Mechanics of Soft Materials (2019) 1:5

/Published online: 22 March 2019

REVIEW PAPER

Electroactive polymer (EAP) actuators—background review

Yoseph Bar-Cohen1& Iain A. Anderson2,3

Received: 2 January 2019 /Accepted: 11 February 2019# Springer Nature Switzerland AG 2019

AbstractCertain polymers can be excited by electric, chemical, pneumatic, optical, or magnetic field to change their shape or size. Forconvenience and practical actuation, using electrical excitation is the most attractive stimulation method and the related materialsare known as electroactive polymers (EAP) and artificial muscles. One of the attractive applications that are considered for EAPmaterials is biologically inspired capabilities, i.e., biomimetics, and successes have been reported that previously were consideredscience fiction concepts. Today, there are many known EAP materials. Some of the EAP materials also exhibit the reverse effectof converting mechanical strain to electrical signal allowing using them as sensors and energy harvesters. Efforts are madeworldwide to turn EAP materials to actuators-of-choice and they involve developing their scientific and engineering foundationsincluding the understanding of their operation principles. These are also involve developing effective computational chemistrymodels, comprehensive material science, and electro-mechanics analytical tools. These efforts have been leading to betterunderstanding the parameters that control their capability and durability. Moreover, effective processing techniques are devel-oped for their fabrication, shaping, electroding, and characterization. While progress have been reported in the research anddevelopment of all the types of EAP materials, the trend in recent years has been growing towards significant development inusing dielectric elastomers.

Keywords EAP . Electroactive polymers . Activatable polymers . Biologically inspired technologies . Biomimetics . Robotics

1 Introduction

Manipulation, mobility, and activation of engineered mechanisms and systems are done by actuators, typically consistingof motors, gearboxes, and associated mechanisms. These are hard, heavy, and noisy in contrast to muscles of biologicalsystems, which are soft, light weight, and quiet. However, living muscles are driven by a complex microscopic ionicmolecular linear motor mechanism that is very difficult to mimic and currently impossible to manufacture. We can emulatemuscle action in a soft mechanism using certain polymers that can be stimulated by electric, chemical, pneumatic, optical,or magnetic field, to cause shape or size change. Polymer actuation using electricity is convenient and practical. Suchelectroactive polymers (EAP) are among the closest to emulate biological muscles [7]. As polymers, they have manyadvantages including mechanical flexibility, low density, as well as being easy to process, and mass produce. In addition totheir mechanical response to electricity, some of the EAP materials also exhibit the ability to sense mechanical strain and toharvest electrical energy from it (e.g., [3, 27, 28, 75, 118, 119, 121]).

* Yoseph Bar-Cohenyosi@jpl.nasa.gov; http://ndeaa.jpl.nasa.gov

Iain A. Andersoni.anderson@auckland.ac.nz

1 Jet Propulsion Laboratory/California Institute of technology, 4800 Oak Grove Drive, M/S 67-119, Pasadena, CA 91109, USA2 Biomimetics Laboratory, Auckland Bioengineering Institute, The University of Auckland, Level 6, 70 Symonds, St., Auckland, New Zealand3 Department of Engineering Science, The University of Auckland, Auckland, New Zealand

https://doi.org/10.1007/s42558-019-0005-1

These characteristics are making them highly attractive for use in muscle-like actuators. Some are biologically inspired (i.e.,biomimetic applications) [8, 9, 11], and all can function without hard metallic gears and mechanisms. Examples of the applica-tions of EAP actuators include a polypyrrole fish [71], ionic polymer–metal composite robot fish [23], miniature dielectricelastomer grippers for satellites [5], ionic conductor loudspeakers [51], an electrostrictive polymer catheter [35], a haptic interface[32], active braille displays [9], a fish-like blimp [45], static electric rotary motors [3], worm-like robots [46], a crawling robotwith no hard electronics [42], facial animatronic devices [11], optical devices [107], biomedical devices, microfluidic devices,and even a wearable device to assist eyelid blinking [103]. The impressive improvements in the field [52, 68] are increasinglyattracting the interest of engineers and scientists from many different disciplines.

