Applications In Soft Robotics Research Articles

Ultra‐Tough, yet Rigid and Healable Supramolecular Polymers with Variable Stiffness for Multimodal Actuators

AbstractVariable stiffness materials have shown considerable application in soft robotics. However, previously reported materials often struggle to reconcile high stiffness, stretchability, toughness, and self‐healing ability, because of the inherently conflicting requisite of these properties in molecular design. Herein, we propose a novel strategy that involves incorporating acid‐base ionic pairs capable of from strong crosslinking sites into a dense and robust hydrogen‐bonding network to construct rigid self‐healing polymers with tunable stiffness and excellent toughness. To demonstrate these distinct features, the polymer was employed to serve as the strain‐regulation layers within a fiber‐reinforced pneumatic actuator (FPA). The exceptional synergy between the configuration versatility of FPA and the dynamic molecular behavior of the supramolecular polymers equips the actuator with simultaneous improvement in motion dexterity, multimodality, loading capacity, robustness, and durability. Additionally, the concept of integrating high dexterity at both macro‐ and micro‐scale is prospective to inspire the design of intelligent yet robust devices across various domains.

Spring-reinforced pneumatic actuator and soft robotic applications

Abstract Compared to rigid-structure robots, soft robots possess higher degrees of freedom and stronger environmental adaptability, which has aroused increasing attention in the robotic field. Among them, soft pneumatic robots have excellent performances in various practical applications. However, the nonlinearity and instability of pressure response of soft actuators caused by lateral expansion come to a great challenge. To address this problem, we proposed to embed a spring constraint layer around each single air chamber. Following the design concept, we obtained single-cavity and multi-DoF pneumatic actuators and evaluated their elongation and bending characteristics. Experimental results demonstrated that our proposed actuators have more linear pressure response as well as higher consistency. Eventually, through robotic applications, including soft robotic hand and gripper our proposed actuators could facilitate flexible manipulation and elaborate performance.

Novel 4D-printed multi-stable metamaterials: programmability of force-displacement behaviour and deformation sequence.

The unique properties of metamaterials are determined by the configuration and spatial arrangement of artificially designed unit structures. However, the configuration and mechanical properties of conventional metamaterials are challenging to reverse and adjust. Based on curved beams, two types of novel three-dimensional (3D) multi-stable metamaterials with reconfigurable deformation and tunable mechanical properties are designed and fabricated using a four-dimensional (4D) printing method. The effects of temperature and curved-beam thickness on the force-displacement curves and multi-stable snapping sequence of the 3D multi-stable metamaterials are investigated by finite-element analysis (FEA) and experiments. In addition, based on the designed four-branch multi-stable metamaterials, three- and six-branched multi-stable structures are designed by changing the number of curved-beam branches. It is shown that, owing to shape memory effects, the 3D multi-stable metamaterials can realize mechanical programmability, and the multi-stable deformation sequence can be precisely regulated by varying the temperature and curved-beam thickness. These 4D-printed multi-stable metamaterials provide valuable contributions to the design of programmable multi-stable metamaterials and their applications in soft robots and intelligent structures. This article is part of the theme issue 'Current developments in elastic and acoustic metamaterials science (Part 1)'.

3D printing of lignin-based supramolecular topological shape-morphing architectures with high strength, toughness, resolution, and fatigue resistance

