Soft Gripper Research Articles

The Role of 3D Printing Technologies in Soft Grippers.

Soft grippers are essential for precise and gentle handling of delicate, fragile, and easy-to-break objects, such as glassware, electronic components, food items, and biological samples, without causing any damage or deformation. This is especially important in industries such as healthcare, manufacturing, agriculture, food handling, and biomedical, where accuracy, safety, and preservation of the objects being handled are critical. This article reviews the use of 3D printing technologies in soft grippers, including those made of functional materials, nonfunctional materials, and those with sensors. 3D printing processes that can be used to fabricate each class of soft grippers are discussed. Available 3D printing technologies that are often used in soft grippers are primarily extrusion-based printing (fused deposition modeling and direct ink writing), jet-based printing (polymer jet), and immersion printing (stereolithography and digital light processing). The materials selected for fabricating soft grippers include thermoplastic polymers, UV-curable polymers, polymer gels, soft conductive composites, and hydrogels. It is conclude that 3D printing technologies revolutionize the way soft grippers are being fabricated, expanding their application domains and reducing the difficulties in customization, fabrication, and production.

Fabrication and Applications of Magnetic Polymer Composites for Soft Robotics.

The emergence of magnetic polymer composites has had a transformative impact on the field of soft robotics. This overview will examine the various methods by which innovative materials can be synthesized and utilized. The advancement of soft robotic systems has been significantly enhanced by the utilization of magnetic polymer composites, which amalgamate the pliability of polymers with the reactivity of magnetic materials. This study extensively examines the production methodologies involved in dispersing magnetic particles within polymer matrices and controlling their spatial distribution. The objective is to gain insights into the strategies required to attain the desired mechanical and magnetic properties. Additionally, this study delves into the potential applications of these composites in the field of soft robotics, encompassing various devices such as soft actuators, grippers, and wearable gadgets. The study emphasizes the transformative capabilities of magnetic polymer composites, which offer a novel framework for the advancement of biocompatible, versatile soft robotic systems that utilize magnetic actuation.

High-Stroke, High-Output-Force, Fabric-Lattice Artificial Muscles for Soft Robots.

Artificial muscles, providing safe and close interaction between humans and machines, are essential in soft robotics. However, their insufficient deformation, output force, or configurability usually limits their applications. Herein, this work presents a class of lightweight fabric-lattice artificial muscles (FAMs) that are pneumatically actuated with large contraction ratios (up to 87.5%) and considerable output forces (up to a load of 20kg, force-to-weight ratio of over 250). The developed FAMs consist of a group of active air chambers that are zigzag connected into a lattice through passive connecting layers. The geometry of these fabric components is programmable to convert the in-plane lattice of FAMs into out-of-plane configurations (e.g., arched and cylindrical) capable of linear/radial contraction. This work further demonstrates that FAMs can be configured for various soft robotic applications, including the powerful robotic elbow with large motion range and high load capability, the well-fitting assistive shoulder exosuit that can reduce muscle activity during abduction, and the adaptive soft gripper that can grasp irregular objects. These results show the unique features and broad potential of FAMs for high-performance soft robots.

Aerosol Jet Printing of Strain Sensors for Soft Robotics

The field of soft robotics is rapidly progressing toward applications including; wearable electronics, prosthetics, and biomedical devices. This is leading to demand for flexible, embedded high‐performance strain sensors to deliver real‐time feedback on the static configurations and dynamic motions of these robotic devices, to ultimately enable the levels of autonomous control and structural monitoring required for intelligent manipulation. Herein, aerosol jet printing (AJP) technology is utilized to generate arbitrary piezoresistive strain sensor layouts on fibrous paper suitable for direct integration into elastomeric soft robots. A custom graphene nanoplatelet ink with a viscosity of around 2.70 cP has been formulated for optimized atomization and patterning of conductive traces via AJP. Single and multilayer printing onto different paper substrates are explored; with the nominal resistance of the printed tracks varying from 272 to 4900 kΩ depending on paper type and number of layers. Maximum gauge factors of 24.2 ± 1.8 and 56.5 ± 4.5 are determined for sensor surfaces under tensile and compression modes, respectively. To demonstrate the possibility for direct integration of this approach for soft robotics, strain sensors are directly printed onto the strain‐limiting layer of a pneumatic soft robotic gripper, to provide continuous feedback of the gripper over curvatures up to 80 m−1.

