Transparent Actuator Research Articles

Single-layer iron network microstructure magnetorheological elastomer for transparent soft actuator

The motion of a soft magnetic magnetorheological elastomer, a composite material consisting of soft magnetic fillers and an elastomeric matrix, is limited by the gradient of the applied magnetic field, making its application as an out-of-plane soft actuator difficult. In this study, an anisotropic soft magnetic magnetorheological elastomer-based transparent soft actuator with large out-of-plane deformation was fabricated using a very low concentration (1 vol%) of carbonyl iron particles as the soft magnetic fillers. An applied AC electric field treatment method was used to manipulate iron particles in silicone oil, assembling and depositing them onto a glass substrate to form a single-layer iron-network microstructure. A single-layer iron-network microstructure-based magnetorheological elastomer (SL-sMRE) film with initial self-buckling was fabricated by exploiting the swelling properties of the residual silicone oil and polydimethylsiloxane. The SL-sMRE film exhibited out-of-plane deformation with a very short reaction time (228 ms) under a uniform magnetic field. When stimulated by a non-uniform magnetic field, the SL-sMRE film deformed away from the permanent magnet, which was not possible with an isotropic film. An optically transparent SL-sMRE film was applied to a transparent flexible gripping device that could detect the surface of a gripped object in real time.

Transparent Localized Haptics: Utilization of PVDF Actuators on Touch Displays

Generating localized haptic feedback on touch displays has been a challenge in recent years. In this study, we introduce a haptic interface using transparent thin-film PVDF actuators to address this issue. The transparency feature can be used to mount the actuators at any location beneath the display, enabling localized haptic feedback as the generated vibration is primarily evident on the mounting area. Two different configurations are designed, simulated and prepared to explore the effectiveness of the proposed approach. The first configuration is used to characterize the haptic interface. Modal and forced-vibration analyses are performed to identify important design characteristics based on human factors. Subsequent 2AFC psychophysics experiments validate the characteristics. In the second configuration, eight actuators are attached to the touch surface in a 2 × 4 matrix formation and excited at different voltage amplitudes. Human experiments are conducted based on the results from corresponding forced-vibration analysis. The results show that subjects demonstrate an accuracy of 96% in identifying locations with haptic feedback when the actuators are excited with 232 Vpp. Overall, our study demonstrates the effectiveness of the proposed transparent haptic interface equipped with PVDF actuators in achieving localized haptic feedback on touch displays.

Ink-based transparent compliant electrode for direct coating on untreated hydrophobic PDMS surface

Polydimethylsiloxane (PDMS) is widely used as the substrate for wearable sensors and elastomeric actuators because of its good dielectric property, stretchability, thermal/chemical resistance, biocompatibility, and transparency. PDMS-based stretchable sensors and electroactive actuators need a coating of thin compliant electrodes. However, PDMS is hydrophobic with water contact angle above 120°. Such low surface energy of PDMS creates issues for uniform coating of ink-based electrodes and/or their adhesion. Similarly, metal thin-film or nanowire-based materials coated on PDMS easily peels off. Surface treatments like UV/plasma exposure can temporarily increase PDMS's surface energy. However, these surfaces return to its original state within hours leaving behind a brittle surface layer that cracks upon stretching. To address these issues, we formulated a viscous composite ink with PEDOT:PSS and PDMS that can easily be coated on untreated PDMS by blade casting, screen printing, and so on. These coatings are highly transparent, stretchable, electrically conductive and work as compliant electrodes. We fabricated transparent dielectric elastomer actuators by coating PEDOT:PSS/PDMS composite ink on Elastosil (PDMS) substrates. The actuation strain and breakdown-fields were slightly lower compared to DEA with conventional graphite electrodes. However, the additional self-clearing capability of PEDOT:PSS/PDMS electrodes enhanced robustness of the DEA in events of localized dielectric breakdown.

