Progresses in Tensile,Torsional, and Multifunctional Soft Actuators

Soft actuators have received intensive attention in the fields of soft robots, sensors, intelligent control, artificial intelligence, and visual intelligence. By combination of tensile and torsional deformations, different types of motions can be realized, such as bending, rolling, and jumping. Soft robotics need soft actuators, such as artificial muscles to lift or move objects to perform some work. Additionally, actuation integrated with functions of sensing, signal transmission, and control is also needed in the development of advanced intelligent systems, which further stimulates the requirement of multifunctional actuators. Here different types of soft actuators that can perform tensile and torsional actuations are summarized, including twisted fiber artificial muscles, shape memory polymers, hydrogels, liquid crystal polymers, electrochemical actuators with conducting polymers, and some natural materials. Examples are also included regarding the bending or rolling deformations of the actuators for lifting objects. Then, recent interesting reports about multifunctional soft actuators combined with sensing and signal transmission performances, are summarized. Last, a summary of different ways to realize tensile and torsional actuations, different materials, and designs for lifting or moving objects, as well as construction of multifunctional actuators with actuation and sensing functions is provided.     

A Versatile Ionomer-Based Soft Actuator with Multi-Stimulus Responses,Self-Sustainable Locomotion,and Photoelectric Conversion

The prospects of endowing artificial robotics or devices with increasingly complex and emergent life-like behaviors have attracted growing interest in the soft functional materials that mimic the versatile motions of living creatures in the iridescent nature. However, despite the flourishing achievements so far, soft actuators capable of sensitive multi-stimulus responses and self-sustainable movements, have been extensively pursued to reduce control complexity yet remains a challenging target. Here, through material-structural synergistic design incorporating stress-mismatching structure, high pseudo-negative coefficient of thermal expansion of perfluoro-sulfonic acid ionomer, comprehensive converting properties of carbon nanotube, and anisotropic large thermal expansion of PE polymer, an ionomer-based bilayer actuator is proposed, presenting high-performance actuation of various forms and nice stability, responsive to light (including sunlight without focusing, LED light), low voltage, mild heating, and humidity/solvent change. With a built-in structural feedback loop, the actuation performances are further explored to realize intelligent systems, including: 1) self-sustainable locomotion under sunlight irradiation with adjustable photophobic and phototropic direction as well as adaption to different topographies and loading conditions, 2) self-sustainable oscillation and solar-electric generating, and 3) bionic floristic reaction according to environmental change. These diversified actuation modes allow promising following-up designs for bio-hybrid soft robotics fueled by and harmonized with natural environments.     

Dry‐Type Artificial Muscles Based on Pendent Sulfonated Chitosan and Functionalized Graphene Oxide for Greatly Enhanced Ionic Interactions and Mechanical Stiffness

Biopolymer‐based artificial muscles are promising candidates for biomedical applications and smart electronic textiles due to their multifaceted advantages like natural abundance, eco‐friendliness, cost‐effectiveness, easy chemical modification and high electical reactivity. However, the biopolymer‐based actuators are showing relatively low actuation performance compared with synthetic electroactive polymers because of inadequate mechanical stiffness, low ionic conductivity and ionic exchange capacity (IEC), and poor durability over long‐term activation. This paper reports a high‐performance electro‐active nano‐biopolymer based on pendent sulfonated chitosan (PSC) and functionalized graphene oxide (GO), exhibiting strong electro‐chemo‐mechanical interations with ionic liquid (IL) in open air environment. The proposed GO‐PSC‐IL nano‐biopolymer membrane shows an icnreased tensile strength and ionic exchange capacity of up to 44.8% and 83.1%, respectively, and increased ionic conductivity of over 18 times, resulting in two times larger bending actuation than the pure chitosan actuator under electrical input signals. Eventually, the GO‐PSC‐IL actuators could show robust and high‐performance actuation even at the very low applied voltages that are required in realistic applications.     

