it is possible to dramatically increase the electrical breakdown and decrease the dielectric losses by controlling processing parameters of the polymer synthesis and fabrication procedure. As a result, an enhancement of around 70% is achieved in both the strain and blocking force. The effects on the dielectric losses of the polymer crystallinity, molecular weight, solvent purity, and crystallization temperature are also investigated. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 1164–1173 [ABSTRACT FROM AUTHOR]

In the field of micromechanics, piezoelectric actuator has attracted great attention for its high-frequency response, high displacement resolution, and high output force. However, its prospect of practical application has been largely limited by the displacement of micrometer. A fluid coupling flexible actuator was proposed, which utilizes resonance to enlarge the output displacement. The actuator uses a piezoelectric oscillator as an excitation source, fluid as the transmission medium and a flexible diaphragm for the displacement output. On the condition that the fluid is inviscid and incompressible, mathematical formulation of the membrane vibration theory has been analyzed. Then, the prototype is made. The displacement is amplified 21 times to 1.106 mm when driving frequency is 127 Hz. The flexible diaphragm appears the largest displacement output when driving frequency is close to one of the system's natural frequency. Then, the points with zero amplitude form a circle on the surface of flexible diaphragm and the movement direction of the flexible diaphragm is opposite on different sides of the circle. In fact, rather than vibrates at the first resonance frequency, the membrane in the essay is vibrating at a certain higher-order resonance frequency. The experimental results are mainly consistent with the theoretical analysis. [ABSTRACT FROM AUTHOR]

To meet the needs of present-day robotics, a family of gripping flexible fingers has been designed. Each of them consists of a number of independent and flexible modules that can be assembled in different configurations. Each module consists of a body with a flexible central rod and three longitudinally positioned shape memory alloy (SMA) wires. When heated by the Joule effect, one to two SMA wires shorten, allowing the module to bend. The return to undeformed conditions is achieved in calm air and is guaranteed by the elastic bias force exerted by the central rod. This article presents the basic concept of the module and a simple mathematical model for the design of the device. Experimental tests were carried out on three prototypes with bodies made of different materials. The results of these tests confirm the need to reduce the antagonistic action of the inactive SMA wires and led to the realization of a fourth prototype equipped with an additional SMA wire-driven locking/unlocking device for these wires. The preliminary results of this last prototype are encouraging.

This paper proposes the development of a piezo-driven flexible actuator that creates displacement to drive a vacuum micropump. This actuator consists of parallel flexible amplification mechanisms and piezoelectric actuators. The flexible mechanism relies on its own deformation to amplify the displacement of the piezoelectric actuators. An elastic model is established to estimate the deflection behavior and the effects of its geometric relationship. The finite element method is employed to validate the design and analysis. An experimental investigation is performed to study the deflection and negative pressure. Conclusion is reached that the deflection of the flexible actuator is sensitive to the initial incline angle of the bridge arm. The displacement amplification ratio is not related to the material. An increase in compliance of the flexure hinges can improve the deflection behavior. The proposed actuator produces a displacement of 12.6 μm at a voltage of 54 V. The central rotational symmetry with multiple linkages increases the robustness against radial parasitic displacement. [ABSTRACT FROM AUTHOR]

Predictable bending deformation, high cycle stability, and multimode complex motion have always been the goals pursued in the field of flexible robots. In this study, inspired by the delicate structure and humidity response characteristics of Selaginella lepidophylla , a new multilevel assisted assembly strategy was developed to construct MXene-CoFe 2 O 4 (MXCFO) flexible actuators with different concentration gradients, to achieve predictable bending deformation and multi-stimulus cooperative control of the actuators, revealing the intrinsic link between the gradient change and the bending deformation ability of the actuator. The thickness of the actuator shows uniformity compared with the common layer-by-layer assembly strategy. And, the bionic gradient structured actuator shows high cycle stability, and it maintains excellent interlayer bonding after bending 100 times. The flexible robots designed based on the predictable bending deformation and the multi-stimulus cooperative response characteristics of the actuator initially realize conceptual models of humidity monitoring, climbing, grasping, cargo transportation, and drug delivery. The designed bionic gradient structure and unbound multi-stimulus cooperative control strategy may show great potential in the design and development of robots in the future.

In the field of micromechanics, piezoelectric actuator has attracted great attention for its high-frequency response, high displacement resolution, and high output force. However, its prospect of practical application has been largely limited by the displacement of micrometer. A fluid coupling flexible actuator was proposed, which utilizes resonance to enlarge the output displacement. The actuator uses a piezoelectric oscillator as an excitation source, fluid as the transmission medium and a flexible diaphragm for the displacement output. On the condition that the fluid is inviscid and incompressible, mathematical formulation of the membrane vibration theory has been analyzed. Then, the prototype is made. The displacement is amplified 21 times to 1.106 mm when driving frequency is 127 Hz. The flexible diaphragm appears the largest displacement output when driving frequency is close to one of the system’s natural frequency. Then, the points with zero amplitude form a circle on the surface of flexible diaphragm and the movement direction of the flexible diaphragm is opposite on different sides of the circle. In fact, rather than vibrates at the first resonance frequency, the membrane in the essay is vibrating at a certain higher-order resonance frequency. The experimental results are mainly consistent with the theoretical analysis.