UV and IR dual light triggered cellulose-based invisible actuators with high sensitivity

Actuators are widely used in bionic devices and soft robots, among which invisible actuators have some unique applications, including performing secret missions. In this paper, highly visible transparent cellulose-based UV-absorbing films were prepared by dissolving cellulose raw materials using N-methylmorpholine-N-oxide (NMMO) and using ZnO nanoparticles as UV absorbers. Furthermore, transparent actuator was fabricated by growing highly transparent and hydrophobic polytetrafluoroethylene (PTFE) film on regenerated cellulose (RC)-ZnO composite film. In addition to its sensitive response to Infrared (IR) light, the as-prepared actuator also shows a highly sensitive response to UV light, which is attributed to the strong absorption of UV light by ZnO NPs. Thanks to the drastic differences in adsorption capacity between the RC-ZnO and PTFE materials for water molecules, the asymmetrically- assembled actuator demonstrates extremely high sensitivity and excellent actuation performance, with a force density of 60.5, a maximum bending curvature of 3.0 cm−1, and a response time of below 8 s. Bionic bug, smart door and the arm of excavator made from the actuator all exhibit sensitive responses to UV and IR lights.

Toward Improving Actuation Transparency and Safety of a Hip Exoskeleton With a Novel Nonlinear Series Elastic Actuator

Actuation transparency and safety are important requirements in the design and control of assistive exoskeletons for individuals who suffer lower limb deficits but still maintain a certain level of voluntary motor control. In recent years, series elastic actuator (SEA) has been regarded as a promising solution for transparent actuation and safe human–robot interaction, thus SEAs are widely developed and applied in assistive exoskeletons. However, existing SEAs designed for assistive exoskeletons still lack both actuation transparency and safety because of high stiffness of the elastic element and the high mechanical impedance of the actuators. To address this problem, a novel nonlinear SEA (nSEA) is presented in this article. The optimized nonlinear series elastic element coupled with a quasi-direct drive motor creates the nSEA with low mechanical impedance, high backdrivability, and less acoustic noise. Besides, a new torque control, based on cascade PI control, is proposed for the nSEA to control the interaction torque with high accuracy and robustness. Finally, an experimental evaluation with human subjects is performed to validate the advantages of the nSEA-driven hip exoskeleton in the realization of actuation transparency and safety. The root-mean-square interaction torque in zero-impedance mode is as low as 0.051 N <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\cdot$</tex-math></inline-formula> m during walking conditions, leading to a negligible negative influence on the hip joint's range of motion, walking speed, and energy expenditure when wearing the hip exoskeleton.

Numerical Investigation of the Effect of Symmetry on Evaporation Triggered Elastocapillary Top-Gathering of High Aspect Ratio Micropillars

High-aspect-ratio (HAR) micropillar arrays offer a wide range of applications in micro-contact printing, switchable transparent optical windows, superhydrophobic surfaces, mechanical sensors, and actuators, due to their properties such as large surface area and excellent mechanical compliance. However, owing to their high aspect ratio, these microstructures are prone to lateral deflection by elastocapillary forces in liquid environments, which is known as top-gathering, limiting their manufacturing processes and applications. Here, the impact of symmetry on evaporation triggered top-gathering of micropillars was studied numerically. The initiation of the micropillar deflection due to capillary forces under varying force distributions was simulated using a COMSOL Multiphysics simulation package. The simulation was carried out for the configurations of two, four, and an array of micropillars. For the four micropillar configuration, a new equation was suggested for calculating the micropillar deflection due to elastocapillary forces, using force distributions around the micropillars. The suggested equation was verified by comparison with the experimental observations. The effect of droplet evaporation on deflection/top-gathering of micropillars was also investigated. It was found that initiation of deflection is due to asymmetry at the rim of the droplet, generating domino-like deflection of the other micropillars. This study provides a new equation/criterion for estimating deflection of the micropillars, suggesting array designs that are resistant to such deflections when interacting with liquids.

