Published in Materials Today. Copyright Elsevier.

Polymer artificial muscles

Tissaphern Mirfakhrai1, John D W Madden1 and Ray H Baughman 2

Abstract

The various types of natural muscle are incredible material systems that enable the production of large deformations by repetitive molecular motions. Polymer artificial muscle technologies are being developed that produce similar strains and higher stresses using electrostatic forces, electrostriction, ion insertion and molecular conformational changes. Materials used include elastomers, conducting polymers, ionically conducting polymers and carbon nanotubes. The mechanisms, performance and remaining challenges associated with these technologies are described. Initial applications are being developed. Further work by the materials community should help make these technologies widely applicable in a range of devices where muscle-like motion is desirable.

1. Introduction A variety of polymers have emerged that exhibit substantial deformations in response to applied voltage. These materials reversibly contract and expand in length and volume, which is the primary similarity with muscle.

Is there a need for new motors 1, 2? Our most common actuators are combustion engines, electric motors and piezoelectrics. Combustion engines are generally most efficient when operated continuously and are therefore not ideal for applications in which motion is frequently interrupted, such as in valves or walking robots. Electric motors are low in torque to mass compared to muscle making them very bulky for medical, robotic and fluidic applications. Piezoceramics achieve very high power densities but strain is small (0.1 %), making massive mechanical amplification necessary if significant displacements are needed. There is currently no technology that is widely used to replace or simulate muscle, providing a strong motivation for research and development.

The performance of the emerging polymer actuators exceeds that of natural muscle in many respects, making them particularly attractive for use anywhere where a muscle-like response is desirable, including in medical devices, prostheses, robotics, toys, bio-mimetic devices and micro/nano-electromechanical systems 1, 2. Commercial application of these materials is at an early stage 3. Challenges remain with many of the technologies, most of which can be overcome via improvements in material properties. In this review we have sought to relate material properties to performance in the hopes of galvanizing the materials community into improving existing materials and inventing new ones.

1 Department of Electrical and Computer Engineering and Advanced Materials and Process Engineering Laboratory, The University of British Columbia, 2332 Main Mall, Vancouver BC, Canada V6T 1Z4 2 NanoTech Institute, The University of Texas at Dallas, Richardson, Texas, 75080

In this review polymer artificial muscles have been divided into two major groups 4. In the first group dimensional change (actuation) is in response to electric field. These are commonly known as electronic or electric electroactive polymers. Some of the technologies that fall under this category are dielectric elastomer actuators (DEAs), relaxor ferroelectric polymers, and liquid crystal elastomers. The second group is a class of materials in which the presence and movement of ions is necessary to make actuation possible. This group is referred to as ionic electroactive polymers. For the ions to be able to move an electrolyte phase is necessary, which is often liquid; so these actuators are also known as wet electroactive polymers. Actuators described in this paper employ conducting polymers, ionic polymer metal composites (IPMC), and carbon nanotubes as the active materials. Gel actuators 4 also fit into this category, but are not covered here. A number of other artificial muscle technologies exist that respond to heat, which have also been omitted. The article also gives a brief description of some exciting directions in actuation in which molecular design is used to create actuation that is either voltage or light driven.

The composition, mechanisms, properties and materials challenges are described for each technology. The key properties of interest are elastic modulus, strain, stress, work density, power density and electromechanical coupling. Work density is the amount of mechanical work per unit volume in one actuator stroke. The convention in work density used in the piezoelectric and dielectric elastomer literature is to report the work done in elastically deforming the actuator materials itself in one dimension, u= 2

21 εE , where E is

the elastic modulus and ε is the strain. In acting on a matched elastic load only half of this work can be extracted, so in fact the work out is the u divided by two to provide an upper bound on the external work done under such loading conditions. Similarly the electromechanical coupling values include mechanical work done on the actuator materials themselves. In achieving high work densities in compliant materials large strains are needed, whereas in materials with small strains the stiffness must be high. The full range of compliances has been explored, from dielectric elastomers (E< 1 MPa) to carbon nanotubes (Etube~640 GPa). Table 1 lists properties of mammalian skeletal muscle for comparison. Most technologies described have higher work densities and similar power densities. High work and power densities are particularly beneficial where space (or mass) limitations exist, as in robots, implantable devices, and microelectromechanical systems.