Many EAP actuators are still emerging and need further advancement in order for them to form part of mass-producedproducts. This requires the use of computational chemistry models, comprehensive material science, electro-mechanic analyticaltools, and material processing research. To maximize their actuation capability and durability, effective fabrication, shaping, andelectroding techniques are being developed. In addition, techniques of characterizing their response as well as documenting themin databases and related standards [21] are being established. Engineers are continually seeking to find niche critical applicationsfor these materials to enable them to see daily use as part of mass-produced products.

2 The development of EAP actuator mechanisms—–historical overview

The first to conduct a documented experiment with EAP materials was [97], who demonstrated electroactive strains in a rubbersheet by spraying electric charge onto it. The spraying of electric charge onto a dielectric was more recently repeated by [51], whodemonstrated how this electrode-free method could be used for eliciting large strains in a simple actuator without sufferingdielectric breakdown.

[100] is credited as the first to formulate the effect of the strain response to electric field activation in polymers. [30] discoveryof the piezoelectric polymer called electret is the next important milestone in the field of EAP. He produced the material usingrosin (carnauba wax) and beeswax that was solidified by cooling while being subjected to a DC bias field. Electrets are electro-active where they deform under electric field and produce electric field when deformed. Since the electric field generates quitesmall strain in electrets, their application has been limited to sensors.

The responsive gels were pioneered with the development by Katchalsky and his co-investigators in Israel [47]. They reportedchemo-mechanical activation in gel polymers causing shrinkage or swelling in the presence of acid or alkaline, respectively.Important milestone studies of responsive gels and their electro-chemical activation have taken place at the Hokkaido University,Japan [83]. The developed gel polymers were demonstrated to create large strain under relatively low activation voltage [82].

The discovery of electrets was followed with Fukada’s work on piezoelectric biopolymers [33] and Kawai’s discovery ofsignificant piezoelectric activity in polyvinylidene fluoride (PVDF) [48]. The investigations of PVDF and its copolymers haveshown strong electromechanical activity in certain noncrystalline polymers with very large dielectric relaxations resulting fromorientation of their molecular dipoles (e.g., [49, 120]). The development of PVDF as an electroactive polymer was followed withextensive search for other polymer systems that exhibit significant response. Mostly during the 1970s and 1980s, extensiveinvestigations related to PVDF have taken place in efforts to improve the performance and seeking applications [34]. The limitedstrain that PVDF produces led to its use mostly as sensors and ultrasonic wave transducers [13].

Successes in developing effective new EAPmaterials were reported mostly in the 1990s and examples include [14, 16, 65, 76,78, 128]. The use of stretchable electrodes to place electrical charge on the surface of a rubbery dielectric, an invention byworkersat SRI, led to the creation of the dielectric elastomer (DE) actuator. These workers demonstrated strains that exceeded 100%witha relatively fast response speed (< 0.1 s) [89].

In 1995, the lead author started his research in the field of EAP and soon he determined that in order to acceleratedevelopment of EAP materials and lead to effective actuators it is critical to form worldwide cooperation. Therefore, heinitiated various forums for exchanging information including the SPIE annual EAP Actuators and Devices (EAPAD)Conference that started in March 1999 as part of the SPIE Smart Structures and Materials Symposium [6]. In 1999, the leadauthor posed an arm wrestling challenge (http://ndeaa.jpl.nasa.gov/nasa-nde/lommas/eap/EAP-armwrestling.htm) in aneffort to promote worldwide development towards realizing the potential of EAP materials. The challenge consists ofhaving an EAP activated robotic arm win against human in a wrestling match. An example of one of the arm wrestlingrobots is provided in Fig. 1.

Choosing to focus on arm wrestling with a human was done in order to emphasize that human muscle is a baseline forperformance comparison. Success in emulating human muscle will allow the use of EAP materials to improve many aspects of

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our life including the development of effective implants and prosthetics, active clothing, realistic biologically inspired robots aswell as fabricating products with unmatched capabilities and dexterity.