The design and fabrication of customized and sustainable elastomers with supramolecular frameworks remain a central focus of scientific research. 3D printing represents an advanced manufacturing technology that has garnered significant attention. Lignin, a naturally abundant polymer, fits well with 3D printing due to its unique aromatic-rich structures that can provide rigidity and structural support. However, challenges persist in developing UV-curable lignin-based inks and fabricating high-strength lignin-based composite hydrogels with tailored shapes and structures through vat photopolymerization (VPP) printing techniques, largely due to lignin’s inherent heterogeneity, fragility, and poor fluidity. Here, we successfully developed a unique lignin-based photosensitive macromonomer resin tailored for VPP 3D printing. Using an ethanol/water fractionation process, heterogeneous lignosulfonate (LS), a by-product of the pulp and paper industry, was treated to isolate highly reactive lignin fractions rich in phenolic hydroxyl and sulfonic groups. These fractions were then chemically modified to synthesize a lignin-based macromonomer known as urethane acrylated lignosulfonate (UALS). The resulting lignin-based macromonomer (15–35 wt%) exhibits excellent compatibility with photosensitive resin formulations, enabling effective VPP 3D printing. The 3D-printed lignin-based supramolecular composite hydrogels exhibit high strength (tensile strength of ∼2.12 MPa, an elongation at break of ∼220.13 %), high resolution, fatigue resistance (up to 10000 cycles), and moisture-induced responsive behavior. The development of 3D-printed lignin-based supramolecular elastomers with defined shapes and patterned structures significantly advances the discovery of robust and environmentally sustainable soft materials with potential applications in soft robotics and tissue engineering.

Buckling-inspired triboelectric sensor for multifunctional sensing of soft robotics and wearable devices

Amidst the rapid development of intelligent robotics and wearable health monitoring devices, there is an urgent demand for flexible sensor with high sensitivity and precise feedback. In this paper, a multifunctional triboelectric sensor inspired by the phenomenon of buckling is introduced, which integrates kirigami structures with triboelectric nanogenerator principles to accurately detect one-dimensional (1D) and two-dimensional (2D) deformations. The sensor employs a contact-separating structure, with its friction layers consisting of 2D laser-cut kirigami electrodes, which deform into three-dimensional (3D) structures through tensile or compressive buckling. The tensile buckling sensor (BS) developed is capable of sensing the bending angle of soft grippers, allowing the detection of the size of grasped objects with a high recognition rate of 93.75 %. Additionally, in its 1D compressive buckling form, the sensor can be used to recognize various human motion patterns effectively, while the 2D compressive version can be used to measure the expansion of curved surfaces with a sensitivity of 1.3307 V/mm. By combining the unique properties of buckling structures, the innovative triboelectric multifunctional sensor proposed offers a new solution for high-precision motion feedback control with potential application in soft robotics and health monitoring.

A soft robotic system imitating the multimodal sensory mechanism of human fingers for intelligent grasping and recognition

The sensory function is crucial for achieving precise feedback control and environmental interaction in soft robots. Therefore, the multimodal sensory capabilities that mimic human fingers have always been a research hotspot. This study proposes a design method for a soft robotic gripper that can emulate the multimodal perception mechanism of human fingers and achieve high-precision recognition of grasped objects using machine learning algorithms. In addition, a finger texture structure inspired by human fingerprints is designed using a negative pressure-induced buckling method to enhance the soft gripper's ability to perceive minute surface textures. Inspired by slow adapting (SA) receptors and fast adapting (FA) receptors of human fingers, we have innovatively designed a capacitive curvature sensor (CS) and a triboelectric texture sensor (TS) to detect the size, material, and texture of objects. Furthermore, a non-contact medium identification sensor (MIS), having a sensitivity of more than 1300 times higher compared to the traditional MISs, is fabricated using the principle of electromagnetic resonance. It is shown that by employing a deep learning-based multi-channel feature fusion network, the soft grasping system with multimodal sensing has achieved a recognition accuracy of 98.43 % for different objects. This work provides an innovative approach for the application of soft robots in various fields, such as logistics sorting and food safety.

