Dielectric Elastomer Based "Grippers" for Soft Robotics

A high-response-frequency bimodal network polyacrylate elastomer with ultrahigh power density under low electric field

Dielectric elastomers, used as driver modules, require high power density to enable fast movement and efficient work of soft robots. Polyacrylate elastomers usually suffer from low power density under low electric fields due to limited response frequency. Here, we propose a bimodal network polyacrylate dielectric elastomer which breaks the intrinsic coupling relationship between dielectric and mechanical properties, featuring relatively high dielectric constant, low Young's modulus, and wide driving frequency bandwidth (~200 Hz) like silicones. Therefore, an ultrahigh power density (154 W kg-1@20 MV m-1, 200 Hz) is realized at low electric field and high resonance frequency, 75 times greater than at 10 Hz. Further, a rotary motor is developed, reaching an impressive speed of 1245 rpm at 19.6 MV m-1 and 125 Hz, surpassing previous acrylate-based motors and entering the high-speed domain of silicone-based motors. These findings offer a versatile strategy to fabricate high-power-density dielectric elastomers for low-electric-field soft actuators.

Review of Flexible Robotic Grippers, with a Focus on Grippers Based on Magnetorheological Materials

Flexible grippers are a promising and pivotal technology for robotic grasping and manipulation tasks. Remarkably, magnetorheological (MR) materials, recognized as intelligent materials with exceptional performance, are extensively employed in flexible grippers. This review aims to provide an overview of flexible robotic grippers and highlight the application of MR materials within them, thereby fostering research and development in this field. This work begins by introducing various common types of flexible grippers, including shape memory alloys (SMAs), pneumatic flexible grippers, and dielectric elastomers, illustrating their distinctive characteristics and application domains. Additionally, it explores the development and prospects of magnetorheological materials, recognizing their significant contributions to the field. Subsequently, MR flexible grippers are categorized into three types: those with viscosity/stiffness variation capabilities, magnetic actuation systems, and adhesion mechanisms. Each category is comprehensively analyzed, specifying its unique features, advantages, and current cutting-edge applications. By undertaking an in-depth examination of diverse flexible robotic gripper types and the characteristics and application scenarios of MR materials, this paper offers a valuable reference for fellow researchers. As a result, it facilitates further advancements in this field and contributes to the provision of efficient gripping solutions for industrial automation.

Adaptive Neural Control for Hysteretic Nonlinear Systems With Hysteresis Neural Direct Inverse Compensator and Its Application

Aiming at high precision control for a class of hysteretic nonlinear systems, a new hysteresis direct inverse compensator-based adaptive output feedback control scheme is designed in this article. First, a novel long short-term memory neural network (LSTMNN)-based hysteresis inverse compensator is established to compensate the asymmetric hysteresis nonlinearity, where the LSTMNN is used as the prediction mechanism for model operator weights, rather than the overall mapping of hysteresis input and output. Second, by designing the modified high-gain K-Filter states observer and the error transformed function, the unmeasurable states are estimated with arbitrarily small estimation error and the prespecified tracking performance is achieved. Lastly, the biconical dielectric elastomer actuator (DEA) motion platform is constructed. Then, the effectiveness of the proposed LSTMNN-based hysteresis inverse compensator and control scheme are verified on the experimental platform. The experimental results illustrate the effectiveness and advantages of proposed control scheme.

Soft Gripping Fingers Made of Multi-Stacked Dielectric Elastomer Actuators with Backbone Strategy

Soft grippers, a rapidly growing subfield of soft robotics, utilize compliant and flexible materials capable of conforming to various shapes. This feature enables them to exert gentle yet, if required, strong gripping forces. In this study, we elaborate on the material selection and fabrication process of gripping fingers based on the dielectric elastomer actuation technique. We study the effects of mixing the silicone elastomer with a silicone thinner on the performance of the actuators. Inspired by nature, where the motion of end-effectors such as soft limbs or fingers is, in many cases, directed by a stiff skeleton, we utilize backbones for translating the planar actuation into a bending motion. Thus, the finger does not need any rigid frame or pre-stretch, as in many other DEA approaches. The idea and function of the backbone strategy are demonstrated by finite element method simulations with COMSOL Multiphysics® 6.5. The paper describes the full methodology from material choice and characterization, design, and simulation to characterization to enable future developments based on our approach. Finally, we present the performance of these actuators in a gripper demonstrator setup. The developed actuators bend up to 68.3° against gravity, and the gripper fingers hold up to 10.3 g against gravity under an actuation voltage of 8 kV.

Advanced Design of Soft Robots with Artificial Intelligence

A comprehensive review focused on the whole systems of the soft robotics with artificial intelligence, which can feel, think, react and interact with humans, is presented. The design strategies concerning about various aspects of the soft robotics, like component materials, device structures, prepared technologies, integrated method, and potential applications, are summarized. A broad outlook on the future considerations for the soft robots is proposed.

In recent years, breakthrough has been made in the field of artificial intelligence (AI), which has also revolutionized the industry of robotics. Soft robots featured with high-level safety, less weight, lower power consumption have always been one of the research hotspots. Recently, multifunctional sensors for perception of soft robotics have been rapidly developed, while more algorithms and models of machine learning with high accuracy have been optimized and proposed. Designs of soft robots with AI have also been advanced ranging from multimodal sensing, human–machine interaction to effective actuation in robotic systems. Nonetheless, comprehensive reviews concerning the new developments and strategies for the ingenious design of the soft robotic systems equipped with AI are rare. Here, the new development is systematically reviewed in the field of soft robots with AI. First, background and mechanisms of soft robotic systems are briefed, after which development focused on how to endow the soft robots with AI, including the aspects of feeling, thought and reaction, is illustrated. Next, applications of soft robots with AI are systematically summarized and discussed together with advanced strategies proposed for performance enhancement. Design thoughts for future intelligent soft robotics are pointed out. Finally, some perspectives are put forward.