Today, many workshops, meetings, and conferences are taking place that are covering the subject of EAP. As informationarchive, the lead author created theWorldWide EAP (WW-EAP)Webhub [10] linking the growing number of related websites. Inaddition, in June 1999, the lead author started publishing the web-based semi-annual WW-EAP Newsletter that provides snap-shots of the advances in the field.

Mostly, EAP materials are still custom-made by researchers but there is a growing effort of various manufacturers to establishmass production methods and commercial products. In order to help making these materials widely available, the authorestablished a website that provides fabrication procedures for the leading types of EAP materials and a website about sourcesfor obtaining these materials and samples [10].

3 The two EAP actuator groups

Based on the mechanism that activates EAP actuators, they can be divided into two major groups including ionic and field-activated EAP. The actuation process of ionic EAPmaterials is involved with diffusion of ions [7, 86]. In addition, the material isin film form containing electrolyte and is covered by two electrodes. Examples of these materials include conductive polymers,ionic polymer gels, polymer-metal composites, and carbon nanotubes. Their advantages include operation at low activationvoltage (1–2 V) and generating large bending displacements. Disadvantages include the need to maintain electrolyte anddifficulty to sustain constant displacement under activation of a DC voltage (except for conductive polymers). In contrast tothe ionic EAP, the field-activated (electronic) EAP materials are driven by Coulomb forces and this can require high voltages (>10-V/μm) [7, 24]. This EAP group is stimulated by the electric field between the electrodes that are applied onto the polymer inthe form of a film. The field-activated type of EAP materials holds the activated displacement when operated by a DC voltage,which is a great benefit to many applications including robotics. In addition, they have higher mechanical energy density and theycan be activated in air with no constraints. However, they require high activation field that may be close to the level of dielectricbreakdown level. A summary of the advantages and disadvantages of the two EAP material groups is given in Table 1.

3.1 Field-activated EAP actuators

Two of the principal causes for electrostriction in an electric field polymer actuator are associated with (1) intrinsic field-inducedmolecular conformational changes to the elastomer (ferroelectric polymers) and (2) extrinsic electronic charge attraction-repulsion at the electrodes on the surfaces of the actuator. While both phenomena are present to a greater or lesser extent in allactuators of this kind, the dominant mode of actuation will be one or the other. Thus, they fall into two main types: ferroelectricpolymer actuators and dielectric elastomer actuators.

Fig. 1 An example of using dielectric elastomers in addressing the armwrestling challenge from EMPA. EAP robot that was on display at theSPIE EAPAD San Diego 2005, described in [60] (a). Schematic of a

spring roll actuator (rolls of DE augmented by helical spring) (b).Groups of these actuators operated antagonistically as depicted in (c).Courtesy of Gabor Kovacs, EMPA, Duebendorf, Switzerland

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3.1.1 Ferroelectric polymer actuators

Ferroelectric polymers are characterized by an electrical polarization that can be changed in an applied electric field [127]. Theexternal field will apply moments to polarized groups within the polymer. The most widely known ferroelectric polymer is thePoly(vinylidene fluoride), also known as PVDF or PVF2 [12, 34, 79, 104], and its copolymers [130].

These polymers have partial crystallinity with an inactive amorphous phase. In 1998, Zhang and his coinvestigators usedelectron radiation to introduce defects into the crystalline structure of the copolymer P(VDF-TrFE) and observed increase in thedielectric constant. The resulting material generates strains as large as 5% and levels of pressure of about 45 MPa under voltagesof about 150 V/μm. The drawback to the irradiation is the introduction of many undesirable defects and formation of cross-linking and chain scission [67]. By producing terpolymers via molecular design, the issue was addressed and the degree ofconformational changes at the molecular level was enhanced. The advantage of terpolymers is that they generate higher electro-mechanical response than the high energy electron irradiated copolymer [129, 130]. To increase this constant, a compositematerial was proposed, using filler that is made of a high dielectric constant (the 2001 edition of [7]). This approach wassuccessfully implemented by Zhang and his coinvestigators [129] who used an all-organic composite that consists of particulateshaving high dielectric constant (K > 10,000). Photographs of such a composite ferroelectric EAP in passive and activated statesare shown in Fig. 2.