Ionic Conductivity Switchable and Shape Changeable Smart Skins with Azobenzene‐Based Ionic Reactive Mesogens

AbstractTo develop smart ionic skins capable of multi‐responsiveness and switchable ionic conductivity, a stimuli‐responsive and ionic conductive azobenzene‐based monomer is newly synthesized, uniaxially oriented, and polymerized for anisotropic liquid crystal elastomers (LCEs). Since the uniaxially oriented monodomain LCE with ionic asymmetric azobenzene monomers (i‐AAM) is prepared by thermal oligomerization, uniaxial stretching, and subsequent photopolymerization, the stimuli‐responsive i‐AAM LCE is thermally contracted by increasing the temperature above TNI as well as is bent along the aligned direction by irradiating it with UV light. In addition to the change of shape, the ionic conductivity of the LCE is reversible in response to heat and light stimuli. Polydomain i‐AAM LCE polymerized without the stretching process exhibits higher ionic conductivity than stretched monodomain LCE due to the initially formed stable ionic pathways. Ionic conductivity can also be switched simultaneously by polarity changes in response to the photoisomerization of i‐AAM, increased mobility from heat, and an elastic mechanoresponse from external stresses. These i‐AAM LCE‐based soft grippers exhibit stimuli‐responsive grasping and releasing actuation and detect ionic conductivity switches finely, suggesting potential as smart skins for advanced soft robots.

Magnetic-field induced asymmetric hydrogel fibers for tough actuators with programmable deformation

Ingenious design of soft actuators with high mechanical performance, rapid actuation, and programmable deformation is necessary for their practical applications in the fields of artificial muscles, smart textiles, and soft robotics. Here we used a facile magnetic-field induction strategy to fabricate programmable actuators composed of tough hydrogel fibers. The anisotropic hydrogel fibers that consisted of dual physically cross-linked networks showed high tensile strength (28.9 MPa), while the asymmetric distribution of magnetic nanoparticles in hydrogel networks endowed the actuators with rapid bending velocity (3.2°·min−1·mm−1) in response to solvents. The pick-and-place manipulation of objects from ethanol to water were demonstrated by using the soft grippers composed of asymmetric hydrogel fibers. Furthermore, smart curtains that were woven by hydrogel fibers automatically rolled up or spread out in response to the change of ambient humidity. This work proved that tough hydrogel fibers fabricated by our magnetic-field induction strategy provided more possibilities in the field of soft actuators.

Highly Stretchable and Conductive MXene‐Encapsulated Liquid Metal Hydrogels for Bioinspired Self‐Sensing Soft Actuators

AbstractAdvanced sensation and actuation abilities of various living organisms in nature have inspired researchers to design bioinspired self‐sensing soft actuators. However, the majority of conventional soft actuators primarily possess actuation capabilities while lacking a real‐time sensing signal feedback. Here, a promising strategy is reported to develop highly stretchable and conductive hydrogels for bioinspired self‐sensing soft actuators, which integrate actuation and strain‐sensing functions into a single materials system. The conductive hydrogels are designed and fabricated by in situ copolymerization of amino‐functionalized MXene‐encapsulated liquid metal nanodroplets (LM@A‐MXene) and poly(N‐isopropylacrylamide) hydrogels with controllable activated nanogels as nano‐cross‐linkers. The resulting hydrogel presents a compacted conducting network and highly porous microstructure, giving rise to robust integration of high conductivity, excellent strain sensitivity, broad stretchability, high stability, and fast response speed. Interestingly, the gradient network structure, formed by self‐precipitation of LM@A‐MXene, endows the hydrogel with shape‐programmable actuation, light‐driven remote control, and self‐sensing function. As a proof‐of‐concept application, the soft gripper based on the self‐sensing hydrogel actuators is developed, which can not only grasp, lift, and release objects, but also perceive every movement state by monitoring resistance changes. The proposed self‐sensing soft actuator can offer new insights for developing smart soft robotics and other artificial intelligent devices.

