Actuation of Three-Dimensional-Printed Nanocolloidal Hydrogel with Structural Anisotropy

Polymer hydrogels exhibit actuation properties that result in reversible shape transformations and have promising applications in soft robotics, drug delivery systems, sensors, and microfluidic devices. Actuation occurs due to differential hydrogel swelling and is generally achieved by modulating hydrogel composition. Here a different approach to hydrogel actuation that originates solely from its structural anisotropy is reported. For 3D-printed single-layer hydrogels formed by cellulose nanocrystals (CNCs) and gelatin methacryloyl it is shown that shear-induced orientation of CNCs results in anisotropic mechanical and swelling properties of the hydrogel. Upon swelling in water, planar hydrogels acquire multiple complex 3D shapes that are achieved by i) varying CNC orientation with respect to the shape on the hydrogel sheet and ii) patterning the hydrogel with the regions of shear-mediated and random CNC orientation. This study shows the capability to generate multiple shapes from the same hydrogel actuator based on the degree of its structural anisotropy. In addition, it introduces a biocompatible nanocolloidal ink with shear-thinning and self-healing properties for additive manufacturing of hydrogel actuators.     

Electrical and Mechanical Self‐Healing in High‐Performance Dielectric Elastomer Actuator Materials

Dielectric elastomers are of interest for actuator applications due to their large actuation strain, high bandwidth, high energy density, and their flexible nature. If future dielectric elastomers are to be used reliably in applications that include soft robotics, medical devices, artificial muscles, and electronic skins, there is a need to design devices that are tolerant to electrical and mechanical damage. In this paper, the first report of self‐healing of both electrical breakdown and mechanical damage in dielectric actuators using a thermoplastic methyl thioglycolate–modified styrene–butadiene–styrene (MGSBS) elastomer is provided. The self‐healing functions are examined from the material to device level by detailed examination of the healing process, and characterization of electrical properties and actuator response before and after healing. It is demonstrated that after dielectric breakdown, the initial dielectric strength can be recovered by up to 67%, and after mechanical damage, a 39% recovery can be achieved with no degradation of the strain–voltage response of the actuators. The elastomer can also heal a combination of mechanical and electrical failures. This work provides a route to create robust and damage tolerant dielectric elastomers for soft robotic and other applications related to actuator and energy‐harvesting systems.     

Biomimetic 4D-Printed Breathing Hydrogel Actuators by Nanothylakoid and Thermoresponsive Polymer Networks

Shape-morphing actuators, which can breathe with the accompany of morphology changes to mimic botanical events, are challenging to fabricate with soft hydrogel materials. Herein, 4D printed-smart hydrogel actuators are reported that can not only dynamically deform but also generate oxygen (O₂) upon external stimulations. The printed breathing actuators featured with spinach leaf-derived thylakoid membrane (nanothylakoid) for photothermal conversion and catalytical O₂ evolution, a poly(N-isopropylacrylamide) (PNIPA) thermoresponsive polymer network for generating deformation forces by swelling/shrinkage (rehydration/dehydration), and an asymmetric bilayer poly(N-isopropylacrylamide)/polyacrylamide (PNIPA/PAA) structure to amplify the mechanical motions. Upon thermal stimulation or laser irradiation, the actuator can reversibly bend/unbend because of the photothermal conversion of nanothylakoid and the printed thermoresponsive asymmetric bilayer structure. Additionally, the catalase-like property of nanothylakoid imparts the actuator with O₂ evolution capability to breathe for further mimicking botanical systems. Notably, 4D printing can greatly facilitate and simplify the actuator fabrication process, including adjusting the size and layer compositions. This artificial breathing actuator with photothermal and catalytical properties provides a strategy in designing intelligent hydrogel systems and proves to be a highly promising material candidates in the fields of 3D/4D printing, automated robotics, and smart biomedical devices.     