Biotissue-Inspired Anisotropic Carbon Fiber Composite Hydrogels for Logic Gates,Integrated Soft Actuators,and Sensors with Ultra-High Sensitivity

Natural biotissues like muscles, ligaments, and nerves have highly aligned structures, which play critical roles in directional signal transport, sensing, and actuation. Inspired by anisotropic biotissues, composite hydrogels with outstanding mechanical properties and conductivity are developed by compositing thermo-responsive poly (N-isopropylacrylamide) (PNIPAM) hydrogels with highly aligned carbon fibers (CFs). The anisotropic hydrogels show superior tensile strength (3.0 ± 0.3), modulus (74 ± 7.0 MPa), excellent electrical conductivity (≈670 S m⁻¹), and ultra-high sensitivity (gauge factor up to 647) along CFs, with an anisotropic ratio (AR) up to 740 over those in perpendicular direction. The extremely high AR in conductivity (more than 400) produces high-level output in parallel direction and low-level output in perpendicular direction with a direct current (DC) power supply, which is used to fabricate AND and OR gates. Moreover, the composite hydrogels are converted into thermo-responsive actuators with CFs twisted before compositing with PNIPAM/clay network. The pre-twisted CF helices impart internal stress that drives reversible actuation of hydrogel helices upon thermo-stimulating. The actuation is self-sensed due to the extremely high sensitivity of the composite hydrogels. Such biomimetic anisotropic self-sensing hydrogel actuators resemble natural biotissues with both actuation and sensing capabilities, and have promise applications for artificial robotics.     

Graphene‐Based Actuator with Integrated‐Sensing Function

Flexible actuators have important applications in artificial muscles, robotics, optical devices, and so on. However, most of the conventional actuators have only actuation function, lacking in real‐time sensing signal feedbacks. Here, to break the limitation and add functionality in conventional actuators, a graphene‐based actuator with integrated‐sensing function is reported, which avoids the dependence on image post‐processing for actuation detection and realizes real‐time measurement of the shape‐deformation amplitudes of the actuator. The actuator is able to show a large bending actuation (curvature of 1.1 cm^?1) based on a dual‐mode actuation mechanism when it is driven by near infrared light. Meanwhile, the relative resistance change of the actuator is ?17.5%. The sensing function is attributed to piezoresistivity and thermoresistivity of the reduced graphene oxide and paper composite. This actuator can be used as a strain sensor to monitor human motions. A smart gripper based on the actuators demonstrates perfect integration of the actuating and sensing functions, which can not only grasp and release an object, but also sense every actuation state of the actuator. The developed integrated‐sensing actuator is hopeful to open new application fields in soft robotics, artificial muscles, flexible wearable devices, and other integrated‐multifunctional devices.     

Highly Conductive MXene/PEDOT:PSS-Integrated Poly(N-Isopropylacrylamide) Hydrogels for Bioinspired Somatosensory Soft Actuators

Sophisticated sensing and actuation capabilities of many living organisms in nature have inspired scientists to develop biomimetic somatosensory soft robots. Herein, the design and fabrication of homogeneous and highly conductive hydrogels for bioinspired somatosensory soft actuators are reported. The conductive hydrogels are synthesized by in situ copolymerization of conductive surface-functionalized MXene/Poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) ink with thermoresponsive poly(N-isopropylacrylamide) hydrogels. The resulting hydrogels are found to exhibit high conductivity (11.76 S m⁻¹), strain sensitivity (GF of 9.93), broad working strain range (≈560% strain), and high stability after over 300 loading–unloading cycles at 100% strain. Importantly, shape-programmable somatosensory hydrogel actuators with rapid response, light-driven remote control, and self-sensing capability are developed by chemically integrating the conductive hydrogels with a structurally colored polymer. As the proof-of-concept illustration, structurally colored hydrogel actuators are applied for devising light-driven programmable shape-morphing of an artificial octopus, an artificial fish, and a soft gripper that can simultaneously monitor their own motions via real-time resistance variation. This work is expected to offer new insights into the design of advanced somatosensory materials with self-sensing and actuation capabilities, and pave an avenue for the development of soft-matter-based self-regulatory intelligence via built-in feedback control that is of paramount significance for intelligent soft robotics and automated machines.     

