Electroactive Actuators Research Articles

Bending and sensing performances of electro-ionic soft actuators based on carboxylated cellulose nanofibers reinforced with graphene nanoplatelets

Abstract Soft robots not only possess greater degrees of freedom and the capability for continuous transformation, but they also offer exceptionally high safety in human–robot interactions, avoiding harm to the human body. Soft actuators are essential for developing high-performance soft robots, offering significant bending deformation, rapid response times, and prolonged operational capabilities. Herein, we present an ionic electroactive soft actuator based on functional cellulose nanofibers, graphene nanoplatelets, and ionic liquid. The proposed actuator achieved a large displacement about ±8 mm under 2.0 V at 0.1 Hz, with long working stability (98% of initial peak displacement maintained after 1260 cycles of cycling). The human–robot interaction applications of this actuator were explored by simulating human fingers. More importantly, the static and dynamic sensing performances of the actuator were investigated, finding that it generated a sensing voltage of 0.37 V at a vibration displacement of only 1.75 mm. The designed actuator provides a promising approach for developing high-performance soft robots, soft actuators, flexible sensors, and flexible active devices.

Exploring the potential of induced dipole-dominated electrorheological effect: advancing ER elastomers

Abstract Prior research has predominantly focused on traditional electrorheological (ER) effects while overlooking the transformative potential of induced dipoles in enhancing the overall performance of ER materials. In this study, we introduced a novel type of ER elastomer called induced dipole-dominated ER elastomer (ID-ERE). Through high-energy ball milling (HEBM) of the filler particles, the oxygen vacancies were produced within the particles that acted as local charge centers. In the presence of an external electric field (E), these oxygen vacancies induced the dipoles with significant dipole moments, thus amplifying the local electric field EL within the particle gaps. The powerful interactions of these dipoles significantly improved the overall performance of elastomer; the phenomenon referred to as the ID-ER effect. The viscoelastic results showed that ID-EREs have high field-induced storage modulus (G’ = 395.7 kPa), a significant increment in storage modulus (ΔG’ = 270.5 kPa) and high relative ER effect (ΔG’/G0 = 217.2%) at 3 kV mm−1. Additionally, after testing ID-EREs viscoelastic properties, it was discovered that excessive powder content leads to a decline in the elastomer’s performance. The results showed that ID-ERE’s viscoelastic, mechanical, dielectric, and overall efficiency is finer than the control ER elastomer (C-ERE) having unmilled TiO2 particles. Besides, the preparation method is straightforward, easily replicated, scalable, and cost effective. Thus, these ID-EREs should be a new generation of elastomer with the potential to be used in various automotive, robotics, construction, and electroactive actuators industries.

Confining Nano-ZrO2 in Nanodomains Leads to Electroactive Artificial Muscle with Large Deformation.

Dielectric elastomers generate muscle-like electroactive actuation, which is applicable in soft machines, medical devices, etc. However, the actuation strain and energy density of most dielectric elastomers, in the absence of prestretch, have long been limited to ∼20% and ∼10 kJ m-3, respectively. Here, we report a dielectric elastomer with ZrO2 nanoparticles confined in nanodomains, which achieves an actuation strain >100% and an energy density of ∼150 kJ m-3 without prestretch. We decorate the surface of each nanoparticle with a layer of a diblock oligomer, poly(acrylic acid-b-styrene). The surface-decorated nanoparticles coassemble with a triblock copolymer elastomer, poly(styrene-b-(2-ethylhexyl acrylate)-b-styrene) during cosolvent casting. Consequently, the nanoparticles are confined in the polystyrene nanodomains, which results in a dielectric elastomer nanocomposite with a low modulus, high breakdown strength, and intense strain-hardening behavior. During the actuation, the nanocomposite avoids the snap-through instability that most elastomers would suffer and achieves a superior actuation performance.

High-Performance Nanocellulose-Based Ionic Electroactive Soft Actuators

High-performance electroactive polymer actuators with large bending, fast response, and high durability have gained attention in the development of micromanipulators and multifunctional bionic soft robots. Herein, we developed high-performance electroactive soft actuators fabricated with ultrathin free-standing microfibrillated cellulose (MFC)-reinforced poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS) with multi-walled carbon nanotube (MWCNT)-doped composite electrode films and ion-exchange Nafion membranes by a hot-pressing method. The prepared PEDOT/PSS-MFC-MWCNT electrodes have good film-forming properties with a Young’s modulus of 448 MPa and an electrical conductivity of 75 S/cm. The proposed PEDOT/PSS-MFC-MWCNT/Nafion soft actuators have a sustained peak displacement of 2.1 mm and a long-term cyclic stability of 94% with no degradation over 1 h at 1.0 V, 0.1 Hz. Furthermore, we fabricated soft micro-grippers based on the actuators for mimicking actual finger actions for grasping, pointing, and counting, which introduces new possibilities for the next-generation development of micromanipulators and bionic soft robotics.

