Artificial Muscle Materials Research Articles

End-block-curing ABA triblock copolymer towards dielectric elastomers with both high electro-mechanical performance and excellent mechanical properties

Dielectric elastomers (DEs) featured with fast reversible and large deformation excited by an external electric field show promising applications as leading artificial muscle materials in many emerging technologies. It remains a great challenge to synthesize a DE simultaneously with high electromechanical performance (large actuation strain, high energy density, free of pre-stretch and bracing) and excellent mechanical properties (low Tg, low viscous loss, low modulus, and high mechanical strength). Herein, we propose and demonstrate that end-block-curing ABA triblock copolymer elastomer with well-designed structures is a new strategy to address the challenge by taking advantage of the microphase separation nature of the block copolymer. Particularly, poly(styrene-b-butyl acrylate-b-styrene) (SBAS) with post-curable end blocks, i.e. polystyrene (PS) blocks, are designed and synthesized. Different levels of cyclohex-3-enylmethyl acrylate (CEA) that contains two asymmetric vinyl groups with very different reactivity are incorporated into the end blocks for end-block curing while the molecular weight for each block is fixed. It is found that curing the end blocks enhances the strain-hardening effect during stretch and thus significantly increases the electro-mechanical performance. Without pre-stretch, the DE actuator of the end block cured SBAS exhibits a maximum actuation strain up to 72%, dielectric breakdown strength up to 154 kV mm−1, and energy density up to 270 kJ/m3, which are three, four, and almost fourty times that of the SBAS free of CEA, respectively. The maximum energy density is over 30 times that of mammalina skeletal muscle while the maximum actuation strain is 3.4 times. Interestingly, the improvement in eletromechanical performance is fulfilled with little increase in glass transition temperature (Tg), mild increase in modulus, and excellent tensile strength due to the microphase structures. The end-block-cured SBAS retains a low Tg of the rubbery phase around −35 °C, relatively low viscous loss of 0.2 at room temperature, a modulus lower than 1 MPa, and tensile strength around 3 MPa, which are attractive in terms of DE device applications.

Using force-displacement relations to obtain actuation parameters from artificial muscles

Many different test methods are currently used to characterise the output of artificial muscle materials but few studies report the full range of possible force and displacements that can be generated by a given material when activated with a given input stimulus but when operated against different external loads. The measurement of the loading and unloading force extension curves in tension in both the un-activated and activated states is investigated as a means for efficiently characterising the full range of outputs for three different types of artificial muscles: pneumatically operated braided muscle and thermally operated shape memory alloy spring and twisted / coiled polymer fiber. A graphical method of analysis was applied whereby the force-extension curves obtained before and after actuator activation were plotted on the same axes. By overlaying the external loading conditions, the graphical method provided the equilibrium starting and finishing forces and displacements and successfully predicted the isotonic strokes, isometric forces and combined force and displacement generated when the actuator was operated against an external spring. Complications in the interpretation of the force-stroke curves were encountered as all three artificial muscles displayed a degree of loading-unloading hysteresis and non-ideal mechanical behavior.

Bionic structure of shark’s gill jet orifice based on artificial muscle

Based on the biological prototype characteristics of shark’s gill jet orifice, the flexible driving characteristics of ionic exchange polymer metal composites (IPMC) artificial muscle materials and the use of sleeve flexible connector, the IPMC linear driving unit simulation model is built and the IPMC material-driving dynamic control structure of bionic gill unit is developed. Meanwhile, through the stress analysis of bionic gill plate and the motion simulation of bionic gill unit, it is verified that various dynamic control and active control of the jet orifice under the condition of different mainstream field velocities will be taken by using IPMC material-driving. Moreover, the large-deflection deformation of bionic gill plate under dynamic pressure and the comparative analysis with that of a rigid gill plate is studied, leading to the achievement of approximate revised modifier from real value to theoretical value of the displacement control of IPMC.

The Imitation of Muscles Stretching Device

This paper presents a review of some of the applications for artificial muscle and several material of artificial muscle. We focus attention on the polymer material artificial muscle, which responds to electrical stimulation with a significant change in shape or size. Through our research on a variety of materials and the analysis of the mechanical properties of muscle movement, finally we designed the artificial muscle device the imitation of muscles stretching device. This article describes the structure and performance of the device.

