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Abstract Actuator plays a significant role in soft robotics. This paper proposed an ultralong stretchable soft actuator (US2A) with a variable and sizeable maximum elongation. The US2A is composed of a silicone rubber tube and a bellows woven sleeve. The maximal extension can be conveniently regulated by just adjusting the wrinkles’ initial angle of the bellows woven sleeve. The kinematics of US2A could be obtained by geometrically analyzing the structure of the bellows woven sleeve when the silicone rubber tube is inflated. Based on the principle of virtual work, the actuating models have been established: the pressure-elongation model and the pressure-force model. These models reflect the influence of the silicone tube’s shell thickness and material properties on the pneumatic muscle’s performance, which facilitates the optimal design of US2A for various working conditions. The experimental results showed that the maximum elongation of the US2A prototype is 257%, and the effective elongation could be variably regulated in the range of 0 and 257%. The proposed models were also verified by pressure-elongation and pressure-force experiments, with an average error of 5% and 2.5%, respectively. Finally, based on the US2A, we designed a pneumatic rehabilitation glove, soft arm robot, and rigid-soft coupling continuous robot, which further verified the feasibility of US2A as a soft driving component.

Typical soft actuators can perform unidirectional bending between pressurized and loose states. Multidirectional motion commonly requires multiple chambers or the bundling of multiple unidirectional actuators. If multidirectional motion can be achieved with a simple mechanism, it will provide considerable flexibility in designing objects such as grippers or wearable devices. Here, we present a new design for a multidirectional soft actuator that uses a combination of a temperature-sensitive material (TSM) and a non-temperature-sensitive material (NTSM). The actuator consists of a fluid chamber, and the bending is controlled by the temperature and pressure of the working fluid to balance the stiffness between the TSM and NTSM. In this study, the concept, design, and fabrication method are introduced, and the basic characteristics are discussed through experiments and modeling using a prototype actuator. The results of the static analysis of the curvature and curve fitting based on the model show that actuator bending in a static state can be represented by a simple linear-spring model. We also discuss the features, potential, and applications of the actuator by demonstrating its grasping capabilities.

Research on small‐scale soft robots has emerged in recent years, and various soft actuators have been developed to implement various actions. Nevertheless, realizing self‐sensing alongside environmental sensing capabilities remains a challenge, largely due to the constraints imposed by compact dimensions and the limited load‐bearing capacity intrinsic to these robots. In this study, an innovative approach is introduced through the development of a visualized sensing integrated soft actuator, which harnesses the actuator's color as a straightforward and real‐time sensing medium. Additionally, a multi‐responsive actuation mechanism is adopted in which the actuator responds concurrently to both electric and magnetic fields. To exemplify the efficacy of this concept, the actuators are engineered as flexible soft grippers, thereby accommodating functions encompassing grasping and transporting. Relying on the color distribution manifested by the actuator, the actuator's self‐sensing ability alongside its capacity to discern object temperatures is demonstrated. Such a soft actuator provides new perspectives in the realm of soft robotics, showing great potential in diverse robotic applications. [ABSTRACT FROM AUTHOR]

The application of soft actuators has become increasingly common in wearable devices. In this study, we investigated the force characteristics of soft actuators made entirely of elastic material, when equipped with solid external chamber plates of varying thickness that can be attached from the outside. This study examines the effect of these plates on the force characteristics of a fully silicone-based fifteen-chamber soft actuator without any non-stretchable internal components. The parameters of the actuator were determined with consideration of wearable applications, such as rehabilitation devices and exoskeletons. The observed differences in the behavior of the actuator at various pressure levels and plate thicknesses were measured. Furthermore, the effect of the externally inserted plates between the chambers on the passive bending of the actuator was examined. The obtained results were evaluated and compared to determine how external chamber plates of given thicknesses affect the operational performance of a soft actuator.

Complex environments, such as those found in surgical and search‐and‐rescue applications, require soft devices to adapt to minimal space conditions without sacrificing the ability to complete dexterous tasks. Stacked balloon actuators (SBAs) are capable of large deformations despite folding nearly flat when deflated, making them ideal candidates for such applications. This paper presents the design, fabrication, modeling, and characterization of monolithic, inflatable, soft SBAs. Modeling is presented using analytical principles based on geometry and then using conventional and real‐time finite‐element methods. Both one and three degree‐of‐freedom (DoF) SBAs are fully characterized with regard to stroke, force, and workspace. Finally, three representative demonstrations show the SBA's small‐aperture navigation, bracing, and workspace‐enhancing capabilities.

