Soft Actuator Materials Research Articles

Construction of MXene interlayer conductive ion transport channel by polypyrrole-coated cellulose nanofibers for ultralow voltage high-performance ionic soft actuators

Ionic soft actuators are extensively utilized in soft robotics and intelligent flexible devices due to their low-voltage actuation capability. MXene(Ti3C2Tx) has potential value as an electrode material for ionic soft actuators owing to its high conductivity and large specific surface area. However, the actuation performance of its ionic soft actuator is limited by poor extensibility and low capacitance caused by the stacking effect. Here, a high-performance ionic soft actuator with MXene\\polypyrrole-coated cellulose nanofibers (CNF@PPy) is reported. While gaining high flexibility of the electrode due to the great mechanical property of CNF, the fast conductive transport channel is constructed by doping CNF@PPy between MXene layers, which not only effectively reduces the loss of conductive properties and drastically improves the ion migration rate, but also prevents MXene from stacking and has additional chemical active sites, significantly increasing its ion storage capacity. Consequently, the actuator exhibits excellent actuation performance at the 0.5 V ultralow voltage: 0.79 % bending strain, 6 s rapid response, and 91.03 % high durability after 5 hours. Additionally, the potential for the actuating core of advanced flexible electronic devices is demonstrated by the application of flexible forceps and the flexible electronic eye.

Fast-Swimming Soft Robotic Fish Actuated by Bionic Muscle.

Soft underwater swimming robots actuated by smart materials have unique advantages in exploring the ocean, such as low noise, high flexibility, and friendly environment interaction ability. However, most of them typically exhibit limited swimming speed and flexibility due to the inherent characteristics of soft actuation materials. The actuation method and structural design of soft robots are key elements to improve their motion performance. Inspired by the muscle actuation and swimming mechanism of natural fish, a fast-swimming soft robotic fish actuated by a bionic muscle actuator made of dielectric elastomer is presented. The results show that by controlling the two independent actuating units of a biomimetic actuator, the robotic fish can not only achieve continuous C-shaped body motion similar to natural fish but also have a large bending angle (maximum unidirectional angle is about 40°) and thrust force (peak thrust is about 14 mN). In addition, the coupling relationship between the swimming speed and actuating parameters of the robotic fish is established through experiments and theoretical analysis. By optimizing the control strategy, the robotic fish can demonstrate a fast swimming speed of 76 mm/s (0.76 body length/s), which is much faster than most of the reported soft robotic fish driven by nonbiological soft materials that swim in body and/or caudal fin propulsion mode. What's more, by applying programmed voltage excitation to the actuating units of the bionic muscle, the robotic fish can be steered along specific trajectories, such as continuous turning motions and an S-shaped routine. This study is beneficial for promoting the design and development of high-performance soft underwater robots, and the adopted biomimetic mechanisms, as well as actuating methods, can be extended to other various flexible devices and soft robots.

Multiresponsive MXene Actuators with Asymmetric Quantum‐Confined Superfluidic Structures

AbstractMXene, which is known for its high electrical/thermal conductivity, surface hydrophilicity, excellent mechanical flexibility, and chemical stability, is a versatile and smart material for soft actuators. However, most MXene actuators are fabricated by combining MXene with other inert materials to form a bilayer or multilayer structure. Considering the strain mismatch at multimaterial interfaces under frequent deformation, MXene‐based actuators are generally associated with poor stability, which limits their practical applications. Herein, inspired by the natural quantum‐confined superfluidic (QSF) effect, a multiresponsive MXene actuator that can be driven by moisture, light, and electricity by engineering an asymmetric QSF structure on both sides of the MXene film is reported. The actuation mechanism of the MXene film can be attributed to nonuniform water adsorption, transport, and desorption within the asymmetric QSF channels under moisture, photothermal, and electrothermal stimuli. Interestingly, MXene actuators can be flexibly formed into various shapes under moisture‐assisted mechanical compression, which not only enhances their multiresponsive actuation, but also permits a more complex deformation. As proof‐of‐concept demonstrations, various intriguing applications including a dual‐role robot, a smart shielding curtain, and a dragonfly robot, are fabricated, revealing the potential of MXene actuators for soft robotics.

