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Electro-Active Polymers (EAPs), also termed artificial muscles due to their operational similarity to biological muscles, refer to a class of materials that has received much attention in the last ten years, owing to the unique set of promising characteristics such large strains in response to an electric stimulus, low density, ease of processing and good mechanical properties. Generally, EAPs are divided into two major categories based on their activation mechanism: electronic (driven by electric field or Coulomb forces) and ionic (involving mobility or diffusion of ions). The electronic polymers, such as electrostrictive, electrostatic, piezoelectric, and ferroelectric require high activation field close to the breakdown level. In contrast, ionic EAP materials, such as gels, polymer-metal composites, conductive polymers, and carbon nanotubes require low driving voltage. However, there is a need to maintain their wetness, and except for conductive polymers it is difficult to sustain dc-induced displacements. The induced displacement of both the electronic and ionic EAP can be designed geometrically to bend, stretch, or contract.
Zhang et al. at the Pennsylvania State University has observed an exceptionally high electrostrictive response in electron-irradiated poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer.2 It is the electric-field-induced change between nonpolar and polar regions that is responsible for the large electrostriction observed in this polymer. As large as 4% electrostrictive strains can be achieved at low frequency drive fields having amplitudes of about 150 V/μm. However, the electron-irradiation process is difficult for mass production.
Dielectric elastomer based EAPs have been developed for the past 10 years.3,4,5 Dielectric elastomer transducers are rubbery polymer materials with compliant electrodes that have a large electromechanical response to an applied electric field. The induced strain is proportional to the square of electric field, multiplied by the dielectric constant and inversely proportional to the elastic modulus. Use of polymers with high dielectric constants and application of high electric fields leads to large forces and strains. The deformation of the polymer film can be used in many ways to produce muscle-like linear actuation. A linear artificial muscle was made based on an acrylic-film double bow-tie actuator. An acrylic film rolled actuator was developed and applied to an insect-inspired legged robot. An insect-inspired flapping-wing robot was demonstrated by using four silicon bow-tie actuators.
A graft-elastomer EAP was developed at NASA Langley Research Center.6that exhibits not only large electric field-induced strain (4%) but also a relatively high mechanical modulus (560 MPa). This electrostrictive polymer consists of two components, a flexible backbone macromolecules and a grafted polymer that can form crystalline. By combining this graft elastomer with a piezoelectric polymer, an electrostrictive- piezoelectric multifunctional polymer blend system has been developed. The critical problem of these electronic EAPs is that they need high voltage for activation, which results in high electrical strain related difficulties, such as voltage breakdown shielding, packaging, miniaturization, and driver configuration in device implementations.
|Dielectric Elastomer EAP actuators|
Ionic polymer gels can be synthesized to produce strong actuators having the potential that matches or is comparable to the force and energy density of biological muscles. Calvert at the University of Arizona has made muscle-like actuators from bilayers of crosslinked polyacrylamide and polyacrylic acid hydrogels sandwiched between electrodes.7 The polyacrylic acid responds to applied positive polarity field by contracting and expelling water which is taken up by the polyacrylamide layer. When activated, these gels bend as the cathode side becomes more alkaline and the anode side more acidic. However, the response of this multilayered gel structure is relatively slow because of the need to diffuse ions through the gel. Nonionic polymer gels containing a dielectric solvent can be made to swell under a dc electric field with a significant strain.
Ionomeric polymer-metal composites (IPMC) is a well-known EAP that bends in response to an electrical activation as a result of mobility of cations in the polymer network.8,9 A relatively low voltage is required to stimulate bending in IPMC, whereas the base polymer provides channels for mobility of positive ions in a fixed network of negative ions on interconneted clusters. However, the slow response and the need of wetness restricts the applications. By utilizing IPMC, dust wipers, grippers, swimming fish and a catheter tube have been demonstrated.
|IPMC EAPap Actuator|
Conducting polymers(CP) typically function via the reversible counter-ion insertion and expulsion that occurs during redox cycling. Most studies to date have investigated the contractile properties of either two CPs, polypyrrole or polyaniline. Madden has investigated the dimensional changes in the CP polypyrrole.10 Strains up to 6 %, strain rates of 4%/s, power to mass ratios of 40 W/kg and forces of up to 34 Pa are achieved. CP actuators based on trilayer have been made by many research groups in Japan, USAand some other countries. Researchers at the Universityof Pisain Italy reported the construction and characterization of a linear actuator prototype made of PANi fibers, a solid polymer electrolyte, and a spiral-shaped copper wire as counter-electrode. Artificial Muscle Research Institute of the University of Mexico is developing a new technique that electrically activates PAN fibers. Linkopings University in Sweden developed a microrobotic arm with individual controllable hinges for an elbow, a wrist and 2-4 fingers by using CP actuators.11 With the microrobotic arm, they successfully moved 0.1 mm glass beads over the surface crossing a distance of 0.25 mm.
Single-walled carbon nanotubes (SWNTs) were shown to generate higher stresses than natural muscle and higher strains than high-modulus ferroelectrics.12Like natural muscles, the macroscopic actuators are assemblies of billions of individual nanoscale actuators. Low operating voltages of a few volts generate large actuator strains. Ionic EAP actuators require drive voltage as low as 1-5 V. However, their response is slow and there is a need to maintain their wetness, and except for conductive polymers it is difficult to sustain dc-induced displacements.
|Microrobotic arm made with conducting polymer.|