Bio-Inspired Transparent Soft Jellyfish Robot.

Jellyfish are among the widely distributed nature creatures that can effectively control the fluidic flow around their transparent soft body, thus achieving movements in the water and camouflage in the surrounding environments. Till now, it remains a challenge to replicate both transparent appearance and functionalities of nature jellyfish in synthetic systems due to the lack of transparent actuators. In this work, a fully transparent soft jellyfish robot is developed to possess both transparency and bio-inspired omni motions in water. This robot is driven by transparent dielectric elastomer actuators (DEAs) using hybrid silver nanowire networks and conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/waterborne polyurethane as compliant electrodes. The electrode exhibits large stretchability, low stiffness, high transmittance, and excellent conductivity at large strains. Consequently, the highly transparent DEA based on this hybrid electrode, with Very-High-Bond membranes as dielectric layers and polydimethylsiloxane as top coating, can achieve a maximum area strain of 146% with only 3% hysteresis loss. Driven by this transparent DEA, the soft jellyfish robot can achieve vertical and horizontal movements in water, by mimicking the actual pulsating rhythm of an Aurelia aurita. The bio-inspired robot can serve multiple functions as an underwater soft robot. The hybrid electrodes and bio-inspired design approach are potentially useful in a variety of soft robots and flexible devices.

All polymer‐based transparent composites for electrothermal actuator and supercapacitor

AbstractWith the rapid development of smart devices, multifunctional integrated systems are greatly developed. Although several multifunctional devices integrated with functions of actuation, sensing, and energy storage have been successfully fabricated, most of these integrated multifunctional systems are opaque. Here, all polymer‐based transparent composites are proposed for the electrothermal actuator, supercapacitor, and intelligent integrated electronic system. The transmittances of the polyaniline@poly(3, 4‐ethylene dioxythiophene): poly(styrenesulfonate)/polyethylene terephthalate/polyethylene hierarchical composites with transmittance over 40% are prepared by the facile method of in‐situ polymerization and “cut‐paste” process, which can be directly used as transparent actuators as well as electrodes for transparent supercapacitors. The transparent actuator exhibits large bending deformation (&gt;0.8 cm−1) and long‐term repeatability under electrical stimulation. Also, the transparent supercapacitor presents a high areal capacitance of 9.9 mF cm−2 and excellent cycling stability. Due to the above advantages, a series of practical applications are designed, such as biomimetic transparent grippers, flexible supercapacitor arrays, and so forth. Finally, an intelligent integrated electronic system with intelligent electrical‐controlled switches and supercapacitor arrays is successfully constructed to demonstrate the practical application of functional polymer film in smart devices.

Highly transparent PZT capacitors on glass obtained by layer transfer process

Transparent ITO/PZT/ITO capacitors were fabricated on 200 mm glass substrate. The PZT films of 1 µm and 2 µm thickness were first grown on platinized Si wafer by sol–gel method, and then transferred onto glass substrate together with ITO electrodes following an innovative process. The obtained PZT based stacks on glass show an average transmission of about 70% in the visible range. PZT films keep their preferred (100) orientation after transfer process. The capacitors exhibit ferroelectric, dielectric and piezoelectric properties comparable to standard non-transparent PZT films with metal electrodes. Transverse piezoelectric coefficient e31,f as high as 16 C/m2 was measured for both PZT film thicknesses. This proof of concept opens the way to the fabrication of transparent piezoelectric actuators on glass for high performances haptic devices, as well as for other emerging applications like self-cleaning or functionalization of smart windows.

