Soft Robotics Research Articles

Slot‐Die‐Printed Electroactive Polymer Actuators with High‐Strain Sensitivity for Soft Robotic Applications

AbstractIonic electroactive polymer gels are widely employed as actuators in soft robotics due to their ability to undergo rapid mechanical deformation under low‐voltage. How to improve the performance of the ionic electroactive polymer actuators is always the focus of research in this field. Optimizing the migration of ions within the actuator is crucial for enhancing the actuation performance. In this regard, this study innovatively introduces slot‐die‐printing technology to fabricate gel actuation layers characterized by high smoothness and uniform particle distribution, achieving seamless integration between the electrodes and gel actuation layers, while mitigating issues associated with ion accumulation due to polymer clustering. Furthermore, this study attempts to add lactic acid to the traditional CS‐PVA‐IL gel, effectively strengthening the connection between CS and PVA, and promoting smooth ion transport within the gel. The results demonstrate that the actuators prepared in this study achieved a high‐bending‐strain of 0.35%, with a retention rate of 91% for actuation displacement after 10 000 cycles, showcasing superior actuation performance compared to existing research. The fabrication method proposed in this study is simple and highly reproducible, making it suitable for widespread industrial application, and providing a new approach for future industrial production of soft robotics.

Programmable Hydrogel-Based Soft Robotics via Encoded Building Block Design

Hydrogels have revolutionized the field of soft robotics with their ability to provide dynamic and programmable responses to different stimuli, enabling the fabrication of highly adaptable and flexible robots. This continual development holds significant promise for applications in biomedical devices, active implants, and sensors due to the biocompatibility of hydrogels. Actuation in hydrogel-based soft robotics relies on variations in material properties, structural design, or a combination of both to generate desired movements and behaviors. While such traditional approaches enable hydrogel actuation, they often rely on complex material design, bringing challenges to hydrogel fabrication and hindering practical use. Therefore, this work seeks to present a simplified and versatile approach for fabricating programmable single-component hydrogel-based soft robotics using an encoded building block design concept and 3D printing. A series of structural building blocks have been designed to achieve various actuation characteristics, including the direction, degree, and kinetics of actuation. By assembling these building blocks into various configurations, a broader range of actuation responses can be encoded, allowing for the fabrication of versatile, programmable soft robotics using a single uniform material through vat photopolymerization 3D printing. This approach enables adaptation to a wide range of applications, providing highly customizable encoding designs.

Biomimetic soft actuator: Rapid response to multiple stimuli with programmable control

Stimuli-responsive soft actuators have attracted increasing attention in multiple fields since they exhibit unique advantages in safe interaction with humans and adaptability to dynamic environments. However, the study of untethered stimuli-responsive soft actuators has been limited by the slow response speed, irreversible deformations or inability in responding to multiple stimuli. Taking inspiration from the growth process of lotus leaves, we propose a novel design for an untethered soft actuator comprising graphene oxide (GO) and polyethylene (PE), which shows sensitive response to multiple stimuli, including moisture, light, heating, cooling, and elective volatile organic compounds (VOCs), and exhibits significant deformation at a rapid rate (up to 90.5°/s). The high sensitivity of the actuators enables them to be powered by the neglected energy in daily life instead of requiring specialized energy sources, paving the way for the green and sustainable development in soft robotics. In addition, the bending modes and bending degrees of actuators can be programmed to meet the personalized needs of complex three-dimensional structures. Based on GO/PE actuators, a variety of multi-stimuli-responsive intelligent devices have been developed to demonstrate their application potential in arts, bionics, soft robotics and wearable devices, including interesting WordArt, Chinese paper-cut art pieces, artificial iris, soft crawling robots and a smart cloth unit with adjustable breathability.

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.

Functional Thermoplastic Polyurethane Elastomers with α, ω-Hydroxyl End-Functionalized Polyacrylates.

Thermoplastic polyurethane (TPU) is an essential class of materials for demanding applications, from soft robotics and electronics to medical devices and batteries. However, traditional TPU development is primarily relied on specific soft segments, such as polyether, polyester, and polycarbonate polyols. Here, a novel method is introduced for developing TPU elastomers with enhanced performance and superior functionalities compared to conventional TPUs, achieved through the use of α,ω-hydroxyl end-functionalized polyacrylates. This approach involves a defect-free synthesis of α,ω-hydroxyl end-functionalized polyacrylates through visible-light-driven photoiniferter polymerization. By strategically blending these functionalized polyacrylates with conventional polyols, TPUs that exhibit exceptional toughness and notable self-healing capabilities, traits rarely found in existing TPUs are engineered. Furthermore, incorporating photo-crosslinkable acrylic monomers has enabled the creation of the first TPU with superior elastomeric properties and photopatterning capabilities. This approach paves the way for a new direction in polyurethane engineering, introducing a novel class of soft segments and unlocking the potential for a wide range of advanced applications.