Biomorphic Actuation Driven Via On-Chip Buckling of Photoresponsive Hydrogel Films

Remotely controllable photoresponsive hydrogel actuators are promising for applications in multiple fields. However, simple deformation mechanisms, which rely on the general swelling/deswelling, limit their performance and application. Herein, we report a displacement amplification mechanism based on the buckling deformation of photoresponsive hydrogel film. The on-chip buckled architecture of the hydrogel enables actuation between a flat 2D shape and tubular 3D buckled shape with remarkable performances, including high deformation ratio (height ratio: ≈360%), tunable cycle motion frequency (0.1—1 Hz) and high cyclic stability. Moreover, localized buckling deformation, such as tube opening and closing, can be controlled in response to photostimulation. Inspired by these biomorphic shapes and motions, an intestine-mimetic device and demonstrate segmentation with substance crushing and peristalsis motion with substance propelling were further fabricated. This study will provide a useful design principle for hydrogel actuators and shed light on diverse applications in soft robotics, dynamic microfluidics and organs-on-chips.     

Air Permeable Vibrotactile Actuators for Wearable Wireless Haptics

Vibrotactile actuators can evoke mechanical stimulations on human skins to induce haptic feedbacks for various human machine interaction applications. However, efforts toward their practical usages encounter several engineering challenges, including wearable comfortability and output abilities. Here, air permeable actuators are developed and embedded in common fabrics for vibrotactile actuation, achieving excellent air permeability of 108 L m⁻² s⁻¹, low preload requirement of 10 mN, high output sensitivity of 0.2 mN/V, and good mechanical durability by surviving 11 million testing cycles. As demonstration examples, a wireless haptic feedback glove is shown to distinguish 32 different English characters and symbols with an overall accuracy of 97.8%, and large size actuators (10 × 10 cm²) are also proved for providing haptic feedback for parts of human body. As such, the proposed system opens a new class of wearable vibrotactile actuators for potential applications in wide fields of metaverse, teleoperation, smart textiles, and robotics.     

Multiresponsive MXene Actuators with Asymmetric Quantum-Confined Superfluidic Structures

MXene, 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.     

Intelligently Actuating Liquid Crystal Elastomer‐Carbon Nanotube Composites

Strategies for obtaining materials that respond to external stimuli by changing shape are of intense interest for the replacement of traditional actuators. Here, a strategy that enables programmable, multiresponsive actuators that use either visible light or electric current to drive shape change in composites comprising carbon nanotubes (CNTs) in liquid crystal elastomers (LCEs) is presented. In the nanocomposites, the CNTs function not only in the traditional roles of mechanical reinforcement and enhancers of thermal and electrical conductivity but also serve as an alignment layer for the LCEs. By controlling the orientation, location, and quantity of layers of CNTs in LCE/CNT composites, programmed, patterned actuators are built that respond to visible light or electrical current. Photothermal LCE/CNT film actuators undergo fast shape change, within 1.2 s using 280 mW cm^?2 light input, and complex, programmed localized deformations. Furthermore, twisting LCE/CNT composite films into a fiber increases uniaxial muscle stroke and work capacity for electrothermal actuation, thereby enabling about 12% actuation strain and 100 kJ m^?3 of work capacity in response to an applied DC voltage of 15.1 V cm^?1.     