Electrostrictive graft elastomers, another ferroelectric type, consist of a chain structure (i.e., backbone) with molecular pendantgroup that can alter alignment due to polarization from the external electric field. These polymers may consist of two compo-nents, a flexible backbone macromolecule and a grafted polymer made of a polarizable molecular or nanocrystalline structure.Subjecting such elastomers to large electric field was reported to produce about 4% strain and about 24 MPa stress [112, 113,129]. A combination of the electrostrictive-grafted elastomer with a piezoelectric poly(vinylidene fluoride-trifluoroethylene)copolymer allows producing various compositions of ferroelectric-electrostrictive molecular composite systems. Such combi-nations can be operated as piezoelectric sensor as well as electrostrictive actuator. The photographs in Fig. 3 show on the right anactivated grafted elastomer-based bimorph actuator while on the left is the EAP at the rest state.

Table 1 Summary of the advantages and disadvantages of the two major EAP groups

EAP type Advantages Disadvantages

Ionic EAP • Produces large bending displacements• Requires low voltage• Natural bi-directional actuation that depends

on the voltage polarity.

• Except for CPs and NTs, ionic EAPs do not hold strain under dc voltage• Slow response (fraction of a second)• Bending EAPs induce a relatively low actuation force• Except for CPs, it is difficult to produce a consistent material• In aqueous systems, ionic EAP suffer electrolysis at > 1.23 V• To operate in air requires attention to the electrolyte.• Low electromechanical coupling efficiency.

Field-activated EAP • Can operate in room conditions for a long time• Rapid response (msec levels)• Can hold strain under dc activation• Induces relatively large actuation forces

• Requires high field strength and this can result in high voltages.For dielectric elastomers fields of order of 150 V/μm for ~ 10% strain.Using composite DE allows for (~ 20 V/μm) [7, 24].

• The electrostriction dictates monopolar actuation that is independentof the voltage polarity.

Activated Rest stateFig. 2 Composite ferroelectric EAP in passive (right) and activated states (left). This EAP material was provided to the lead author as a courtesy ofQiming Zhang, Penn State University

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Recent developments in ferroelectric type actuators include a P(VDF-TrFE) for micropump applications [125].

3.1.2 Dielectric elastomer EAP actuators

Dielectric elastomers actuators are currently the subject of wide research interest. Their mechanism of actuation involves theplacing of electric charge on the surfaces a thin rubbery dielectric (Fig. 4). Earlier, we described the experiment of [97] whosprayed electric charge unto a rubber membrane and this resulted in a shape change to the membrane and how the charge sprayingmethod of Roentgen was repeated more recently by [50], demonstrating how large electroactive strains could be produced usingthis approach. The practical application to actuation became possible in the 1990’s when workers at SRI (Menlo Park Cal.)applied the surface charge using a flexible electrode material that coated the surface of thin dielectric materials [90]. Thesematerials included a stretchable acrylic and silicone rubber. Electroactive strains of greater than 100% were achieved [89]. Themechanism for actuation was surface electrostatics mathematically described as the Maxwell Pressure (Eq. 1) that has beenexperimentally validated by several researchers including ([56, 88]; and [123]):

σMaxwell ¼ ε0εrVd

� �2

ð1Þ

Fig. 3 An electrostrictive grafted elastomer-based bimorph actuator in its rest state (left) and in one of the two directions of the activated state (right).Courtesy of Ji Su, NASA LaRC, VA

Fig. 4 a Under electrical activation, a dielectric elastomer film withcompliant electrodes on both surfaces expands laterally whilecontracting in thickness. b An expanding dot dielectric elastomer

actuator, consisting of stretchable carbon electrodes sandwiching anacrylic dielectric [36]