Programmable Chemical Reactions Enable Ultrastrong Soft Pneumatic Actuation.

Soft pneumatic actuation is widely used in wearable devices, soft robots, artificial muscles, and surgery machines. However, generating high-pressure gases in a soft, controllable, and portable way remains a substantial challenge. Here, a class of programmable chemical reactions that can be used to controllably generate gases with a maximum pressure output of nearly 6MPa is reported. It is proposed to realize the programmability of the chemical reaction process using thermoelectric material with programmable electric current and employing preprogrammed reversible chemical reactants. The programmable chemical reactions as soft pneumatic actuation can be operated independently as miniature gas sources (∼20-100g) or combined with arbitrary physical structures to make self-contained machines, capable of generating unprecedented pressures of nearly 6MPa or forces of about 18kN in a controllable, portable, and silent manner. Striking demonstrations of breaking a brick, a marble, and concrete blocks, raising a sightseeing car, and successful applications in artificial muscles and soft assistive wearables illustrate tremendous application prospects of soft pneumatic actuation via programmable chemical reactions. The study establishes a new paradigm toward ultrastrong soft pneumatic actuation.

Drop-on-Demand 3D Printing of Programable Magnetic Composites for Soft Robotics

Soft robotics have become increasingly popular as a versatile alternative to traditional robotics. Magnetic composite materials, which respond to external magnetic fields, have attracted significant interest in this field due to their programmable two-way actuation and shape-morphing capabilities. Additive manufacturing (AM), also known as 3D printing, allows for the incorporation of different functional composite materials to create active components for soft robotic systems. However, current AM methods have limitations, especially when it comes to printing smart composite materials with high functional material content. This is a key requirement for enhancing responsiveness to external stimuli. Commonly used AM methods for smart magnetic composites, such as direct ink writing (DIW), confront challenges in achieving discontinuous printing, and enabling multi-material control at the voxel level, while some AM techniques are not suitable for producing composite materials. To address these limitations, we employed high-viscosity drop-on-demand (DoD) jetting and developed versatile programmable magnetic composites filled with micron-sized hard magnetic particles. This method bridges the gap between conventional ink-jetting and DIW, which require inks with viscosities at opposite ends of the spectrum. This high-viscosity DoD jetting enables continuous, discontinuous, and non-contact printing, making it a versatile and effective method for printing functional magnetic composites even with micron-sized fillers. Furthermore, we demonstrated stable magnetic domain programming and two-way shape-morphing actuations of printed structures for soft robotics. In summary, our work highlights high-viscosity DoD jetting as a promising method for printing functional magnetic composites and other similar materials for a wide range of applications.

Design and Demonstration of Hingeless Pneumatic Actuators Inspired by Plants.

Soft robots have often been proposed for medical applications, creating human-friendly machines, and dedicated subject operation, and the pneumatic actuator is a representative example of such a robot. Plants, with their hingeless architecture, can take advantage of morphology to achieve a predetermined deformation. To improve the modes of motion, two pneumatic actuators that mimic the principles of the plants (the birds-of-paradise plant and the waterwheel plant) were designed, simulated, and tested using physical models in this study. The most common deformation pattern of the pneumatic actuator, bending deformation, was utilized and hingeless structures based on the plants were fabricated for a more complex motion of the lobes. Here, an ABP (actuator inspired by the birds-of-paradise plant) could bend its midrib downward to open the lobes, but an AWP (actuator inspired by the waterwheel plant) could bend its midrib upward to open the two lobes. In both the computational and physical models, the associated movements of the midrib and lobes could be observed and measured. As it lacks multiple parts that have to be assembled using joints, the actuator would be simpler to fabricate, have a variety of deformation modes, and be applicable in more fields.