Skin-inspired, sensory robots for electronic implants

Drawing inspiration from cohesive integration of skeletal muscles and sensory skins in vertebrate animals, we present a design strategy of soft robots, primarily consisting of an electronic skin (e-skin) and an artificial muscle. These robots integrate multifunctional sensing and on-demand actuation into a biocompatible platform using an in-situ solution-based method. They feature biomimetic designs that enable adaptive motions and stress-free contact with tissues, supported by a battery-free wireless module for untethered operation. Demonstrations range from a robotic cuff for detecting blood pressure, to a robotic gripper for tracking bladder volume, an ingestible robot for pH sensing and on-site drug delivery, and a robotic patch for quantifying cardiac function and delivering electrotherapy, highlighting the application versatilities and potentials of the bio-inspired soft robots. Our designs establish a universal strategy with a broad range of sensing and responsive materials, to form integrated soft robots for medical technology and beyond.

Ultrarobust Actuator Comprising High-Strength Carbon Fibers and Commercially Available Polycarbonate with Multi-Stimulus Responses and Programmable Deformation

Polymer-based actuators have gained extensive attention owing to their potential applications in aerospace, soft robotics, etc. However, poor mechanical properties, the inability of multi-stimuli response and programmable deformation, and the costly fabrication procedure have significantly hindered their practical application. Herein, these issues are overcome via a simple and scalable one-step molding method. The actuator is fabricated by hot-pressing commercial unidirectional carbon fiber/epoxy prepregs with a commodity PC membrane. Notable CTE differences between the CF and PC layers endow the bilayer actuator with fast and reliable actuation deformation. Benefiting from the high strength of CF, the actuator exhibits excellent mechanical performance. Moreover, the anisotropy of CF endows the actuator with design flexibility. Furthermore, the multifunction of CF makes the actuator capable of responding to thermal, optical, and electrical stimulation simultaneously. Based on the bilayer actuator, we successfully fabricated intelligent devices such as light-driven biomimetic flowers, intelligent grippers, and gesture-simulating apparatuses, which further validate the programmability and multi-stimuli response characteristics of this actuator. Strikingly, the prepared gripper possesses a grasping capacity approximately 31.2 times its own weight. It is thus believed that the concept presented paves the way for building next-generation robust robotics.

Repeatedly Programmable Liquid Crystal Dielectric Elastomer with Multimodal Actuation

Dielectric elastomers (DEs) are actuatable under an electric field, whose large strain and fast response speed compare favorably with natural muscles. However, the actuation of DE-based devices is generally limited to a single mode and cannot be reconfigured after fabrication, which pales in comparison to biological counterparts given the ability to alter actuation modes according to external conditions. To address this, liquid crystal dielectric elastomers (LC-DEs) that can alter the dielectric actuation modes based on the thermally triggered shape-changing are prepared. Specifically, the two shapes through the LC phase transition possess different bending stiffness, which leads to distinct actuation modes after an electric field is applied. Moreover, the two shapes can be individually programmed/reprogrammed, that is, the one before the transition is regulated through force-directed solvent evaporation and the one after the transition is via bond exchange-enabled stress relaxation. As such, the multimodal dielectric actuation behaviors upon temperature change can be readily diversified. Meanwhile, the space charge mechanism endows LC-DEs with the significantly reduced driving e-field (8 V µm-1) and bidirectional actuation manners. It is believed this unique adaptivity in the actuation modes under a low electric field shall offer versatile designs for practical soft robots.

Active Materials for Functional Origami

In recent decades, origami has been explored to aid in the design of engineering structures. These structures span multiple scales and have been demonstrated to be used toward various areas such as aerospace, metamaterial, biomedical, robotics, and architectural applications. Conventionally, origami or deployable structures have been actuated by hands, motors, or pneumatic actuators, which can result in heavy or bulky structures. On the other hand, active materials, which reconfigure in response to external stimulus, eliminate the need for external mechanical loads and bulky actuation systems. Thus, in recent years, active materials incorporated with deployable structures have shown promise for remote actuation of light weight, programmable origami. In this review, active materials such as shape memory polymers (SMPs) and alloys (SMAs), hydrogels, liquid crystal elastomers (LCEs), magnetic soft materials (MSMs), and covalent adaptable network (CAN) polymers, their actuation mechanisms, as well as how they have been utilized for active origami and where these structures are applicable is discussed. Additionally, the state-of-the-art fabrication methods to construct active origami are highlighted. The existing structural modeling strategies for origami, the constitutive models used to describe active materials, and the largest challenges and future directions for active origami research are summarized.

Soft Sensors and Actuators for Wearable Human-Machine Interfaces

Haptic human-machine interfaces (HHMIs) combine tactile sensation and haptic feedback to allow humans to interact closely with machines and robots, providing immersive experiences and convenient lifestyles. Significant progress has been made in developing wearable sensors that accurately detect physical and electrophysiological stimuli with improved softness, functionality, reliability, and selectivity. In addition, soft actuating systems have been developed to provide high-quality haptic feedback by precisely controlling force, displacement, frequency, and spatial resolution. In this Review, we discuss the latest technological advances of soft sensors and actuators for the demonstration of wearable HHMIs. We particularly focus on highlighting material and structural approaches that enable desired sensing and feedback properties necessary for effective wearable HHMIs. Furthermore, promising practical applications of current HHMI technology in various areas such as the metaverse, robotics, and user-interactive devices are discussed in detail. Finally, this Review further concludes by discussing the outlook for next-generation HHMI technology.

Muscle-inspired soft robots based on bilateral dielectric elastomer actuators

Muscle groups perform their functions in the human body via bilateral muscle actuation, which brings bionic inspiration to artificial robot design. Building soft robotic systems with artificial muscles and multiple control dimensions could be an effective means to develop highly controllable soft robots. Here, we report a bilateral actuator with a bilateral deformation function similar to that of a muscle group that can be used for soft robots. To construct this bilateral actuator, a low-cost VHB 4910 dielectric elastomer was selected as the artificial muscle, and polymer films manufactured with specific shapes served as the actuator frame. By end-to-end connecting these bilateral actuators, a gear-shaped 3D soft robot with diverse motion capabilities could be developed, benefiting from adjustable actuation combinations. Lying on the ground with all feet on the ground, a crawling soft robot with dexterous movement along multiple directions was realized. Moreover, the directional steering was instantaneous and efficient. With two feet standing on the ground, it also acted as a rolling soft robot that can achieve bidirectional rolling motion and climbing motion on a 2° slope. Finally, inspired by the orbicularis oris muscle in the mouth, a mouthlike soft robot that could bite and grab objects 5.3 times of its body weight was demonstrated. The bidirectional function of a single actuator and the various combination modes among multiple actuators together allow the soft robots to exhibit diverse functionalities and flexibility, which provides a very valuable reference for the design of highly controllable soft robots.