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where the Maxwell pressure σMaxwell is calculated from the absolute and relative permittivities, the voltage V and the membranethickness, d. This simple equation that relates Maxwell stress to field strength is at the heart of the analysis of strain. However,given that the strains are large, the question of how much deformation strain will occur under a particular field strength will begoverned by non-linear material properties of elastomers. This complex issue is discussed in the analysis by [114]. Other workershave shown how extremely large actuation strains can be achieved through elimination of electromechanical instability (thisinstability is manifested through formation of wrinkles and dielectric breakdown) by lateral pre-stretch [57, 58].

Commercial uptake of DE is being preceded by a search for better materials for dielectric and electrode, ways of improvingperformance, and means of manufacturing [59]. For the past two decades, the common dielectric material for prototyping andtesting newDE concept actuators has been the 0.5 mm and 1-mm thick very high bond (VHB) acrylic tapes from 3M: VHB 4905and 4910 respectively. The use of VHB in laboratories for making DE actuator demonstrators is gradually being replaced bysilicone-based elastomers (polysiloxanes) [70]. The dielectric constants for off-the-shelf silicones are relatively low (2–3),roughly less than half some of the values measured for VHB [72]. However, these elastomers do not suffer the high viscoelasticityof VHB and, therefore, can demonstrate fast response with excellent reliability for millions of cycles. Actuators built fromsilicone elastomers are also less fragile than those built from VHB and can demonstrate vastly better reliability over time andmillions of cycles. Research is currently focused on producing silicone elastomers with high dielectric constant for high fieldwhile maintaining low modulus for good actuation and high dielectric strength. Two methods for doing this include blending inpolymers with high dielectric permittivity or using chemically modified silicones. The latter is achieved through grafting organicdipoles to the silicone elastomer backbone. For reviews of how polysiloxanes have been modified see [70]. In another morerecent review by [81], the opportunities from chemical modification to polysiloxanes as well as other polar elastomers arereviewed and clear directions have been put forward for workers in this area with the end goal of producing actuators that areoperational at very low voltages (~ 24 V).

For electrode materials, laboratory workers have used several forms of carbon particles suspended in silicone grease. This hasbeen relatively easy to apply and effective for fast prototyping of actuators but suffers the drawback of being messy and difficultto protect from damage. The review of DE electrodes by [98] describes many advances in this area that now include loose carbonpowder stamped onto the surface, surface printing a carbon in a fluid, formation of carbon/silicone composite electrodes, anddeposition of compliant thin-film metal electrodes. The latter can be made compliant by the formation of micro-wrinkles due tobuckling induced by DE compression or through metal deposition on a micro-corrugated silicone surface.

A number of DE actuator mechanisms have been demonstrated over the past 20 years [90]. They involve either in-planeexpansion of a stretched membrane (Fig. 5) with in-plane electrode expansion that can also lead to out-of-plane displace-ment with some biasing mechanism (air pressure, spring, etc.). On the other hand, out-of-plane contraction usually requires

Fig. 5 In-plane actuation example. The dielectric elastomer inverter, acombination of DE actuator and DE switch enables fundamental signalprocessing function upon generation of patterned signals. b Photographof Trevor the Caterpillar—containing no conventional electronic parts.Charge control is provided by interaction between DE actuators and

piezoresistive DE switches. b–d A traveling wave generated within therobot’s DE network is translated into a forwardmotion by its flexible legs,which simultaneously connect the robot to its driving voltage and thesurface [42]

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the stacking of many membranes in order to provide a large enough contraction (Fig. 6). Reviews of DE actuators can befound in ([59] [38] [3, 99. 17]) and the book edited by [20]. Some recent research interest includes the use of an encasedionic fluid and/or the fluid within which the DE robot swims [26, 63], non-linear springs coupled to DE for enhancedactuation [40, 41], lab-on-a-chip devices for peristaltic pumps [111], haptic feedback [91] and cell stretchers [92], opticalapplications (including tunable grating, lenses, laser speckle reducers, and tunable windows) [37, 93, 107], couplingactuators with piezoresistive switches [42], and hydraulically amplified self-healing electrostatic actuators (HASEL) [1].The latter use a contained fluid dielectric that is pumped out from between the electrode plates. Electrostatics can also becombined with DE actuation to enhance the ability to grip and handle objects [39, 106]. As an aside, the electrostaticmechanism has recently been combined with the principles of origami to produce a new type of high strength, highdisplacement ribbon actuator [116].