Capacitive flexible haptic sensor based on micro-cylindrical structure dielectric layer and its decoupling study

With the advancement of robotics, flexible sensors have undergone significant development. Extensive research has been conducted on the sensor structure to enhance its sensing capabilities. However, due to the absence of a definitive theory on the relationship between input force and output signal, and the unclear mechanism of force and deformation, decoupling the force remains challenging. To address this, we propose a capacitive flexible tactile sensor that utilizes a dielectric layer with a microcylindrical structure for detecting external loads. The micro-cylindrical structure of the dielectric layer enhances the strain capacity, thereby improving the sensor's performance. We also present a decoupling model based on the deformation mechanism by analyzing the sensor coupling. Furthermore, we validate the sensor's application in tangential detection and object grasping by integrating it into a soft rehabilitation glove and a soft gripper. This research lays the foundation for future self-aware operations.

Corrugated V-fold soft actuator with large deformation and high force density

Soft pneumatic actuators with a corrugated structure have the advantage of low inflation pressure and a large actuation range and have great potential for use in grippers. However, they are generally difficult to have both large deformation and high force density performance. Inspired by the hydraulic joints of spider legs, this paper designs a multi-cavity soft-pneumatic actuator with a corrugated V-fold structure, which realizes bending through the deformation of the air cavity structure instead of the traditional way of squeezing the air cavities against each other, and has significantly large curvature high force density compared with actuators such as PneuNets. At an inflation pressure of 40 kPa, the proposed soft actuator achieves a bending angle of 306° with a curvature of 0.114 rad mm−1, while at an inflation pressure of 45 kPa, the actuator achieves an actuation force of 5.15 N with a force density of 0.0101 mN (mm3.kPa)−1. The modeling approach based on continuum mechanics modeling of soft materials combined with finite element analysis (FEA) (or experiment) is proposed to achieve accurate modeling of the bending angle and actuation force of actuators with complex corrugated structures. In addition, the proposed actuator has made initial explorations into the application of soft grippers and has achieved fairly good gripping results compared to other types of grippers.

A 3D Printing‐Enabled Artificially Innervated Smart Soft Gripper with Variable Joint Stiffness

AbstractThe manufacturing industry has witnessed advancements in soft robotics, specifically in robotic grippers for handling fragile or irregular objects. However, challenges remain in balancing shape compliance, structural rigidity, weight, and sensor integration. To address these limitations, a 3D‐printed multimaterial gripper design is proposed. This approach utilizes a single, nearly fully automated 3D printing process to create a universal gripper with almost no assembly work. By processing functional polymer, polymer nanocomposite, and metal wire simultaneously, this technique enables multifunctionality. The gripper achieves different gripping configurations by adjusting joint stiffness through Joule heating of conductive polylactic acid material, ensuring shape conformance. Embedded metal wires, created using an in‐house wire embedding technique, form reliable high‐current‐loading interconnections for the conductive joints acting as the heater. Additionally, an integrated soft sensor printed in thermoplastic polyurethane (TPU) and conductive TPU detects compression levels and discerns handled samples. This study showcases the potential of 3D multimaterial printing for on‐demand fabrication of a smart universal gripper with variable stiffness and integrated sensors, benefiting the automation industry. Overall, this work presents an effective strategy for designing and fabricating integrated multifunctional structures using soft, rigid, and conductive materials, such as polymer, polymer nanocomposite, and metal through multimaterial 3D printing.

Wang tiles enable combinatorial design and robot-assisted manufacturing of modular mechanical metamaterials

In this paper, we introduce a novel design paradigm for modular architectured materials that allows for spatially nonuniform designs from a handful of building blocks, which can be robotically assembled for efficient and scalable production. The traditional, design-limiting periodicity in material design is overcome by utilizing Wang tiles to achieve compatibility among building blocks. We illustrate our approach with the design and manufacturing of an L-shaped domain inspired by a scissor-like soft gripper, whose internal module distribution was optimized to achieve an extreme tilt of a tip of the gripper’s jaw when the handle part was uniformly compressed. The geometry of individual modules was built on a 3 × 3 grid of elliptical holes with varying semi-axes ratios and alternating orientations. We optimized the distribution of the modules within the L-shaped domain using an enumeration approach combined with a factorial search strategy. To address the challenge of seamless interface connections in modular manufacturing, we produced the final designs by casting silicone rubber into modular molds automatically assembled by a robotic arm. The predicted performance was validated experimentally using a custom-built, open-hardware test rig, Thymos, supplemented with digital image correlation measurements. Our study demonstrates the potential for enhancing the mechanical performance of architectured materials by incorporating nonuniform modular designs and efficient robot-assisted manufacturing.