Table 1: Properties of mammalian skeletal muscle 2 PROPERTY TYPICAL MAXIMUM

Strain (%) 20 > 40 Stress (MPa) 0.1 (sustainable) 0.35 Work Density (kJ⋅m-3) 8 Density (kg⋅m-3) 1037 Strain Rate (%⋅s-1) 500 Power to Mass (W⋅kg-1) 50 200 Efficiency (%) 40 Cycle Life 109

Modulus (MPa) 10 - 60

2. Electronic artificial muscle The simplest field driven actuation mechanism is the result of the electrostatic interactions between electrodes. This mechanism dominates in low modulus materials such as dielectric elastomers (section 2.1) in which extremely large strains (> 40 %) can be obtained. A second mechanism is observed in electrostrictive polymers, where electric dipoles within the polymer are pushed or pulled by the applied field, resulting in displacement. The highest strains are achieved in ferroelectric polymers in which imperfections are introduced to disrupt long range order such that field applied to a paraelectric phase leads to conformational changes in the backbone, induced polarization, and large strains (up to 7 %). Graft copolymers in which polar side chains form crystalline domains also produce substantial strains, as do liquid crystal elastomers in which polar groups are oriented by the applied field. A perceived drawback with this class of actuators is their use of high voltages (> 1 kV typical) due to the very high fields that are needed (~ 100 MV/m). This challenge is being addressed by reducing film dimensions and increasing dielectric constant. These materials feature good electromagnetic coupling coefficients, and high work densities (100 x that of muscle).

2.1 Dielectric elastomer actuators (DEAs) Dielectric elastomer actuators are capable of generating large strains and strain rates. They are one of the most studied polymer actuators and numerous applications are being developed including electroactive fluid pumps, conformal skins for Braille screens, insect-like robots and Artificial Muscles Inc.'s auto focus lens positioner (Figure 1), which can focus a photography camera by moving the lens by as much as 350 µm 5, 6.

a b Figure 1: (a) Artificial Muscles Inc.'s DLP-95 auto focus lens positioner 6 at right, and (b) cutaway drawing of the diaphragm mechanism in which bottom and top ring actuators alternately push or pull. Reproduced with permission from Artifical Muscle Inc.

The actuators are capacitors with very compliant dielectrics and electrodes. When a voltage is applied across these materials the attraction between opposite charges and the repulsion of like charges generates stress in the dielectric, known as Maxwell stress, which compresses and elongates the dielectric as shown in Figure 2. This stress is proportional to the square of the applied field and to the dielectric constant 7, 8. Low modulus (~ 1 MPa) and high dielectric strength (> 100 MV/m) can lead to strains of up to 380 % at high applied fields 5. More typically strains are 10 to 100 %, which compares favorably with the 20 % observed in our skeletal muscle.

Figure 2: The mechanism of actuation in dielectric elastomer actuators. The application of the voltage V between the two electrodes results in the generation of a Maxwell stress of σ , compressing the dielectric and resulting in its lateral expansion.

The elastomer employed is often a silicone or acrylic elastomer, sometimes loaded with heavy particles such as Titanium Dioxide 9 to increase the dielectric constant. The compliant electrodes can be made of conductive carbon or silver pastes, spin-coated conductive rubbers, sprayed graphite particles or superelastic carbon nanotube sheets 8-10. The key to large strain is to make both the electrodes and the elastomer highly compliant, without sacrificing dielectric strength and conductivity.

(a)

(b)

Figure 3: (a) A spring roll dielectric elastomer actuator made by rolling a planar DEA around a compliant compression spring11 and (b) SRI's FLEX 2 6-legged robot operating with spring roll actuators such as those in (a) 7, (© 2002, 2003 SPIE).