3D-Printed Photoresponsive Liquid Crystal Elastomer Composites for Free-Form Actuation

Direct ink writing of liquid crystal elastomers (LCEs) offers a new opportunity to program geometries for a wide variety of shape transformation modes toward applications such as soft robotics. So far, most 3D-printed LCEs are thermally actuated. Herein, a 3D-printable photoresponsive gold nanorod (AuNR)/LCE composite ink is developed, allowing for photothermal actuation of the 3D-printed structures with AuNR as low as 0.1 wt.%. It is shown that the printed filament has a superior photothermal response with 27% actuation strain upon irradiation to near-infrared (NIR) light (808 nm) at 1.4 W cm⁻² (corresponding to 160 °C) under optimal printing conditions. The 3D-printed composite structures can be globally or locally actuated into different shapes by controlling the area exposed to the NIR laser. Taking advantage of the customized structures enabled by 3D printing and the ability to control locally exposed light, a light-responsive soft robot is demonstrated that can climb on a ratchet surface with a maximum speed of 0.284 mm s⁻¹ (on a flat surface) and 0.216 mm s⁻¹ (on a 30° titled surface), respectively, corresponding to 0.428 and 0.324 body length per min, respectively, with a large body mass (0.23 g) and thickness (1 mm).     

Leveraging Bioinspired Structural Constraints for Tunable and Programmable Snapping Dynamics in High-Speed Soft Actuators

Creating high-speed soft actuators will have broad engineering and technological applications. Snapping provides a power-amplified mechanism to achieve rapid movements in soft actuators that typically show slow movements. However, precise control of snapping dynamics (e.g., speed and direction of launching or jumping) remains a daunting challenge. Here, a bioinspired design principle is presented that harnesses a reconfigurable constraint structure integrated into a photoactive liquid crystal elastomer actuator to enable tunable and programmable control over its snapping dynamics. By reconfiguring constrained fin-array-shaped structure, the snapping dynamics of the structured actuator, such as launching or jumping angle and height, motion speed, and release force can be on-demand tuned, thus enabling controllable catapult motion and programmable jumping. Moreover, the structured actuators exhibit a unique combination of ultrafast moving speed (up to 2.5 m s⁻¹ in launching and 0.22 m s⁻¹ in jumping), powerful ejection (long ejection distance of ≈20 cm, 35 mg ball), and high jumping height (≈8 cm, 40 times body lengths), which few other soft actuators can achieve. This study provides a new universal design paradigm for realizing controllable rapid movements and high-power motions in soft matter, which are useful for building high-performance soft robotics and actuation devices.     

Bioinspired and Hierarchically Textile-Structured Soft Actuators for Healthcare Wearables

Soft pneumatic actuators possess the increasing potential for various healthcare applications, such as smart wearable devices, safe human-robot interaction, and flexible manipulators. However, it is difficult to translate the existing technologies to commercial applications due to their inefficient volumetric power, sophisticated control with high operation pressure, slow production, and high cost. To overcome these issues, herein, a caterpillar-inspired actuator using hierarchical textile architectures based on simple fabrication and low-cost strategy is designed. Unlike the existing textile-based pneumatic actuators, the designed actuators are constructed by combining boucle fancy yarns with a novel trilayer-knit architecture. The as-prepared actuators concurrently possess fast response (1100° s⁻¹), large bending actuation strain (1080° m⁻¹), high-power density (272 W m⁻³), mechanical robustness, easy-programmable motions, and human-tactile comfort, which outperforms currently reported textile-based pneumatic actuators. Furthermore, due to the geometrical transition of the engineered hierarchical structure, the developed actuators exhibit superior dual-stiffness effect with stress evolution, providing a facile approach to addressing the conflict of flexibility and force output in soft fluidic actuators. This concept as a paradigm provides new insights to develop soft actuators with outstanding design flexibility, adaptability, and multifunctionality using engineered textile-structure, which has great potential for real-world applications in medical rehabilitation, physiotherapy, and soft robotics.     