Harnessing the Actuation Potential of Solid‐State Intercalation Compounds

High‐strain, high‐force mechanical actuation technologies are desirable for numerous applications ranging from microelectromechanical systems (MEMS) to large‐scale “smart structures” that are able to change shape to optimize performance. Here we show that electrochemical intercalation of inorganic compounds of high elastic modulus offers a low‐voltage mechanism (less than 5 V) with intrinsic energy density approaching that of hydraulics and more than a hundred times greater than that of existing field‐operated mechanisms, such as piezostriction and magnetostriction. Exploitation of the reversible crystallographic strains (several percent) of intercalation compounds while under high stress is key to realization of the available energy. Using a micromachined actuator design, we test the strain capability of oriented graphite due to electrochemical lithiation under stresses up to 200 MPa. We further demonstrate that simultaneous electrochemical expansion of the LiCoO₂/graphite cathode/anode couple can be exploited for actuation under stresses up to ～ 20 MPa in laminated macroscopic composite actuators of similar design to current lithium‐ion batteries. While the transport‐limited actuation mechanism of these devices results in intrinsically slower actuation compared to most ferroic materials, we demonstrate up to 6.7 mHz (150 s) cyclic actuation in a laminated actuator designed for a high charge/discharge rate. The potential for a new class of high‐strain, high‐force, moderate‐frequency actuators suitable for a broad range of applications is suggested.     

On the design of large degree-of-freedom digital mechatronic devices based on bistable dielectric elastomer actuators

Binary actuation has been proposed to reduce complexity in robotic and mechatronic systems. However, a relatively large number of binary actuators are required to achieve the accuracy necessary for practical applications. Conventional actuators are not practical for such large degree-of-freedom (DoF) devices. Here, a dielectric elastomer (DE) actuator is developed for these applications. It is shown that DE actuators have high energy densities, light weight, low cost, and large displacements. Hence they could potentially make large DoF binary systems practical. DE actuators proposed here consist of thin electrically sensitive elastomer films that are mounted in a flexible frame that incorporates a passive bistable element. The frame prestrains the film and provides a restoring force that allows the actuator to operate bidirectionally. A simple experimental prototype 6-DoF binary manipulator demonstrates the concept.     

Design of a bidirectional MEMS actuator with high displacement resolution, large driving force and power-free latching

This paper describes the design and ANSYS modeling of a bidirectional thermal inchworm MEMS actuator featuring high actuation resolution (0.1 μm) combined with large driving force (100 mN) and power-free latching. A promising application of this device is for precision in-package positioning of optical fibers, but the actuator also has potential for wider use. The inchworm mechanism includes two E-shaped metallic actuators facing each other and a pusher between them that couples to a moveable object such as an optical fiber. Nickel is chosen as the material of the actuators due to its desirable electrical, thermal, and mechanical properties, and availability in fabrication facilities. Electrothermal actuation is used to move the pusher as well as the optical fiber in the desired direction by inputting electric currents in a particular sequence. A transient thermal-mechanical analysis shows the feasibility of the input sequence that drives the inchworm actuation.     

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.     

3D Printing of Interdigitated Dielectric Elastomer Actuators

Dielectric elastomer actuators (DEAs) are soft electromechanical devices that exhibit large energy densities and fast actuation rates. They are typically produced by planar methods and, thus, expand in‐plane when actuated. Here, reported is a method for fabricating 3D interdigitated DEAs that exhibit in‐plane contractile actuation modes. First, a conductive elastomer ink is created with the desired rheology needed for printing high‐fidelity, interdigitated electrodes. Upon curing, the electrodes are then encapsulated in a self‐healing dielectric matrix composed of a plasticized, chemically crosslinked polyurethane acrylate. 3D DEA devices are fabricated with tunable mechanical properties that exhibit breakdown fields of 25 V µm^?1 and actuation strains of up to 9%. As exemplars, printed are prestrain‐free rotational actuators and multi‐voxel DEAs with orthogonal actuation directions in large‐area, out‐of‐plane motifs.     