Tunable force sensor based on carbon nanotube fiber for fine mechanical and acoustic technologies

Design of new smart prosthetics or robotic grippers gives a major impetus to low-cost manufacturing and rapid prototyping of force sensing devices. In this paper, we examine piezoresistive force sensors based on carbon nanotube fibers fabricated by a novel wet pulling technique. The developed sensor is characterized by an adjustable force range coupled with high sensitivity to enable the detection of a wide range of forces and displacements limited by the experimental setup only. We have demonstrated the applicability of the developed unit in tactile sensing, displacement sensing, and nanophone vibration monitoring system and evaluated its force sensing characteristics, i.e. displacement/force input and resistance/mechanical response. In the experiments it measures 0–115 N force range within 2.5 mm displacement. Moreover, the sensor demonstrates good linearity, low hysteresis, and stability when tested over 10 000 cycles. The developed sensor suits multiple applications in the field of soft and transparent sensors, nanophones, actuators, and other robotics devices for both regular and extreme environments, e.g. deep underwater and radioactive environment.

Ultraflexible transparent conductive films based on Ag nanowires for use in quick thermal response transparent heater

This work reports on the fabrication of an ultraflexible transparent conductive hybrid films via embedding silver nanowires (AgNWs) into a thin UV-curable resin film. This AgNW/UV-curable resin hybrid film exhibits excellent performance in terms of conductivity (sheet resistance: 9.9 Ω/sq.) and transparency (optical transmittance: 86.4%). After bending 1000 times at a 100 μm curvature radius or even undergoing an ultrasonication test, it still shows excellent electrical performance and negligible decay. The hybrid film has environmental endurance, and there is no significant performance degradation after being kept at high temperature (85 °C) and high humidity (85% humidity) for 60 h. Finally, the successful implementation of the hybrid film into a transparent film heater is demonstrated. The efficient Joule heating can increase the temperature of the heaters to 121 °C in ∼12 s at an input voltage of 6 V. The fast-heating characteristics at low voltages associated with its transparency and flexibility properties make the heaters a potential candidate in electrothermal applications, such as transparent soft actuators, wearable heaters, and medical thermotherapy pads.

Transparent Soft Electrothermal Actuators with Integrated Cu Nanowire Heater for Soft Robotics

AbstractSoft actuators with high flexibility have received widespread attention. Here, a transparent electrothermal soft actuator is designed and manufactured via sandwiched copper nanowires (Cu NWs) between two kinds of flexible substrates with different thermal expansion characteristics (polyethylene‐terephthalate (PET) and low‐density polyethylene (LDPE)). In order to improve bending characteristics of the soft actuator, heating performance of heaters with different widths is studied, and the bending characteristics of strain gauges under different voltages are also studied. The fabricated soft actuator can produce a bend of 90° and provide about 1 mN of force at a low voltage of 6 V. The application of this type of soft actuator is demonstrated toward biomimetic robotics, a two‐finger flexible transparent gripper is designed, where two soft actuators are utilized to simulate the movements of human fingers and realizes the operation of picking and placing fine objects. In addition, the topography of the object can be feasibly observed during clamping process of the soft actuator with certain light transmittance, which is favorable for the soft gripper application.

Highly transparent RCF/PTFE humidity and IR light dual-driven actuator with high force density, sensitivity and stability

Transparent actuators have received great interest among researchers due to their potential applications in tactical displays, optical switches, zoom lenses, etc. In this work, we designed and developed a highly transparent cellulose-based actuator, made of regenerated cellulose film (RCF) and hydrophobic polytetrafluoroethylene (PTFE) two-layered film, which was fabricated by using a simple sputtering method to grow PTFE film on the RCF film. The RCF/PTFE actuator exhibits high sensitivity towards both humidity- and IR light-driving responses, which can be attributed to thermal expansion and hygroscopic expansion differences between RCF film and PTFE film. The stability of the actuator was greatly improved by O plasma etching of the RCF film surface. Bi-directional actuation of the actuator can be achieved by controlling the ambient humidity and IR light irradiation, with the bending angle changing from 0° to 360° within 14 s and 10 s, respectively. Numerical simulations show that the sensitivity and stress of RCF/PTFE actuators are more than one order of magnitude higher than petroleum-based actuators, which can be attributed to the unique hygroscopic effect of cellulose. The as-obtained actuator may have potential applications in the fields of smart window, bionic robot, optical devices, sensing and camouflage.