Electromechanical coupling is typically 60 - 80% for acrylic and up to 90 % for silicone elastomers 7, 12. The large strains resulting from the actuation lead to high work per unit volume per stroke in these actuators; the 3.4 MJ/m3 maximum is four hundred times that of our muscle) 2. The electrostatic energy stored within a dielectric elastomer increases in proportion to the field squared while the mechanical energy density is roughly proportional to the fourth power of field. As a result mechanical energy density and electromechanical coupling are maximized by operating close to breakdown and using materials with the maximum dielectric strength. Dielectric strength can be increased by prestraining the elastomers, and this prestraining also serves to amplify strain 13.

The high voltage used to drive DEAs (> 1 kV) could be provided using relatively compact ferroelectric DC-to-DC converters 14. Some safety issues may arise in large devices where both high voltages and currents are present. Since it is the field in the dielectric and not the voltage itself that drives the actuation, it is possible to maintain the same field while reducing the required voltage by using thinner sheets of elastomer 15 or by increasing dielectric constant 9. Stacking in layers 15 or rolling in tubes produces larger forces by making multiple thin layers of DEAs apply their actuation forces in parallel. These actuators can be used in devices, including the robotic insect shown in Figure 3. The advantage of high voltage

operation is that currents are very low, and this advantage is lost as multiple thin layers are used. As currents increase it becomes increasingly important to have a high conductivity in the compliant electrode layer – which is a materials challenge in itself.

2.2 Electrostrictive relaxor ferroelectric polymers Ferroelectric materials, like ferromagnets, have dipoles that can be aligned, and a Curie point above which they lose their permanent polarization. Inorganic ferroelectrics such as barium titanate are known to undergo dimensional changes in response to field, but these are only on the order of 0.1 %, much smaller than those in ferroelectric polymers, which can strain by an incredible 10 % 16. Like inorganic piezo and ferroelectrics, the ferroelectric polymer actuators are fast and have a high work density (1 MJ/m3) 17. The best actuator performance to date is obtained from poly(vinylidene fluoride)-based, (PVDF), polymers that are co-polymerized with trifluoroethylene, forming P(VDF-TrFE). The electronegativity of the fluorine makes these polymer backbones highly polar. Field driven alignment of polar groups produces reversible conformational changes which are made use of in actuation, as depicted in Figure 4 for PVDF 17. The application of field perpendicular to the chains leads to a transition between the non-polar forms (alpha phase) and the polar (ferroelectric beta phase). The result is a contraction in the direction of polarization and an expansion perpendicular to it. In order to facilitate this transition P(VDF-TrFE) is irradiated or a small mass fraction of a bulky monomer such as chlorofluoroethylene (< 10 %) is added in order to disrupt long range order. The disruption effectively eliminates the formation of domains and the hysteretic behavior characteristic of ferroelectrics. The resulting material is known as a relaxor ferroelectric. Strain is maximum at stresses of approximately 20 MPa, and electromechanical coupling reaches 0.42 17.

Figure 4: Alpha and beta phases in PVDF17, looking along the chains (top) and perpendicular to the chains (bottom). A similar alpha to beta phase changes is induced in P(VDF-TrFE) relaxor ferroelectrics which begin in the non-polar alpha phase and switch to the polar beta phase upon the application of an electric field. This results in dimensional changes (expansion along the chain length and contraction perpendicular to the chain direction).

The elastic modulus of the ferroelectric relaxors (0.3-1.2 GPa) is 1000 X higher than in DEAs, the dielectric constant is ~ 20 X higher, and the dielectric strength is similar. Overall this leads to smaller strains but similar energy densities (~ 1 MJ/m3) and coupling. As in dielectric elastomers, fields and voltages are very high (~100 MV/m, > 1 kV). As in DEAs increases in dielectric constant and reductions in thickness are options for lowering voltage17. In general, polymers with high polarizability associated with a large (and preferably fast) conformational change in the polymer backbone are desirable. Two other electrostrictive materials are now described.