Perceptual Soft End-Effectors for Future Unmanned Agriculture

As consumers demand ever-higher quality standards for agricultural products, the inspection of such goods has become an integral component of the agricultural production process. Unfortunately, traditional testing methods necessitate the deployment of numerous bulky machines and cannot accurately determine the quality of produce prior to harvest. In recent years, with the advancement of soft robot technology, stretchable electronic technology, and material science, integrating flexible plant wearable sensors on soft end-effectors has been considered an attractive solution to these problems. This paper critically reviews soft end-effectors, selecting the appropriate drive mode according to the challenges and application scenarios in agriculture: electrically driven, fluid power, and smart material actuators. In addition, a presentation of various sensors installed on soft end-effectors specifically designed for agricultural applications is provided. These sensors include strain, temperature, humidity, and chemical sensors. Lastly, an in-depth analysis is conducted on the significance of implementing soft end-effectors in agriculture as well as the potential opportunities and challenges that will arise in the future.

Constitutive formulations for intrinsic anisotropy in soft electroelastic materials

Inspired by biology and engineered soft active material systems, we propose a new constitutive formulation for a soft material consisting of soft contractile fibers embedded in a soft matrix. The mathematical implementation of the model is based on a multi-field invariant formulation within a nonlinear continuum mechanics framework. The coupled constitutive formulation highlights a new electromechanical coupling term that describes the intrinsic (or active) anisotropy due to the contractile units. The model demonstrates the relative role that intrinsic anisotropy plays in the overall stress response. The resulting formulation could be used to design and inspire the development of new soft material systems that seek to replicate three dimensional biological motion.

A stiffness-tunable soft actuator inspired by helix for medical applications

PURPOSE

This paper introduces the stiffness-tunable soft actuator (STSA), a novel device that combines a silicone body with a thermoplastic resin structure (TPRS). The STSA's design allows for the variable stiffness of soft robots, significantly increasing their potential for use in medical scenarios such as minimally invasive surgeries (MIS). By adjusting the stiffness of the STSA, it is possible to enhance the robot's dexterity and adaptability, making it a promising tool for performing complex tasks in narrow and delicate spaces.

METHODS

The stiffness of the STSA can be modulated by altering the temperature of the TPRS, which has been inspired by the helix and is integrated into the soft actuator to achieve a broad range of stiffness modulation while maintaining flexibility. The STSA has been designed with both diagnostic and therapeutic functions in mind, with the hollow area of the TPRS serving as an instrument channel for delivering surgical instruments. Additionally, the STSA features three uniformly arranged pipelines for actuation by air or tendon, and can be expanded with more functional chambers for endoscopy, illumination, water injection, and other purposes.

RESULTS

Experimental results show that the STSA can achieve a maximum 30-fold stiffness tuning, providing a significant improvement in load capacity and stability when compared to pure soft actuators (PSAs). Of particular importance, the STSA is capable of achieving stiffness modulation below 45 °C, thereby ensuring a safe entry into the human body and creating an environment conducive to the normal operation of surgical instruments such as endoscope.

CONCLUSION

The experimental findings indicate that the soft actuator with TPRS can achieve a broad range of stiffness modulation while retaining flexibility. Moreover, the STSA can be designed to have a diameter of 8-10 mm, which satisfies the diameter requirements of a bronchoscope. Furthermore, the STSA has the potential to be utilized for clamping and ablation in a laparoscopic scenario, thereby demonstrating its potential for clinical use. Overall, these results suggest that the STSA has significant promise for use in medical applications, particularly in the context of minimally invasive surgeries.

Grasping through dynamic weaving with entangled closed loops

Pick-and-place is essential in diverse robotic applications for industries including manufacturing, and assembly. Soft grippers offer a cost-effective, and low-maintenance alternative for secure object grasping without complex sensing and control systems. However, their inherent softness normally limits payload capabilities and robustness to external disturbances, constraining their applications and hindering reliable performance. In this study, we propose a weaving-inspired grasping mechanism that substantially increases payload capacity while maintaining the use of soft and flexible materials. Drawing from weaving principles, we designed a flexible continuum structure featuring multiple closed-loop strips and employing a kirigami-inspired approach to enable the instantaneous and reversible creation of a woven configuration. The mechanical stability of the woven configuration offers exceptional loading capacity, while the softness of the gripper material ensures safe and adaptive interactions with objects. Experimental results show that the 130 g·f gripper can support up to 100 kg·f. Outperforming competitors in similar weight and softness domains, this breakthrough, enabled by the weaving principle, will broaden the scope of gripper applications to previously inaccessible or barely accessible fields, such as agriculture and logistics.

Methods for numerical simulation of soft actively contractile materials

Soft materials that can demonstrate on demand reconfigurability and changing compliance are highly sought after as actuator materials in many fields such as soft robotics and biotechnology. Whilst there are numerous proof of concept materials and devices, rigorous predictive models of deformation have not been well-established or widely adopted. In this paper, we discuss programming complex three-dimensional deformations of a soft intrinsically anisotropic material by controlling the orientation of the contractile units and/or direction of the applied electric field. Programming is achieved by patterning contractile units and/or selectively activating spatial regions. A new constitutive model is derived to describe the soft intrinsic anisotropy of soft materials. The model is developed within a continuum mechanics framework using an invariant-based formulation. Computational implementation allows us to simulate the complex three-dimensional shape response when activated by electric field. Several examples of the achievable Gauss-curved surfaces are demonstrated. Our computational analysis introduces a mechanics-based framework for design when considering soft morphing materials with intrinsic anisotropy, and is meant to inspire the development of new soft active materials.

Sustainable Elastomers for Actuators: "Green" Synthetic Approaches and Material Properties

Elastomeric materials have great application potential in actuator design and soft robot development. The most common elastomers used for these purposes are polyurethanes, silicones, and acrylic elastomers due to their outstanding physical, mechanical, and electrical properties. Currently, these types of polymers are produced by traditional synthetic methods, which may be harmful to the environment and hazardous to human health. The development of new synthetic routes using green chemistry principles is an important step to reduce the ecological footprint and create more sustainable biocompatible materials. Another promising trend is the synthesis of other types of elastomers from renewable bioresources, such as terpenes, lignin, chitin, various bio-oils, etc. The aim of this review is to address existing approaches to the synthesis of elastomers using "green" chemistry methods, compare the properties of sustainable elastomers with the properties of materials produced by traditional methods, and analyze the feasibility of said sustainable elastomers for the development of actuators. Finally, the advantages and challenges of existing "green" methods of elastomer synthesis will be summarized, along with an estimation of future development prospects.