Tremendous Stiffness‐Changing Polymer Networks Enabled by Touch‐Induced Crystallization

AbstractStiffness‐changing polymers are critically needed for intelligent and adaptive applications in soft robotics and wearable electronics; however, they suffer from the requirement of persistent external stimulation. Herein, a stiffness‐changing polymer network impregnated with a supercooled ionic liquid is introduced, which undergoes touch‐induced crystallization from a soft ionogel to a stiff plastic. The network exhibits a remarkable tunability in stiffness change, with the highest value exceeding 190 000 times between the soft and stiff states (3.5 kPa vs 691.1 MPa). This tremendous transition is achieved through the first‐order liquid–solid phase transition of the supercooled ionic liquid, which is highly reversible and isochoric via a heating‐cooling process. Unlike traditional stimuli, only a single touch is needed without sustained stimulation to trigger the stiffening from soft to stiff states. Importantly, the touch‐responsive polymer network can be extended to various types of ionic liquids and monomers, indicating excellent strategic versatility. Moreover, this unique ionogel exhibits switchable adhesiveness and ionic conductivity, effectively demonstrating smart adhesives and ionic switches with enhancements of more than 66 and 5800 times, respectively. This work provides a facile and versatile strategy for designing stable stiffness‐changing polymers, unlocking significant potential applications for next‐generation smart devices.

Thermoresponsiveness Across the Physiologically Accessible Range: Effect of Surfactant, Cross-Linker, and Initiator Content on Size, Structure, and Transition Temperature of Poly(N-isopropylmethacrylamide) Microgels.

The influence of surfactant, cross-linker, and initiator on the final structure and thermoresponse of poly(N-isopropylmethacrylamide) (pNIPMAM) microgels was evaluated. The goals were to control particle size (into the nanorange) and transition temperature (across the physiologically accessible range). The concentration of the reactants used in the synthesis was varied, except for the monomer, which was kept constant. The thermoresponsive suspensions formed were characterized by dynamic light scattering, small-angle X-ray scattering, atomic force microscopy, and rheology. Increasing surfactant, sodium dodecyl sulfate content, produced smaller microgels, as expected, into the nanorange and with greater internal entanglement, but with no change in phase transition temperature (LCST), which is contrary to previous reports. Increasing cross-linker, N,N-methylenebis acrylamide, content had no impact on particle size but reduced particle deformability and, again contrary to previous reports of decreases, progressively increased the LCST from 39 to 46 °C. The unusual LCST trends were confirmed using different rheological techniques. Initiator, potassium persulfate, content was found to weakly influence the outcomes. An optimized content was identified that provides functional nanogels in the 100 nm (swollen) size range with controlled LCST, just above physiological temperature. The study contributes chemistry-derived design rules for thermally responsive colloidal particles with physiologically accessible LCST for a variety of biomedical and soft robotics applications.

In-situ characterization of dynamic electromechanical properties for PVC gel actuators

PVC gel actuator possesses the advantages including low driving voltage, substantial deformation, and high output force, making it ideal for applications in micro-lenses, assistive rehabilitation, and soft robotics. However, there remains a lack of in-situ characterization and quantitative analysis of plasticizer migration and charge transfer within PVC gel, resulting in an unclear understanding of changes in the plasticizer content, mechanical and electrical properties of the enrichment layer. In this study, we conducted in-situ characterization of plasticizer migration and charge transfer within PVC gel, and analyzed the change of plasticizer content, mechanical and electrical properties. In-situ Raman spectroscopy results revealed that plasticizer migration takes time to reach equilibrium, resulting in a gradient distribution of plasticizer content within the enrichment layer. Furthermore, in-situ broadband impedance analysis demonstrated that stimulation voltage induces changes in the material's electrical properties, transitioning from uniform electrical characteristics to distinct properties between the gel layer and the enrichment layer. Notably, we established a mathematical relationship between modulus and plasticizer content, revealing a gradient distribution of modulus within the enrichment layer. This study not only contributes to understanding the underlying mechanisms of mechanical and electrical properties variations within PVC gel, but also provides valuable insights for optimizing the actuation performance of PVC gel actuators.

Fast Responsive and High-Strain Electro-Ionic Soft Actuator Based on the 3D-Structure MXene-EGaIn/MXene Bilayer Composite Electrode.