Electromechanical Actuators Based on Graphene and Graphene/Fe3O4 Hybrid Paper

Exceptionally high specific surface area, mechanical strength, electrical conductivity, and a special two‐dimensional structure make graphene a highly promising material for electromechanical actuators. Electromechanical actuators are fabricated using flexible graphene‐based paper prepared via a filtration process, and the stroke of these graphene‐based actuators is directly measured during electrochemical double‐layer charge injection. Actuation strain up to 0.064% was obtained for pristine graphene paper in response to an applied potential of –1 V in 1 M NaCl solution. Double‐layer charge injection in graphene sheets is believed to induce actuation strain through a combination of coulombic and quantum‐chemical‐based expansion. To increase electrochemical‐double‐layer capacitance and actuator performance, Fe₃O₄ nanoparticles were used to partially prevent graphene sheets from restacking and allow the electrolyte ions to infiltrate the resulting magnetic graphene paper more easily. The magnetic graphene paper exhibits actuation strain as large as 0.1% at –1 V applied potential, which is about 56% higher than that of the pristine graphene paper.     

A Moisture-Driven Actuator Based on Polydopamine-Modified MXene/Bacterial Cellulose Nanofiber Composite Film

As a new 2D material, MXene (Ti₃C₂T_x) shows great potential as a smart multifunctional humidity-responsive actuator due to its high hydrophilicity and conductivity but suffers from ambient oxidation and mechanical brittleness. Inspired by the mussels, the authors overcome these weaknesses by designing and fabricating a nacre-like and lamellar-structured composite film that consists of polydopamine-modified MXene and bacterial cellulose nanofibers, which shows improved properties as a moisture-driven actuator. The actuator has high conductivity (2848 S cm^–1), excellent tensile strength (406 MPa), and toughness (15.3 MJ m^–3). Moreover, the actuator is highly sensitive to moisture with the advantages of fast response (1.6 s), large deformation (176°), and high actuation force output (6.5 N m^–2). It is additionally demonstrated that the actuator works as the electrical switch, robotic arm, and motor in a moisture-driven manner. Overall, it is believed that this work improves the drawbacks of current MXene-based actuators, laying the groundwork for their wider applications as moisture-driven devices.     

Bioinspired Electro‐Thermo‐Hygro Reversible Shape‐Changing Materials by 4D Printing

Hygromorph composites are moisture‐induced shape‐changing materials that are increasingly studied to develop autonomously actuated deployable structures. The morphing mechanism is based on the high affinity for moisture and the hygroexpansive nature of at least one component, combined with a bilayer microstructure. Among available hygromorphs, those consisting of cellulosic or hydrogel material‐based actuators trigger fast responses to moisture. Their stiffness however decreases significantly with the moisture content and that restricts their potential application as soft actuators. This work proposes a novel 4D printed multistimuli‐responsive structural material based on conductive carbon reinforcements and combined with a moisture sensitive polymer. These 4D printed materials possess a microstructure that provides the capability of natural actuators like pine cones. The actuation of these functional materials could be either triggered passively by the variation of the ambient moisture, or by electroheating, with the latter leading to the control of the moisture content in initially wet samples via Joule effects. This new class of functional materials shows an increase of the actuation speed by a factor 10 compared to other existing hygromorphs with the same responsiveness. When the electrical heating is turned off, passive cooling and moisture driven actuation is triggered in a full reversible mode.     

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.     