(

(

)n

)n

0.462 nmAlpha Phase

0.516 nmBeta Phase

0.496 nm 0.964 nm

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2.3 Electrostrictive graft elastomers These polymers feature polar groups attached as side chains to a flexible backbone of a polymer. The polar groups aggregate to form polar crystalline regions. Application of a field aligns these regions, and this reorientation leads to dimensional changes within the polymer, as depicted in Figure 5. Applied fields on the order of 10 MV/m are needed to induce strains reaching 2.5 %, and provide an elastic energy density of ~ 0.5 MJ/m3. These results are obtained using a chlorotrifluoroethylene and trifluoroethylene backbone having P(VDF-TrFE) polar sidechains, producing a comparatively stiff matrix (E = 0.6 GPa) 17. Fields are lower than those required for relaxor ferroelectrics, but so are actuator strains. Due to the size of the polar grafts actuation rates are lower than for relaxor ferroelectrics. Figure 5: Depiction of a graft elastomer with no field applied. Colored lines enable different flexible polymer backbones to be distinguished.

Polarization of crystallized grafts Polar grafted unit Polymer chains

2.4 Liquid crystal elastomers Liquid crystals are known to change phase and orientation under the influence of applied field. By incorporating such mesogens into a compliant polymer backbone or as side chains, field induced changes in phase can be used to produce actuation 18. A recent implementation is depicted in Figure 6 in which a 4 % strain was obtained at 133 Hz using field amplitudes of 1.5 MV/m 19. The modulus of these polymers is low and the combination of low modulus and relatively low actuator strains leads to low work density. Work density has recently been improved by using a stiffer polymer (2 % strains at 25 MV/m with a work density of 0.02 MJ/m3 20). This performance still does not match

that of high performance relaxor ferroelectrics, but investigations are still at an early stage. Minimizing dielectric loss is important for high frequency operation in liquid crystal elastomers, as in other field driven actuators, since high loss reduces efficiency and leads to heating and failure.

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Figure 6: An example of a ferroelectric liquid crystal elastomer. (a) The polymer backbones (rectangles) are siloxane, from which mesogens (ovals) extend. A small fraction of the mesogens are crosslinked to neighboring polymer chains. In the system shown the application of a field normal to the long axis of the mesogens produces a reversible rotation, as depicted in (b), resulting in dimensional changes. Adapted from Lehman et al 19. (© 2004 IEEE) 2.

3. Ionic artificial muscle An alternative means of producing actuation in a polymer is to employ ions that are mobile within the polymer phase. Applied field drives the motion of these ions and entrained solvent, leading to swelling or contraction when these ions enter or leave regions of the polymer. If the polymer phase is electronically conducting (as in conducting polymers and carbon nanotubes) then ions serve to balance charge generated on these conductors as potential is changed, creating very strong local fields (but overall low voltage). The voltages employed in these materials are low (1 – 7 V) but the energies are nonetheless high because of the close spacing between ions and electronic charges, and the large amount of charge that can be transferred.

3.1 Carbon nanotube (CNT) actuators Carbon single-wall nanotubes (SWNTs) can be thought of as a single layer of graphite (graphene) rolled into a cylinder of nanometer diameter (Figure 7a). Carbon multi-wall nanotubes (MWNTs) are nested SWNTs (Figure 7b). Individual SWNTs or very long MWNTs are materials with exceptional mechanical properties. The tensile modulus of single-wall carbon nanotubes (640 GPa) approaches that of diamond, while their tensile strength is thought to be between 20 and 40 GPa, about ten times higher than for continuous fibers of any type 21. While mechanical properties in this range are observed for individual SWNTs, observed properties are much lower for assemblies of untold trillions of these nanotubes in nanotube yarns and sheets, and this restricts the performance of actuators based on nanotube yarns or sheets. At high levels of charge injection into the carbon nanotubes, the predominant cause of actuation is electrostatic, like for the dielectric elastomer actuators. However, the electrostatic forces are repulsive interactions between like charges injected into the nanotubes, rather than between two electrodes. A voltage is applied between an actuating nanotube electrode and a counter electrode, through an ion containing solution, as depicted in Figure 8a (where the counter-electrode is another CNT), leading to charging (Figure 8a, b). Electrostatic repulsive forces between like charges on the carbon nanotubes work against the stiff carbon-carbon bonds in the nanotubes to elongate and expand the nanotubes, though quantum mechanical effects can predominate over electrostatic forces at low decreases of charge injection 22. Unlike dielectric elastomers the strains are low (< 2 % 21) since carbon nanotubes are extremely stiff.

