Toward a Unified Naming Scheme for Thermo-Active Soft Actuators: A Review of Materials, Working Principles, and Applications

Soft robotics is a rapidly growing field that spans the fields of chemistry, materials science, and engineering. Due to the diverse background of the field, there have been contrasting naming schemes such as "intelligent," "smart," and "adaptive" materials, which add vagueness to the broad innovation among literature. Therefore, a clear, functional, and descriptive naming scheme is proposed in which a previously vague name-Soft Material for Soft Actuators-can remain clear and concise-Phase-Change Elastomers for Artificial Muscles. By synthesizing the working principle, material, and application into a naming scheme, the searchability of soft robotics can be enhanced and applied to other fields. The field of thermo-active soft actuators spans multiple domains and requires added clarity. Thermo-active actuators have potential for a variety of applications spanning virtual reality haptics to assistive devices. This review offers a comprehensive guide to selecting the type of thermo-active actuator when one has an application in mind. In addition, it discusses future directions and improvements that are necessary for implementation.

Triple Stimuli-Responsive Flexible Shape Memory Foams with Super-Amphiphilicity

Highly porous multi-responsive shape memory foams have unique advantages in designing 3D materials with lightweight for varied applications. Herein, a facile and efficient approach to fabricating a thermo-, electro-, and photo-responsive shape memory composite foam is demonstrated. A specific multi-step carbonization protocol is adopted for transforming commercial melamine sponge (MS) to highly porous carbon foam (CF) with robust elastic resilience, efficient electrothermal/photothermal conversions, and super-amphiphilicity. It is a novel proposal for CF to take the dual role of the elastic supporting framework and 3D energy conversion/transmission network without any functional fillers. The composite foam cPCL@CF incorporates the CF skeleton with in situ crosslinked polycaprolactone (PCL) layers, which exhibits high conductivity (≈140 S m^-1 ) and excellent light absorption (≈97.7%) in the range of 250-2500 nm. By triggering the crystalline transition of PCL, the composite foam displays sensitive electro- and photo-induced shape memory effect (SME) with outstanding shape fixation ratio (R_f ) and recovery ratio (R_r ). Thanks to the super-amphiphilicity and high electrical conductivity, the cPCL@CF composite foam can give rapid and distinguishable electric signals upon tiny drips of salt solutions or lithium-ion battery (LIB) electrolytes, making it a new type of sensor for detecting electrolyte leakage.

A new aggregation induced emission enhancement (AIEE) dye which self-assembles to panchromatic fluorescent flowers and has application in sensing dichromate ions

We report for the very first time the crystal structure and self-assembly of a new aggregation-induced emission enhancement (AIEE) dye 4-(5-methoxythiazolo[4,5-b]pyridin-2-yl)-N,N-dimethylaniline (TPA) and its application in sensing dichromate ions. TPA reveals cyan blue emission under UV and visible light. The self-assembly properties of TPA were studied extensively by scanning electron microscopy (SEM) which revealed the formation of beautiful flower-like morphologies. These structures revealed both green and red fluorescence under FITC and rhodamine filters respectively when observed through fluorescence microscopy connoting the panchromatic emission properties of TPA from blue to red. The interactions which cause self-assembled structure formation in TPA were also validated theoretically using density functional theory (DFT) calculations. Crystal and molecular structure analysis of TPA was carried out via single-crystal X-ray diffraction to visualize the intermolecular interactions occurring in the solid-state and to study the structure-photophysical property relationship in the aggregated state. The photophysical properties of TPA were also studied extensively by UV-visible and fluorescence spectroscopy and its quantum yield and fluorescence lifetime were calculated by time-correlated single-photon counting (TCSPC). Interestingly, TPA could efficiently sense dichromate (Cr₂O₇^2-) ions in an acidic medium and an interesting morphological transition from a fluorescent flower to non-fluorescent disassembled structures could also be observed. The limit of detection of TPA for Cr₂O₇^2- ions was found to be as low as 5.5 nM, suggesting its exceptional sensitivity. More importantly, TPA could selectively sense Cr₂O₇^2- ions in real water samples even in the presence of other metal ions routinely present in polluted water, hence making it practically useful for water quality monitoring.