Enabling liquid crystal elastomers with tunable actuation temperature

Liquid crystalline elastomers are regarded as a kind of desirable soft actuator material for soft robotics and other high-tech areas. The isotropization temperature (Ti) plays an important role as it determines the actuation temperature and other properties, which in turn has a great effect on their applications. In the past, the common physical methods (e.g. annealing) to tune Ti is not applicable to tune the actuation temperature. The new Ti obtained by annealing immediately goes back to the old one once it is heated to a temperature above Ti, while actuation needs a temperature higher than Ti. For a fully cross-linked LCE material, once it is synthesized, the actuation temperature is fixed. Accordingly, the actuation temperature can not be tuned unless the chemical structure is changed, which usually needs to start from the very beginning of the molecular design and material synthesis. Here, we found that different Ti achieved by annealing can be preserved by reversible reactions of dynamic covalent bonds in covalently adaptable LC networks including LC vitrimers. Thus, a variety of soft actuators with different actuation temperatures can be obtained from the same fully cross-linked LCE material. As the tuning of Ti is also reversible, the same actuator can be adjusted for applications with different actuation temperature requirements. Such tuning will also expand the application of LCEs.

Multimaterial Printing of Liquid Crystal Elastomers with Integrated Stretchable Electronics.

Liquid crystal elastomers (LCEs) have grown in popularity in recent years as a stimuli-responsive material for soft actuators and shape reconfigurable structures. To make these material systems electrically responsive, they must be integrated with soft conductive materials that match the compliance and deformability of the LCE. This study introduces a design and manufacturing methodology for combining direct ink write (DIW) 3D printing of soft, stretchable conductive inks with DIW-based "4D printing" of LCE to create fully integrated, electrically responsive, shape programmable matter. The conductive ink is composed of a soft thermoplastic elastomer, a liquid metal alloy (eutectic gallium indium, EGaIn), and silver flakes, exhibiting both high stretchability and conductivity (order of 105 S m-1). Empirical tuning of the LCE printing parameters gives rise to a smooth surface (<10 μm) for patterning the conductive ink with controlled trace dimensions. This multimaterial printing method is used to create shape reconfigurable LCE devices with on-demand circuit patterning that could otherwise not be easily fabricated through traditional means, such as an LCE bending actuator able to blink a Morse code signal and an LCE crawler with an on/off photoresistor controller. In contrast to existing fabrication methodologies, the inclusion of the conductive ink allows for stable power delivery to surface mount devices and Joule heating traces in a highly dynamic LCE system. This digital fabrication approach can be leveraged to push LCE actuators closer to becoming functional devices, such as shape programmable antennas and actuators with integrated sensing.

Electrically Controlled Liquid Crystal Elastomer Surfaces for Dynamic Wrinkling

Liquid crystal elastomers (LCEs) are becoming increasingly popular as a shape memory material for soft robot actuators that operate in a contractile or flexural mode. There have been previously studies on the use of LCEs for reversible changes in surface topography. However, surface protrusions have typically been limited to the order of 1 μm or depend on light, heat, or electrical stimulation that are difficult to locally control or require relatively high voltage. This article presents a novel operation mode of LCE actuators based on the wrinkling behavior of an LCE‐elastomer bilayer architecture. Embedding a liquid‐metal‐based conductive ink within the LCE enables electrical control of surface wrinkling through Joule heating. The actuator cells can generate wrinkles with amplitudes ranging from 17 to 45 μm within 30 s under an input power of 2 W and a voltage on the order of 1 V. As the bilayer is composed entirely of soft materials, it is highly deformable, flexible, and can be integrated into a multi‐cell array capable of bending on curved surfaces.