High-strength self-healing multi-functional hydrogels with worm-like surface through hydrothermal-freeze-thaw method

Soft self-healing materials are promising candidates for flexible electronic devices due to their exceptional compatibility, extensibility, and self-restorability. Generally, these materials suffer from low tensile strength and susceptibility to fracture because of the restricted microstructure design. Herein, we propose a hydrothermal-freeze-thaw method to construct high-strength self-healing hydrogels with even interconnected networks and distinctive wrinkled surfaces. The integration of the wrinkled outer surface with the three-dimensional internal network confers the self-healing hydrogel with enhanced mechanical strength. This hydrogel achieves a tensile strength of 223 kPa, a breaking elongation of 442%, an adhesion strength of 57.6 kPa, and an adhesion energy of 237.2 J m-2. Meanwhile, the hydrogel demonstrates impressive self-repair capability (repair efficiency: 93%). Moreover, the density functional theory (DFT) calculations are used to substantiate the stable existence of hydrogen bonding between the PPPBG hydrogel and water molecules which ensures the durability of the PPPBG hydrogel for long-term application. The measurements demonstrate that this multifunctional hydrogel possesses the requisite sensitivity and durability to serve as a strain sensor, which monitors a spectrum of motion signals including subtle vocalizations, pronounced facial expressions, and limb articulations. This work presents a viable strategy for healthcare monitoring, soft robotics, and interactive electronic skins.

Customizable metamaterial design for desired strain-dependent Poisson’s ratio using constrained generative inverse design network

Inverse design of metamaterial structures with customized strain-dependent Poisson’s ratio has significant potential across various applications. However, achieving precise control over these mechanical properties presents a challenge due to the complex relationship between geometry and mechanical performance. Here, we present a novel data-driven approach utilizing a constrained generative inverse design network (CGIDN) to address this challenge. The CGIDN uses backpropagation to efficiently navigate the design space and achieve target mechanical properties with high accuracy. Our method starts by generating a comprehensive dataset of Poisson’s ratio-strain curves for various geometries incorporating cuts. These curves are then compressed using principal component analysis (PCA) to reduce dimensionality while preserving essential features. A deep neural network (DNN) is then trained to map input geometric parameters to these principal components, with the architecture optimized using grid search. The CGIDN facilitates the inverse design process by recommending geometric parameters for unit cell designs that match specified target Poisson’s ratio-strain curves. We validated the effectiveness of our approach through Finite Element Analysis (FEA) and experimental verification. The FEA results for the designed unit cells showed high agreement with the target and predicted curves, demonstrating the accuracy of the CGIDN model. Further, tensile tests on specimens confirmed that the inverse-designed structures reproduced the desired mechanical behavior upon scale-up. Our method, which enables efficient and accurate design of metamaterials with tailored mechanical properties, holds promise for applications in wearable devices, soft robotics, and advanced sensor systems

Rapid 3D printing of electro-active hydrogels

Electroactive hydrogels (EAH) have gained prominence for their unique ability to change shape or size under an electric field, finding applications in biosensors and soft actuators. This study focuses on a tunnelable EAH tailored for high-resolution 3D printing via the micro continuous liquid interface production (µCLIP) process. The resin formulations mainly involve acrylic acid (AA) and 4-hydroxybutyl acrylate (4-HBA). AA, with its carboxyl groups, introduces electroactuation, generating osmotic pressure in the hydrogel matrix. This results in swelling, inducing bending towards the cathode, accentuating the material’s responsiveness. In contrast, 4-HBA offers mechanical resilience, providing an elastic backbone, and ensuring the hydrogel’s applicability. Our results illustrate the hydrogel’s strength, flexibility, and bending capabilities across varied compositions and electric field strengths. Notably, the µCLIP process enabled the 3D printing of our EAH into sophisticated structures, like the lattice. The combination of AA’s electro-responsive traits with 4-HBA’s durability has birthed a material with vast practical implications. This study provides extended potential breakthroughs in soft robotics, wearable electronics, and medical devices, marking a significant stride in the realm of electroactive materials.

Deformation and enclosed volume change of axisymmetric non-linear dielectric elastomer membrane pump: a theoretical study

Abstract This work investigates the change in the enclosed volume of an axisymmetric non-linear hyper-elastic membrane pump subjected to dielectric actuation. The equilibrium equations in cylindrical coordinates, coupled with non-linear constitutive relations, are solved numerically to obtain the deformed membrane geometry under pressure loading (using Variational Calculus). The effect of dielectric actuation is then incorporated by considering the change in material properties and deformation induced by the applied electric field. The deformed geometry under dielectric actuation is determined, and the change in enclosed volume is calculated by comparing the deformed states with and without the applied electric field. The proposed approach enables quantifying the volumetric change in non-linear hyper-elastic membrane pumps due to dielectric actuation for potential applications in soft robotics, adaptive optics, and microfluidics. By non-dimensionalizing variables, key dimensionless parameters governing the problem are identified for broader applicability. Furthermore, this methodology can estimate volume changes for different electric actuations when a specific material is chosen with experimentally derived parameter trends, facilitating material selection or synthesis to meet targeted volumetric requirements.