Ionic Liquid Crystal-Polymer Composite Electromechanical Actuators: Design of Two-Dimensional Molecular Assemblies for Efficient Ion Transport and Effect of Electrodes on Actuator Performance.

We present the development of free-standing ionic liquid crystal-polymer composite electrolyte films aimed at achieving high-frequency response electromechanical actuators. Our approach entails designing novel layered ionic liquid-crystalline (LC) assemblies by complexing a mesomorphic dimethylphosphate with either a lithium salt or a room-temperature ionic liquid through the formation of ion-dipole interactions or hydrogen bonds. These electrolytes, exhibiting room-temperature ionic conductivities on the order of 10-4 S cm-1 and wide LC temperature ranges up to 77 °C, were successfully integrated into porous polymer networks. We systematically investigated the impact of ions and electrodes on the performance of ionic electroactive actuators. Specifically, the Li+-based liquid crystal-polymer composite actuator with PEDOT:PSS electrodes demonstrated the highest bending deformation, achieving a strain of 0.68% and exhibiting a broad frequency response up to 110 Hz, with a peak-to-peak displacement of 3 μm. In contrast, the ionic-liquid-based liquid crystal-polymer composite actuator with active carbon electrodes showcased a bending response at a maximum frequency of 50 Hz and a force generation of 0.48 mN, without exhibiting the back relaxation phenomenon. These findings offer valuable insights for advancing high-performance electromechanical systems with applications ranging from soft robotics to haptic interfaces.

Interpenetrating Liquid Crystal Elastomer and Ionogel as Tunable Electroactive Actuators and Sensors

AbstractElectroactive liquid crystal elastomers (eLCEs) are used to make actuators and soft robotics. However, most eLCEs are monofunctional with one type of deformation (bending or contraction). Recently, a trilayer eLCE are reported by combining ion‐conducting LCE and ionic electroactive polymer device (i‐EAD). This i‐EAD‐LCE is bifunctional and performs either bending or contractile deformation by controlling low‐voltage stimulation. Nevertheless, it has a Young's modulus of only 1.63 MPa. To improve the mechanical performance, the i‐EAD‐IPN‐LCE is prepared here, whose central membrane is composed of interpenetrating LCE and ionogel (i‐IPN‐LCE) instead of a single ion‐conducting LCE. This i‐EAD‐IPN‐LCE with a typical thickness of 0.5 mm can function not only as linear and bending actuators, but also as a sensor. As a linear actuator, its Young's modulus, actuation stress, and strain are 51.6 MPa, 0.14 MPa and 9%, respectively, reaching skeletal muscles’ values. As a bending actuator, its bending strain difference Δε is 1.18% with 3 mN output force. It can also operate as a sensor producing 0.4 mV Open‐Circuit‐Voltage to respond to bending deformation (Δε = 9%). Therefore, this i‐EAD‐IPN‐LCE is a promising system for the fabrication of robust electroactive devices and sensors with multiple degrees of freedom.

Design Optimization of Printed Multi-Layered Electroactive Actuators Used for Steerable Guidewire in Micro-Invasive Surgery.

To treat cardiovascular diseases (i.e., a major cause of mortality after cancers), endovascular-technique-based guidewire has been employed for intra-arterial navigation. To date, most commercially available guidewires (e.g., Terumo, Abbott, Cordis, etc.) are non-steerable, which is poorly suited to the human arterial system with numerous bifurcations and angulations. To reach a target artery, surgeons frequently opt for several tools (guidewires with different size integrated into angulated catheters) that might provoke arterial complications such as perforation or dissection. Steerable guidewires would, therefore, be of high interest to reduce surgical morbidity and mortality for patients as well as to simplify procedure for surgeons, thereby saving time and health costs. Regarding these reasons, our research involves the development of a smart steerable guidewire using electroactive polymer (EAP) capable of bending when subjected to an input voltage. The actuation performance of the developed device is assessed through the curvature behavior (i.e., the displacement and the angle of the bending) of a cantilever beam structure, consisting of single- or multi-stack EAP printed on a substrate. Compared to the single-stack architecture, the multi-stack gives rise to a significant increase in curvature, even when subjected to a moderate control voltage. As suggested by the design framework, the intrinsic physical properties (dielectric, electrical, and mechanical) of the EAP layer, together with the nature and thickness of all materials (EAP and substrate), do have strong effect on the bending response of the device. The analyses propose a comprehensive guideline to optimize the actuator performance based on an adequate selection of the relevant materials and geometric parameters. An analytical model together with a finite element model (FEM) are investigated to validate the experimental tests. Finally, the design guideline leads to an innovative structure (composed of a 10-stack active layer screen-printed on a thin substrate) capable of generating a large range of bending angle (up to 190°) under an acceptable input level of 550 V, which perfectly matches the standard of medical tools used for cardiovascular surgery.