Enhanced Rupture Force in a Cut-Dispersed Double-Network Hydrogel

The Kirigami approach is an effective way to realize controllable deformation of intelligent materials via introducing cuts into bulk materials. For materials ranging from ordinary stiff materials such as glass, ceramics, and metals to soft materials, including ordinary hydrogels and elastomers, all of them are all sensitive to the presence of cuts, which usually act as defects to deteriorate mechanical properties. Herein, we study the influence of the cuts on the mechanical properties by introducing "dispersed macro-scale cuts" into a model tough double network (DN) hydrogel (named D-cut gel), which consists of a rigid and brittle first network and a ductile stretchable second network. For comparison, DN gels with "continuous cuts" having the same number of interconnected cuts (named C-cut gel) were chosen. The fracture tests of D-cut gel and C-cut gel with different cut patterns were performed. The fracture observation revealed that crack blunting occurred at each cut tip, and a large wrinkle-like zone was formed where the wrinkles were parallel to the propagation direction of the cut. By utilizing homemade circular polarizing optical systems, we found that introducing dispersed cuts increases the rupture force by homogenizing the stress around the crack tip surrounding every cut, which reduces stress concentration in one certain cut. We believe this work reveals the fracture mechanism of tough soft materials with a kirigami cut structure, which should guide the design of advanced soft and tough materials along this line.

Shape Memory Alloys in Textile Platform: Smart Textile-Composite Actuator and Its Application to Soft Grippers

In recent years, many researchers have aimed to construct robotic soft grippers that can handle fragile or unusually shaped objects without causing damage. This study proposes a smart textile-composite actuator and its application to a soft robotic gripper. An active fiber and an inactive fiber are combined together using knitting techniques to manufacture a textile actuator. The active fiber is a shape memory alloy (SMA) that is wire-wrapped with conventional fibers, and the inactive fiber is a knitting yarn. A knitted textile structure is flexible, with an excellent structure retention ability and high compliance, which is suitable for developing soft grippers. A driving source of the actuator is the SMA wire, which deforms under heating due to the shape memory effect. Through experiments, the course-to-wale ratio, the number of bundling SMA wires, and the driving current value needed to achieve the maximum deformation of the actuator were investigated. Three actuators were stitched together to make up each finger of the gripper, and layer placement research was completed to find the fingers' suitable bending angle for object grasping. Finally, the gripping performance was evaluated through a test of grasping various object shapes, which demonstrated that the gripper could successfully lift flat/spherical/uniquely shaped objects.

Effects of Electrode Materials and Compositions on the Resistance Behavior of Dielectric Elastomer Transducers

Dielectric elastomer (DE) transducers possess various advantages in comparison to alternative actuator technologies, such as, e.g., electromagnetic drive systems. DE can achieve large deformations, high driving frequencies, and are energy efficient. DEs consist of a dielectric membrane sandwiched between conductive electrodes. Electrodes are especially important for performance, as they must maintain high electrical conductivity while being subjected to large stretches. Low electrical resistances allow faster actuation frequencies. Additionally, a rate-independent, monotonic, and hysteresis-free resistance behavior over large elongations enables DEs to be used as resistive deformation sensors, in contrast to the conventional capacitive ones. This paper presents a systematic study on various electrode compositions consisting of different polydimethylsiloxane (PDMS) and nano-scaled carbon blacks (CB). The experiments show that the electrode resistance depends on the weight ratio of CB to PDMS, and the type of CB used. At low ratios, a high electrical resistance accompanied by a bimodal behavior in the resistance time evolution was observed, when stretching the electrodes cyclic in a triangular manner. This phenomenon decreases with increasing CB ratio. The type of PDMS also influences the resistance characteristics during elongation. Finally, a physical model of the observed phenomenon is presented.

Dielectric Polymer with Designable Large Motion under Low Electric Field

Dielectric elastomers (DEs) can demonstrate fast and large in-plane expansion/contraction due to electric field (e-field)-induced Maxwell stress. For robotic applications, it is often necessary that the in-plane actuation is converted into out-of-plane motions with mechanical frames. Despite their performance appeal, their high driving e-field (20-100 V µm^-1 ) demands bulky power accessories and severely compromises their durability. Here, a dielectric polymer that can be programmed into diverse motions actuated under a low e-field (2-10 V µm^-1 ) is reported. The material is a crystalline dynamic covalent network that can be reconfigured into arbitrary 3D geometries. This gives rise to a geometric effect that markedly amplifies the actuation, leading to designable large motions when the dielectric polymer is heated above its melting temperature to become a DE. Additionally, the crystallization transition enables dynamic multimodal motions and active deployability. These attributes result in unique design versatility for soft robots.

On the Evolution of Additive Manufacturing (3D/4D Printing) Technologies: Materials, Applications, and Challenges

The scientific community is and has constantly been working to innovate and improve the available technologies in our use. In that effort, three-dimensional (3D) printing was developed that can construct 3D objects from a digital file. Three-dimensional printing, also known as additive manufacturing (AM), has seen tremendous growth over the last three decades, and in the last five years, its application has widened significantly. Three-dimensional printing technology has the potential to fill the gaps left by the limitations of the current manufacturing technologies, and it has further become exciting with the addition of a time dimension giving rise to the concept of four-dimensional (4D) printing, which essentially means that the structures created by 4D printing undergo a transformation over time under the influence of internal or external stimuli. The created objects are able to adapt to changing environmental variables such as moisture, temperature, light, pH value, etc. Since their introduction, 3D and 4D printing technologies have extensively been used in the healthcare, aerospace, construction, and fashion industries. Although 3D printing has a highly promising future, there are still a number of challenges that must be solved before the technology can advance. In this paper, we reviewed the recent advances in 3D and 4D printing technologies, the available and potential materials for use, and their current and potential future applications. The current and potential role of 3D printing in the imperative fight against COVID-19 is also discussed. Moreover, the major challenges and developments in overcoming those challenges are addressed. This document provides a cutting-edge review of the materials, applications, and challenges in 3D and 4D printing technologies.