This paper presents the bio-mimetic design approach, the dynamic model, and potential applications for a hybrid soft actuator. The proposed hybrid soft actuator consists of two main parts: a cylinder-shaped rigid core and soft silicone spikes wrapped around the core’s surface. The key idea of the proposed design approach is to mimic the movement of a grass-spike at a functional level by converting the vibration force generated by a small electric motor with a counterweight in the rigid core into a propulsion force produced by the elastic restoration of the spikes. One advantage of this design approach is that the hybrid soft actuator does not need to be tethered by a tube line from an air compressor and is more amenable to fine control. In addition, the hybrid soft actuator can be modularized with a wire and a tubular passage, which in turn work as a linear actuator. The dynamic model of the hybrid soft actuator can be derived by applying Lagrangian mechanics, and unknown system parameters can be identified by the optimization process based on the empirical data. Two applications—an elbow manipulator and a robotic hand grasper—demonstrate the feasibility of the proposed actuator to perform a muscle-tendon action successfully.

Haptics involves human touch sensing and tactile feedback and plays a crucial role in physical interactions of humans with their environment. There is an ever‐increasing interest in development of haptic technologies, due to their role in various applications such as robotics, virtual and augmented reality, healthcare, and smart electronics. Electrically driven actuation mechanisms for soft materials like dielectric elastomer actuators (DEAs), electrohydraulic soft actuators (ESAs), ionic polymer−metal composites (IPMCs), and liquid crystal elastomers (LCEs) hold the potential for the development of the next generation of haptic feedback devices due to a variety of advantages such as light weight and compact design, untethered activation and control, large actuation strains, and distributed and localized actuation. Herein, a detailed look is taken at the advancement in material designs for these electrically driven soft actuators. A detailed analysis of the different strategies for improving the electromechanical performance of existing material systems is presented. Approaches adopted to synthesize novel material systems are explained. Advancements in compliant electrode materials for the electrically driven soft actuators are also described. The conclusion reflects on the main challenges in the field and provides perspectives on recent advancements expected to have a significant impact.

The internal channel networks embedded within a soft structure can be a fruitful mechanism to create and activate actuators in the research fields of soft robotics. The deformation of the supporting elastic structure from the pressurized viscous fluid into the channels needs an accurate investigation. In this paper, accurate modeling and dynamic analysis of this nonlinear soft actuator is our goal. In this modeling, the soft actuator is considered the Euler-Bernoulli beam with large deflection and nonlinear strain. After implementing Hamilton's principle, the assumed mode method is used to achieve the mathematical model in terms of the multi-mode system that is more similar to the flexible nature of the actuation system. Steady-state dynamics is investigated by a combination of the complex averaging method with arc-length continuation. The accuracy of the proposed modeling is validated by comparing simulation results to those obtained with a nonlinear finite element method and numerical method. It shows that only one-third of the degree-of-freedoms used for the finite element method are sufficient to obtain equivalent converged solutions with the proposed model. The effect of nonlinear strain and multi-mode consideration in the analysis of the proposed modeling is investigated. It is advantageous to analyze the system performance by looking into the geometrical parameters and fluid properties. [ABSTRACT FROM AUTHOR]

There is a growing interest in creating biomimetic actuators that can perform human‐like actions in response to external stimuli. Despite the significant advancements in soft actuators, using high‐energy UV light in photoresponsive actuators remains a great challenge. Herein, we develop a film composed of polyvinyl alcohol and photo responsive Disperse Red 1 (PVA/DR1) that actuate in response to visible light. The photoresponsive film starts to actuate in the presence of green light due to trans→cis isomerization of photoresponsive dye. The actuation response of the films was investigated by varying concentrations of DR1 dye and the intensity of green light. Based on these features, we demonstrated a four‐arm soft‐gripper that can mimic human‐hand‐like motions in response to green light. This kind of actuator may find potential applications in soft robotics and biomimetic microgrippers. [ABSTRACT FROM AUTHOR]