Single-Material Solvent-Driven Polydimethylsiloxane Sponge Bending Actuators.

Soft robots mimic the agility of living organisms without rigid joints and muscles. Continuum bending (CB) is one type of motion living organisms can display. CB can be achieved using pneumatic, electroactive, or thermal actuators prepared by casting an active layer on a passive layer. The corresponding input actuates only the active layer in the assembly resulting in the bending of the structure. These two different layers must be laminated well during manufacturing. However, the formed bilayer can still delaminate later, and the detachment hampers the actuator's reversible, long-time use. An approach to creating a single material bending actuator was previously reported, for which spatial gradient swelling was used. This authentic approach allows a single material to be manufactured as a bending actuator, allowing easy access to such actuators without lamination. In this study, we show spatial porosity differences in the sponges of polydimethylsiloxane (PDMS) (a common material in soft robotics) can be used to create the required anisotropy for bending. The spongy polymers are manufactured through table sugar templates and actuated by (organic) solvent absorption/desorption. This enables some versatility in the mechanical properties, shape, actuation force, and actuation speed. The one-material system's straightforward production and seamless nature are advantageous for reversible and repetitive bending. This simple method can further be developed in hydrogels and polymers for soft robotics and functional materials.

Soft Actuator with Biomass Porous Electrode: A Strategy for Lowering Voltage and Enhancing Durability.

Electroactive artificial muscles with deformability have attracted widespread interest in the field of soft robotics. However, the design of artificial muscles with low-driven voltage and operational durability remains challenging. Herein, novel biomass porous carbon (BPC) electrodes are proposed. The nanoporous BPC enables the electrode to provide exposed active surfaces for charge transfer and unimpeded channels for ion migration, thus decreasing the driving voltage, enhancing time durability, and maintaining the actuation performances simultaneously. The proposed actuator exhibits a high displacement of 13.6 mm (bending strain of 0.54%) under 0.5 V and long-term durability of 99.3% retention after 550,000 cycles (∼13 days) without breaks. Further, the actuators are integrated to perform soft touch on a smartphone and demonstrated as bioinspired robots, including a bionic butterfly and a crawling robot (moving speed = 0.08 BL s-1). This strategy provides new insight into the design and fabrication of high-performance electroactive soft actuators with great application potential.

Intelligent electroactive material systems with self-adaptive mechanical memory and sequential logic

By synthesizing the requisite functionalities of intelligence in an integrated material system, it may become possible to animate otherwise inanimate matter. A significant challenge in this vision is to continually sense, process, and memorize information in a decentralized way. Here, we introduce an approach that enables all such functionalities in a soft mechanical material system. By integrating nonvolatile memory with continuous processing, we develop a sequential logic-based material design framework. Soft, conductive networks interconnect with embedded electroactive actuators to enable self-adaptive behavior that facilitates autonomous toggling and counting. The design principles are scaled in processing complexity and memory capacity to develop a model 8-bit mechanical material that can solve linear algebraic equations based on analog mechanical inputs. The resulting material system operates continually to monitor the current mechanical configuration and to autonomously search for solutions within a desired error. The methods created in this work are a foundation for future synthetic general intelligence that can empower materials to autonomously react to diverse stimuli in their environment.