Electro-ionic soft actuators have garnered significant attention owing to their promising applications in flexible electronics, wearable devices, and soft robotics. However, achieving high actuation performance (large bending strain and fast response time) of these soft actuators under low voltage has been challenging due to issues related to ion diffusion and accumulation. In this study, an electro-ionic soft actuator is fabricated using Ti3C2Tx MXene and eutectic gallium-indium (EGaIn) composite material as the bilayer electrode and methylammonium formate/1-ethyl-3-methylimidazolium tetrafluoroborate/poly(vinylidene fluoride) (MAF-EMIMBF4/PVDF) as the ionic liquid-type electrolyte. The research results indicate that the prepared soft actuator exhibits excellent actuation performance with a peak-to-peak displacement of 35 mm and a bending strain of 0.69% (a peak-to-peak strain of 1.38%) under a low voltage (3 V). The electro-ionic soft actuator shows a wide frequency range (0.1-10 Hz), fast response time (0.35 s), and a rise time of 7.5 s. Furthermore, it demonstrates good cyclic durability, with a retention rate of 92.5% of its performance for 10 000 cycles. These excellent performances are attributed to the 3D structure of the Ti3C2Tx-EGaIn/Ti3C2Tx bilayer composite electrode, as well as the characteristics of the low viscosity, high conductivity, small ion volume, and larger volume difference between cations and anions in MAF ionic liquid. The high-performance electro-ionic soft actuator can be used in various fields such as artificial muscles, tactile devices, and soft robots.

An afferent nerve-like electronic device with somatic mechanical perception and sensation management

Tactile and pain perception are essential for biological skin to interact with the external environment. This complex interplay of sensations allows for the detection of potential threats and appropriate responses to stimuli. However, the challenge is to enable flexible electronics to respond to mechanical stimuli such as biological skin, and researchers have not clearly reported the successful integration of somatic mechanical perception and sensation management functions into neuro-like electronics. In this work, an afferent nerve-like device with a pressure sensor and a perception management module is proposed. The pressure sensor comprises two conductive fabric layers and an ionic hydrogel, forming a capacitor structure that emulates the swift transition from tactile to pain perception under mechanical stimulation. Drawing inspiration from the neuronal “gate control” mechanism, the sensation management module adjusts signals in response to rubbing, accelerating the discharge process and reducing the perception duration, thereby replicating the inhibitory effect of biological neurons on pain following tactile interference. This integrated device, encompassing somatic mechanical perception and sensation management, holds promise for applications in soft robotics, prosthetics, and human-machine interaction.

Integrated 3D printing of reconfigurable soft machines with magnetically actuated crease-assisted pixelated structures

Reconfigurable magnetic soft machines have a wide range of applications in soft robotics, aerospace, medical devices and flexible electronics. However, the high-precision reconfiguration of magnetically actuated soft machines is hindered by the absence of rational structural design and integrated manufacturing techniques. Here, we report an innovative design and manufacturing approach to realize magnetically actuated high-precision reconfigurability. We present a crease-assisted pixelated design using a mixture of magnetic particles and phase-change medium as the pixel points, with an elastomer as the crease to connect the pixel points, simulating origami. The design of elastomer creases improves pixel stiffness without loss of reconfigurable accuracy, thereby significantly improves the magnetic particle concentration in the pixel. An effective multi-material 3D printing technique is used to achieve the integrated printing of the designed structure. The resulting magnetically actuated soft machines exhibit remarkable capabilities, achieving small curvature bending and precise reconfiguration with weaker actuating magnetic fields (100mT). The superior performance of this design has been confirmed through demonstrations of crawling and rolling machines, magnetically controlled switches, and logic circuits. The work enhances the reconfiguration accuracy of magnetically actuated soft machines, increasing their potential for soft robotics and flexible electronics applications.