Poly (vinyl alcohol) based gradient cross-linked and reprogrammable humidity-responsive actuators

Actuators are a kind of energy-conversion devices which can perceive external stimulus and undergo reversible deformation or motion to produce a mechanical response. Developing actuators with advanced structures rather than typical bilayer configuration is essential for stable actuations, and installing reprogrammability into actuators for on-demand complex shape transformation within the same piece of material is more convenient and cost-effective for new actuation modes. However, actuators that satisfy these requirements have rarely been reported yet. Herein, a facile and low-cost design strategy for a transparent, gradient cross-linked, patternable and reprogrammable humidity-responsive actuator based on commercial available poly (vinyl alcohol) (PVA) is presented. The actuator is fabricated by grafting PVA with 9-anthracenecarboxylic acid (9-AN) via esterification and then blending with a UV-screening agent 2,2′,4,4′-tetrahydroxybenzophenone (THBP) followed by photo-crosslinking. In such design, the abundant hydroxyl groups in grafted PVA endow the actuator with humidity-responsive ability, the THBP enables the gradient cross-linking of actuator due to the attenuated UV intensity through the film thickness, and the photo-controlled reversible cross-linking of anthracene groups allows the patterning and reprogramming of the actuator. The chemical structure, gradient morphology, and humidity-responsiveness of actuators are thoroughly investigated. The fast actuation speed and large actuation amplitude, the realization of complex shape deformation by photo-controlled programming/reprogramming, and the biomimetic applications such as the soft walking robot are also demonstrated. This study is expected to provide new opportunity for facilely developing high-performance, stable and reprogrammable actuators.

Tunable Soft Lens of Large Focal Length Change.

Tunable lens technology inspired by the human eye has opened a new paradigm of smart optical devices for a variety of applications due to unique characteristics such as lightweight, low cost, and facile fabrication over conventional lens assemblies. The fast-growing demands for tunable optical lenses in consumer electronics, medical diagnostics, and optical communications require the lens to have a large focal length modulation range and high compactness. Herein, for the first time, an all-solid tunable soft lens driven by highly transparent dielectric elastomer actuators (DEAs) based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and waterborne polyurethane (PEDOT:PSS/WPU) transparent electrodes is developed. The deformation of the tunable soft lens is achieved by the actuation of DEAs, mimicking the change of the surface profile of the human eye to achieve remarkable focal length variations. Upon electrical activation, this tunable soft lens can vary its original focal length by 209%, which is one of the highest among current tunable soft lenses and far beyond that of the human eye. This study demonstrates that transparent DEAs are capable of achieving focus-variation functions, and potentially useful in artificial robotic vision, visual prostheses, and adjustable glasses, which will induce significant effects on the future development of tunable optics.

Highly Transparent, Stretchable, and Conducting Ionoelastomers Based on Poly(ionic liquid)s.

The rapid development of soft electronics has revitalized the research of conducting elastomers. However, the design of conducting elastomers having high stretchability and good transparency still remains a considerable challenge. In this study, we develop a highly transparent, stretchable, and conducting ionoelastomer based on a poly(ionic liquid) in which cations are fixed to a stretchable elastomeric network and counter anions are mobile. The ionoelastomer solves the dilemma of simultaneous transparency and stretchability in the design of traditional conducting elastomers, possessing good transparency (96%) with an extraordinarily high stretchability, up to a limiting strain of 1460%. Moreover, this novel material is completely nonvolatile and nonhygroscopic, endowing the ionoelastomer with highly stable thermal, environmental, electrochemical, and mechanoelectrical properties. An underwater sensor based on the ionoelastomer is developed with good performance in an aqueous environment. Also, a transparent dielectric elastomer actuator (DEA) is demonstrated using the ionoelastomer. It is believed that the ionoelastomer would pave the way to develop exceptional conducting elastomers toward next-generation soft electronics.