Solar-Driven Soft Robots

Stimuli-responsive materials have been lately employed in soft robotics enabling new classes of robots that can emulate biological systems. The untethered operation of soft materials with high power light, magnetic field, and electric field has been previously demonstrated. While electric and magnetic fields can be stimulants for untethered actuation, their rapid decay as a function of distance limits their efficacy for long-range operations. In contrast, light-in the form of sunlight or collimated from an artificial source (e.g., laser, Xenon lamps)-does not decay rapidly, making it suitable for long-range excitation of untethered soft robots. In this work, an approach to harnessing sunlight for the untethered operation of soft robots is presented. By employing a selective solar absorber film and a low-boiling point (34 °C) fluid, light-operated soft robotic grippers are demonstrated, grasping and lifting objects almost 25 times the mass of the fluid in a controllable fashion. The method addresses one of the salient challenges in the field of untethered soft robotics. It precludes the use of bulky peripheral components (e.g., compressors, valves, or pressurized gas tank) and enables the untethered long-range operation of soft robots.

Transparent low-voltage-driven soft actuators with silver nanowires Joule heaters

Transparent soft actuators with silver nanowire Joule heaters embedded in liquid crystal elastomer and PDMS layer was prepared, and it can perform reversible large bending deformation driven by low voltage.

Design and Fabrication of a Low-Cost Silicone and Water-Based Soft Actuator with a High Load-to-Weight Ratio

Traditional actuators, such as motors as well as hydraulic or pneumatic artificial muscles, demonstrate excessive noise, a heavy weight, and a large size, which limit their practical application in many areas. Therefore, for many decades, scientists have worked to develop new types of silent, small, and light actuators. In this article, a novel soft actuator (actuator₃) with a high load-to-weight ratio from silicone and a low-boiling liquid (ethanol: actuator_3E or water: actuator_3W) is presented and is compared with two actuators (actuator₁ and actuator₂) fabricated according to a method described in the literature. Compared with actuator₁ and actuator₂, actuator₃ shows a larger volume expansion, output force, and load-to-weight ratio when heated. Owing to the weaker stability and repeatability of actuator_3E, many different kinds of applications based on actuator_3W are proposed, such as robotic hands and underwater rolling robots with color variations. The experimental results demonstrate that the method proposed in this article may be a viable alternative for fabricating low-cost soft actuators with high load-to-weight ratios that can be useful for future applications of soft robots.

Soft Actuators Based on Liquid-Vapor Phase Change Composites

Liquid-vapor phase change materials (PCMs), capable of significant volume change, are emerging as attractive actuating components in forming advanced soft composites for robotic applications. However, the novel and functional design of these PCM composites is significantly limited due to the lacking of the fundamental understanding of the mechanical properties, which further inhibits the broad applications of PCM based materials in the engineering structures requiring large deformation and high loading capacity. In this study we fabricate PCM-elastomer composites exhibiting large deformation and high output stress. Thermomechanical properties of these composites are experimentally and theoretically investigated, demonstrating enhanced deformation and loading capacity due to the induced vapor pressure. By controlling the distribution and content of the PCM inclusions, structures with tunable deformability under a relatively small strain in comparison with traditional soft materials are fabricated. Accompanying with the asymmetrical friction and deformation, complex locomotion and adaptable grabbing function are achieved with excellent performance.