Dynamic braille display based on surface-structured PVC gel

Braille displays are a class of human–computer interaction electromechanical devices that display dynamic braille through an array of actuators. However, existing actuators for braille displays suffer from issues such as bulky size, heavy weight, and small tactile displacement, leading to difficulties in improving their resolution and readability. To address the above issues, we developed a novel electroactive artificial muscle actuator and applied it to braille displays. The novel actuator consists of a surface-structured PVC gel and planar electrodes. To investigate the effect of surface structure on the performance of novel PVC gel actuators, four types of surface-structured PVC gels were fabricated by a casting process, and their actuation performance was tested. The results show that the conical and frustum conical array structures are more favorable for improving the displacement of novel PVC gel actuators, while the cylindrical and quadrangular array structures are more favorable for improving their recovery forces. We observed both surface structure and dynamic electrical actuation, suggesting that the actuation of the novel actuator is mainly caused by the deformation of the surface structure of the array. We also analyzed electrowetting effects in PVC gels using the Lippmann–Young equation, to explain the differences in the performance of surface-structured PVC gels with different contact angles. Moreover, six multilayer actuators composed of PVC gels with a conical surface array structure are applied to the braille display unit to display the braille digits from 0 to 9. It has been shown that the novel PVC gel actuator has excellent mechanical properties, which makes it an ideal solution for braille displays.

Functionally antagonistic polyelectrolyte for electro-ionic soft actuator

Electro-active ionic soft actuators have been intensively investigated as an artificial muscle for soft robotics due to their large bending deformations at low voltages, small electric power consumption, superior energy density, high safety and biomimetic self-sensing actuation. However, their slow responses, poor durability and low bandwidth, mainly resulting from improper distribution of ionic conducting phase in polyelectrolyte membranes, hinder practical applications to real fields. We report a procedure to synthesize efficient polyelectrolyte membranes that have continuous conducting network suitable for electro-ionic artificial muscles. This functionally antagonistic solvent procedure makes amphiphilic Nafion molecules to assemble into micelles with ionic surfaces enclosing non-conducting cores. Especially, the ionic surfaces of these micelles combine together during casting process and form a continuous ionic conducting phase needed for high ionic conductivity, which boosts the performance of electro-ionic soft actuators by 10-time faster response and 36-time higher bending displacement. Furthermore, the developed muscle shows exceptional durability over 40 days under continuous actuation and broad bandwidth below 10 Hz, and is successfully applied to demonstrate an inchworm-mimetic soft robot and a kinetic tensegrity system.

Self-standing bacterial cellulose-reinforced poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) doped with graphene oxide composite electrodes for high-performance ionic electroactive soft actuators.

Flexible electrode films with good film-forming properties, large deformation ability, high conductivity, and strong charge and discharge capabilities are crucial for ionic electroactive polymer soft actuators. However, there are still challenges in preparing high-quality electrode films that can combine well with the intermediate polyelectrolyte to form high-performance soft actuators. Herein, we propose an advanced sandwich ionic electroactive actuator utilizing self-standing bacterial cellulose (BC) reinforced poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PP) doped with graphene oxide (GO) conductive composite electrodes and a Nafion ion-exchange membrane via a hot-pressing method. The prepared BC-PP-GO electrodes have good film-forming properties with a Young's modulus of 1360 MPa and a high conductivity of 150 S cm-1. The hot-pressed BC-PP-GO/Nafion ionic actuator exhibited a large bending displacement of 6.2 mm (1 V, 0.1 Hz) with a long-term actuation stability up to 95% over 360 cycles without degradation. Furthermore, we introduced the actuator's potential applications including bionic grippers, flies, and fish, providing more opportunities for the development of next-generation micromanipulators and biomimetic microrobots in cm-scale space.

Electroactive Soft Bistable Actuator With Adjustable Energy Barrier and Stiffness

Electroactive Soft Bistable Actuator With Adjustable Energy Barrier and Stiffness

Electroactive actuator composites using volume change induced by insulator-to-metal transition in VO2

This report describes the actuator functionality of a ceramic composite that uses the volume change caused by the insulator–metal (I–M) transition of VO2. When an electrical field was applied at 295 K, 50 vol. %-VO2/Al2O3 ceramic composite underwent the I–M transition because the VO2 powder dispersed in the composite became metallic. Simultaneous measurement of I–V characteristics and linear strain demonstrated that this I–M transition was accompanied by a positive strain. This strain increased with increase in the limit current value, which limits excessive current flow after transition. The strain exceeded 1300 ppm at maximum. Its magnitude corresponded to the volume change obtained from thermal expansion measurements on the composite. This composite performs a volume-change-driven actuator function that is distinct from the strain-driven counterpart of piezoelectric materials.

Analysis of the Impact of Electrochemical Properties of Copper-Doped Electrode Membranes on the Output Force of Biomimetic Artificial Muscles.