Electric Field-Driven Dielectrophoretic Elastomer Actuators

Dielectrophoresis is the electro-mechanical phenomenon where a force is generated on a dielectric material when exposed to a non-uniform electric field. It has potential to be exploited in smart materials for robotic manipulation and locomotion, but to date it has been sparsely studied in this area. Herein, a new type of dielectrophoretic actuator exploiting a novel electroactive polymer is described, termed as dielectrophoretic elastomer (DPE), which undergoes electric field-driven actuation through dielectrophoresis. Unique deflection and morphing behavior of the elastomer induced by controlling the dielectrophoretic phenomenon, such as out-of-plane deformation and independence of electric field polarity, are illustrated. The dielectric and mechanical properties of the DPE are studied to gain insight into the influence of materials composition on deformation. Actuation performance using different electrode parameters is experimentally investigated with supplementary analysis through finite element simulation, revealing the relationship between electric field inhomogeneity and deflection. The applications of DPE actuators in a range of robotic devices is demonstrated, including a pump, an adjustable optical lens, and a walking robot. This diverse range of applications illustrates the wide potential of these new soft-and-smart electric field-driven materials for use in soft robotics and soft compliant devices.     

Enhancement Of Dielectric Permittivity And Electromechanical Response In Silicone Elastomers: Molecular Grafting Of Organic Dipoles To The Macromolecular Network

A novel method is established for permittivity enhancement of a silicone matrix for dielectric elastomer actuators (DEAs) by molecular level modifications of the elastomer matrix. A push‐pull dipole is synthesized to be compatible with the silicone crosslinking chemistry, allowing for direct grafting to the crosslinker molecules in a one‐step film formation process. This method prevents agglomeration and yields elastomer films that are homogeneous down to the molecular level. The dipole‐to‐silicone network grafting reaction is studied by FTIR. The chemical, thermal, mechanical and electrical properties of films with dipole contents ranging from 0 wt% to 13.4 wt% were thoroughly characterized. The grafting of dipoles modifies the relative permittivity and the stiffness, resulting in the actuation strain at a given electrical field being improved by a factor of six.     

Selective Stiffening in Soft Actuators by Triggered Phase Transition of Hydrogel-Filled Elastomers

Nature has inspired a new generation of robots that not only imitate the behavior of natural systems but also share their adaptability to the environment and level of compliance due to the materials used to manufacture them, which are typically made of soft matter. In order to be adaptable and compliant, these robots need to be able to locally change the mechanical properties of their soft material-based bodies according to external feedback. In this work, a soft actuator that embodies a highly controllable thermo-responsive hydrogel and changes its stiffness on direct stimulation is proposed. At a critical temperature, this stimulation triggers the reversible transition of the hydrogel, which locally stiffens the elastomeric containment at the targeted location. By dividing the actuator into multiple sections, it is possible to control its macroscopic behavior as a function of the stiffened sections. These properties are evaluated by arranging three actuators into a gripper configuration used to grasp objects. The results clearly show that the approach can be used to develop soft actuators that can modify their mechanical properties on-demand in order to conform to objects or to exert the required force.     

MRI Compatibility of Silicone-Made Contractile Dielectric Elastomer Actuators

Actuators based on dielectric elastomers, a specific class of electroactive polymers, appear to be suitable candidates for new MRI-compatible technologies, due to their intrinsic material properties and working principle. This paper presents the first investigation into the MRI compatibility of a recently developed linear contractile actuator made of a silicone elastomer. The apparent absence of any degradation of both the actuator electromechanical performance in the MRI environment and the quality of images acquired from a phantom demonstrates the MRI compatibility of the actuator. These results suggest the suitability of this soft actuation technology as a possible new entry in the class of MRI-compatible mechatronic systems.     

Microtentacle Actuators Based on Shape Memory Alloy Smart Soft Composite

Recent advances in miniature robotics have brought promising improvements in performance by leveraging the latest developments in soft materials, new fabrication schemes, and continuum actuation. Such devices can be used for applications that need delicate manipulation such as microsurgery or investigation of small‐scale biological samples. The shape memory effect of certain alloys is one of the promising actuation mechanisms at small scales because of its high work density and simple actuation mechanism. However, for sub‐millimeter devices, it is difficult to achieve complex and large displacement with shape memory alloy actuators because of the limitation in the fabrication process. Herein, a fabrication scheme for miniaturized smart soft composite actuator is proposed by utilizing two‐photon polymerization. The morphing modes are varied by changing the direction of the scaffold lamination. In addition, the actuation is controlled via local resistive heating of a carbon nanotube layer deposited inside of the actuators. The proposed design can generate a 390 µN force and achieve a bending angle up to 80°. Applications of the actuators are demonstrated by grasping small and delicate objects with single and two finger devices.     