Actuation is generally achieved in films or yarns (Figure 8c) composed of many nanotubes. The porous nature of the films and fibres enables fast ion transport with response times of < 10 ms, with effective strain rates of 19 %/s, and effective power to mass ratios of 270 W/kg (half that of a high revving electric motor) 23. The achievable response rate decreases with increasing nanotube yarn or sheet thickness, increasing inter-electrode separation, and decreasing ionic conductivity of the electrolyte.

Work densities in CNT fibers and yarns are calculated from elastic modulus and strain to be ~ 1 MJ/m3 (as in DEAs and ferroelectric polymers). Measured values are somewhat lower due to creep 24. The energy densities thought achievable for individual nanotube actuators are unmatched by any actuator technology, perhaps reaching 108 J/m3. Achieving such incredible work densities in a bulk material will require using extremely long nanotubes, or ones that are cross-linked together, so that the mechanical properties of nanotube yarns and sheets approach those of the individual SWNTs. These approaches could reduce creep (which is presently a problem), and lead to high electromechanical coupling. Coupling is presently less than 1 %, likely the result of the tremendously large energy needed to deform individual nanotubes (E=640 GPa) and the fact that this energy cannot presently be effectively utilized for actuation because of low bulk modulus (E~ 15 GPa) compared with the individual nanotube modulus 24. High work density combined with good temperature stability (> 450 oC in air, > 1000 oC inert environment) make CNTs prime candidates for situations where weight and

temperature are important, as in aerospace applications. Strains are relatively small compared to other polymer actuators (but an order of magnitude larger than is found in typical piezoceramics). Strain might be increased by employing electrolytes such as highly purified ionic liquids that can withstand very large potentials without reacting electrochemically.

(a)

(b)

Figure 7: (a) A graphene sheet rolled into a nanotube25 (b) and a multi-walled carbon nanotube (MWNT)26.

(c) Figure 8: Charge injection in a CNT-based electromechanical actuator (a) an applied potential injects charges in the two nanotube electrodes in solution 24 (red +’ve, yellow –‘ve ions) (b) Charge injection at the surface of a nanotube bundle 22, (c) A scanning electron micrograph of a twist-spun MWNT yarn, which provides a 0.6 % actuator stroke at loads of 100 MPa 24.

100 μm

CNT actuators have recently been shown to actuate when used as electrodes in a fuel cell 27. This possibility is exciting because the energy density of fuel cells is much higher than that of batteries, helping enable autonomous applications of CNTs and possibly other artificial muscle technologies 28.

3.2 Conducting Polymers Conducting polymers are polymeric materials that are typically semiconducting when undoped and highly conducting when doped with donor or acceptor ions 29. Doping is typically achieved chemically or electrochemically. Some applications that have long been of interest include polymer light emitting diodes, electrochromic windows, energy storage, sensing and actuation 29. Polypyrrole and polyaniline are two common conducting polymers (Figure 9)

HN

NH

**

n

a

HN

NH

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Figure 9: The structure of two common conducting polymers: (a) polypyrrole and (b) polyaniline Though the use of conducting polymers as artificial muscles was first published by Baughman and coworkers in 1990 30, advances in performance have been continuously made up to the present and further improvements are both desirable and achievable. These conducting polymer artificial muscles use the dimensional changes resulting from electrochemical ion insertion and deinsertion, possibly along with associated solvating species. Since both electrodes can comprise conducting polymers, both can be used as artificial muscles. Depending upon the conducting polymer/electrolyte system used, the initial state, and the rate of potential change used for actuation, electron insertion into one electrode can be accompanied by a volume increase as cations are inserted or a volume decrease as anions are removed and similar processes can occur at the counter-electrode. This feature, as well as possible time-dependent changes of the intercalated/deintercalated species can complicate control of actuation 31, 32. Expansion appears to be primarily perpendicular to the polymer chain orientation for oriented polymers, suggesting to first order that ions and accompanying solvent are slotted between chains 33-35, as depicted in Figure 10, which also shows the suggested apparent structure of polypyrrole doped with PF6