A Versatile Topology-Optimized Compliant Actuator for Soft Robotic Gripper and Walking Robot.

The remarkable interaction capabilities of soft robots within various environments have captured substantial attention from researchers. In recent years, bionics has provided a rich inspiration for the design of soft robots. Nevertheless, predicting the locomotion of soft actuators and determining material layouts solely based on intuition or experience remain a formidable challenge. Previous actuators predominantly targeted separate applications, leading to elevated costs and diminished interchangeability. The objective of this article is to extract the common requirements of diverse application domains and develop a versatile compliant actuator. A mathematical model of the compliant mechanism is proposed under the framework of topology optimization, resulting in an optimal distribution of both structure and material. Through comparison with empirical and semioptimal designs, the results show that the proposed versatile actuator has the advantages of both stiffness and flexibility. We propose an associative design strategy for soft grippers and walking robots. The soft gripper can perfectly complete adaptive grasping of objects with varying sizes, shapes, and masses. The successful in-water gripping experiment underscores the robust cross-medium operational capabilities of the soft gripper. Notably, our experimental results show that the walking robot can move quickly for 5 cycles in 8.25 s and can guarantee the control accuracy of continuous motion. Moreover, the robot swiftly switches walking directions within a mere 0.45 s. The optimization and design strategy presented in this article can furnish novel insights for shaping the next generation of soft robots.

A stretchable all-nanofiber iontronic pressure sensor

Flexible pressure sensors with high stretchability, sensitivity, and stability are undoubtedly urgently required for potential applications in intelligent soft robots, human-machine interaction, health monitoring, and other fields. However, most current flexible pressure sensors are unable to endure large deformation and are prone to performance degradation or even failure during frequent operation due to their multilayered structures. Here, we propose a stretchable all-nanofiber iontronic pressure sensor that is composed of ionic nanofiber membranes used as dielectric layers and liquid metal used as electrodes. This sensor exhibits a high sensitivity of 1.08 kPa-1 over a wide range of 0-300 kPa, with a fast response-relaxation time of about 18/22 ms and excellent stability. The high sensitivity comes from the electric double layer formed at the ionic film/electrode interface, while high stretchability and stability are enabled by in-situ encapsulated all-nanofiber structures. As a proof of concept, a prototype sensor array is integrated into a soft pneumatic gripper, demonstrating its capability of pressure perception and object recognition during the grasping process. Thus, the scheme provides another excellent strategy to fabricate stretchable pressure sensors with superb performance in terms of high stretchability, sensitivity, and stability.

Intelligent Soft Robotic Grippers for Agricultural and Food Product Handling: A Brief Review with a Focus on Design and Control

Advances in material sciences, control algorithms, and manufacturing techniques have facilitated rapid progress in soft grippers, propelling their adoption in various fields. In this review article, a comprehensive overview of the design and control aspects of intelligent soft robotic grippers tailored specifically for agricultural product handling is provided. Soft grippers have emerged as a promising solution for handling delicate and fragile objects. In this article, the recent progress in various gripper design, including fluidic and mechanical grippers, is elucidated and the role of advanced control approaches in enabling intelligent functions, such as object classification and grasping condition evaluation, is explored. Moreover, the challenges and opportunities pertaining to implementation of soft grippers in the agricultural industry are thoroughly discussed. While most demonstrations of soft grippers and their control strategies remain at the experimental stage, in this article, it is aimed to provide insights into the potential applications of soft grippers in agricultural product handling, thereby inspiring future research in this field.

Templated Laser-Induced-Graphene-Based Tactile Sensors Enable Wearable Health Monitoring and Texture Recognition via Deep Neural Network.