Bending electromechanical actuation mechanism and properties of nanostructured dielectric poly (styrene-b- (ethylene-co-butylene)-b-styrene) / white mineral oil (SEBS/WO) blend elastomers

Dielectric elastomers (DEs) as one class of electronic type of electroactive polymers (EAPs) can be deformed under electrical stimulation, and have important applications in the soft robots, wearable electronic devices, and medical rehabilitation machinery so on. In this work, the bending actuation mechanism, and electromechanical properties of nanostructured dielectric poly (styrene-b-(ethylene-co-butylene)-b-styrene)/white mineral oil (SEBS/WO) blend films were investigated under free electrodes condition. The morphology and mechanical properties of SEBS/WO films were adjusted by the content of WO, where the WO selectively swelled the poly (ethylene-co-butylene) (PEB) domains. The phase transition of lamellar (LAM) structure to hexagonal array packed cylinder (HEX) of SEBS/WO films was observed when added WO content larger than 17 wt%. The modulus and the polystyrene (PS) domain radius of SEBS/WO blend films with HEX structure were decreased as increase of WO weight fraction. The bending displacement of SEBS/WO films was almost linearly dependent on the applied electric voltages, and the single cantilever model based calculated bending force was almost linearly dependent on inverse of PS domain radius for SEBS/WO films with HEX morphology. The bending actuation mechanism of SEBS/WO films with HEX morphology was suggested that the bending actuation of SEBS/WO films was caused by electrostatic attraction force between inter-PS domains induced squeezing deformation of poly (ethylene-co-butylene) (PEB) domains. The SEBS/WO blend films with 50 wt% of WO content showed 6.54 mm (corresponding to 23.56°) at 2 kV (50 V/mm of nominal electric field). Our results can provide new insight on development of nanostructured electroactive elastomers for applications in the soft actuator materials.

Wireless electrochemical actuation of soft materials towards chiral stimuli.

Different areas of modern chemistry, require wireless systems able to transfer chirality from the molecular to the macroscopic event. The ability to recognize the enantiomers of a chiral analyte is highly desired, since in the majority of cases such molecules present different physico-chemical properties that could lead, eventually, to dangerous or harmful interactions with the environment or the human body. From an electrochemical point of view, enantiomers have the same electrochemical behavior except when they interact in a chiral environment. In this Feature Article, different approaches for the electrochemical recognition of chiral information based on the actuation of conducting polymers are described. Such a dynamic behavior of π-conjugated materials is based on an electrochemically induced shrinking/swelling transition of the polymeric matrix. Since all the systems, described so far in the literature, are achiral and require a direct connection to a power supply, new strategies will be presented in the manuscript, concerning the implementation of chirality in electrochemical actuators and their use in a wireless manner through bipolar electrochemistry. Herein, the synergy between the wireless unconventional actuation and the outstanding enantiorecognition of inherent chiral oligomers is presented as an easy and straightforward read out of chiral information in solution. This approach presents different advantages in comparison to classic electrochemical systems such as its wireless nature and the possible real-time data acquisition.

Optomechanical computing in liquid crystal elastomers.

Embodied decision-making in soft, engineered matter has sparked recent interest towards the development of intelligent materials. Such decision-making capabilities can be realized in soft materials via digital information processing with combinational logic operations. Although previous research has explored soft material actuators and embedded logic in soft materials, achieving a high degree of autonomy in these material systems remains a challenge. Light is an ideal stimulus to trigger information processing in soft materials due to its low thermal effect and remote use. Thus, one approach for developing soft, autonomous materials is to integrate optomechanical computing capabilities in photoresponsive materials. Here, we establish a methodology to embed combinational logic circuitry in a photoresponsive liquid crystal elastomer (LCE) film. These LCEs are designed with embedded switches and integrated circuitry using liquid metal-based conductive traces. The resulting optomechanical computing LCEs can effectively process optical information via light, thermal, and mechanical energy conversion. The methods introduced in this work to fabricate a material capable of optical information processing can facilitate the implementation of a sense of sight in soft robotic systems and other compliant devices.