Multi-functional materials in actuator applications have been developed toward reversibility and sensitivity under various actuating fields. In this work, magneto-electro-responsive materials consisting of a polyurethane (PU) matrix and its composites embedded with magnetite nanoparticles (MNP) as a dispersed phase were fabricated to tailor the electromechanical properties and bending performance under electric, magnetic, and electromagnetic fields. Due to the superior characteristics of MNP over other magnetic materials, the composites fabricated with electronic polarization were highly responsive under electric field. The highest storage modulus sensitivity belonged to the 1.0% v/v MNP/PU composite which possessed the value of 3.46 at the electric field 2 kV mm − 1 . Moreover, all of the PU composites behaved as an electrostrictive material in which the stress depended quadratically on the electric field. It was demonstrated that the PU composites also possessed very good recoverability, fast response (< 15 s) and large bending angle relative to that of pristine PU under applied electric field. Interestingly, the steady state storage modulus response was attained within the first electrical actuation cycle and the PU composite was a fully reversible material. In addition, it was shown that superparamagnetism was a common characteristic of all fabricated composites under magnetic field. The 3.0% v / v MNP/PU composite provided the largest bending distance up to 23.60 mm, and 14.10 mm under the magnetic field of 5000 G, and the electromagnetic field of 320 G, respectively. In summary, the MNP/PU composite material is a potential candidate to be used as a smart material under the influences of electric and/or magnetic fields over other existing dielectric materials. [ABSTRACT FROM AUTHOR]

It is a well-known fact that minimally invasive surgery (MIS) has many benefits, e.g., unnecessary trauma and blood loss can be minimized by opening tiny surgery scars, recovery process of patients can be lessened, and the possibilities of complications can be decreased. Although robots are widely used in MIS due to their fundamental superiorities such as high precision, current surgery robots mainly suffer from absence/limitation of haptic feedback during operation. This paper proposes a novel electrohydraulic haptic transmission system to improve the haptic feedback of a surgery robot. This new device replaces the highly nonlinear mechanical friction of thrust wire by viscous friction and avoids the backlash due to the incompressibility of fluid. Moreover, since the fluid is constantly occupying the entire volume of the hydraulic tube, the system behavior during bending of the tube can be modeled much easier than the thrust wire, for which position, shape, and bending angles of the inner wire inside the outer tube are hardly predictable. The validity of the proposal and its superiorities over conventional transmission mechanisms, such as thrust wire, are experimentally verified. [ABSTRACT FROM PUBLISHER]

This paper describes a flexible Shape Memory Alloy (SMA) actuator designed to increase the limited displacement that these alloys can induce. The SMA actuator has been designed so that it can be bent up to about 180°, providing more freedom of movements and a better integration in wearable robots, specially in soft wearable robots, than standard rigid solutions. Although the actuator length is relatively short, this original design allows a great linear displacement, because it can have one or more loops of the same SMA wire inside the actuator. This implies that the length of the SMA wire is at least two times greater than the length of the actuator. The adopted strategy for both position and speed control that overcomes the hysteresis and prevents overheating the actuator is also described. The control algorithm has been implemented in a rapid control prototyping (RCP) system based on a low cost hardware platform. Finally, the application of this novel actuator in a wrist exoskeleton prototype is shown to demonstrate the feasibility of using the flexible SMA actuator in a real soft wearable robot. [ABSTRACT FROM AUTHOR]

Compliant pneumatic actuators have recently attracted the interests of the robotics community, especially for soft robotic applications where large strokes are needed in delicate environments. To date, these actuators have demonstrated the ability to generate contracting, expanding and bending strokes. This paper introduces a new actuator design that complements the existing actuators by generating twisting deformations upon pressurization. A fabrication process is developed that uses accessible PDMS soft lithography techniques. With this process, prototype actuators with a width of 7 mm and a thickness of 0.65 mm have been made that achieved a twisting rotation of 6.5°/mm actuator length at a pressure of 178 kPa. This paper also presents the integration of four twisting actuators into a 2 DOF tilting mirror platform which is capable of deflecting a mirror over 25° at a pressure of 231 kPa. [ABSTRACT FROM AUTHOR]

Magneto-Rheological Elastomers (MREs) are functional magnetic polymers filled with magnetic particles. MREs with fixed magnetic polarization within the particles, or H-MREs, develop a torque in response to an applied magnetic field. This ability leads to unique actuation properties which are critical for designs and applications of flexible actuators employing H-MREs. Thus, the primary goal of this paper is to characterize the relationship between torque generated and volume concentration of magnetic particles, an essential design parameter in H-MRE fabrication. To this end, a torque magnetometer based on a parallel plate capacitor was designed, and used to measure the torque response from H-MRE samples. The H-MRE samples were fabricated with varying percent volume of magnetic particles. The results from torque testing paralleled theoretical trends expected for the samples. An empirical correlation between volume content and maximum torque was found for samples above 10% volume composition and followed a linear trend. The equation governing the relationship was found from experimental results. [ABSTRACT FROM AUTHOR]