Polymer Actuators Research Articles

Rapid and spatially programmed electrostatic actuation of anisotropic polymers

Liquid crystal elastomers (LCEs) are a fascinating class of anisotropic soft actuating materials that can respond to various stimuli. However, achieving both fast actuation and a high degree of freedom (DOF) in the LCE is one of the main challenges to overcome. In this study, we demonstrate rapid actuation (∼25 Hz) of LCEs driven by electrostatic force via direct charging. Notably, the end of the LCE film in our system is physically separated from the electrode, unlike conventional dielectric elastomer actuators, which allow a high DOF actuation when applying the DC or AC voltage in a spatially controlled manner. As a proof-of-concept, various functional actuators, including popup letters, a Kirigami-inspired spring, a dynamic segments device, a freeform oscillator, and a flapping motion of beetle, are successfully demonstrated. We envision our electrostatically actuating LCEs with sophisticatedly engineered electric fields in 1D, 2D, and 3D environments as a valuable platform for investigating the fundamental mechanics of electric field-driven soft actuators.

Regulation of electrically responsive shape recovery behavior of 4D printed polymers for application in actuators

Four-dimensional (4D) printing, an additive manufacturing technology for intelligent materials and structures, has received increasing scholarly attention for its ability to achieve variable shapes, properties and functions. However, recent research has primarily focused on developing new materials and structures, neglecting the regulation of deformation behavior. We hereby propose a novel photocuring 4D printing strategy for electrically responsive shape memory polymers (E-SMPs) actuators embedded with a dual-layered electrothermal layer (ETL) and channel. One way to achieve tunable shape recovery behavior in E-SMPs is to regulate the heat generation behavior of ETL by introducing a coolant into the channel. The electric heating and water cooling were sequentially and simultaneously activated to achieve suspension of the shape recovery process and reduction in the shape recovery rate, respectively. E-SMPs embedded with single and double ETLs were utilized to evaluate their electrically-induced heating behavior, electrically responsive shape recovery behavior, and shape recovery force. Additionally, sequential shape recovery behavior was achieved by serially interconnecting three (3) ETLs with varying thicknesses. Furthermore, the channel integrated within the SMPs were employed to realize remote thermally responsive shape recovery (T-SMPs), for which the shape recovery rate was regulated by adjusting the flow rate of hot water and channel dimensions. This strategy was successfully demonstrated for manufacturing E-SMPs actuators with adjustable shape recovery rate, sequential recovery behavior, and high-load deformation capability.

NARMAX Approach for the Identification of a Dielectric Electroactive Polymer Actuator

NARMAX Approach for the Identification of a Dielectric Electroactive Polymer Actuator

Fabricating 3D Moisture- and NIR Light-Responsive Actuators by a One-Step Gradient Stress-Relaxation Process.

Polymeric materials that can actuate under the stimulation of environmental signals have attracted considerable attention in fields including artificial muscles, soft robotics, implantable devices, etc. To date, the improvement of shape-changing flexibility is mainly limited by their unchangeable shapes and structural and compositional distributions. In this work, we report a one-step treatment process to convert 2D poly(ethylene oxide)/sodium alginate/tannic acid thin films into 3D-shaped moisture- and NIR light-responsive actuators. Spatial surface wetting of the film leads to the release of residual stress generated in film formation in a gradient manner, which drives the wetted regions to bidirectionally bend. By controlling the position and bending amplitude of the wetted regions, designated 3D shapes can be obtained. Moreover, Fe3+ ions in the aqueous solution used for surface wetting can coordinate with carboxylate groups in sodium alginate chains to form a gradient cross-linking network. This gradient network can not only stabilize the resulting 3D shape but also render the film with moisture-responsive morphing behaviors. Fe3+ ions can also self-assemble with tannic acid molecules to form photothermal aggregates, making the film responsive to NIR light. We further show that films with versatile 3D shapes and different modes of deformation can be fabricated by a one-step treatment process. This strategy is convenient and extendable to develop 3D-shaped polymer actuators with flexible shape-changing behaviors.