Highly flexible polypyrrole electrode with acanthosphere-like structures for energy storage and actuator applications

Polypyrrole (PPy) is a promising electrode material for energy storage devices and soft ionic actuators owing to its high theoretical capacitance, excellent electrolyte tolerance, and easy low-cost synthesis. However, large-scale production of flexible and self-standing pristine PPy electrodes with hierarchical structures and high capacitance for practical applications remains challenging. Herein, we report a large-scale flexible PPy membrane (15 cm in diameter) with acanthosphere-like vesicle structures (PPy-V) formed via nonionic surfactant–assisted interfacial polymerization. The unique structure could offer the PPy-V electrode more electrochemical active sites for high electrolyte loading and enable fast ion/electron transfer, resulting in improved electrochemical performance. The PPy-V electrode showed a high specific capacitance of 1434.21 mF cm−2 at a current density of 1 mA cm−2, with a capacitance retention rate of 87.2 % after 5000 cycles. The symmetrical supercapacitor assembled using the PPy-V electrode exhibited an energy density of 35.34 μW h cm−2 at a power density of 0.12 μW cm−2, and the curved actuator showed a reversible deformation with a low power consumption per unit strain of 7.45 mW cm−2 %−1. These superior properties of the PPy-V electrode with the bioinspired hierarchical structure make it a promising candidate for flexible supercapacitors and soft ionic actuators.

Fabrication and characteristics of flexible thermoplastic polyurethane filament for fused deposition modeling three‐dimensional printing

AbstractIn the era of artificial intelligence (AI), materials with soft and strong properties are becoming increasingly important. Various three‐dimensional (3D) fused deposition modeling (FDM) filaments have been developed as soft actuator materials. In the present study, a soft actuator‐grade thermoplastic polyurethane (TPU) was developed and then processed into a filament for FDM 3D printers during a melt‐extrusion process using commercial (65, 75, 80, and 85 Shore A) pellets. Rheological, chemical, thermal, and mechanical properties of the manufactured TPU filaments were characterized. Auxetic re‐entrant TPU samples were then printed with the manufactured filament. Results showed that the 75 Shore A pellet was the most suitable for auxetic re‐entrant FDM 3D printing.

Low-intensity near-infrared light-tunable gold nanorod-incorporated hydrogel actuator system for remotely controlled human–robot interface applications

Light-responsive soft actuators have recently drawn attention as the need for lightweight remotely controlled actuator systems with high environmental adaptability and safe human–robot interaction interface characteristics has increased. In this study, we focused on a near-infrared (NIR) light-tunable hydrogel soft actuator system capable of fast and precisely controllable mechanical actuation even under the irradiation of NIR light with low intensity. We designed a bilayer type hydrogel soft actuator consisting of a poly(N-isopropylacrylamide) (PNIPAAm) active layer containing gold nanorods (AuNRs) as a photothermal agent and poly(acrylamide) (PAAm) passive layer. AuNRs with an aspect ratio of 4.12 showed the largest longitudinal plasmon resonance absorption peak in the NIR 880 nm range, which was used as a photothermal agent. The AuNR-incorporated PNIPAAm/PAAm hydrogel with a bilayer structure (PNIPAAm-AuNR/PAAm) exhibited more than 70% gel fraction and temperature-sensitive swelling behaviors. In particular, rod-shaped PNIPAAm-AuNR/PAAm hydrogels showed rapid bending deformation by a photothermally induced phase transition of PNIPAAm upon 880 nm laser irradiation, but a PNIPAAm/PAAm bilayer hydrogel without AuNRs did not show a light-induced bending movement. The NIR-responsive bending actuation of the hydrogel can be precisely controlled by cross-linking density, content of photothermal agent, NIR laser intensity, and thickness ratio of the active/passive layer. It was observed that the full bending deformations of the hydrogel into a ring shape through an arc occurred within a minute under NIR irradiation with a lower intensity of less than 1.0 W/cm2. Hydrogels showed a cell viability of more than 95% in the biocompatibility test, indicating no significant cytotoxicity. Therefore, these PNIPAAm-AuNR/PAAm hydrogels are promising soft actuator materials that can be controlled by low-intensity NIR irradiation for remotely controlled human–robot interaction interface applications.