New Silicone Composites for Dielectric Elastomer Actuator Applications In Competition with Acrylic Foil

Several polydimethylsiloxane elastomers were developed and investigated regarding their potential use as materials in dielectric elastomer actuators (DEA). A hydroxyl end‐functionalized polydimethylsiloxane was reacted with different crosslinkers and the electromechanical properties of the resulting elastomers were investigated. The silicone showing the best actuation at the lowest electric field was further used as matrix and compounded with encapsulated conductive polyaniline particles. These composites have enhanced properties including increased strain at break, higher dielectric constant as well as, gratifyingly, breakdown fields higher than that of the matrix. One of the newly synthesized composites is compared to the commercially available acrylic foil VHB 4905 (3M) which is currently the most commonly used elastomer for DEA applications. It was found that this material has little hysteresis and can be activated at lower voltages compared to VHB 4905. For example, when the newly synthesized composite was 30% prestrained, a lateral actuation strain of about 12% at 40 V μm^?1 was measured while half of this actuation strain at the same voltage was measured for VHB 4905 film that was 300% prestrained. It also survived more than 100 000 cycles at voltages which are close to the breakdown field. Such materials might find applications wherever small forces but large strains at low voltages are required, in, for example, tactile displays.     

Stimulating Acrylic Elastomers by a Triboelectric Nanogenerator – Toward Self‐Powered Electronic Skin and Artificial Muscle

Dielectric elastomers are a type of actuator materials that exhibit excellent performance as artificial muscles, but a high driving voltage is required for their operation. By using the amazingly high output voltage generated from a triboelectric nanogenerator (TENG), a thin film dielectric elastomer actuator (DEA) can be directly driven by the contact‐separation motion of TENG, demonstrating a self‐powered actuation system. A TENG with a tribo surface area of 100 cm² can induce an expansion strain of 14.5% for the DEA samples (electrode diameter of 0.6 cm) when the system works stably within the contact‐separation velocity ranging from 0.1 to 10 cm s^?1. Finally, two simple prototypes of an intelligent switch and a self‐powered clamper based on the TENG and DEA are demonstrated. These results prove that the dielectric elastomer is an ideal material to work together with TENG and thereby the fabricated actuation system can potentially be applied to the field of electronic skin and soft robotics.     

Biomimetic Color Changing Anisotropic Soft Actuators with Integrated Metal Nanowire Percolation Network Transparent Heaters for Soft Robotics

To add more functionalities and overcome the limitation in conventional soft robots, highly anisotropic soft actuators with color shifting function during actuation is demonstrated for the first time. The electrothermally operating soft actuators with installed transparent metal nanowire percolation network heater allow easy programming of their actuation direction and instantaneous visualization of temperature changes through color change. Due to the unique direction dependent coefficient of thermal expansion mismatch, the suggested actuator demonstrates a highly anisotropic and reversible behavior with very large bending curvature (2.5 cm^?1) at considerably low temperature (≈40 °C) compared to the previously reported electrothermal soft actuators. The mild operating heat condition required for the maximum curvature enables the superior long‐term stability during more than 10 000 operating cycles. Also, the optical transparency of the polymer bilayer and metal nanowire percolation network heater allow the incorporation of the thermochromic pigments to fabricate color‐shifting actuators. As a proof‐of‐concept, various color‐shifting biomimetic soft robots such as color‐shifting blooming flower, fluttering butterfly, and color‐shifting twining tendril are demonstrated. The developed color‐shifting anisotropic soft actuator is expected to open new application fields and functionalities overcoming the limitation of current soft robots.