The Ogden and the extended tube model as backbone in describing electroactive polymers: advancements in modelling nonlinear behaviour and fracture

Hyperelastic constitutive relations form the basis of advanced models for novel materials. Such elastic deformation potentials are the backbone for complex material formulations at elastic and inelastic deformations, especially when embedded into powerful frameworks like generalized standard materials, as well as multiphysical and multiscale formulations. With the focus on electroactive polymers, the article at hand demonstrates the derivation of a variational, rate-dependent electromechanical model for quasi-incompressible polymers and the derivation of an electromechanical model for regularized fracture mechanics by means of the phase-field method. Starting at the prominent Ogden and the extended tube model, some developments from the last decades are revisited and presented via the principle of virtual power, for instance, the established mixed element formulation, nonlinear viscoelasticity and electromechanical coupling. An electromechanically fully coupled representative crack element is used to derive a novel phase-field model for fracture. A key property of the proposed model is the ability to describe the electrical free-space behaviour inside the crack gap, which is demonstrated by adopting three common crack-face conditions. This article is part of the theme issue 'The Ogden model of rubber mechanics: Fifty years of impact on nonlinear elasticity'.

Soft Gripper with Electro-Thermally Driven Artificial Fingers Made of Tri-layer Polymers and a Dry Adhesive Surface

Soft grippers have attracted great interest in the soft robotics research field. Due to their lack of deformability and control over compliance, it can be challenging for them to pick up objects that are too large or too small in size. In particular, compliant objects are vulnerable to the large grasping force. Therefore, it is crucial to be able to adjust the stiffness of the gripper materials. In this study, a soft gripper consisting of three artificial fingers is reported on. Each of the artificial fingers is made of a tri-layer polymer structure. An exterior layer, made of an ecoflex–graphene composite is embedded with electric wires as a heating source, by applying direct-current potential. The Joule heat not only allows for deformation of the exterior layer, but also transfers heat to the middle layer of the thermoplastic polyurethane (TPU) elastomer. As a result, the stiffness of the TPU layer can be adjusted using electro-thermal heating. Meanwhile, the third layer consists of a polydimethylsiloxane replica as a supporting layer with a gecko-inspired dry adhesive structure. By applying voltage through electric wires, the artificial fingers can bend and, thus, the soft gripper can hold the objects, with the help of the dry adhesive layer. Finally, objects like a shuttlecock, tennis ball and a glass beaker, can be picked up by the soft gripper. This research may provide an insight for the design and fabrication of soft robotic manipulators.

Smart Film Actuators for Biomedical Applications

Taking inspiration from the extremely flexible motion abilities in natural organisms, soft actuators have emerged in the past few decades. Particularly, smart film actuators (SFAs) demonstrate unique superiority in easy fabrication, tailorable geometric configurations, and programmable 3D deformations. Thus, they are promising in many biomedical applications, such as soft robotics, tissue engineering, delivery system, and organ-on-a-chip. In this review, the latest achievements of SFAs applied in biomedical fields are summarized. The authors start by introducing the fabrication techniques of SFAs, then shift to the topology design of SFAs, followed by their material selections and distinct actuating mechanisms. After that, their biomedical applications are categorized in practical aspects. The challenges and prospects of this field are finally discussed. The authors believe that this review can boost the development of soft robotics, biomimetics, and human healthcare.

Bioinspired Ultra Tear-Resistant Elastomer with a Slidable Double-Network Structure

Tear resistance is of vital importance in the fabrication and application of synthetic soft materials. However, the paradox of simultaneously improving the tearing energy and elasticity remains a huge challenge for conventional approaches. Here, inspired by the skin, we successfully constructed an extraordinary tear-resistant, superelastic elastomer by the introduction of nanosized polycyclodextrin into the elastomer network to form a slidable interpenetrate double network structure. The tearing energy of the SDEP elastomer is up to 274 KJ/m², which is comparable to metals and alloys and increased more than 100 times compared with the chemically cross-linked elastomer. The fracture strain exceeded 3300%, which is hardly achieved by other materials with high tearing energy. This comprehensive improvement of antitearing and super elasticity property was achieved by (i) a slide ring effect to dissipate energy and blunt a crack tip; (ii) straightening and reorientation of the slidable double network to deflect the advancing of a crack tip; (iii) a double network sharing the load. These results provide a novel strategy to fabricate elastic, tear-resistant soft material, which may contribute to the practical application as tear-resistant flexible electronics and irregular-shaped stretchable devices.

Design, Fabrication, and Performance Test of a New Type of Soft-Robotic Gripper for Grasping

This investigation presents a novel soft-robotic pneumatic gripper that consists of three newly proposed soft actuators. The newly proposed soft actuators adopt a composite structure of two kinds of pneumatic networks which can work independently and play their respective roles in grasping. The design, analyses, and fabrication of the proposed soft actuators are introduced systematically, and then an experimental system is built to examine the output characteristics of the soft actuator. Compared with the conventional single pneumatic network-based soft actuator, the newly proposed one combines the advantages of the two pneumatic networks, and it employs a larger output force and retains desired bending deformation ability at the same time. The grasping performance test results show that the new soft gripper constituted by the proposed soft actuators has high reliability and stability whether in pinching or in enveloping grasping, and it is also competent for grasping heavier or irregular objects, demonstrating the feasibility and effectiveness of the newly proposed soft actuator, and giving it a good and wide application prospect.

Ratchetlike motion of helical bilayers induced by boundary constraints

We show that application of boundary constraints generates unusual folding behaviors in responsive (swellable) helical bilayer strips. Unlike the smooth folding trajectories typical of free helical bilayers, the boundary-constrained bilayers exhibit intermittent folding behaviors characterized by rapid, steplike movements. We experimentally study bilayer strips as they swell and fold, and we propose a simple model to explain the emergence of ratchetlike behavior. Experiments and model predictions are then compared to simulations, which enable calculation of elastic energy during swelling. We investigate the dependence of this steplike behavior as a function of elastic boundary condition strength, strip length, and strip shape; interestingly, "V-shape" strips with the same boundary conditions fold smoothly.