Abstract Soft actuators exhibit activeness and flexibility and are widely used as next‐generation intelligent devices. However, their locomotion depends on friction with contact surfaces that restrict their movement. To overcome this limitation, a noncontact‐type multiresponsive soft actuator that levitates in a magnetic field is proposed. This soft actuator can respond to humidity, heat, and diamagnetic repulsion force stimuli, resulting in high degrees of freedom and multiple motions. The soft actuator is fabricated by coating a highly hygroscopic membrane onto a diamagnetic graphite film, which enables the actuator to levitate in the magnetic field. Bending actuation is induced by the swelling mismatch between the two layers via the hygrothermal response. The translational force driven by local concentrated heating of the actuator leads to the realization of high‐speed linear and curvilinear motions. Frictionless rotational motion is also realized remotely with broad heating of an asymmetrically bent soft actuator, generating nonzero torque acting on the floating soft actuator. The proposed levitating soft actuators are applied to a fast and reliable capsule‐delivery gripper and a remotely controllable levitated motor. The soft actuator exhibits the potential to be applied in a wide range of applications, such as soft robotics and smart mechanical devices.

In this study, a method for fabricating actuators made of various silicone materials is proposed to improve the flexural performance of soft-body actuators. Specifically, the redundant deformation part of the soft actuator was replaced with a material with higher hardness to limit the redundant deformation of the soft actuator. Materials with lower hardness were used to produce the main deformation part of the soft actuator, so that the soft body actuator could perform greater bending under the same air pressure and create a greater bending force. In addition, the fabricated actuator was divided into three regions in this study: the periphery of the chamber, the chamber wall (the main curved part), and the bottom surface of the actuator. The impact on the overall performance of soft-body actuators when using silicone materials with different hardness in these three regions was explored in this study. According to the idea of the multi-hardness silicone structure, an actuator with seven chambers was fabricated, and the performance of the actuator was improved by 90.72% compared with the uniform material actuator.

Soft actuators are a promising option for the advancing fields of human-machine interaction and dexterous robots in complex environments. Shape memory alloy wire actuators can be integrated into fiber rubber composites for highly deformable structures. For autonomous, closed-loop control of such systems, additional integrated sensors are necessary. In this work, a soft actuator is presented that incorporates fiber-based actuators and sensors to monitor both deformation and temperature. The soft actuator showed considerable deformation around two solid body joints, which was then compared to the sensor signals, and their correlation was analyzed. Both, the actuator as well as the sensor materials were processed by braiding and tailored fiber placement before molding with silicone rubber. Finally, the novel fiber-rubber composite material was used to implement closed-loop control of the actuator with a maximum error of 0.5°.

Shape memory materials have been widely used as programmable soft matter for developing multifunctional hybrid actuators. Several challenges of fabrication and effective modelling of these soft actuating systems can be addressed by implementing novel 3D printing techniques and simulations to aid the designer. In this study, the temperature-dependent recovery of an embedded U-shaped Shape Memory Alloy (SMA) and the shape fixity of a 3D-printed Shape Memory Polymer (SMP) matrix were exploited to create a bi-state Shape Memory Composite (SMC) soft actuator. Electrical heating allowed the SMA to achieve the bi-state condition, undergoing phase transformation to a U shape in the rubbery phase and a flat shape in the glassy phase of the SMP. A COMSOL Multiphysics model was developed to predict the deformation and recovery of the SMC by leveraging the in-built SMA constitutive relations and user-defined material subroutine for the SMP. The bi-state actuation model was validated by capturing the mid-point displacement of the 80 mm length × 10 mm width × 2 mm-thick 3D-printed SMC. The viability of the SMC as a periodic actuator in terms of shape recovery was addressed through modelling and simulation. Results indicated that the proposed COMSOL model was in good agreement with the experiment. In addition, the effect of varying the volume ratio of the SMA wire in the SMC on the maximum and recovered deflection was also obtained. Our model can be used to design SMC actuators with various performance profiles to facilitate future designs in soft robotics and wearable technology applications.