Sophisticated means for DE actuator fabrication are also under development that can be used for mass-production [59]. Theseinclude stack actuator fabrication from pre-existing commercially available dielectric membrane material [66], spin coating [64],and screen printing of the electrode [31].

3.2 Ionic EAP

3.2.1 Ionomeric polymer-metal composites

The ionomeric polymer–metal composites (IPMC) have been widely studied as an ionic EAP material [52]. In 1992, threedifferent groups of researchers independently reported the development of IPMC as an EAP material including [78] in Japan, aswell as [101, 105] in the USA. The attractive characteristic of IPMC is the significant bending in response to a relatively lowelectrical voltage (Fig. 7), where the base polymer provides mobility channels for positive ions to migrate through fixed networkof negative ions on interconnected clusters [77, 86]. The response of IPMC is relatively slow (< 10 Hz) because of the need forions to physically travel though the polymer.

In recent years, research related to IPMC has been focused on improvements in the modeling [2, 80] and fabrication methods[22, 52, 110] as well as the performance as actuation material [54]. These are done as part of the efforts to understand how IPMCmaterials function and to enable broad range of IPMC based designs and applications. Using 3D manufacturing, researchers areseeking to create shapes that are not possible with commercially available ionomer options. The development of the relatedprocesses is involved with great challenges since the Nafion, which is the leading base material of IPMC, needs to be kept hotduring the printing process in order to assure that each of the layers are kept adhering to each other. Researchers at the Universityof Nevada, Las Vegas, and their collaborators have made significant progress in addressing the related challenges [110].

3.2.2 Conducting polymers

Conducting polymer EAP materials offer mechanical energy densities of over 20 J/cm3 that is relatively high and have thepotential of highly effective actuation materials [68]. A sandwich of two conducting polymer electrodes (e.g., polypyrrole) withan electrolyte between them forms a bending EAP actuator. They typically function via reversible counter-ion insertion and

Fig. 6 Multilayered dielectric elastomer in passive (left) and activated states (right) [61]. Courtesy of Gabor Kovacs, EMPA, Duebendorf, Switzerland

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expulsion that occurs during redox cycling [4, 84, 85, 87, 102, 108]. Voltage applied between the electrodes causes oxidation atthe anode and reduction at the cathode and result in a volume change mainly due to exchange of ions with the electrolyte [68].The electric charge is balanced by migration of ions between the electrolyte and the electrodes. The added ions cause swelling ofthe polymer while their removal results in shrinkage and therefore bending of the sandwich. Conducting polymer actuatorsrequire voltages in the range of 1–5 V (Fig. 8) and the speed increases with the voltage. The efficiency of conducting polymers isrelatively low (~ 1% if no electrical energy is recovered) [69].

Advances in conducting polymers (CP) over the last two decades have been made resulting from the efforts to applynovel modeling and fabrication techniques [44, 68]. Nanocomposites of polymers, carbon nanotubes, graphene, andinorganic compounds have been incorporated to obtain special structure and properties. In addition, progress has beenmade in applying conducting polymers as sensors and actuators with potential biomimetics applications. The capabilityof CP in the form of trilayers has been improved to the level that researchers are considering it as potential alternative to

Fig. 7 IPMC in passive (left) and activated states (right)

Fig. 8 Conducting polymer in reference and activated states (about 1.5 V)

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piezoelectric and electrostatic actuators [117]. Efforts are also continuing towards making all solid-state CP actuators andrecent progress has been reported by Ribeiro and his research collaborators [96]. In addition, Tan and his collaboratorsused solid polymer electrolyte sandwiched between two poly(3,4-ethylenedioxythiophene) (PEDOT) electrodes andfabricated and characterized the produced actuator.