From passive to emerging smart silicones

Amassing remarkable properties, silicones are practically indispensable in our everyday life. In most classic applications, they play a passive role in that they cover, seal, insulate, lubricate, water-proof, weather-proof etc. However, silicone science and engineering are highly innovative, seeking to develop new compounds and materials that meet market demands. Thus, the unusual properties of silicones, coupled with chemical group functionalization, has allowed silicones to gradually evolve from passive materials to active ones, meeting the concept of “smart materials”, which are able to respond to external stimuli. In such cases, the intrinsic properties of polysiloxanes are augmented by various chemical modifications aiming to attach reactive or functional groups, and/or by engineering through proper cross-linking pattern or loading with suitable fillers (ceramic, magnetic, highly dielectric or electrically conductive materials, biologically active, etc.), to add new capabilities and develop high value materials. The literature and own data reflecting the state-of-the art in the field of smart silicones, such as thermoplasticity, self-healing ability, surface activity, electromechanical activity and magnetostriction, thermo-, photo-, and piezoresponsivity are reviewed.

Model Analysis and Experimental Investigation of Soft Pneumatic Manipulator for Fruit Grasping

With the superior ductility and flexibility brought by compliant bodies, soft manipulators provide a nondestructive manner to grasp delicate objects, which has been developing gradually as a rising focus of soft robots. However, the unexpected phenomenon caused by environmental effects, leading to high internal nonlinearity and unpredictable deformation, makes it challenging to design, model, and control soft manipulators. In this paper, we designed a soft pneumatically actuated manipulator consisting of four soft actuators, as well as a flange, and investigated the influence of structural parameters on the output characteristics of the manipulator through finite element analysis (FEA). To enhance the bending deformation of the soft actuator, annular rings were employed on the soft actuator. A mathematical model for the bending deformation of air cavities was established to explore the relationship between the driving pressure and the bending angle based on the Yeoh strain energy function. Moreover, an end-output force model was established to depict the variation of the force output with the bending angle of the soft actuator, which was then experimentally validated by adopting the manufactured manipulator. The soft actuator studied in this paper can bend from 0° to 110° under an applied pressure of 0-60 kPa, and the maximum grasping load of the soft manipulator is 5.8 N. Finally, practical tests were conducted to assess the adaptability of the soft manipulator when grasping delicate fruits, such as apples, pears, tomatoes, and mangoes, demonstrating its broad application prospects in nondestructive fruit harvesting.

Glowing Sucker Octopus (Stauroteuthis syrtensis)-Inspired Soft Robotic Gripper for Underwater Self-Adaptive Grasping and Sensing

A soft gripper inspired by the glowing sucker octopus (Stauroteuthis syrtensis)' highly evolved grasping capability enabled by the umbrella-shaped dorsal and ventral membrane between each arm is presented here, comprising of a 3D-printed linkage mechanism used to actuate a modular mold silicone-casting soft suction disc to deform. The soft gripper grasp can lift objects using the suction generated by the pump in the soft disc. Moreover, the protruded funnel-shaped end of the deformed suctorial mouth can adapt to smooth and rough surfaces. Furthermore, when the gripper contacts the submerged target objects in a turbid environment, local suctorial mouth arrays on the suction disc are locked, causing the variable flow inside them, which can be detected as a tactile perception signal to the target objects instead of visual perception. Aided by the 3D-printed linkage mechanism, the soft gripper can grasp objects of different shapes and dimensions, including flat objects, objects beyond the grasping range, irregular objects, scattered objects, and a moving turtle. The results report the soft gripper's versatility and demonstrate the vast application potentials of self-adaptive grasping and sensing in various environments, including but are not limited to underwater, which is always a key challenge of grasping technology.

Deformation Modeling and Simulation of a Novel Bionic Software Robotics Gripping Terminal Driven by Negative Pressure Based on Classical Differential Algorithm

A general pneumatic soft gripper is proposed in this paper. Combined with the torque balance theory, the mathematical theoretical model of bending deformation of soft gripper is established based on Yeoh constitutive model and classical differential geometry. Assuming that the pressure in each inner cavity is evenly distributed, the input gas is in an ideal state, which is approximately treated as an isothermal condition, and all orifices experience blocked flow. In addition, compared with the mechanical work of gas, the energy related to gas flow and heat transfer is negligible. The nonlinear mechanical properties of silicone rubber are studied. It is regarded as isotropic and incompressible material, which is characterized by strain energy per unit volume. The material constant coefficients C10 and C20 are determined through the uniaxial tensile test, and the software gripper is simulated on the ABAQUS platform. The bending deformation models of grippers with three different force-bearing cavity structures are analyzed and compared, and the software clamping structure with the bending deformation most in line with the application conditions is selected. The limit input air pressure of the gripper and the situation of enveloping the clamping target object are analyzed. Through the bending deformation experiment, the maximum deformation angle is 72.4°. The relative error between the simulation analysis data and the prediction results of the mathematical model is no more than 3.5%, which verifies the effectiveness of the simulation and the correctness of the mathematical theoretical model of bending deformation. The soft manipulator proposed in this paper has good adaptability to grasping objects of different shapes and sizes. The minimum diameter of the target object that can be clamped is 0.1 mm. It can clamp the object weighing up to 1 kg. It has compact size, light weight, high ductility, and flexibility.

Bistable and Multistable Actuators for Soft Robots: Structures, Materials, and Functionalities

Snap-through bistability is often observed in nature (e.g., fast snapping to closure of Venus flytrap) and the life (e.g., bottle caps and hair clippers). Recently, harnessing bistability and multistability in different structures and soft materials has attracted growing interest for high-performance soft actuators and soft robots. They have demonstrated broad and unique applications in high-speed locomotion on land and under water, adaptive sensing and fast grasping, shape reconfiguration, electronics-free controls with a single input, and logic computation. Here, an overview of integrating bistable and multistable structures with soft actuating materials for diverse soft actuators and soft/flexible robots is given. The mechanics-guided structural design principles for five categories of basic bistable elements from 1D to 3D (i.e., constrained beams, curved plates, dome shells, compliant mechanisms of linkages with flexible hinges and deformable origami, and balloon structures) are first presented, alongside brief discussions of typical soft actuating materials (i.e., fluidic elastomers and stimuli-responsive materials such as electro-, photo-, thermo-, magnetic-, and hydro-responsive polymers). Following that, integrating these soft materials with each category of bistable elements for soft bistable and multistable actuators and their diverse robotic applications are discussed. To conclude, perspectives on the challenges and opportunities in this emerging field are considered.