Recently, attempts have been made to develop ionic polymer–metal composite (IPMC), which is garnering growing interest for ionic artificial muscle, as a soft actuator and sensor due to its inherent properties of low weight, flexibility, softness, and particularly, its efficient transformation of electrical energy into mechanical energy, with large bending strain response under a low activation voltage. In this paper, we focused on several current deficiencies of IPMC that restrict its application, such as non-standardized preparation steps, relaxation under DC voltage, solvent evaporation, and poor output force. Corresponding solutions to overcome the abovementioned problems have recently been proposed from our point of view and developed through our research. After a brief introduction to the working mechanism of IPMC, we here investigate the key factors that influence the actuating performance of IPMC. We also review the optimization strategies in IPMC actuation, including those for preparation steps, additive selection for a thick casting membrane, solvent substitutes, water content, encapsulation, etc. With consideration of the role of the interface electrode, its effects on the performance of IPMC are revealed based on our previous work. Finally, we also discuss IPMCs as potential sensors theoretically and experimentally. The elimination of the deficiencies of IPMC will promote its applications in soft robotics.

Low-voltage soft actuators with large displacement, long actuation durability, and fast response time have aroused great attention in soft robotics, wearable devices, haptic devices, and implantable or disposal biomedical devices. Herein, we report a low-voltage electroactive ionic soft actuator based on the sulfonated cellulose nanowhisker (SCN), microfibrillated cellulose (MFC), ionic liquids (IL), and graphene nanoplatelet (GN). The proposed SCN/MFC-IL-GN (0.1wt%) actuator demonstrated a large bending displacement (6.6 mm under ± 1 V sinusoidal input signal at 0.1 Hz, low driving voltage (as low as 0.25 V), wide actuation frequency (0.1 to 5.0 Hz), and long actuation durability (96.7% retention for 1 h), all of which stemmed from the strong crosslinking and ionic interactions among the functional sulfonated groups of SCN, hydroxyl groups of MFC, IL, and GN. Furthermore, the designed actuator was successfully employed to imitate the human finger's behaviors including turning on/off the flashlight and sliding electronic photographs on a smart phone screen. Therefore, the proposed SCN/MFC-IL-GN actuator has great potential in artificial muscles, soft robots, haptic devices, and wearable devices because of its low excitation voltage, large bending displacement, long actuation durability, and biofriendly property. [ABSTRACT FROM AUTHOR]

The prospects of endowing artificial robotics or devices with increasingly complex and emergent life‐like behaviors have attracted growing interest in the soft functional materials that mimic the versatile motions of living creatures in the iridescent nature. However, despite the flourishing achievements so far, soft actuators capable of sensitive multi‐stimulus responses and self‐sustainable movements, have been extensively pursued to reduce control complexity yet remains a challenging target. Here, through material‐structural synergistic design incorporating stress‐mismatching structure, high pseudo‐negative coefficient of thermal expansion of perfluoro‐sulfonic acid ionomer, comprehensive converting properties of carbon nanotube, and anisotropic large thermal expansion of PE polymer, an ionomer‐based bilayer actuator is proposed, presenting high‐performance actuation of various forms and nice stability, responsive to light (including sunlight without focusing, LED light), low voltage, mild heating, and humidity/solvent change. With a built‐in structural feedback loop, the actuation performances are further explored to realize intelligent systems, including: 1) self‐sustainable locomotion under sunlight irradiation with adjustable photophobic and phototropic direction as well as adaption to different topographies and loading conditions, 2) self‐sustainable oscillation and solar‐electric generating, and 3) bionic floristic reaction according to environmental change. These diversified actuation modes allow promising following‐up designs for bio‐hybrid soft robotics fueled by and harmonized with natural environments. [ABSTRACT FROM AUTHOR]