A Planar Coil-Based Inductive Bend Sensor for Soft Robotic Applications

A Planar Coil-Based Inductive Bend Sensor for Soft Robotic Applications

Sensory Nervous System‐Inspired Self‐Classifying, Decoupled, Multifunctional Sensor with Resistive‐Capacitive Operation Using Silver Nanomaterials

AbstractSelf‐classification technology has remarkable potential for autonomously discerning various stimuli without any circuit or software assistance, enabling it to realize electronic skin. In conventional self‐classification systems that rely on complex circuitry for operation, integrating the sensing and algorithm processing units inevitably leads to bulkiness in devices and bottlenecks in signal processing. In this study, the novel double‐sided structure inspired by the human nervous system is newly designed for a self‐classifying sensor (SCS) without the need for additional circuits. The sensor is layered with Ag nanocomposites that have been mechanically enhanced via interface engineering and surface treatment techniques. This structure enables the resistance‐capacitance hybrid operation, facilitating the detection and distinguishment of changes in strain, pressure, and temperature within a single device, which mimics the human sensing recognition process. Moreover, the intensity of the applied stimuli is determined by analyzing the detected signal, and precise localization of the stimuli is achieved by arraying the sensors. With its self‐classification capabilities, SCS opens promising avenues for applications in soft robotics and advanced multifunctional sensor platforms, providing a sensing system characterized by simplicity and efficiency.

Soft robotic actuators with asymmetrically engineered liquid crystal elastomers

Physical intelligence uses material properties and structural design for actuation. This study designed an asymmetrical cylinder with metal needles, springs, and inorganic salt. When heated, it generates a twisting force that induces rolling by shifting the center of gravity. Cylindrical liquid crystal elastomers (CLCEs) were used as wheels in a cargo transfer device. Additionally, actuators capable of grabbing were developed using particle-doped CLCEs with salt and PDA particles. These asymmetrical CLCEs show potential for soft robot applications, including AI components and polymeric light/heat control systems. BackgroundFew reports exist on shifting the center of gravity in liquid crystal elastomers with dopants, enabling rotation or bending. MethodsTo fabricate CLCEs with asymmetric constructions, different dopants and particles were mixed with an LCE mixture and injected into a cylindrical glass substrate for polymerization. Significant findingsThis study demonstrates the creation of CLCEs with shifted centers of gravity through various dopants and twisting angles. These elastomers can roll, making them ideal for cargo transfer, with movement speed controllable by adjusting temperature, dopants, and twisting angles. They also hold promise for developing robotic arms that can grasp objects, controlled through heat or light exposure.

Fluid Ejections in Nature.

From microscopic fungi to colossal whales, fluid ejections are universal and intricate phenomena in biology, serving vital functions such as animal excretion, venom spraying, prey hunting, spore dispersal, and plant guttation. This review delves into the complex fluid physics of ejections across various scales, exploring both muscle-powered active systems and passive mechanisms driven by gravity or osmosis. It introduces a framework using dimensionless numbers to delineate transitions from dripping to jetting and elucidate the governing forces. Highlighting the understudied area of complex fluid ejections, this review not only rationalizes the biophysics involved but also uncovers potential engineering applications in soft robotics, additive manufacturing, and drug delivery. By bridging biomechanics, the physics of living systems, and fluid dynamics, this review offers valuable insights into the diverse world of fluid ejections and paves the way for future bioinspired research across the spectrum of life.