Quick Thermal Response‐Transparent‐Wearable Heater Based on Copper Mesh/Poly(Vinyl Alcohol) Film

High‐performance, transparent, wearable heaters with fast thermal response are developed. To accomplish this, highly conductive and transparent copper (Cu) mesh films are transfer printed onto flexible poly(vinyl alcohol) (PVA) substrates of different thicknesses. The thermal response characteristics of the heaters, with different PVA substrate thicknesses, are analyzed and results are recorded. The Cu mesh/PVA film heater shows a favorable figure of merit value of 228.4 and outstanding durability in response to mechanical stress tests for bending, crumpling, and detaching. Through the test results, it is concluded that the thermal response time of the heater is directly related to the thickness of the PVA substrate. The optimized Cu mesh/PVA film heater, with a substrate thickness of ≈60 μm and input voltage of 3 V (DC), exhibits a fast Joule heating effect with a response time of ≈12 s and a ramping rate of ≈7.5 °C s−1. These are excellent results, compared with the previously reported studies with other metal‐based flexible transparent heaters. A flexible transparent heater with low applied voltage and fast response time is expected to have innovative electrothermal applications, such as transparent soft actuators, wearable heaters, and medical thermotherapy pads.

Assistance of a Person with Muscular Weakness Using a Joint-Torque-Assisting Exoskeletal Robot

Robotic systems for gait rehabilitation have been actively developed in recent years; many of the rehabilitation robots have been commercialized and utilized for treatment of real patients in hospitals. The first generation of gait rehabilitation robots was a tethered exoskeleton system on a treadmill. While these robots have become a new trend in rehabilitation medicine, there are several arguments about the effectiveness of such robots due to the passiveness of the motions that the robots generate, i.e., the continuous passive motions may limit the active involvement of patients’ voluntary motion control. In order to let a patient be more actively involved by requiring the self-control of whole-body balance, untethered powered exoskeletons, wearable robots that patients can wear and walk on the ground, are receiving great attention. While several powered exoskeletons have been commercialized already, the question about their effectiveness has not been cleared in the viewpoint of rehabilitation medicine because most of the powered exoskeletons provide still continuous passive motions, even though they are on the ground without tethering. This is due to their control strategy; the joints of a powered exoskeleton are position-controlled to repeatedly follow a predefined angle trajectory. This may be effective when a wearer is completely paraplegic such that the powered exoskeleton must generate full actuation power for walking. For people with muscular weakness due to various reasons, the powered exoskeleton must assist only the lack of muscular force without constraining human motion. For assistance and rehabilitation of people with partial impairment in walking ability, Angel Legs is introduced in this paper. The proposed powered exoskeleton system is equipped with a transparent actuation system such that the assistive force is accurately generated. The overall design and control of Angel Legs are introduced in this paper, and a clinical verification with a human subject is also provided.

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.

Transparent Soft Actuators/Sensors and Camouflage Skins for Imperceptible Soft Robotics.

The advent of soft robotics has led to great advancements in robots, wearables, and even manufacturing processes by employing entirely soft-bodied systems that interact safely with any random surfaces while providing great mechanical compliance. Moreover, recent developments in soft robotics involve advances in transparent soft actuators and sensors that have made it possible to construct robots that can function in a visually and mechanically unobstructed manner, assisting the operations of robots and creating more applications in various fields. In this aspect, imperceptible soft robotics that mainly consist of optically transparent imperceptible hardware components is expected to constitute a new research focus in the forthcoming era of soft robotics. Here, the recent progress regarding extended imperceptible soft robotics is provided, including imperceptible transparent soft robotics (transparent soft actuators/sensors) and imperceptible nontransparent camouflage skins. Their principles, materials selections, and working mechanisms are discussed so that key challenges and perspectives in imperceptible soft robotic systems can be explored.