Soft actuators exhibit activeness and flexibility and are widely used as next‐generation intelligent devices. However, their locomotion depends on friction with contact surfaces that restrict their movement. To overcome this limitation, a noncontact‐type multiresponsive soft actuator that levitates in a magnetic field is proposed. This soft actuator can respond to humidity, heat, and diamagnetic repulsion force stimuli, resulting in high degrees of freedom and multiple motions. The soft actuator is fabricated by coating a highly hygroscopic membrane onto a diamagnetic graphite film, which enables the actuator to levitate in the magnetic field. Bending actuation is induced by the swelling mismatch between the two layers via the hygrothermal response. The translational force driven by local concentrated heating of the actuator leads to the realization of high‐speed linear and curvilinear motions. Frictionless rotational motion is also realized remotely with broad heating of an asymmetrically bent soft actuator, generating nonzero torque acting on the floating soft actuator. The proposed levitating soft actuators are applied to a fast and reliable capsule‐delivery gripper and a remotely controllable levitated motor. The soft actuator exhibits the potential to be applied in a wide range of applications, such as soft robotics and smart mechanical devices. [ABSTRACT FROM AUTHOR]

Matching the rich multimodality of natural organisms, i.e., the ability to transition between crawling and swimming, walking and jumping, etc., represents a grand challenge in the fields of soft and bio‐inspired robotics. Here, a multimodal soft robot locomotion using highly compact and dynamic bistable soft actuators is achieved. These actuators are composed of a prestretched membrane sandwiched between two 3D printed frames with embedded shape memory alloy (SMA) coils. The actuator can swiftly transform between two oppositely curved states and generate a force of 0.3 N through a snap‐through instability that is triggered after 0.2 s of electrical activation with an input power of 21.1 ± 0.32 W (i.e., electrical energy input of 4.22 ± 0.06 J. The consistency and robustness of the snap‐through actuator response is experimentally validated through cyclical testing (580 cycles). The compact and fast‐responding properties of the soft bistable actuator allow it to be used as an artificial muscle for shape‐reconfigurable soft robots capable of multiple modes of SMA‐powered locomotion. This is demonstrated by creating three soft robots, including a reconfigurable amphibious robot that can walk on land and swim in water, a jumping robot (multimodal crawler) that can crawl and jump, and a caterpillar‐inspired rolling robot that can crawl and roll. [ABSTRACT FROM AUTHOR]

SUMMARY: In recent years, soft robotics is widely considered as the most promising field for both research and application. First of all, the actuator is fundamental for designing, modeling, and controlling of soft robots. This paper presents a new type of pneumatic trunk-like soft actuator, which contains a chamber for stiffness adjustment in addition to three chambers for driving. Thus, the salient feature of the proposed actuator is the ability of stiffness self-regulation. The structure of the proposed actuator is described in detail. Then the theoretical models for elongation and bending motion of the actuator are established. The elongation as well as single-chamber and multi-chamber driving bending of the actuator were tested to verify the mathematical models. Finally, a dual-segment soft robot based on the proposed trunk-like soft actuator was developed and tested by experiments, which implies its potential application in practice. [ABSTRACT FROM AUTHOR]

Recently, self‐driven soft robotics based on biomimetics, capable of mimicking biological motion, has attracted attention. Soft actuators using intrinsically soft organic materials are expected to be applied to haptic devices, artificial muscles, and micropumps. Ferroelectric polymers can aid in the realization of such soft actuators. However, actuators using such materials encounter problems in terms of the response frequency to an applied voltage. In this study, a soft actuator is fabricated by a printing process using a unique composite material comprising P(VDF‐TrFE), nano‐carbon material (single‐walled carbon nanotubes (SWCNT) and graphene oxide (GO)), and conductive polymer. To characterize the actuator using a minimum substrate thickness of 25 µm, hysteresis curves in the ferroelectric properties and driving characteristics according to the applied frequency are clarified. In addition, the mechanical life of the actuator under continuous voltage sweep is clarified considering it as a mechanical property. Subsequently, a simple haptics system is constructed using the fabricated actuators, and a human‐sensitive actuator demonstration system is constructed wherein the phase of the sweep frequency is variable. [ABSTRACT FROM AUTHOR]

A cylindrical soft actuator is suitable for applications in which pneumatic or hydraulic cylinders are conventionally used. In this report, we discuss the force output model of a spring-reinforced-type cylindrical soft actuator. This type of actuator outputs a larger force than the air pressure multiplied by the pressure-receiving area. We construct a quasi-static model to explore the reason for this phenomenon, based on the strength of materials. A thick-walled cylinder model with three boundary conditions was defined and analyzed. The model indicates that the rubber cylinder itself transmits pneumatic pressure and contributes to the output force. We also modeled the relationship between the pressure and the elongation of the soft actuator. Experiments were conducted to evaluate the proposed models.