3.2.3 Carbon nanotubes

Carbon nanotubes (CNT) consist of nanometer-size tubes that are able to produce strains of about 1% [73]. The use of carbonnanotubes as EAP was first reported in 1999 ([16, 94, 109]). Compared with other types of actuators, carbon nanotube (CNT)actuators have the potential to produce very high work/cycle and this possibility is the result of their very high Young modulus.However, this potential of high energy density and force has not been realized yet in devices at the macro-scale yet. Using CNTasactuator can be made to operate as a bending device using a layered structure or in a single sheet/fiber. As a bender, a carbonnanotube actuator can be constructed by laminating two narrow strips of carbon nanotube sheet with an electrically insulatedintermediate adhesive layer. The resulting three-layer strip is then immersed in an electrolyte. Application of ~ 1 V is sufficient tocause bending, and the direction depends on the polarity of the field with a response that is approximately quadratic relationshipbetween the strain and charge. The carbon–carbon bond in nanotubes (NT), which are suspended in the electrolyte, and thechange in bond length are responsible for the actuation mechanism. A network of conjugated bonds connects all carbons andprovides a path for the flow of electrons along the bonds. The electrolyte forms an electric double layer with the nanotubes andallows injection of large charges that affect the ionic charge balance between the NTand the electrolyte (Fig. 9). Themore chargesare injected into the bond the larger the dimension changes. Removal of electrons causes the nanotubes to carry a net positivecharge, which is spread across all the carbon nuclei causing repulsion between adjacent carbon nuclei and increasing the C–Cbond length.

The advantages of CNT materials are quite attractive for many applications. This is the result of the CNT potential for highlyeffective supercapacitors, actuators, and lightweight electromagnetic shields [53]. Advances in the development of CNT materialsand their applications have been reported in all its aspects including synthesis, purification, and chemical modification [29]. Theinterest in the commercial use of carbon nanotubes (CNTs) has been steadily growing worldwide reaching levels and it is now atthe level of several thousand tons per year. The advances have enabled the integration of CNTmaterials in thin-film electronics andlarge-area coatings. Such products as rechargeable batteries, automotive parts, and sporting goods are being produced using bulkCNT powders. Development in graphene synthesis and characterization is expected to lead to significant improvement to theirfabrication and performance and to lead to potential related commercial products in the coming years [29, 43].

3.2.4 Ionic polymer gels

Ionic polymer gels are generally activated by a chemical reaction where changing from an acid to an alkaline environmentcauses the gel to shrink or swell, respectively. This chemo-mechanical behavior was first reported in 1955 by Katchalskyand his co-investigators [47, 62]. The mechanical change results either from the displacement of water out of the gel, or theredistribution of water within the gel. Since the 1980s, researchers at the Hokkaido University, Japan, [83] have

Fig. 9 Schematic illustration of the charge injection in a nanotube-basedEAP actuator. a Applied potential injects opposite sign charge in the twonanotube electrodes that are immersed in an electrolyte (blue color). bCharge injected ion at the surface of a nanotube bundle is illustrated. It is

balanced by the surface layer of the electrolyte cations. c Edge view of acantilever-based actuator operated in aqueous NaCl at ± 1 V. Courtesy ofRay Baughman, University of Texas at Dallas, TX

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extensively studied ionic polymer gels towards developing EAP actuators. These studies were followed by numerous otherinvestigations worldwide (e.g., [18, 19]). Generally, ionic polymer gels generate very large strains but with relatively lowactuation force [95].

Limited progress in advancing the capability of ionic polymer gels has been reported in recent years. These include theuse of ionic liquids combined with macromolecules [122] to enable making actuator that is less sensitive to open air. Theresearchers focused on the use of ionic liquid-based polymer electrolytes that consist of block copolymers and polyimides.These combinations were demonstrated to enable ionic polymer actuators that are easier to produce and have higherperformance and durability.