Actuation of untethered pneumatic artificial muscles and soft robots using magnetically induced liquid-to-gas phase transitions

Pneumatic artificial muscles have been widely used in industry because of their simple and relatively high-performance design. The emerging field of soft robotics has also been using pneumatic actuation mechanisms since its formation. However, these actuators/soft robots often require bulky peripheral components to operate. Here, we report a simple mechanism and design for actuating pneumatic artificial muscles and soft robotic grippers without the use of compressors, valves, or pressurized gas tanks. The actuation mechanism involves a magnetically induced liquid-to-gas phase transition of a liquid that assists the formation of pressure inside the artificial muscle. The volumetric expansion in the liquid-to-gas phase transition develops sufficient pressure inside the muscle for mechanical operations. We integrated this actuation mechanism into a McKibben-type artificial muscle and soft robotic arms. The untethered McKibben artificial muscle generated actuation strains of up to 20% (in 10 seconds) with associated work density of 40 kilojoules/meter3, which favorably compares with the peak strain and peak energy density of skeletal muscle. The untethered soft robotic arms demonstrated lifting objects with an input energy supply from only two Li-ion batteries.

Versatile high-performance inkjet-printed paper photo-actuators based on 2D materials

In this paper, we present high-performance and versatile inkjet-printed paper photo-actuators based on two-dimensional (2D) nanomaterials. As a rapid fabrication method, inkjet printing of 2D materials is used to promptly fabricate photo-actuators in a bi-layer paper/polymer structure. Water-based and biocompatible inks based on graphene and molybdenum disulfide are developed based on liquid phase exfoliation and differential centrifugation technique. It is shown that incorporation of 2D materials with inkjet printing techniques and liquid phase exfoliation can lead to rapid fabrication of photo-actuators with huge opto-mechanical energy release and versatility with a broad range of applications due to specific design and methods presented in this paper.

Shape programming of polymeric based electrothermal actuator (ETA) via artificially induced stress relaxation

Electrothermal actuators (ETAs) are a new generation of active materials that can produce different motions from thermal expansion induced by Joule heating. It is well-known that the degree of deformation is determined by the amount of Joule heating and the coefficient of thermal expansion (CTE) of the material. Previous works on polymeric ETAs are strongly focused on increasing electrical conductivity by utilizing super-aligned carbon nanotube (CNT) sheets. This allows greater deformation for the same drive voltage. Despite these accomplishments with low-voltage actuation, many of the ETAs were constructed to have basic geometries such as a simple cantilever shape. In this paper, it was discovered that shape of polymeric ETA can be programmed into a desired configuration by applying an induced stress relaxation mechanism and post secondary curing. By utilizing such effects, an ETA can be programmed into a curled resting state which allows the actuator to achieve an active bending angle over 540°, a value far greater than any previous studies. This shape programming feature also allows for tailoring the actuator configuration to a specific application. This is demonstrated here by fabricating a small crawling soft robot similar to mimic an inchworm motion.

Laser fabrication of graphene-based electrothermal actuators enabling predicable deformation

Electrothermal actuators (ETAs) that can convert electric energy into mechanical works have been extensively studied for their great potential in artificial muscles and robotics. However, the production of ETAs that enable complex and predictable deformation is still challenging. In this Letter, an ETA based on reduced graphene oxide (RGO) and polyethylene (PE) bimorph is developed through a facile laser-scribing method. Since the laser-scribing technology permits flexible patterning, conductive RGO electrodes with complex circuit patterns can be readily produced on a thermally active PE film, forming an ETA capable of fast and reversible deformation. In addition, the laser-scribed ETA demonstrated orientation-defined bending performance, enabling more sophisticated deformation control. The laser scribing of graphene oxide has opened up a new way to produce ETAs towards cutting-edge applications such as soft robotics and intelligent systems.

Carbon nanotubes-elastomer actuator driven electrothermally by low-voltage

Electroactive polymers (EAPs) have attracted attention in many fields such as robotics, sensors devices and biomedical devices. However, the practical application of these actuators has still problems due to incomplete reversibility and high applied voltage. In order to overcome these problems, in this study, we have shown actuator based on phase transition that is consisted of the carbon nanotubes yarn infiltrated with the mixture of elastomer and methanol. Our electrothermally driven hybrid coiled yarn muscle provides a work capacity of 0.49 kJ kg^-1 and a tensile contraction of 30.5% within ∼3 s on an applied stress of 3.1 MPa at an applied DC voltage of 5 V. The maximum work capacity is under isobaric 23.4 MPa, which is 110 times that of typical mammalian skeletal muscles. This actuator may serve as a promising candidate for the practical use in soft robotics.