Fracture-driven power amplification in a hydrogel launcher.

Robotic tasks that require robust propulsion abilities such as jumping, ejecting or catapulting require power-amplification strategies where kinetic energy is generated from pre-stored energy. Here we report an engineered accumulated strain energy-fracture power-amplification method that is inspired by the pressurized fluidic squirting mechanism of Ecballium elaterium (squirting cucumber plants). We realize a light-driven hydrogel launcher that harnesses fast liquid vapourization triggered by the photothermal response of an embedded graphene suspension. This vapourization leads to appreciable elastic energy storage within the surrounding hydrogel network, followed by rapid elastic energy release within 0.3 ms. These soft hydrogel robots achieve controlled launching at high velocity with a predictable trajectory. The accumulated strain energy-fracture method was used to create an artificial squirting cucumber that disperses artificial seeds over metres, which can further achieve smart seeding through an integrated radio-frequency identification chip. This power-amplification strategy provides a basis for propulsive motion to advance the capabilities of miniaturized soft robotic systems.

An adhesive, highly stretchable and low-hysteresis alginate-based conductive hydrogel strain sensing system for motion capture

A strain sensor stands as an indispensable tool for capturing intricate motions in various applications, ranging from human motion monitoring to electronic skin and soft robotics. However, existing strain sensors still face difficulties in simultaneously achieving superior sensing performance sufficing for practical applications like high stretchability and low hysteresis, as well as seamless device fabrication like desirable interfacial adhesion and system-level integration. Herein, we develop a highly stretchable and low-hysteresis strain sensor with adhesive poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/polyacrylamide (PAAm)‑sodium alginate (SA) composite hydrogel, allowing the successful construction of a wireless motion capture sensing system that can provide precise data collection within a large deformation range. The resultant composite hydrogel displays favorable interfacial adhesion and robust mechanical stability, and the fabricated strain sensor demonstrates a wide working strain range (up to 500 %) with high sensitivity (gauge factor = 11) and ultra-low hysteresis (1.52 %), outperforming previous PEDOT-based hydrogel strain sensors. Enabled by the intriguing material properties and high sensing performance, we further demonstrate the fabrication and integration of a wireless motion capture sensing system for diverse applications like human motion monitoring, gesture recognition, and interactive communication.

Effect of isotropy and anisotropy: Toward understanding the fracture behavior of magnetoactive elastomers

Magnetoactive elastomers (MAEs) exhibit magneto-mechanical coupling and typically contain micron-sized spherical ferromagnetic particles embedded in an elastomeric matrix. Anisotropic particle arrangement in MAEs, achieved by curing the material in a magnetic field, offers tailored and enhanced magneto-mechanical coupling compared to isotropic configurations. While tunable MAE properties such as the effective modulus and vibrational response have been relatively well studied, a thorough understanding of their fracture mechanisms, particularly in MAEs with microstructural anisotropy or composites with similar structures remains almost completely unexplored. Here, we characterize the fracture mechanisms of anisotropic and isotropic unmagnetized MAEs using experiments and simulations, focusing on the effects of spherical particles in anisotropic chain-like configurations. Specifically, we experimentally measure the fracture toughness of MAEs containing different volume fractions of particles, and with particles arranged both randomly and aligned at 0°, 45°, and 90° to the loading direction. Results show that anisotropic MAEs with particle chains aligned with the load direction can improve fracture toughness by up to almost 600%, whereas isotropic MAEs increase the fracture toughness by only 420%, compared to the pure elastomer. Scanning electron microscopy of the post-fractured surface reveals toughening mechanisms at micro-scale, such as chain waviness, particle agglomeration, and particle distribution. Simulations using a finite element method-based decoupled phase field-cohesive zone model on simplified geometries qualitatively support imaging interpretations. Overall, this work explains how internal geometry, including waviness in particle chains, chain alignment, and particle agglomerations affects fracture of MAEs and generally soft composites with chain-like geometries of spherical particles. These results have engineering applications in improving fracture properties of smart soft composites relevant to soft robotics, tunable vibration absorbers, and noise attenuators.