Electrically Triggered Actuation of Plasticized Thermoplastic Polyurethane Gels

Thermoplastic polyurethanes (TPUs) are copolymers with hard and soft segments. They have excellent physical properties and good biocompatibility. In particular, their shape memory properties and compatibility with plasticizers make TPU gels suitable for use as soft actuator materials. In this work, a plasticizer, dibutyl adipate (DBA), is added to four types of TPU to form gels. All the TPUs form transparent soft gels with different DBA contents. Their dielectric and mechanical properties are significantly improved by DBA addition, and the TPUs are changed from insulators to electrical actuation materials. Polyether TPU/DBA gels show the most significant changes. The bending distance of a TPU/6DBA gel tip (dimensions 20 × 5 × 0.3 mm3) is about 15 mm when the gel is sandwiched between soft electrodes and an electric field of less than 3 V μm–1 is applied. The developed materials show large deformations at low energies, and will make a significant contribution to the field of actuation materials. It is believed that TPU/DBA gels can be used as photovoltaic, dielectric, artificial muscle, robotic, and other smart materials. image

Reverse adhesion of a gecko-inspired synthetic adhesive switched by an ion-exchange polymer-metal composite actuator.

Inspired by how geckos abduct, rotate, and adduct their setal foot toes to adhere to different surfaces, we have developed an artificial muscle material called ion-exchange polymer-metal composite (IPMC), which, as a synthetic adhesive, is capable of changing its adhesion properties. The synthetic adhesive was cast from a Si template through a sticky colloid precursor of poly(methylvinylsiloxane) (PMVS). The PMVS array of setal micropillars had a high density of pillars (3.8 × 10(3) pillars/mm(2)) with a mean diameter of 3 μm and a pore thickness of 10 μm. A graphene oxide monolayer containing Ag globular nanoparticles (GO/Ag NPs) with diameters of 5-30 nm was fabricated and doped in an ion-exchanging Nafion membrane to improve its carrier transfer, water-saving, and ion-exchange capabilities, which thus enhanced the electromechanical response of IPMC. After being attached to PMVS micropillars, IPMC was actuated by square wave inputs at 1.0, 1.5, or 2.0 V to bend back and forth, driving the micropillars to actively grip or release the surface. To determine the adhesion of the micropillars, the normal adsorption and desorption forces were measured as the IPMC drives the setal micropillars to grip and release, respectively. Adhesion results demonstrated that the normal adsorption forces were 5.54-, 14.20-, and 23.13-fold higher than the normal desorption forces under 1.0, 1.5, or 2.0 V, respectively. In addition, shear adhesion or friction increased by 98, 219, and 245%, respectively. Our new technique provides advanced design strategies for reversible gecko-inspired synthetic adhesives, which might be used for spiderman-like wall-climbing devices with unprecedented performance.

Electrical bending actuation of gold-films with nanotextured surfaces

An actuating material system comprising a gold-film with nanotextured surface was fabricated. Using electroless gold plating onto a substrate of porous anodized aluminum oxide, a thin film of gold with a high density of short gold nanofibers on its surface was made. When one end of such a film was connected to an ion generator, bending was achieved upon electrical charging in air. Experiments showed that the free end of an 8 mm film could be displaced by more than 1.6 mm with a bending strain of 0.08%. In contrast with other types of thin-film artificial muscle materials, the present Au-film did not require any electrolyte to function. With the relatively easy fabrication method, this nanotextured film shows promising actuation behavior in air.

Rotaxane-based molecular muscles.