Flexible tactile sensors show great potential for portable healthcare and environmental monitoring applications. However, challenges persist in scaling up the manufacturing of stable tactile sensors with real-time feedback. This work demonstrates a robust approach to fabricating templated laser-induced graphene (TLIG)-based tactile sensors via laser scribing, elastomer hot-pressing transfer, and 3D printing of the Ag electrode. With different mesh sandpapers as templates, TLIG sensors with adjustable sensing properties were achieved. The tactile sensor obtains excellent sensitivity (52260.2 kPa-1 at a range of 0-7 kPa), a broad detection range (up to 1000 kPa), a low limit of detection (65 Pa), a rapid response (response/recovery time of 12/46 ms), and excellent working stability (10000 cycles). Benefiting from TLIG's high performance and waterproofness, TLIG sensors can be used as health monitors and even in underwater scenarios. TLIG sensors can also be integrated into arrays acting as receptors of the soft robotic gripper. Furthermore, a deep neural network based on the convolutional neural network was employed for texture recognition via a soft TLIG tactile sensing array, achieving an overall classification rate of 94.51% on objects with varying surface roughness, thus offering high accuracy in real-time practical scenarios.

Programmable and Variable‐Stiffness Robotic Skins for Pneumatic Actuation

Pneumatic soft actuators possess relatively fast response, inherent high flexibility, and achieve extraordinary shape morphing under large deformations. Conventionally, the entire body of soft pneumatic robots needs to be designed for specific applications. Herein, a soft pneumatic actuator design with structured fabrics as actuator skins, which has high bending stiffness variation that can accomplish multiple tasks and different deformation modes in a single body, is proposed. By adjusting the structured skin, the developed soft actuator can be tailored to achieve various deformations. It is experimentally shown that the bending stiffness of the actuator can be adjusted from 108 to 5654 N m−1. The blocking force of the actuator in bending is comparable with that of conventional fabric‐reinforced pneumatic actuators, while the actuator skins are reusable and programmable. In application, the actuators are used to construct a bionic soft gripper with multiple degrees of freedom. By switching between three grasping modes, the gripper successfully fulfills a series of tasks including lifting weights up to 1 kg and grasping objects ranging from a grain of grape to a large iron basin. This work opens up an avenue for designing structured skins for pneumatic robots with programmable deformation modes and versatile functions.

3D‐Architected low melting point alloys foam microstructure‐reinforced polymer composite with superior stiffness‐switchable for soft actuator

AbstractStiffness variable materials have aroused extensive research interest in smart devices, especially in soft actuators. However, achieving materials with high stiffness switch range remains challenging. Here, a novel three‐dimensional (3D) lightweight Field's metal (FM) foam was developed. Meanwhile, the polymer composite (SUFF) with superior stiffness switchable capacity was fabricated by embedding FM foam into a kind of designed stiffness variable polymer. The SUFF shows outstanding conductivity under relatively low FM volume fraction of 20.6%. Impressively, because of the extremely high modulus and fast transition of FM foam, the stiffness switch ratio of the SUFF are able to reach an ultra‐high value of 6987.5‐folds. Apart from its exceptional stiffness switchable capacity, the SUFF also possesses remarkable shape memory and self‐healing characteristics. Next, the SUFF was then fastened to a soft actuator as the stiffness changing units. The obtained actuator was able to exhibit a short heating–cooling cycle time of 44 s while using 10 A of current and 4°C water for cooling. Moreover, the actuator can reach remarkable values of 973 mN/mm and 4729 mN, respectively, for its stiffness and net force. Then, a soft robot gripper made up of three obtained actuators mounted on a base demonstrates its excellent load and stiffness switchable ability. It can lift a variety of objects with various forms with weights up to more than 3 kg. This study might provide a reference for application of stiffness variable materials in soft actuators.Highlights The FM foam were designed and synthetized by lost‐wax casting method. The polymer composite exhibits large stiffness switching range. The polymer composite achieves 6987.5‐folds rigid/soft stiffness ratio. A 3D‐printing soft actuator was fabricated using obtained polymer composite. The assembled robotic gripper can lift weights up to more than 3 kg.