Latest Advances in Development of Smart Phase Change Material for Soft Actuators

Phase change materials with considerable energy absorption and release show promise for not only energy consumption but also stiffness-changing materials. To improve properties for adapting to a changeable environment, the functional materials have been attracted, which will change in nature or shape under the external stimulus, such as thermal, electricity, magnetic, and PH. Further details can be found in the article number 2100863 by Yuchuan Cheng, Yong Ren and co-workers.

Actuation characteristics and experimental identification of IPMC actuator for underwater biomimetic robotic application

Traditionally underwater robotic devices are driven with the help of hydraulic systems or motors. However, with the current advancement in technology and inclination towards smart structures and miniature designs, various new actuators are being utilized. Ionic polymer-metal composites (IPMC) are the smart materials that come under the category of Ionic Electro-active polymers are considered as the best suitable soft actuator material for robotic devices in the field of military, aerospace and medical applications. The soft actuator’s actuation characteristics are explored in an underwater environment using data acquisition from LabView and laser vibrometer for providing the input voltage to the IPMC and to measure the tip deflection of IPMC actuator respectively. Proportional, Integral and Derivative (PID) closed-loop response using Ziegler Nichols tuning method for getting controller gains is presented through MATLAB/Simulink simulation for better actuation based on desired output response of the actuated IPMC. An evolutionary optimization technique, Particle swarm optimization (PSO) technique has been used to tune the PID controller gains to get a snappier response. The results suggested that the IPMC actuator achieved the desired actuation response of 4 mm under the applied sinusoidal voltage. Also, it is concluded that the proposed experimental model can help researchers in getting proper actuation response from different RC characteristics of IPMC under electric potential in an underwater environment that can be used for application in the underwater biomimetic robotic devices.

Latest Advances in Development of Smart Phase Change Material for Soft Actuators

Inspired from the nature, soft actuators have been attracting the incremental attention on account of the benefits in terms of high flexibility and safety for human operators, but material selection and manufacturing process remain challenging. Compared with the conventional actuators in solid state, smart phase change materials (PCMs) are superior for the soft actuation applications because of the capability to change their shapes or properties in response to a wide range of stimulants, such as heat, light, pH, or electrical field. This review provides a comprehensive analysis of smart PCM applications on soft actuators, including metal alloys and their polymer counterparts, shape memory materials and liquid crystalline polymers. Such materials generally exhibit large deformation degrees, high complexity in movement and diverse versatility, which are compliant and well suited for soft actuators in the vast applications of soft mechatronics and robots. Since stiffness variable plays a significant role on actuator transformation, this review focuses on smart materials with an emphasis on the different forms of phase transformation among solid, liquid, and gas phase. This review enables a better understanding of phase transformation, performances, and limitations of smart PCMs, and provides insights into the potential applications for soft actuators.

Reprogrammable 3D Liquid‐Crystalline Actuators with Precisely Controllable Stepwise Actuation

Liquid‐crystalline elastomers (LCEs) are considered ideal soft actuator materials for a wide range of applications, especially the thriving soft robotics. However, 3D LCE actuators capable of precisely controllable stepwise actuation, which can enhance functionality and versatility of LCE robots for multifarious complicated applications, are still in urgent need for the reported LCE actuators mainly exploit the one‐step actuation upon the liquid‐crystallin (LC)‐isotropic phase transition temperature (Ti). Herein, a catalyst‐free LC‐vitrimer actuator with supercritical behavior is designed, which can perform precisely controllable stepwise actuation with extraordinary shape stability over a broad temperature range of about 70 °C. Moreover, supercritical behavior enables the actuator to be used in nematic phase, imparting the actuator with some extra advantages, such as higher mechanical strength and actuation stability, over the one used above Ti. Furthermore, the LCE can be reprogrammable into arbitrary 3D actuators, which can further be integrated into single‐material actuators with complex stepwise actuation, offering a generalized strategy of LCE actuators for sophisticated practical soft robots.