Direct Sun‐Driven Artificial Heliotropism for Solar Energy Harvesting Based on a Photo‐Thermomechanical Liquid‐Crystal Elastomer Nanocomposite

Inspired by heliotropism in nature, artificial heliotropic devices that can follow the sun for increased light interception are realized. The mechanism of the artificial heliotropism is realized via direct actuation by the sunlight, eliminating the need for additional mechatronic components and resultant energy consumption. For this purpose, a novel reversible photo‐thermomechanical liquid crystalline elastomer (LCE) nanocomposite is developed that can be directly driven by natural sunlight and possesses strong actuation capability. Using the LCE nanocomposite actuators, the artificial heliotropic devices show full‐range heliotropism in both laboratory and in‐field tests. As a result, significant increase in the photocurrent output from the solar cells in the artificial heliotropic devices is observed.     

Multi-Stimuli-Responsive Weldable Bilayer Actuator with Programmable Patterns and 3D Shapes

Soft actuators can harvest environmental energy and convert it into kinetic energy for motions like bending, twisting, stretching, and contracting. However, it remains challenging to design soft film actuators for complex and programmable deformation in three dimensions. Herein, a weldable and patternable multi-stimuli-responsive bilayer soft actuator is developed by a mask-assisted spraying coating process, and its 3D geometries are achieved by welding the sodium alginate (SA) layer using water. The intrinsic hygroscopicity of SA film and the magnetic and photothermal properties of Fe₃O₄ nanoparticles enable reversible deformation of the bilayer actuator under three different external stimuli: moisture, magnetic field, and sunlight. Based on these properties, a variety of multi-stimuli-responsive intelligent devices are developed including smart curtains, smart grippers, biomimetic walkers, rolling actuators, swimmers, and windmill rotators. All these actuating stimulations are derived from naturally renewable energy without the consumption of any artificial energy, providing important enlightenment for green and sustainable applications of soft actuators.     

Programmable Water/Light Dual-Responsive Hollow Hydrogel Fiber Actuator for Efficient Desalination with Anti-Salt Accumulation

An intelligent fiber actuator that can sense, adapt, and interact with environmental stimuli is highly desirable for numerous applications. However, conventional fiber sensors have difficulty in responding to complex environments with fast sensing and actuation. Herein, water/light dual-responsive double-twisted RGO@HHF (hollow hydrogel fiber loaded with reduced graphene oxide) actuators are successfully developed based on dynamic self-contraction/elongation via twisting, folding, plying, and re-plying process. Under stimuli by water and light, the twisted RGO@HHF provides a forward and reverse torsional stroke of 3168 and 60° cm⁻¹, respectively. The excellent swelling properties endow the fiber with highly effective water-responsive ability, and the hollow structure can reduce the water required for fiber to swell. Meanwhile, the RGO loading further accelerates water evaporation and enhances the light response rate of the fiber. After twisting and re-plying, the double-twisted RGO@HHF exhibits highly increased elongation and contraction under light/water stimulation. On this basis, a new strategy is proposed for efficient desalination and anti-salt accumulation. The double-twisted RGO@HHF can elongate to absorb water and contract to evaporate seawater under sunlight/water stimulation, respectively. The accumulated salt can be dissolved in the seawater during the dynamic process. This study provides a promising approach for realizing sustainable seawater desalination.     

Multiresponsive Bidirectional Bending Actuators Fabricated by a Pencil‐on‐Paper Method

Recently, actuating materials based on carbon nanotubes or graphene have been widely studied. However, present carbon‐based actuating materials are mostly driven by a single stimulus (humidity, light, electricity, etc.), respectively, which means that the application conditions are limited. Here, a new kind of multiresponsive actuating material which can be driven by humidity, light, and electricity is proposed, so it can be used in various conditions. The fabrication is based on the simplest pencil‐on‐paper method, in which the pencil and paper are both low‐cost and easily obtained daily materials. The actuation effect is more remarkable due to a dual‐mode actuation mechanism, which leads to an ultralarge actuation (bending curvature up to 2.6 cm^?1). Elaborately designed, the actuator can further exhibit a bidirectional bending actuation, which is a significant improvement compared with previous reported thermal actuators. What is more, a colorful biomimetic flower and a smart curtain are also fabricated, fully utilizing the printable characteristic of the paper and multiresponsive characteristic of the actuator. It is assumed that the newly designed actuating material has great potential in the fields of lab‐on‐paper devices, artificial muscles, robotics, biomimics, and smart household materials.     