Programming Soft Shape-Morphing Systems by Harnessing Strain Mismatch and Snap-Through Bistability: A Review

Multi-modal and controllable shape-morphing constitutes the cornerstone of the functionalization of soft actuators/robots. Involving heterogeneity through material layout is a widely used strategy to generate internal mismatches in active morphing structures. Once triggered by external stimuli, the entire structure undergoes cooperative deformation by minimizing the potential energy. However, the intrinsic limitation of soft materials emerges when it comes to applications such as soft actuators or load-bearing structures that require fast response and large output force. Many researchers have explored the use of the structural principle of snap-through bistability as the morphing mechanisms. Bistable or multi-stable mechanical systems possess more than one local energy minimum and are capable of resting in any of these equilibrium states without external forces. The snap-through motion could overcome energy barriers to switch among these stable or metastable states with dramatically distinct geometries. Attributed to the energy storage and release mechanism, such snap-through transition is quite highly efficient, accompanied by fast response speed, large displacement magnitude, high manipulation strength, and moderate driving force. For example, the shape-morphing timescale of conventional hydrogel systems is usually tens of minutes, while the activation time of hydrogel actuators using the elastic snapping instability strategy can be reduced to below 1 s. By rationally embedding stimuli-responsive inclusions to offer the required trigger energy, various controllable snap-through actuations could be achieved. This review summarizes the current shape-morphing programming strategies based on mismatch strain induced by material heterogeneity, with emphasis on how to leverage snap-through bistability to broaden the applications of the shape-morphing structures in soft robotics and mechanical metamaterials.

A unimorph nanocomposite dielectric elastomer for large out-of-plane actuation

Dielectric elastomer actuators (DEAs) feature large, reversible in-plane deformation, and stacked DEA layers are used to produce large strokes in the thickness dimension. We introduce an electrophoretic process to concentrate boron nitride nanosheet dispersion in a dielectric elastomer precursor solution onto a designated electrode surface. The resulting unimorph nanocomposite dielectric elastomer (UNDE) has a seamless bilayer structure with 13 times of modulus difference. The UNDE can be actuated to large bending curvatures, with enhanced breakdown field strength and durability as compared to conventional nanocomposite dielectric elastomer. Multiple UNDE units can be formed in a simple electrophoretic concentration process using patterned electrode areas. A disc-shaped actuator comprising six UNDE units outputs large bidirectional stroke up to 10 Hz. This actuator is used to demonstrate a high-speed lens motor capable of varying the focal length of a two-lens system by 40 times.

Highly stretchable silicone elastomer applied in soft actuators

In this work, a highly stretchable silicone elastomer is incorporated into dielectric elastomer actuators (DEAs) in order to decrease operation voltages by applying high prestretches. Results show that the fabricated DEAs (5-mm-diameter circle active region) can be actuated to a lateral strain of 30% at 4.3 kV for a 122 μm-thick prestretched film, and to a lateral strain of 2.5% at only 250 V for a 6.9 μm-thick prestretched film. Due to the significant viscous component of the silicone elastomer, the DEAs respond more slowly (2-14 s to reach 90% of full strain) and show greater strain changes over time compared to conventional silicone-based DEAs. While this inherent viscosity is not universally favorable, it can be advantageous in applications where actuator damping is desirable. The studied DEAs' mean lifetimes under DC actuation range significantly-from 0.9 h to more than 123.0 h-depending mainly on initial electrical fields (17.8-36.3 V/μm). For instance, DEAs with a 150 μm initial thickness and a prestretch ratio of 3 show 1.4-2.6% lateral strains for the mean lifetime (123.0 h) at only 300 V. Given the strains achieved at low voltage, such DEAs show promise for applications that do not require fast response speeds. This article is protected by copyright. All rights reserved.

Photoswitchable Covalent Adaptive Networks Based on Thiol-Ene Elastomers

Covalent adaptive networks combine the advantages of cross-linked elastomers and dynamic bonding in a single system. In this work, we demonstrate a simple one-pot method to prepare thiol-ene elastomers that exhibit reversible photoinduced switching from an elastomeric gel to fluid state. This behavior can be generalized to thiol-ene cross-linked elastomers composed of different backbone chemistries (e.g., polydimethylsiloxane, polyethylene glycol, and polyurethane) and vinyl groups (e.g., allyl, vinyl ether, and acrylate). Photoswitching from the gel to fluid state occurs in seconds upon exposure to UV light and can be repeated over at least 180 cycles. These thiol-ene elastomers also exhibit the ability to heal, remold, and serve as reversible adhesives.

Data-driven geometric system identification for shape-underactuated dissipative systems

Modeling system dynamics becomes challenging when the properties of individual system components cannot be directly measured, and often requires identification of properties from observed motion. In this paper, we show that systems whose movement is highly dissipative have features which provide an opportunity to more easily identify models and more quickly optimize motions than would be possible with general techniques. Geometric mechanics provides means for reduction of the dynamics by environmental homogeneity, while the dissipative nature minimizes the role of second order (inertial) features in the dynamics. Here we extend the tools of geometric system identification to 'shape-underactuated dissipative systems (SUDS)'-systems whose motions are more dissipative than inertial, but whose actuation is restricted to a subset of the body shape coordinates. Many animal motions are SUDS, including micro-swimmers such as nematodes and flagellated bacteria, and granular locomotors such as snakes and lizards. Many soft robots are also SUDS, particularly robots that incorporate highly damped series elastic actuators to reduce the rigidity of their interactions with their environments during locomotion and manipulation. We motivate the use of SUDS models, and validate their ability to predict motion of a variety of simulated viscous swimming platforms. For a large class of SUDS, we show how the shape velocity actuation inputs can be directly converted into torque inputs, suggesting that systems with soft pneumatic or dielectric elastomer actuators can be modeled with the tools presented. Based on fundamental assumptions in the physics, we show how our model complexity scales linearly with the number of passive shape coordinates. This scaling offers a large reduction on the number of trials needed to identify the system model from experimental data, and may reduce overfitting. The sample efficiency of our method suggests its use in modeling, control, and optimization in robotics, and as a tool for the study of organismal motion in friction dominated regimes.