A stretchable and self-powered strain sensor with elastomeric electret

Stretchable deformation sensors play an important role in the perception and autonomy of soft robots. While various sensors have been reported, the mechanical sensor with self-powered capability and stretchability to suit extreme environment is urgently required. In this study, a stretchable, self-powered, sensitivity adjustable, and long-term stability strain sensor based on the stretchable electret is researched for deformation monitoring. This stretchable and self-powered sensor is composed of the dielectric elastomer with designed elastic modulus gradient and the stretchable electret with elastomeric and Nano-particles. The electro-mechanical capability of this designed sensor is researched and optimized by multi-objective optimization approach. The sensor exhibits self-powered capability, stretchability (tensile strain from 0% to 20%), tunable sensitivity (optimized from 5.7 to 5.9 pC mm−1), and stability, where electromechanical performance remains stable after 8000 stretching cycles. The feasibility of its autonomous sensing which promotes a wide range of potential application in soft robotics is demonstrated. This elastic modulus gradient-induced and net charge design significantly broadened the potential applications with large deformation and low-attached stiffness.

Amyloid‐based functional materials and their application in flexible sensors

AbstractFlexible electronic devices have garnered increasing attention for their applications in wearable devices, biomedical systems, soft robots, and flexible displays. However, the current sensors face limitations regarding low sensitivity, poor stability, and inadequate adhesion bonding between stimuli‐responsive functional materials and flexible substrates. To overcome these challenges and enable the further development of sensor devices, surface modification of stimuli‐responsive materials with amyloid aggregates has emerged as a promising approach to enhance functionality and create superior multifunctional sensors. This review presents recent research advancements in the flexible sensors based on protein amyloid aggregation. The article begins by explaining the basic principles of protein amyloid aggregation, followed by outlining the process of preparing 1D to 3D amyloid‐based composite materials. Finally, it discusses the utilization of protein amyloid aggregation as a surface modification technique for developing flexible sensors. Based on this foundation, we identify the shortcomings associated with protein amyloid aggregate composites and propose possible solutions to address them. We believe that comprehensive investigations in this area will expedite the development of high‐performance flexible sensors with high sensitivity, high structural stability, and strong interface adhesion, especially the implantable flexible sensors for health monitoring.

Self-rotation-eversion of an anisotropic-friction-surface torus

Recent experiments have revealed a new end-to-end ribbon spiral structure capable of demonstrating two interconnected periodic zero-energy-mode self-rotation-eversion in reaction to constant temperature or constant light sources, yet its fabrication is challenging due to its intricate nature. Differently, this paper develops a light-fueled self-rotating and everting liquid crystal elastomer torus on an isotropic frictional plane, by imparting anisotropic frictional properties to the surface of the torus. Based on a mature dynamic liquid crystal elastomer model, we constructed the theoretical model of the torus system. The torus is capable of absorbing the illumination energy to counteract damping dissipation and produce zero-energy-mode self-eversion-rotation. Theoretical findings demonstrate that the angular velocity of self-eversion is influenced by light intensity, light penetration depth, gravitational acceleration and anisotropic friction surface. Moreover, there is a proportional relationship between the self-rotation and self-eversion angular velocities, which is determined by the anisotropic frictional surface. The detailed criterion for the self-rotation direction is also established. Theoretical findings exhibit several similar phenomena to experimental observations. This paper proposes a new simple strategy that utilizes anisotropic friction surface to control self-eversion-rotation, which has guiding significance for the application of soft robots, active devices, and energy harvesters.

FlexScale: Modeling and Characterization of Flexible Scaled Sheets

We present a computational approach for modeling the mechanical behavior of flexible scaled sheet materials---3D-printed hard scales embedded in a soft substrate. Balancing strength and flexibility, these structured materials find applications in protective gear, soft robotics, and 3D-printed fashion. To unlock their full potential, however, we must unravel the complex relation between scale pattern and mechanical properties. To address this problem, we propose a contact-aware homogenization approach that distills native-level simulation data into a novel macromechanical model. This macro-model combines piecewise-quadratic uniaxial fits with polar interpolation using circular harmonics, allowing for efficient simulation of large-scale patterns. We apply our approach to explore the space of isohedral scale patterns, revealing a diverse range of anisotropic and nonlinear material behaviors. Through an extensive set of experiments, we show that our models reproduce various scale-level effects while offering good qualitative agreement with physical prototypes on the macro-level.