Large-Magnitude Transformable Liquid-Metal Composites

Most of the existing robots would find it difficult to stretch and transform all parts of their body together due to rigid components and complex actuation mechanisms inside. Here, we presented a highly transformable liquid-metal composite (LMC) that is easy to change shape in large magnitude and resume its original state again according to need. When subject to heating, part of the ethanol droplets embedded in the composite would change phase and then actuate. We demonstrate the flexible transformation of LMC-made octopus from a two-dimensional shape into several predictable three-dimensional shapes freely on a large scale (even up to 11 times its initial height) through remote wireless heating, which needs no sophisticated operating system at all. Further, several designed behaviors, such as movement of octopus and entangling objects of soft robots, are also realized. Theoretical analysis of the heating-induced liquid-vapor transition of the embedded ethanol droplet interprets the mechanisms involved. The present findings open a new way to fabricate functional transformable composites that would find significant applications in developing future generation soft robots.

Photomechanical and Chemomechanical Actuation Behavior of Graphene-Poly(dimethylsiloxane)/Gold Bilayer Tube for Multimode Soft Grippers and Volatile Organic Compounds Detection Applications

Graphene polymers-based soft actuators driven by infrared (IR) light have attracted wide attention recently. However, the scientific fraternity is striving hard in unraveling the area of actuators that could be triggered by IR light along with chemicals. The fabricating methodology of multiresponsive soft actuators based on graphene nanoplatelets (GNPs)-poly(dimethylsiloxane) (PDMS) nanocomposite/gold bilayers, ensuring large, fast, and reversible response, has been illustrated. The actuators display a novel dual-mode operation as photomechanical and chemomechanical actuation. The actuators are realized by depositing a thin film (100 nm) of gold on GNP-PDMS nanocomposite films resulting tubular structure on account of thermal residual stress. The actuation response of this structure upon its exposure to IR light and chemicals was measured in terms of percentage opening and degree of unscroll, respectively. The three-dimensional tubular structure is transformed into a two-dimensional sheet within 8 s under IR light irradiation. The same structures were also tested in various organic solvents like methanol, ethanol, acetone, isopropyl alcohol, and aldehydes, but the actuation has been observed only in acetone and aldehydes. This tubular actuator unscrolls completely and then scrolls in opposite direction along with tube axis shift through 90° during its exposure to acetone (liquid/vapors) and aldehydes. Few applications of these actuators, such as multimode soft grippers for on-demand capture/release of objects (with weight 1.2 times the actuator's own weight) and volatile organic compounds detection module, have been demonstrated. The combination of surface micromachining techniques of microelectromechanical systems process with this smart material may find applications in drug-delivery systems with precise control, soft robotics, and noninvasive diagnosis of diabetes and breast/lung cancers.

Sunlight-driven eco-friendly smart curtain based on infrared responsive graphene oxide-polymer photoactuators

Photomechanical actuation is the conversion of light energy into mechanical energy through some smart materials. Infrared-responsive smart materials have become an emerging field of research due to easy availability and eco-friendly nature of their stimulus in the form of sunlight, which contains about 50% of near-infrared(nIR) making these materials useful at macro-scale photoactuator applications. Here, we demonstrate fabrication of highly versatile nIR triggered photoactuators based on graphene oxide/polycarbonate bilayers that offers fast, low-cost fabrication, large deflection, reversible actuation and wavelength-selective response. The photoactuators are realized by vacuum filtration of graphene oxide/water dispersion through polycarbonate membrane resulting graphene oxide/polymer bilayer structure. The photoactuation response was measured in the form of deflection from equilibrium position as a result of infrared-irradiation. The deflection is caused by the generated thermal stress at the interface of bilayers due to mismatch of thermal expansion coefficient as a results of nIR absorption by graphene oxide and subsequent temperature rise. A maximum deflection of 12 mm (circular-shaped structure with diameter 28 mm) with corresponding bending curvature of 0.33 cm-1 was shown by this photoactuator for illumination intensity of 106 mW/cm2. Few applications of these photoactuators such as sunlight-driven smart curtain, infrared actuated curtain and self-folding box are also demonstrated.