Continuous textile manufacturing method for twisted coiled polymer artificial muscles

Twisted, coiled polymer actuators (TCPAs) are a promising type of fiber-based actuators with high energy density, low material costs, and good recyclability; however, current manufacturing methods limit the length and stability of TCPAs, hampering their potential for large-scale textile applications. To overcome this limitation, we propose a textile manufacturing method based on the false-twisting principle, allowing for continuous and rapid production of highly twisted monofilaments. Additionally, this process enables the plying of two or more twisted monofilaments together, as well as the integration of wires for heating and sensing purposes. The resulting twist-stable plies can then be mandrel-coiled and annealed to create a new class of TCPAs with three superimposed levels of helicity, in contrast to the usual two levels. In this study, we investigate the impact of the additional helix level and various factors, including twist density, annealing temperature, cooling speed, and chirality, on the contractility of these TCPAs. Furthermore, due to the twist-stability of the plied yarns, they can be processed on standard textile machines, enabling the manufacture of TCPAs with multiple active yarns that can form contracting artificial muscles using a circular braiding machine. Our key findings reveal that the twisted monofilament coils can contract up to 60%, and higher twist density leads to improved performance for monofilament TCPAs. Notably, this phenomenon is not observed in plied-yarn TCPAs, where varying levels of twist on the monofilament and yarn helix level result in enhanced contractile performance. Overall, this work presents a novel textile manufacturing method for producing twist-stable TCPAs with good contractile performance, providing insights into the design and fabrication of advanced fiber-based actuators for potential applications in large-scale textiles, robotics, and biomedical devices.

Improved performance in temperature and speed of TCP artificial muscles for soft wearables robots by length modification

Artificial muscles provide a unique solution for wearable rehabilitation robots (WRRs) because they are compliant, compact, and lightweight. Twisted and coiled polymer actuators (TCPs) are artificial muscles from thermally activated polymer fibres. They present high power density, linearity, stress and strain compared to other artificial muscles. Nevertheless, as TCPs require heat to start, their main barrier for widespread use in WRRs are their slow reaction times and the high temperatures they reach. Previous studies have analysed different parameters, like fibre material, fibre diameter, and various cooling systems, to improve TCP frequency response and working temperature. Nevertheless, the length of the actuator has not been explored as a possible parameter to enhance the actuation performance in this regard. This work focuses on studying the behaviour of TCPs with different lengths and how the performance in frequency response and temperature can be improved using the length as a primary parameter, as they are critical for wearable robots. First, a characterisation of the TCPs was performed. Then, a method to improve frequency response, based on offsets on long actuators was implemented and validated using a chirp signal. The experimental results show that the mechanical characteristics are similar regardless of the actuator’s length. They reached a strain of 10% with a power of 0.16 W cm−1. However, the electrothermal properties changed as the power needed to increase temperature was higher when the actuator was enlarged. Therefore, an improvement in the required temperature was found, able to reduce the temperature with the same frequency response. Regarding the technique to enhance the speed of the actuator, it was possible to increase the frequency by 0.0006 Hz for each mm applied as an offset. Hence, the frequency response for the same displacement was increased linearly as the actuator was elongated.

Comparison of the Hysteresis Response of Super-Coiled Polymer Actuators

Abstract In recent years, super-coiled polymer (SCP) actuators have been investigated as a higher yield, low-cost alternative to conventional actuators. A major challenge has been finding appropriate general and industry uses for these actuators due to their hysteretic response. This paper seeks to compare the response for SCP actuators based on different thread weights and sizes and thus suggest possible directions for their use. The actuators were produced through coiling and heat training controlled via Arduino. The responses were obtained by supplying a controlled voltage to the system. The response was filmed and processed using matlab; the resulting displacement and strain were used to observe the hysteretic characteristics. Based on the characteristics noted, different applications were recommended for these SCP actuators. It is recommended that this research into SCP actuators be furthered by developing hysteresis models for compensation, control, and exploring their use in robotic systems and tactile mechanisms.

New Soft Actuating Way by Rapidly Temperature Change of Electrocaloric Effect.

Compared to traditional temperature control methods, the electrocaloric (EC) effect offers several advantages such as small size, rapid response, and environmental friendliness. However, current EC effects are generally used for the cooling area rather than heating. Here, poly(vinylidenefluorideter-trifluoroethylene-ter-chlorofluoroethylene) [P(VDF-TrFE-CFE)] film is combined with an electrothermal actuator (ETA) composed of polyethylene (PE) film and carbon nanotube (CNT) film. The heating and cooling process of the EC effect is used to help drive the ETA. The P(VDF-TrFE-CFE) film can produce a temperature change (ΔT) of 3.7 °C at 90 MV/m, and this process occurs within 0.1 s. With this ΔT, the composite film actuator can produce a deflection of 10°. In addition, due to the electrostrictive effect of P(VDF-TrFE-CFE), the composite film can also be used as an actuator. At 90 MV/m, the composite film actuator can produce a deflection over 240° within 0.05 s. Apart from other current driving modes for thermally responsive actuators, in this paper, a new type of soft actuating composite film by the temperature change of the EC effect is proposed. Except from ETAs, the EC effect can also have a wide application prospect in other thermally responsive actuators, including shape memory polymer actuators, shape memory alloy actuators, and so on.