Bio-inspired soft pneumatic actuator based on a kresling-like pattern with a rigid skeleton

IntroductionBiomimetic soft pneumatic actuators (SPA) with Kresling origami patterns have unique advantages over conventional rigid robots, owing to their adaptability and safety. ObjectivesInspired by cloning and moving behaviors observed from salps, we proposed an SPA based on a Kresling-like pattern with a rigid skeleton. The elongation and output force were tested, and the effectiveness of the applications with the SPA was evaluated. MethodsThe proposed SPA consists of rigid skeletons and a soft skin. The rigid skeletons are constructed using layers of Kresling-like patterns, while a novel extensible inserting structure is devised to replace the folds found in conventional Kresling patterns. This innovative approach ensures that the SPA exhibits axial contraction/expansion motion without any twisting movement. To mimic the bionic characteristics of swimming and ingesting progress of salps, the proposed SPA can perform an axial contraction motion without twisting and a controllable bending motion based on multi-layered Kresling-like patterns; to mimic the cloning and releasing life phenomena of salps, the number of layers of Kresling-like patterns is changeable by adding or reducing skeleton components according to the practical needs. ResultsThe experimental elongation results on the SPA with multiple layers of Kresling-like patterns show that the elongation can increase to above 162% by adding layers; the experimental output force results show that the three-layer SPA can provide 6.36 N output force at an air flow rate of 10 L/min, and the output force will continue to increase as the number of layers of Kresling-like pattern increases or the air flow rate increases. Further, we demonstrate the applications of the SPA in soft grippers, scissor grippers, claw grippers and pipe crawlers. ConclusionOur proposed SPA can avoid twisting in the radial contraction motion with high elongation and output force, and provide the practical guidance for bio-inspired soft robotic applications.

Stiffness-Tunable Soft Bellows Actuators by Cross-Fiber Jamming Effect for Robust Grasping

Soft actuators typically exhibit low stiffness and low load-bearing properties due to the intrinsic limitations of soft materials. Stiffness modulation is an effective means to improve the performance of soft actuators. However, most stiffness-tunable mechanisms are nonstretchable, and have difficulty in independently adjusting the tensile stiffness of soft elongation actuators. Here, a universal cross-fiber jamming mechanism with both elongation and bending stiffness-tunable capability is proposed for soft bellows actuators. It is composed of two semicircular symmetrical fiber bundles in an up-and-down crossed arrangement, and can follow the tensile state of the soft bellows actuator through the passive staggered motion of the layered fiber bundles. Furthermore, a soft gripper with wide grasping range and sufficient grasping stability is also developed by applying soft bellows actuators with cross-fiber jamming mechanisms. Experiments are conducted to show the related performance of the proposed cross-fiber jamming mechanism. The results show that it has fast antiimpact damping response, strong shaping ability and adjustable stiffness. Such efficient performance will promote the ability of grasping robots, and is expected to be used in daily life and industry.

Development of a phase-change material-based soft actuator for soft robotic gripper functionality: In-depth analysis of material composition, ethanol microbubble distribution and lifting capabilities

Soft actuators have numerous applications in soft robotics due to their capability of mimicking quite complex motions observed in living things with deformable organs. In the present research, a programmable, low-cost soft actuator based on phase-change material (PCM) composites is developed, using silicon rubber as the matrix, ethanol as the phase-change fluid, and carbon fabric as the conductive path which generates the required heat for actuation. The required pressure for actuation is provided by the electrically-induced phase transition of the PCM (30W electrical power supply). FESEM images illustrate the homogenous distribution of ethanol microbubbles within the active area, with an average diameter of 14.66 μm, and their absence inside the passive layer. The temperature is monitored by an internal thermal sensor placed inside the composite, and controlled to prevent overheating (T > 80 °C) in the active area. The gripper fabricated by using this composite produces considerable motion using a low-voltage driving force (∼30 V). Moreover, it undergoes a sizeable displacement (∼9 cm) and bending angle (∼50°), considering the low amount of power input (30W). Additionally, the tip of the gripper reaches the maximum velocity of ∼1 mm s−1, and the designed sample is capable of lifting objects with varying shapes and weights (from ∼10g to ∼300g).