Ultrarobust Ti3C2Tx MXene-Based Soft Actuators via Bamboo-Inspired Mesoscale Assembly of Hybrid Nanostructures

Soft actuation materials are highly desirable in flexible electronics, soft robotics, etc. However, traditional bilayered actuators usually suffer from poor mechanical properties as well as deteriorated performance reliability. Here, inspired by the delicate architecture of natural bamboo, we present a hierarchical gradient structured soft actuator via mesoscale assembly of micro-nano-scaled two-dimentional MXenes and one-dimentional cellulose nanofibers with molecular-scaled strong hydrogen bonding. The resultant actuator integrates high tensile strength (237.1 MPa), high Young's modulus (8.5 GPa), superior toughness (10.9 MJ/m3), and direct and fast hygroscopic actuation within a single body, which is difficult to achieve by traditional bilayered actuators. The proof-of-concept prototype robot is demonstrated to emphasize its high mechanical robustness even after bending 100 000 times, kneading, or being trampled by an adult (7 500 000 times heavier than a crawling robot). This bioinspired mesoscale assembly strategy provides an approach for soft materials to the application of next-generation robust robotics.

Fast, Light-Responsive, Metal-Like Polymer Actuators Generating High Stresses at Low Strain

Summary Producing lightweight polymeric actuators able to generate high stresses typical of hard metals and/or ceramics remains challenging. The photo-mechanical responses of ultra-drawn ultra-high molecular weight polyethylene (UHMWPE) actuators containing azobenzene photo-switches with symmetrically attached polyethylene (PE) side chains are reported. Long PE side chains promote dispersion within the apolar UHMWPE matrix, and the ultra-drawn films are highly aligned. The ultra-drawn azobenzene-doped UHMWPE films have high Young's moduli (∼100 GPa) and are viscoelastic at room temperature at strains below 1%. The photo-mechanical response of the films is fast ( 6 × 104 Pa (kg m−3)−1) to UV or visible light at a low strain (∼0.06%). The actuator responds to rotating linearly polarized light, causing a photo-induced stress wave response. Such rapid, high-stress, low-strain, photo-mechanical responses are unique in soft polymer systems with physical values approaching hard metals/ceramics.

Wireless manipulation using magnetic polymer composites

The risk of damage when handling delicate objects can be reduced by using soft actuators. This paper presents a novel wireless magnetic polymer composite-driven system for soft robotic manipulation. Magnetic polymer composites, or MPCs, are wireless soft material actuators capable of large deformation and fast response. In this work, an antagonistic pair of MPCs were utilized to control the position and stiffness of a wireless end-effector. Experimental testing revealed that the proposed system could achieve low error position tracking (root mean square error ≤0.438 mm) and variable stiffness across the workspace. This research confirms the feasibility of using MPC for robotic manipulation in applications where displacement and stiffness are essential performance requirements.

Multipolar spatial electric field modulation for freeform electroactive hydrogel actuation

Electroactive hydrogels that exhibit large deformation in response to an electric field have received significant attention as a potential actuating material for soft actuators and artificial muscle. However, their mechanical actuation has been limited in simple bending or folding due to uniform electric field modulation. To implement complex movements, a pre-program, such as a hinge and a multilayer pattern, is usually required for the actuator in advance. Here, we propose a reprogrammable actuating method and sophisticated manipulation by using multipolar three-dimensional electric field modulation without pre-program. Through the multipolar spatial electric field modulator, which controls the polarity/intensity of the electric field in three-dimensions, complex three-dimensional (3D) actuation of single hydrogels are achieved. Also, air bubbles generated during operation in the conventional horizontal configuration are not an issue in the proposed new vertical configuration. We demonstrate soft robotic actuators, including basic bending mechanics in terms of controllability and reliability, and several 3D shapes having positive and negative curvature can easily be achieved in a single sheet, paving the way for continuously reconfigurable materials.