Effect of the polyaniline/carbon black additive on the dispersion state of carbon nanotubes and polymer actuator performance

Effect of the polyaniline/carbon black additive on the dispersion state of carbon nanotubes and polymer actuator performance

Magnetic Polymer Actuators with Self-Sensing Resistive Bending Response Based on Ternary Polymer Composites.

A multifunctional polymer-based composite has been designed based on poly(vinylidene fluoride) (PVDF) as polymer matrix and cobalt ferrite (CoFe2O4, CFO) and multiwalled carbon nanotubes (MWCNTs) as fillers, allowing to combine magnetic and electrical responses. The composites were prepared by solvent casting with a fixed 20 wt % concentration of CFO and varying the MWCNTs content between 0 and 3 wt %, allowing to tailor the electrical behavior. The morphology, polymer phase, and thermal and magnetic properties are nearly independent of the MWCNT filler content within the polymer matrix. On the other hand, the mechanical and electrical properties strongly depend on the MWCNT content and a maximum d.c. electrical conductivity value of 4 × 10-4 S·cm-1 has been obtained for the 20 wt %CFO-3 wt %MWCNT/PVDF sample, which is accompanied by an 11.1 emu·g-1 magnetization. The suitability of this composite for magnetic actuators with self-sensing strain characteristics is demonstrated with excellent response and reproducibility.

Polymer actuation using a Joule-heating-induced ferroelectric phase transition.

Polymer actuation using a Joule-heating-induced ferroelectric phase transition.

A TEC Cooling Soft Robot Driven by Twisted String Actuators.

Similar to biological muscles in nature, artificial muscles have unique advantages for driving bionic robots. However, there is still a large gap between the performance of existing artificial muscles and biological muscles. Twisted polymer actuators (TPAs) convert rotary motion from torsional to linear motion. TPAs are known for their high energy efficiency and large linear strain and stress outputs. A simple, lightweight, low-cost, self-sensing robot powered using a TPA and cooled using a thermoelectric cooler (TEC) was proposed in this study. Because TPA burns easily at high temperatures, traditional soft robots driven by TPAs have low movement frequencies. In this study, a temperature sensor and TEC were combined to develop a closed-loop temperature control system to ensure that the internal temperature of the robot was 5 °C to cool the TPAs quickly. The robot could move at a frequency of 1 Hz. Moreover, a self-sensing soft robot was proposed based on the TPA contraction length and resistance. When the motion frequency was 0.01 Hz, the TPA had good self-sensing ability and the root-mean-square error of the angle of the soft robot was less than 3.89% of the measurement amplitude. This study not only proposed a new cooling method for improving the motion frequency of soft robots but also verified the autokinetic performance of the TPAs.

Actuation Behavior of Hydraulically Amplified Self-Healing Electrostatic (HASEL) Actuator via Dimensional Analysis

Electroactive polymer (EAP) actuators are an example of a novel soft material device that can be used for several applications including artificial muscles and lenses. The field of EAPs can be broken down into a few fields; however, the field that will be discussed in this study is that of Soft Electrohydraulic (SEH or EH) actuators. The device that will specifically be studied is the Hydraulically Amplified Self-Healing Electrostatic (HASEL) actuator. The design of the HASEL actuator is simple. There are two compliant films that house a dielectric liquid, and with the application of a voltage potential, there is an output displacement and force. However, the actuation mechanism is more complex, thus there is a need to understand theoretically and experimentally how the actuator works. This study analytically describes the electrode closure and the experimental testing of the actuators. Then, dimensional analysis techniques are used to determine what factors are contributing to the function of the actuator. For this study, eight dimensionless Π groups were found based on the derived analytical equation. These Π groups were determined based on the input voltage, density, viscosity, and elastic modulus of the materials; these were chosen because of their major contribution to the experimental data. The Π groups that are of particular importance are related to the characteristic length, which is directly related to the displacement of the fluid, the fluid velocity, the fluid pressure, and the dielectric constant. From this study, relationships between the output force, the electrostatic contributions, and other parameters were determined. All in all, this type of analysis can provide guidance on the development of high-performance HASEL actuators.

Fully Room Temperature Reprogrammable, Recyclable, and Photomobile Soft Actuators from Physically Cross-Linked Main-Chain Azobenzene Liquid Crystalline Polymers.

