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Scientific background

Electromechanically Active Polymer (EAP) transducers & artificial muscles

The term ‘artificial muscles’ is today broadly used for several types of smart materials, actuation devices and systems, used to generate motion for a variety of uses, which might be grouped as follows:

1) Materials and/or devices with muscle-like structural or functional properties. These can be key enablers of applications in many disciplines, including mechatronics, robotics, automation, medicine, biomimetics, haptics, biotechnology, fluidics, optics and acoustics. Specific examples of uses include compliant and light-weight drive mechanisms, intrinsically safe robots, anthropomorphic robots and humanoids, bioinspired and biomimetic systems, robotic hands/arms/legs/wings/fins, locomotion systems, grippers, manipulators, haptic devices, variable-stiffness devices and linkages, active vibration dampers, minimally-invasive interventional/diagnostic medical tools, mechanical stimulators for cells and tissues, controlled drug delivery devices, fluidic valves and pumps, tunable optical and acoustic systems, as well as systems to convert mechanical energy into electrical energy for sensing and energy harvesting.

2) Functional or physical substitutes or supports for natural muscles. These can be used for prosthetics, orthotics, and artificial organs. Specific examples of applications include artificial skeletal, smooth, and cardiac muscles, artificial hearts and blood vessels, ventricular assist devices, artificial bladders and sphincters, prosthetic hands/arms/legs and articular joints, powered orthoses, exoskeletons and augmenting systems, and wearable systems for motor rehabilitation and personal assistance.

All of these fields, though very different, share today a common need for new actuation materials and devices with large mechanical compliance, effective down-scalability, high power-to-weight and power-to-volume ratios and high efficiency, typically precluded to conventional actuation technologies (namely electrostatic, electromagnetic, hydraulic, pneumatic and thermo-chemical motors).

Aimed at satisfying such requirements, new types of electromechanical transducers based on so-called Electromechanically Active Polymers (EAPs) represent today a well established and promising scientific field of research and development [1-5]. EAPs consist of materials capable of changing dimensions and/or shape in response to opportune electrical stimuli. EAP based actuators show useful properties, such as sizable active strains and/or stresses in response to electrical stimuli, high mechanical flexibility (compliance), light-weight, structural simplicity and versatility, ease of material processing, scalability, no acoustic noise, no generation of heat, and, in most cases, low costs [1-5].

EAPs are commonly classified in two major families: ionic EAPs (activated by an electrically-induced transport of ions and/or molecules) and electronic EAPs (activated by electrostatic forces) [1-5]. Table 1 presents such a classification, reporting for each group the most relevant types, as well as a fundamental reference.

 

Table 1. EAP classification

EAP class

Materials

Ref.

Ionic EAPs

Polymer gels (PG)

[6]

Ionic polymer-metal composites (IPMC)

[7]

Conjugated polymers (CP)

[8]

Carbon nanotubes (CNT)

[9]

Electronic EAPs

Piezoelectric polymers (PP)

[10]

Electrostrictive polymers (EP)

[11]

Dielectric elastomers (DE)

[12]

Liquid crystal elastomers (LCE)

[13]

Carbon nanotube aerogels

[14]

 

Each EAP category shows today specific electromechanical properties, typically suitable for different needs.

Although many EAP materials have been known since many years, they have found very limited applications until a few years ago, despite their greater potential, due to a scarce technological development. Such a trend has changed in the last few years, owing to a higher concentration of efforts for real exploitations. As a result, after several years of basic research, today the EAP field is just starting to undergo transition from academia into commercialization, with large industrial companies starting to invest in this technology.

Within such a context, the EuroEAP community is aimed at contributing to the development of the EAP field in this crucial phase.

 

References


[1] P. Brochu, Q. Pei, Advances in dielectric elastomers for actuators and artificial muscles, Macromol Rapid Comm 31(1), 10-36, 2009.

[2] T. Mirfakhrai et al., Polymer artificial muscles, Mater Today 10(4), 30-38, 2007.

[3] J. Madden et al., Artificial muscle technology: physical principles and naval prospects, IEEE J. Oceanic Eng 29(3),706-728, 2004.

[4] Y. Bar-Cohen (Ed.), Electroactive polymer (EAP) actuators as artificial muscles, SPIE, 2004.

[5] F. Carpi and E. Smela (Ed.), Biomedical applications of electroactive polymer actuators, Wiley, 2009.

[6] T. Tanaka et al. Collapse of gels in an electric field, Sci. 218, 467-469, 1982.

[7] K. Asaka et al., Bending of polyelectrolyte membrane-platinum composites by electric stimuli Polym. J. 27(4), 436-440, 1995.

[8] R. H. Baughman, Conducting polymer artificial muscles, Synth. Met. 78, 339-353, 1996.

[9] R. H. Baughman et al., Carbon nanotube actuators, Sci. 284, 1340, 1999.

[10] H. S. Nalwa, Ferroelectric Polymers, Marcel Dekker, 1995.

[11] Q. M. Zhang et al., Giant electrostriction and relaxor ferroelectric behaviour in electron-irradiated poly(vinylidene fluoride-trifluoroethylene) copolymer, Sci. 280, 2101-2103, 1998.

[12] R. Pelrine et al., High-speed electrically actuated elastomers with strain greater than 100%, Sci. 287, 836-839, 2000.

[13] W. Lehmann et al., Giant lateral electrostriction in ferroelectric liquid-crystalline elastomers, Nat. 410, 447-450, 2001.

[14] A. Aliev et al., Giant-Stroke, Superelastic Carbon Nanotube Aerogel Muscles, Sci. 323, 1575-1578, 2009.