CONSPECTUS: More than two decades of investigating the chemistry of bistable mechanically interlocked molecules (MIMs), such as rotaxanes and catenanes, has led to the advent of numerous molecular switches that express controlled translational or circumrotational movement on the nanoscale. Directed motion at this scale is an essential feature of many biomolecular assemblies known as molecular machines, which carry out essential life-sustaining functions of the cell. It follows that the use of bistable MIMs as artificial molecular machines (AMMs) has been long anticipated. This objective is rarely achieved, however, because of challenges associated with coupling the directed motions of mechanical switches with other systems on which they can perform work. A natural source of inspiration for designing AMMs is muscle tissue, since it is a material that relies on the hierarchical organization of molecular machines (myosin) and filaments (actin) to produce the force and motion that underpin locomotion, circulation, digestion, and many other essential life processes in humans and other animals. Muscle is characterized at both microscopic and macroscopic length scales by its ability to generate forces that vary the distance between two points at the expense of chemical energy. Artificial muscles that mimic this ability are highly sought for applications involving the transduction of mechanical energy. Rotaxane-based molecular switches are excellent candidates for artificial muscles because their architectures intrinsically possess movable filamentous molecular components. In this Account, we describe (i) the different types of rotaxane "molecular muscle" architectures that express contractile and extensile motion, (ii) the molecular recognition motifs and corresponding stimuli that have been used to actuate them, and (iii) the progress made on integrating and scaling up these motions for potential applications. We identify three types of rotaxane muscles, namely, "daisy chain", "press", and "cage" rotaxanes, and discuss their mechanical actuation driven by ions, pH, light, solvents, and redox stimuli. Different applications of these rotaxane-based molecular muscles are possible at various length scales. On a molecular level, they have been harnessed to create adjustable receptors and to control electronic communication between chemical species. On the mesoscale, they have been incorporated into artificial muscle materials that amplify their concerted motions and forces, making future applications at macroscopic length scales look feasible. We emphasize how rotaxanes constitute a remarkably versatile platform for directing force and motion, owing to the wide range of stimuli that can be used to actuate them and their diverse modes of mechanical switching as dictated by the stereochemistry of their mechanical bonds, that is, their mechanostereochemistry. We hope that this Account will serve as an exposition that sets the stage for new applications and materials that exploit the capabilities of rotaxanes to transduce mechanical energy and help in paving the path going forward to genuine AMMs.

Synthesis and Characterization of Vertically Aligned Carbon Nanotube Forest for Solid State Fiber Spinning

Continuous carbon nanotubes (CNT) fibers were directly spun from a vertically aligned CNT forest grown by a plasma-enhanced chemical vapor deposition (PECVD) process. The correlation of the CNT structure with Fe catalyst coarsening, reaction time, and the CNTs bundling phenomenon was investigated. We controlled the diameters and walls of the CNTs and minimized the amorphous carbon deposition on the CNTs for favorable bundling and spinning of the CNT fibers. The CNT fibers were fabricated with an as-grown vertically aligned CNT forest by a PECVD process using nanocatalyst an Al2O3 buffer layer, followed by a dry spinning process. Well-aligned CNT fibers were successfully manufactured using a dry spinning process and a surface tension-based densification process by ethanol. The mechanical properties were characterized for the CNT fibers spun from different lengths of a vertically aligned CNT forest. Highly oriented CNT fibers from the dry spinning process were characterized with high strength, high modulus, and high electrical as well as thermal conductivities for possible application as ultralight, highly strong structural materials. Examples of structural materials include space elevator cables, artificial muscle, and armor material, while multifunctional materials include E-textile, touch panels, biosensors, and super capacitors.

Preparation and experimental investigation of thermo-electro-mechanical behavior of Ag-IPMC actuator

Traditional ionic polymer metal composites (IPMCs) composed of Nafion as the base polymer and mostly precious metal platinum/gold as the surface electrode. The proposed work deal with fabrication and investigation of thermo-electromechanical properties of artificial muscles i.e. ionic polymer-metal composites of non-precious metal silver (Ag) as surface electrode. Chemical decomposition method is followed and in-depth fabrication procedures are outlined. The microscopic and morphological analyses show dense packed silver (Ag) particles on the surface of the Nafion membrane. Several experiments are carried out such as bending test for measuring tip force and surface resistance. Dynamic-mechanical analysis (DMA) and thermo-mechanical analysis (TMA) are performed to evaluate the viscoelastic properties and performance of the actuator. The developed pure Ag-IPMC is inexpensive, exhibits superior performance and can be used as artificial muscle materials.

Electro-mechanical and thermal characteristics of silver-electroded ionic polymer–metal composite actuator

The proposed work is in line with the evaluation of electro-mechanical and thermal characteristics of silver-electroded ionic polymer–metal composite (IPMC). IPMCs are fabricated first using Nafion-117 as base polymer and non-precious metal silver as surface electrode by chemical decomposition method. Several testings are performed on fabricated IPMC to evaluate its thermo-mechanical and micro-structural properties. The characteristics of the electrode layer and deposited particles on IPMC surface are studied using scanning electron microscope. The bending experiment of the actuator is conducted by applying direct current potential and the tip displacement measured. Thermo-gravimetric analysis and differential scanning calorimetry test are carried out, and thermal stability of the actuator is investigated. The crystal structure of IPMC is investigated by X-ray diffraction analysis. Micro-tensile test of the specimen is carried out to ascertain the stress–strain relationship and comparison is made with the base polymer, Nafion. The experimental investigations, characterization, and performance of the IPMC demonstrate its effectiveness to be used as actuator and artificial muscle materials.