Fully room temperature three-dimensional (3D) shape-reprogrammable, recyclable, and photomobile azobenzene (azo) polymer actuators hold much promise in many photoactuating applications, but their development is challenging. Herein, we report on the efficient synthesis of a series of main-chain azo liquid crystalline polymers (LCPs) with such performances via Michael addition polymerization. They have both ester groups and two kinds of hydrogen bond-forming groups (i.e., amide and secondary amino groups) and different flexible spacer length in the backbones. Such poly(ester-amide-secondary amine)s (PEAsAs) show low glass transition temperatures (Tg ≤ 18.4 °C), highly ordered smectic liquid crystalline phases, and reversible photoresponsivity. Their uniaxially oriented fibers fabricated via the melt spinning method exhibit good mechanical strength and photoinduced reversible bending/unbending and large stress at room temperature, which are largely influenced by the flexible spacer length of the polymers. Importantly, all these fibers can be easily reprogrammed under strain at 25 °C into stable fiber springs capable of showing a totally different photomobile mode (i.e., unwinding/winding), mainly owing to the presence of low Tg and both dynamic hydrogen bonding and stable crystalline domains (induced by the uniaxial drawing during the fiber formation). They can also be recycled from a solution at 25 °C. This work not only presents the first azo LCPs with 3D shape reprogrammability, recyclability, and photomobility at room temperature, but also provides some important knowledge of their structure-property relationship, which is useful for designing more advanced photodeformable azo polymers.

Modular Design of a Polymer‐Bilayer‐Based Mechanically Compliant Worm‐Like Robot (Adv. Mater. 18/2023)

Soft Robotics Wormlike robots can navigate through uneven terrain and constricted spaces, but it has proved difficult to completely avoid rigid components, limiting the compliance of such machines. In article 2210409, Hillel J. Chiel, Roger D. Quinn, Christoph Weder, Kathryn A. Daltorio, and co-workers report an earthworm-inspired robot that is based on soft polymer actuators. The robot moves through peristaltic locomotion and can wiggle itself through small openings.

Teleoperation of Soft Robots with Real‐Time Fingertip Haptic Feedback Using Small Batteries

AbstractSoft robots are promising candidates for robust grasping and manipulation of different types of objects; however, their development has been a scientific challenge because of the lack of compact, yet precise, controllable soft actuators. In this work, innovative haptic interactive systems based on sensor‐integrated actuators powered by small batteries are reported. A liquid‐metal strain sensor with a versatile form factor and robust sensing performance is incorporated into a fast‐switching high‐force ionic polymer actuator. The structure of the sensor‐integrated actuator is rationally designed through encapsulation technology to achieve the best sensitivity and actuation while maintaining compactness, allowing for accurate, real‐time estimation and tracking of the actuation process. In a closed loop between the user and sensor‐integrated actuator, the motion and manipulation dynamics of a soft gripper made of sensor‐integrated actuators can be controlled by hand motions during a task. By combining with a thimble‐type haptic feedback device, the states of the gripper can be transmitted back to the user in real‐time via the proprioception of the integrated sensor. The approach opens a prospective avenue for the development of future robotic technologies without requiring camera‐based sensors for versatile, wearable, and scalable haptic systems.

High-performance ionic polymer actuators with Triple-layered multifunctional electrodes

Ionic polymer actuators have attracted attention in recent years due to their remarkably large strain under low-voltage stimulation. However, a key challenge for fabricating high-performance ionic polymer actuators is to develop a firm electrode with high electrical conductivity and electrochemical performance. Herein, inspired by the structure of zoysia grass, we proposed an efficient method to prepare the triple-layered multifunctional electrodes capable of low surface resistance (∼3.4 Ω/□), high adhesion (∼328 KPa) and good electrochemical performance for ionic polymer actuators. Through the spraying and impregnation-electroplating (IEP), three functional layers of multifunctional electrodes are obtained, including MWCNTs/Nafion interfacial layer, Ag NWs fixed layer and Ag NWs@Au layer, which respectively contribute to the charge storage in the electrode, high strength and the uniform and fast charge distribution. The synergic effect of these three functional layers maintains a high stability of multifunctional electrodes after being immersed in DI water for 7 days or 1500 bending cycles, and greatly enhances the actuation performance of ionic polymer actuators, resulting in an extreme bending deformation (increasing by 460%). This work represents an important step towards ionic polymer actuators where the synergic effect in electrodes plays an important role in promoting electrochemical actuation performance.