Hopping potential wells and gait switching in a fish-like robot with a bistable tail

Fish outperform current underwater robots in speed, agility, and efficiency of locomotion, in part due to their flexible appendages that are capable of rich combinations of modes of motion. In fish-like robots, actuating many different modes of oscillation of tails or fins can become a challenge. This paper presents a highly underactuated (with a single actuator) fish-like robot with a bistable tail that features a double-well elastic potential. Oscillations of such a tail depend on the frequency and amplitude of excitation, and tuning the frequency-amplitude can produce controllable oscillations in different modes leading to different gaits of the robot. This robot design is inspired by recent work on underactuated flexible swimming robots driven by a single rotor. The oscillations of the rotor can propel and steer the robot, but saturation of the rotor makes performing long turns challenging. This paper demonstrates that by adding geometric bistability to the flexible tail, turns can be performed by controllably exciting single-well oscillations in the tail, while exciting double-well oscillations of the tail produces average straight-line motion. The findings of this paper go beyond the application to a narrow class of fish-like robots. More broadly we have demonstrated the use of periodic excitation to produce bistable response that generate different gaits including a steering gait. The mechanics demonstrated here show the feasibility of applications to other mobile soft robots.

4D printing of liquid crystal elastomer composites with continuous fiber reinforcement

Multifunctional composites have been continuously developed for a myriad of applications with remarkable adaptability to external stimuli and dynamic responsiveness. This study introduces a 4D printing method for liquid crystal elastomer (LCE) composites with continuous fibers and unveils their multifunctional actuation and exciting mechanical responses. During the printing process, the relative motion between the continuous fiber and LCE resin generates shear force to align mesogens and enable the monodomain state of the matrix materials. The printed composite lamina exhibits reversible folding deformations that are programmable by controlling printing parameters. With the incorporation of fiber reinforcement, the LCE composites not only demonstrate high actuation forces but also improved energy absorption and protection capabilities. Diverse shape-changing configurations of 4D composite structures can be achieved by tuning the printing pathway. Moreover, the incorporation of conductive fibers into the LCE matrix enables electrically induced shape morphing in the printed composites. Overall, this cost-effective 4D printing method is poised to serve as an accessible and influential approach when designing diverse applications of LCE composites, particularly in the realms of soft robotics, wearable electronics, artificial muscles, and beyond.

Sim-to-Real of Soft Robots With Learned Residual Physics

Sim-to-Real of Soft Robots With Learned Residual Physics

High-performance magnetic artificial silk fibers produced by a scalable and eco-friendly production method

Flexible magnetic materials have great potential for biomedical and soft robotics applications, but they need to be mechanically robust. An extraordinary material from a mechanical point of view is spider silk. Recently, methods for producing artificial spider silk fibers in a scalable and all-aqueous-based process have been developed. If endowed with magnetic properties, such biomimetic artificial spider silk fibers would be excellent candidates for making magnetic actuators. In this study, we introduce magnetic artificial spider silk fibers, comprising magnetite nanoparticles coated with meso-2,3-dimercaptosuccinic acid. The composite fibers can be produced in large quantities, employing an environmentally friendly wet-spinning process. The nanoparticles were found to be uniformly dispersed in the protein matrix even at high concentrations (up to 20% w/w magnetite), and the fibers were superparamagnetic at room temperature. This enabled external magnetic field control of fiber movement, rendering the material suitable for actuation applications. Notably, the fibers exhibited superior mechanical properties and actuation stresses compared to conventional fiber-based magnetic actuators. Moreover, the fibers developed herein could be used to create macroscopic systems with self-recovery shapes, underscoring their potential in soft robotics applications.

Perovskite nanocrystals photo-initiated in situ encapsulation for optical tracking

Metal halide perovskite (CsPbX3, X = Cl, Br, I) nanocrystals (MHP NCs) have been widely investigated for a variety of optoelectronic applications due to their superior photophysical characteristics. However, the inherent poor stability has limited broad applications. Herein, a facile in situ photopolymerization strategy using MHP NCs as photo-initiators is designed to fabricate homogeneous perovskite-silicone composites with outstanding stability, flexibility, and scalability. The fabrication process is completed upon exposure to 30 W ultraviolet light for 5 min under ambient conditions without additional protections and additives. The composites have exceptional stability in air and can even withstand harsh conditions, such as a basic pH of 13 for six months. The mechanical properties of composites are tunable from rigid to elastic by regulating the composition, ratios of polymerization monomers, and reaction time. Their superb properties enable optical visualization tracking for magnetically driven soft robots. This convenient and effective approach opens a new pathway for future applications in optical tracking, soft displays, biological detection, and bioimaging of perovskite nanocrystals.