Electrically and Sunlight‐Driven Actuator with Versatile Biomimetic Motions Based on Rolled Carbon Nanotube Bilayer Composite

Designing multistimuli responsive soft actuators which can mimic advanced and sophisticated biological movements through simple configuration is highly demanded for the biomimetic robotics application. Here, inspired by the human's flick finger behavior which can release large force output, a soft jumping robot mimicking the gymnast's somersault is designed based on the rolled carbon nanotube/polymer bilayer composite actuator. This new type of rolled bilayer actuator with tubular shape is fabricated and shows electrically and sunlight‐induced actuation with remarkable performances including ultralarge deformation from tubular to flat (angel change >200° or curvature >2 cm^?1), fast response (<5 s), and low actuation voltage (≤10 V). Besides jumping, the uniquely reversible rolling–unrolling actuation can lead to other smart soft robots with versatile complex biomimetic motions, including light‐induced tumbler with cyclic wobbling, electrically/light‐induced crawling‐type walking robots and grippers, electrically induced mouth movement, and ambient‐sunlight‐induced blooming of a biomimetic flower. These results open the way for using one simple type of actuator structure for the construction of various soft robots and devices toward practical biomimetic applications.     

Water‐Soluble Conjugated Molecule for Solar‐Driven Hydrogen Evolution from Salt Water

Photocatalytic hydrogen evolution is an attractive method for the acquisition of clean and sustainable energy with the use of solar power. Most reported studies have been carried out in scarce pure water. Therefore, the development of an artificial photosynthesis system that works perfectly with the earth's abundant seawater would be attractive. Herein, a supramolecular strategy for photocatalytic hydrogen production from the simulated seawater under sunlight irradiation (AM 1.5G, 100 mW cm^?2) is presented using a water‐soluble, conjugated molecule as the photosensitizer and the photodeposited Pt nanoparticles as the catalyst. Inspired by the natural photosynthesis system, unprecedented advantage of the chloride ions in seawater is taken and the formation of supramolecular structure is promoted by electrostatic interactions between chloride ions and the fine‐designed PorFN, which further facilitates the loading of Pt nanoparticles and multielectron transfer. As a result, a hydrogen evolution rate of 10.8 mmol h^?1 g^?1 is achieved in the simulated seawater. Moreover, the photocatalytic activity shows relatively low dependence on the light intensity, which is of great importance for practical applications.     

Photothermal Bimorph Actuators with In‐Built Cooler for Light Mills,Frequency Switches,and Soft Robots

Photothermal bimorph actuators are widely used for smart devices, which are generally operated in a room temperature environment, therefore a low temperature difference for actuation without deteriorating the performance is preferred. The strategy for the actuator is assembling a broadband‐light absorption layer for volume expansion and an additional water evaporation layer for cooling and volume shrinkage on a passive layer. The response time and temperature‐change‐normalized bending speed under NIR, white, and blue light illumination are at the same level of high performance, fast photothermal actuators based on polymer or polymer composites. The classical beam theory and finite element simulations are also conducted to understand the actuation mechanism of the actuator. A new type of light mill is designed based on a wing‐flapping mechanism and a light‐modulated frequency switch. A fast‐walking robot (with a speed of 26 mm s^?1) and a fast‐and‐strong mechanical gripper with a large weight‐lifting ratio (≈2142), respectively, are also demonstrated.