Thermo-Electro-Mechanical Simulation of Electro-Active Composites

In this contribution, a computational thermo-electro-mechanical framework is considered, to simulate coupling between the mechanical, electrical and thermal fields, in nonhomogeneous electro-active materials. A thermo-electro-mechanical material model and a mixed Q1P0 finite element framework are described and used for the simulations. Finite element simulations of the response of heterogeneous structures consisting of a soft matrix and a stiff incluison are considered. The behavior of the composite material is studied for varying initial temperatures, different volume fractions and various aspect ratios of the inclusion. For some of the examples, the response of the structure beyond a limit point of electro-mechanical instability is traced. Regarding the soft matrix of the composite, thermal properties of silicone rubber at normal conditions have been obtained by molecular dynamics (MD) simulations. The material parameters obtained by MD simulations are used within the finite element simulations.

Materials for Smart Soft Actuator Systems

In contrast to conventional hard actuators, soft actuators offer many vivid advantages, such as improved flexibility, adaptability, and reconfigurability, which are intrinsic to living systems. These properties make them particularly promising for different applications, including soft electronics, surgery, drug delivery, artificial organs, or prosthesis. The additional degree of freedom for soft actuatoric devices can be provided through the use of intelligent materials, which are able to change their structure, macroscopic properties, and shape under the influence of external signals. The use of such intelligent materials allows a substantial reduction of a device's size, which enables a number of applications that cannot be realized by externally powered systems. This review aims to provide an overview of the properties of intelligent synthetic and living/natural materials used for the fabrication of soft robotic devices. We discuss basic physical/chemical properties of the main kinds of materials (elastomers, gels, shape memory polymers and gels, liquid crystalline elastomers, semicrystalline ferroelectric polymers, gels and hydrogels, other swelling polymers, materials with volume change during melting/crystallization, materials with tunable mechanical properties, and living and naturally derived materials), how they are related to actuation and soft robotic application, and effects of micro/macro structures on shape transformation, fabrication methods, and we highlight selected applications.

A Pneumatic Novel Combined Soft Robotic Gripper with High Load Capacity and Large Grasping Range

Pneumatic soft grippers have been widely studied. However, the structures and material properties of existing pneumatic soft grippers limit their load capacity and manipulation range. In this article, inspired by sea lampreys, we present a pneumatic novel combined soft gripper to achieve a high load capacity and a large grasping range. This soft gripper consists of a cylindrical soft actuator and a detachable sucker. Three internal air chambers of the cylindrical soft actuator are inflated, which enables them to hold objects. Under vacuum pressure, the cylindrical soft actuator and the detachable sucker can both adsorb objects. A finite element model was constructed to simulate three inflation chambers for predicting the grasping range of the cylindrical soft actuator. The validity of the finite element model was established by an experiment. The mechanism of holding force and adsorption force were analyzed. Several groups of experiments were conducted to determine adsorption range, holding force, and adsorption force. In addition, practical applications further indicated that the novel combined soft gripper has a high load capacity (10.85 kg) at a low pressure (16 kPa) and a large grasping range (minimum diameter of the object: d = 6 mm), being able to lift a variety of objects with different weights, material properties, and shapes.

Advances in 4D-printed physiological monitoring sensors

Physiological monitoring sensors have been critical in diagnosing and improving the healthcare industry over the past 30 years, despite various limitations regarding providing differences in signal outputs in response to the changes in the user's body. Four-dimensional (4D) printing has been established in less than a decade; therefore, it currently offers limited resources and knowledge. Still, the technique paves the way for novel platforms in today's ever-growing technologies. This innovative paradigm of 4D printing physiological monitoring sensors aspires to provide real-time and continuous diagnoses. In this perspective, we cover the advancements currently available in the 4D printing industry that has arisen in the last septennium, focusing on the overview of 4D printing, its history, and both wearable and implantable physiological sensing solutions. Finally, we explore the current challenges faced in this field, translational research, and its future prospects. All of these aims highlight key areas of attention that can be applied by future researchers to fully transform 4D printed physiological monitoring sensors into more viable medical products.

Stretch-Induced Bending of Soft Ribbed Strips

We show that ribbed elastic strips under tension present large spontaneous curvature and may close into tubes. In this single material architectured system, transverse bending results from a bilayer effect induced by Poisson contraction as the textured ribbon is stretched. Surprisingly, the induced curvature may reverse if ribs of different orientations are considered. Slender ribbed structures may also undergo a nontrivial buckling transition. We use analytical calculations to describe the evolution of the morphology of the ribbon and the transitions between the different experimental regimes as a function of material properties, geometrical parameters, and stretching strain. This scale-independent phenomenon may help the manufacturing of tubular textured structures or easily controllable grippers at small scale.

Soft Robotic Gripper Based on Multi-Layers of Dielectric Elastomer Actuators

Dielectric elastomer actuators (DEAs) are a promising technology for soft robotics. The use of DEAs has many advantages, including light weight, resilience, and fast response for its applications, such as grippers, artificial muscles, and heel strike generators. Grippers are commonly used as grasping devices. In this study, we focus on DEA applications and propose a technology to expand the applicability of a soft gripper. The advantages of gripper-based DEAs include light weight, fast response, and low cost. We fabricated soft grippers using multiple DEA layers. The grippers successfully held or gripped an object, and we investigated the response time of the grippers and their angle characteristics. We studied the relationship between the number of DEA layers and the performance of our grippers. Our experimental results show that the multi-layered DEAs have the potential to be strong grippers.

Grasping with kirigami shells

The ability to grab, hold, and manipulate objects is a vital and fundamental operation in biological and engineering systems. Here, we present a soft gripper using a simple material system that enables precise and rapid grasping, and can be miniaturized, modularized, and remotely actuated. This soft gripper is based on kirigami shells-thin, elastic shells patterned with an array of cuts. The kirigami cut pattern is determined by evaluating the shell's mechanics and geometry, using a combination of experiments, finite element simulations, and theoretical modeling, which enables the gripper design to be both scalable and material independent. We demonstrate that the kirigami shell gripper can be readily integrated with an existing robotic platform or remotely actuated using a magnetic field. The kirigami cut pattern results in a simple unit cell that can be connected together in series, and again in parallel, to create kirigami gripper arrays capable of simultaneously grasping multiple delicate and slippery objects. These soft and lightweight grippers will have applications in robotics, haptics, and biomedical device design.