Soft material for soft actuators

Inspired by natural muscle, a key challenge in soft robotics is to develop self-contained electrically driven soft actuators with high strain density. Various characteristics of existing technologies, such as the high voltages required to trigger electroactive polymers ( > 1KV), low strain ( < 10%) of shape memory alloys and the need for external compressors and pressure-regulating components for hydraulic or pneumatic fluidicelastomer actuators, limit their practicality for untethered applications. Here we show a single self-contained soft robust composite material that combines the elastic properties of a polymeric matrix and the extreme volume change accompanying liquid-vapor transition. The material combines a high strain (up to 900%) and correspondingly high stress (up to 1.3 MPa) with low density (0.84 g cm-3). Along with its extremely low cost (about 3 cent per gram), simplicity of fabrication and environment-friendliness, these properties could enable new kinds of electrically driven entirely soft robots.The development of self-contained electrically driven soft actuators with high strain density is difficult. Here the authors show a single self-contained soft robust composite material that combines the elastic properties of a polymeric matrix and the extreme volume change accompanying liquid vapour transition.

Electroactive Ionic Soft Actuators with Monolithically Integrated Gold Nanocomposite Electrodes

Electroactive ionic gel/metal nanocomposites are produced by implanting supersonically accelerated neutral gold nanoparticles into a novel chemically crosslinked ion conductive soft polymer. The ionic gel consists of chemically crosslinked poly(acrylic acid) and polyacrylonitrile networks, blended with halloysite nanoclays and imidazolium-based ionic liquid. The material exhibits mechanical properties similar to that of elastomers (Young's modulus ≈ 0.35 MPa) together with high ionic conductivity. The fabrication of thin (≈100 nm thick) nanostructured compliant electrodes by means of supersonic cluster beam implantation (SCBI) does not significantly alter the mechanical properties of the soft polymer and provides controlled electrical properties and large surface area for ions storage. SCBI is cost effective and suitable for the scaleup manufacturing of electroactive soft actuators. This study reports the high-strain electromechanical actuation performance of the novel ionic gel/metal nanocomposites in a low-voltage regime (from 0.1 to 5 V), with long-term stability up to 76 000 cycles with no electrode delamination or deterioration. The observed behavior is due to both the intrinsic features of the ionic gel (elasticity and ionic transport capability) and the electrical and morphological features of the electrodes, providing low specific resistance (<100 Ω cm^-2 ), high electrochemical capacitance (≈mF g^-1 ), and minimal mechanical stress at the polymer/metal composite interface upon deformation.

High-Performance Multiresponsive Paper Actuators

There is an increasing demand for soft actuators because of their importance in soft robotics, artificial muscles, biomimetic devices, and beyond. However, the development of soft actuators capable of low-voltage operation, powerful actuation, and programmable shape-changing is still challenging. In this work, we propose programmable bilayer actuators that operate based on the large hygroscopic contraction of the copy paper and simultaneously large thermal expansion of the polypropylene film upon increasing the temperature. The electrothermally activated bending actuators can function with low voltages (≤ 8 V), low input electric power per area (P ≤ 0.14 W cm^-2), and low temperature changes (≤ 35 °C). They exhibit reversible shape-changing behavior with curvature radii up to 1.07 cm^-1 and bending angle of 360°, accompanied by powerful actuation. Besides the electrical activation, they can be powered by humidity or light irradiation. We finally demonstrate the use of our paper actuators as a soft gripper robot and a lightweight paper wing for aerial robotics.