3.3 Other polymer-based actuation materials

3.3.1 Twisted and coiled polymers

Twisted and coiled polymer actuators via twisting nylon threads or fishing wires have been a significant development in polymer-based soft actuators in recent years [15, 74, 124]. The activation is done thermally leading to sizeable deformation and highpower/mass ratios. The material can be activated electrically by causing Joule heating. To address the need to control the responseusing feedback from anti-windup compensator, [115] developed an algorithm that they validated through numerical simulationsand experiments, where the maximum overshoot and the setting time decreased by the effect of the anti-windup compensator.

4 Concluding remarks

Since the early 1990s, new EAP materials have been developed that generate large strains making them highly attractive for usein actuators. Their operational similarity to biological muscles, including resilience, damage tolerance, and ability to induce largeactuation strains makes them unique compared with other electroactive materials. However, the application of EAP materials asactuators still involves many challenges.

Addressing these challenges requires continuing technology development and the growth in multidisciplinary cooperationamong experts from various fields including chemists, materials scientist, roboticists, computer, and electronic engineers.Researchers are increasingly making improvements in the various related areas including better understanding of the operationmechanism of the various EAP material types. The processes of synthesizing, fabricating, electroding, shaping, and handling arebeing refined to maximize the actuation capability and durability. These include the development of 3-D printing of EAP as wellas magnetically activated materials [55]. Methods of reliably characterizing the response of these materials are being developedand efforts are being made to establish process standards and databases with documented material properties to support engineersthat are considering the use of these materials. The general trend in recent years has been towards the application of dielectricelastomers and significant advances have been reported.

Applying EAP materials as actuators of manipulation, mobility, and robotic devices involves multidisciplinary effortsfor new materials, chemistry, electromechanics, computers, and electronics. Even though the actuation force of the existingmaterials requires further improvement, there have already been successes in the development of mechanisms that aredriven by EAP actuators. However, seeing EAP replace existing actuators in commercial devices and engineering mech-anisms would require identifying niche applications where EAP materials would not need to compete with existingtechnologies. It is quite encouraging to see the growing number of researchers and engineers who are pursuing career inEAP-related disciplines. Hopefully, the growth in the research and development activity will lead to making these materi-als becoming the actuators of choice.

Acknowledgements Some of the research reported in this manuscript was conducted at the Jet Propulsion Laboratory (JPL), California Institute ofTechnology, under a contract with National Aeronautics and Space Administration (NASA). The authors would like thank Samuel Rosset, TheUniversity of Auckland, New Zealand, for his comments and suggestions that helped improve the paper. Also, the authors would like thank the manyindividuals’ who contributed to the field of EAP and apologize to those whose publications have not been referenced.

Compliance with ethical standards

Conflict of interest The authors declare that they have no conflict of interest.

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Internet references

Books and proceedings: http://ndeaa.jpl.nasa.gov/nasa-nde/yosi/yosi-books.htmWW-EAP Webhub: http://eap.jpl.nasa.govWW-EAP Newsletter: http://ndeaa.jpl.nasa.gov/nasa-nde/lommas/eap/WW-EAP-Newsletter.htmlEAP Conferences: http://ndeaa.jpl.nasa.gov/nasa-nde/lommas/eap/eap-conferences.htmArmwrestling Challenge: http://ndeaa.jpl.nasa.gov/nasa-nde/lommas/eap/EAP-armwrestling.htmInformation about the process of making the leading EAP materials http://ndeaa.jpl.nasa.gov/nasa-nde/lommas/eap/EAP-recipe.htmSources of obtaining EAP materials http://ndeaa.jpl.nasa.gov/nasa-nde/lommas/eap/EAP-material-n-products.htmResearch at Xuanhe Zhao’s lab (Magnetic 3-D-printed structures crawl, roll, jump, and play catch): http://news.mit.edu/2018/magnetic-3-d-printed-

structures-crawl-roll-jump-play-catch-0613

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