In this study, a biomimetic artificial muscle electroactive actuator was fabricated using environmentally friendly sodium alginate extract. Ultrasonic agitation was employed to embed ultrafine copper powder within a mesh-like structure formed by multi-walled carbon nanotubes (MWCNTs), aimed at reducing the internal resistance of the composite electrode membrane and enhancing its output force performance. Focused gallium ion beam-scanning electron microscopy observations, energy-dispersive X-ray spectroscopy (EDS) analysis, and surface morphology imaging confirmed the successful incorporation of the ultrafine copper powder into the MWCNT network. Additionally, we designed and constructed an output force measurement apparatus to assess the output performance of biomimetic artificial muscles (BMAMs) doped with varying quantities of ultrafine copper powder. Electrochemical testing results demonstrated that the artificial muscles exhibited optimal performance when doped with a mass of 1.5 g, yielding a maximum output force of 6.96 mN, an output force density of 30.64 mN/g, and a peak average rate of 0.059 mN/s. These values represented improvements of 224%, 189%, and 222% compared to the electrode membrane without the addition of ultrafine copper powder, respectively.

Design and characterization of electro-active soft actuator based on phase transition of microencapsulated silicon-ethanol composite

In this study, we demonstrate the development of a fast-response soft composite actuator fabricated by mechanical mixing of silicone elastomer (matrix) and ethanol (as phase-change fluid). Joule heating generated by electrical actuation causes a liquid-vapor transition of ethanol embedded in silicone microcapsules; provides the required pressure for mimicking the natural movements. We discuss the critical design variables to improve the temperature response, chemical composition, mechanical properties, force generated, and microstructure of soft actuator. Our soft actuator produces a high load-to-weight ratio (1000 times greater than its own) using a low-voltage source (˂20V). The stability of the soft actuator under multiple working cycles would bring the application to different robotic areas. The actuator using the mechanism presented here is easy to manufacture, automate, and recyclable. It can be used in a range of robotic applications.

Bioinspired Artificial Muscles Based on Sodium Alginate-Wrapped Multi-Walled Carbon Nanotubes and Molybdenum Disulfide Composite Electrode Membrane.

Using a naturally extracted polymer sodium alginate extracted from natural seaweed as the primary raw material, we have successfully developed an electroactive actuator known as biomimetic artificial muscle (BMAM). In comparison to conventional synthetic materials, this BMAM aligns more coherently with the prevailing principles of environmentally friendly development. During the preparation of the BMAM electrode membrane, we employed ultrasonic oscillation to adsorb varying quantities of MoS2 onto a reticulated structure formed by multi-walled carbon nanotubes (MWCNTs), thus enhancing the mechanical and electrochemical performance of the BMAM. Scanning electron microscopy and energy-dispersive X-ray spectroscopy (EDS) confirmed the successful encapsulation of MoS2 by the MWCNTs network in the composite. To measure the output force of the BMAM fabricated with different masses of MoS2 doping, we established a self-built experimental platform and conducted tests on the electrode membranes doped with varying quantities of MoS2 using an electrochemical workstation. The results revealed that the BMAM exhibited optimal mechanical performance when doped with 1.5 g of MoS2, with a maximum output force of 7.81 mN, an output force density of 34.36 mN/g, and a response rate of 0.09 mN/s. These performances were improved by 309%, 276%, and 175%, respectively, compared to the samples without MoS2 doping, with a mass-specific capacitance enhancement of 151%.

High performance fiber-constrained plasticized PVC gel actuators for soft robotics

Plasticized Poly (vinyl chloride) (PVC) gel actuators, a type of electroactive soft actuator, have recently attracted increasing interest in soft robotics. Herein, we report a novel planar PVC gel actuator with flexible electrodes and a fiber-constrained structure for electroactive artificial muscles. PET adhesive fibers were evenly affixed on both sides of the unidirectional pre-stretched PVC gel membranes in one direction, resulting in a human-muscle-like unidirectional contraction and expansion movement. The fibers can constrain the bending motion of the PVC gel membranes, and the unidirectional pre-stretch in the direction of the attached fibers further makes the membrane anisotropic, leading to easy and fast actuation in the direction normal to the pre-stretch. The developed fiber-constrained plasticized PVC gel actuator (FCPPGA) unit exhibited high strain (>40%) and stress (>0.5 MPa) with a fast response time of about 0.1 s under a relatively low driving DC field (<15 V/μm). The simulated data were in good agreement with the experimental results. Furthermore, the designed FCPPGA was successfully used to demonstrate the mimicry of lower jaw motions in a skeleton model.

Mechanical stimulation of cells with electroactive polymer-based soft actuators

Mechanical stimulation of cells with electroactive polymer-based soft actuators