Shape reconfiguration is ubiquitous in nature and widely used in many applications such as soft robotics, metamaterials, energy absorption and tissue engineering. Shape reconfigurable soft actuators, due to their ability to adapt and adjust in complex and unpredictable working environment, have been designed by the use of various delicate structures and active materials. However, soft actuators that exhibit reconfigurable helical deformation have not been proposed; they have the advantage of integrating both bending and twisting actuations in one deformation mode. In this work, we present a thermal-induced shape reconfigurable soft actuator that shows reversible actuations with vastly shape differences under thermal stimulus. It exhibits helical deformation at lower temperature and mainly in-plane bending at relatively higher temperature. The reversible shape transition is controlled by a thermal stimulus that changes the anisotropy of the structure, which consists of shape memory polymer fibers embedded in a homogeneous elastic matrix. A theoretical model is proposed based on the minimum potential energy that incorporates the thermomechanical behavior of the shape memory polymer fibers. Experiments are conducted and the results agree well with the theoretical modeling. Using the theoretical model, we establish design principles for reconfigurable soft actuators whose functional response is programmable given the architecture and external stimulus. A six-handed helical soft actuator, constructed to demonstrate its programmable deformation, is utilized to catch a living fish in water. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Conventional model theories are not suitable to control soft-bodied robots as deformable materials present rapidly changing behaviors. Neuromorphic electronics are now entering the field of robotics, demonstrating that a highly integrated device can mimic the fundamental properties of a sensory synaptic system, including learning and proprioception. This research work focuses on the physical implementation of a reservoir computing-based network to actuate a soft-bodied robot. More specifically, modeling the hysteresis of a shape memory alloy (SMA) using echo state networks (ESN) in real-world situations represents a novel approach to enable soft machines with task-learning. In this work, we show that not only does our ESN model enable our SMA-based robot with locomotion, but it also discovers a successful strategy to do so. Compared to standard control modeling, established either by theoretical frameworks or from experimental data, here, we gained knowledge a posteriori, guided by the physical interactions between the trained model and the controlled actuator, interactions from which striking patterns emerged, and informed us about what type of locomotion would work best for our robot. [ABSTRACT FROM AUTHOR]

The application of soft actuators has become increasingly common in wearable devices. In this study, we investigated the force characteristics of soft actuators made entirely of elastic material, when equipped with solid external chamber plates of varying thickness that can be attached from the outside. This study examines the effect of these plates on the force characteristics of a fully silicone-based fifteen-chamber soft actuator without any non-stretchable internal components. The parameters of the actuator were determined with consideration of wearable applications, such as rehabilitation devices and exoskeletons. The observed differences in the behavior of the actuator at various pressure levels and plate thicknesses were measured. Furthermore, the effect of the externally inserted plates between the chambers on the passive bending of the actuator was examined. The obtained results were evaluated and compared to determine how external chamber plates of given thicknesses affect the operational performance of a soft actuator. [ABSTRACT FROM AUTHOR]

Nanocellulose is currently in the limelight of extensive research from fundamental science to technological applications owing to its renewable and carbon‐neutral nature, superior biocompatibility, tailorable surface chemistry, and unprecedented optical and mechanical properties. Herein, an up‐to‐date account of the recent advancements in nanocellulose‐derived functional materials and their emerging applications in areas of chiral photonics, soft actuators, energy storage, and biomedical science is provided. The fundamental design and synthesis strategies for nanocellulose‐based functional materials are discussed. Their unique properties, underlying mechanisms, and potential applications are highlighted. Finally, this review provides a brief conclusion and elucidates both the challenges and opportunities of the intriguing nanocellulose‐based technologies rooted in materials and chemistry science. This review is expected to provide new insights for nanocellulose‐based chiral photonics, soft robotics, advanced energy, and novel biomedical technologies, and promote the rapid development of these highly interdisciplinary fields, including nanotechnology, nanoscience, biology, physics, synthetic chemistry, materials science, and device engineering. [ABSTRACT FROM AUTHOR]