Fabrication and Thermo-Mechanical Analysis of Pure Silver-Electrode Ionic Polymer-Metal Composite (IPMC) Actuator

Till to date, fabrication of Ionic Polymer-Metal Composites (IPMC) are carried out successfully using noble metal such as platinum/gold as the surface electrode. In this work we have proposed cost effective fabrication method for IPMC actuator using non-precious metal electrode of silver (Ag). Chemical decomposition method is followed using Nafion as the ion exchange membrane to fabricate pure Ag-electrode IPMC. Microscopic and morphological analyses reveal that, silver particles penetrate well through the surface of Nafion membrane. The bending deformation measurement and analysis of the thermo-mechanical properties of the fabricated IPMC is carried out. The experiment results and performance of the IPMC actuator confirm that the fabrication of pure Ag-IPMC is feasible and can be used as artificial muscle material.

External stimulus driven variable-step grating in a nematic elastomer

We report on the creation of micro-patterns in an oriented nematic elastomer (an artificial muscle material) by photopolymerization of surface aligned nematic liquid crystal monomers. We demonstrate that microscopic techniques are able to create accurate patterns in rubber-like liquid crystal materials. Two approaches, based on one and two-photon excitations respectively, are implemented using a microscope-based setup. Due to its high spatial selectivity, the two-photon excitation mode yields finer patterns. Benefitting from the intrinsic, thermally-induced, contractile properties of the material, gratings with variable steps in response to temperature changes were fabricated.

Voltage-switchable artificial muscles actuating at near neutral pH

Novel artificial muscle materials were developed, which can be electrically actuated at near neutral pH environments by low applied potential (3V or less). The artificial muscle material is based on a hydrogel that is composed of acrylamide and a carboxylic acid derivative, such as acrylic acid, and is doped with a polypyrrole/carbon black composite. The electroactuation response of the artificial muscle is optimized in terms of the constituents of the hydrogel precursor solution and the polymerization conditions. It is shown that the introduction of the dicarboxylic acid monomer maleic acid increases the osmotic pressure arising at the hydrogel/electrolyte interface, increasing the bending response of the material. Additionally, the increase of the polymerization temperature and the use of the crosslinking agent N,N′-methylenebisacrylamide, which promotes intermolecular interactions within the hydrogel network, result in the development of artificial muscles with increased and reproducible electroactuation.

Artificial Muscle Material with Fast Electroactuation under Neutral pH Conditions

A novel artificial muscle material based on an acrylic acid/acrylamide hydrogel blended with a conductive polypyrrole/carbon black composite was prepared. The material was optimized in terms of its electroactuation response by varying the acrylic acid content, the blending concentration of the conductive composite, and the intensity of the electric field. The artificial muscle material showed a fast and reversible electroactuation when a low potential was applied. Further, the artificial muscle was incorporated into a microfabricated pumping device that exhibited reproducible flow rates, thus demonstrating its potential usefulness in bio-micro-electro−mechanical systems (BioMEMS) applications.

Ultra-High Strain Response of Elastomeric Polymer Dielectrics

AbstractThe strain response of dielectric elastomers sandwiched between compliant electrodes was studied. These electroactive polymer artificial muscle (EPAM) materials show excellent overall performance and appear more attractive than many competing actuator technologies. Based on the available data, the actuation mechanism is due to the free charge interaction of the compliant electrodes, enhanced by the dielectric properties of the elastomer (Maxwell stress). Strains over 200%, actuation pressures up to 8 MPa, and energy densities up to 3.4 J/cm3have been demonstrated with silicone rubber and acrylic elastomers. Response time is rapid, and the potential efficiency is high. The fabrication of EPAM actuators can be simple and low cost. A wide range of small devices have been made, to demonstrate the potential of the technology and reveal more about performance and fabrication issues. These devices include bending beam actuators for scanners and clamps, diaphragm actuators for pumps and valves, stretched-film actuators for electro-optics, and bow actuators for muscle-like actuators for small robots and other micro machines.