Active Damping, Vibration Isolation, and Shape Control of Space Structures: A Tutorial

This tutorial reviews the author’s contributions to the active control of precision space structures over the past 35 years. It is based on the Santini lecture presented at the IAC-2022 Astronautical Congress in Paris in September 2022. The first part is devoted to the active damping of space trusses with an emphasis on robustness. Guaranteed stability is achieved by using decentralized collocated actuator–sensor pairs. The so-called integral force feedback (IFF) is simple, robust, and effective, and the performances can be predicted easily with simple formulae based on modal analyses. These predictions have been confirmed by numerous experiments. The damping strategy for trusses has been extended to cable structures, and also confirmed experimentally. The second part addresses the problem of vibration isolation: isolating a sensitive payload from the vibration induced by the spacecraft (i.e., the unbalanced mass of attitude control reaction wheels and gyros). A six-axis isolator based on a Gough–Stewart platform is discussed; once again, the approach emphasizes robustness. Two different solutions are presented: The first one (active isolation) uses a decentralized controller with collocated pairs of the actuator and force sensor, with IFF control. It is demonstrated that this special implementation of the skyhook, unlike the classical one, has guaranteed stability, even if the two substructures it connects are flexible (typical of large space structures). A second approach (passive) discusses an electromagnetic implementation of the relaxation isolator where the classical dash-pot of the linear damper is substituted by a Maxwell unit, leading to an asymptotic decay rate of −40 dB/decade, similar to the skyhook (although much simpler in terms of electronics). The third part of the lecture summarizes more recent work done on the control of flexible mirrors: (i) flat mirrors for adaptive optics (AO) controlled by an array of piezoelectric ceramic (PZT) actuators and (ii) spherical thin shell polymer reflectors controlled by an array of piezoelectric polymer actuators (PVDF-TrFE) aimed at being deployed in space.

Coupled Chemo-Mechanical Swelling Behavior of PH-Sensitive Hollow Cylinder Hydrogels under Extension–Torsion and Internal Pressure: Analytical and 3D FEM Solutions

The coupled transient chemo-mechanical behavior as well as the large deformation behavior under various complex load conditions must be taken into account when designing a functional responsive polymer actuator or sensor. One sort of deformation that can be used to characterize the properties of materials with complicated behavior, like soft hydrogels, is coupled extension and torsion with internal pressure. It is important to thoroughly research the complex kinetics of pH-hydrogels with coupled diffusion and massive deformation behavior. The transient behavior of cylindrical hydrogels under coupled extension–torsion with internal pressure under indifferent conditions is proposed in this work using a reliable semi-analytical method. In this regard, an analytical solution is offered to inspect this problem, which is used as a common experimental methodology for the characterization and modeling of polymeric materials. The results show that the rate of deformation and the physical characteristics of the material have a substantial impact on the cylindrical hydrogel’s transient behavior under coupled extension–torsion and internal pressure. For the same problem, a 3D finite element study was done to assess the analytical solution. The accuracy of our method is supported by the results’ agreement in both the FE analysis and the proposed approach. However, offering such a solution for this complex problem is of tremendous relevance given the significantly cheaper computational cost of analytical methods when compared to FEM. Additionally, the calculations indicate a complex reaction force and moment because the hydrogel experiences nonlinear Poynting-type effects in this deformation domain. The suggested semi-analytical procedure’s resilience behavior is demonstrated by the visualization of the effects of various material properties. This method can be used to calibrate constitutive models and to develop and improve hydrogel structures.

Free and Forced Vibration Behaviors of Magnetodielectric Effect in Magnetorheological Elastomers

This paper is concerned with the free and forced vibration responses of a magneto/electroactive dielectric elastomer, emphasizing the chaotic phenomena. The dielectric elastomers under external magnetic and electrical excitations undergo large elastic deformation. The magnetodielectric elastomer is modeled based on the Gent–Gent strain energy function to incorporate the influence of the second invariant and the strain stiffening. The viscoelasticity of the active polymer is also considered in the form of Rayleigh’s dissipation function. The equation of motion is governed with the aid of the Lagrangian equation in terms of a physical quantity, namely, the stretch of the elastomer. An energy-based approach is utilized to re-evaluate the static and DC voltage instabilities of the resonator. Time-stretch response (time history behavior), phase plane diagram, Poincaré map, and fast Fourier transform are numerically obtained and presented to explore the chaotic oscillation behavior of the active polymer actuators. The results reveal that the magnetic field may tune the stability and instability regions of the active polymeric membrane. It has also been shown that the applied magnetic field may lead to chaotic vibration responses when a sinusoidal voltage is applied simultaneously to the system. The results presented in this paper can be effectively used to design magnetic and electrical soft robotic actuators and elastomer membranes under electrical and magnetic stimulants.