- 36, 37. The polymers most often used in actuation

are polypyrrole, polyaniline and polythiophene derivatives.

a

b

Figure 10: (a) Proposed structure of polypyrrole and (b) possible mechanism of actuation via ion (yellow/purple & A-) and solvent (red/blue/gray) insertion between chains 34, 36, 37.

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Strains are typically between 2 and 10 %, though recent work by Kaneto and coworkers show that actuator strokes can reach 40 % 2, 38-41. Rates of actuation tend to be low (<< 1 %/s) because of the relatively slow transport of ions within the polymer and the large degree of doping sometimes utilized 42, 43, but they can exceed 10 %/s by using metal contacts, porous polymers, fast charging methods or thin films & fibers 44-47. High quality

electrochemically grown films and solution spun fibers have tensile strengths on the order of 100 MPa or higher, and added carbon nanotubes can further increase strength and reduce problematic creep 48. Small strains (< 0.2 %) are achieved at loads of 100 MPa 48, with typical operating stresses being more typically 1-34 MPa 32. Work densities approach 100 MJ/m3

49 . Operating voltages are ~ 2 V, though higher voltages of up to 10

V are sometimes used to increase actuation rate. Such high voltages will ultimately cause degradation of the electrolyte and/or polymer unless the voltage profile as a function of time is properly provided 44, so that excess voltage over the redox stability range is resistively dissipated at the beginning of a voltage pulse and the applied voltage is progressively decreased as the current decreases. The advantage of conducting polymers over electronic EAPs is their low operating voltage. They feature higher strains and are lower in cost than CNTs. Like CNTs the electromechanical coupling is low (< 1 %). Much of the input electrical energy can be recovered, but the need to shunt relatively large amounts of electrical energy can slow actuation and push the limits of power supplies, making large scale applications (e.g. robotic arms) challenging 1, 42. One promising means of improving coupling is to design conjugated molecules that fold and expand in response to changes oxidation state, bringing artificial muscle a little closer to the elegance of real muscle 50, 51. Applications being considered include blood vessel reconnection, dynamic Braille displays, valves and actuated catheters 52-55.

3.3 Ionic Polymer-Metal Composites (IPMCs) This type of electrochemical artificial muscle employs a polyelectrolyte as an ion-conducting layer, wherein polymer chains provide one ion type and the opposite ion type comprises mobile solvated ions. The polyelectrolyte is sandwiched between two high-surface-area flexible metallic electrodes, which interpenetrate the polyelectrolyte. The metal electrodes are typically composed of percolated Pt nanoparticles or Au 56-58. When a voltage is applied between the two electrodes, as depicted in Figure 11a, solvated mobile cations move towards the oppositely charged electrode, resulting in swelling near this negative electrode, shrinkage near the positive electrode, and resulting bending of the actuator tri-layer (Figure 11b) 59. The polyelectrolytes used are commonly either i) perfluorinated alkenes with short side-chains terminated with ionic groups, such as SO3

- (e.g. Nafion) or COO- (e.g. Flemion), and ii) styrene/divinylbenzene-based polymers in which the ionic groups are substituted on the phenyl rings 2, 56. These materials are also used as separators in fuel cells. When a step voltage input is applied to Nafion-based IPMCs an original fast actuation is followed by a slow relaxation in the direction opposite to the original actuation. However, in Flemion-based IPMCs under the same circumstances, the original fast actuation is followed by a slow actuation in the same direction as the original actuation. This is believed to be because of the interaction between the cations and the strongly-polar sulfonate groups in Nafion; the cations originally repel the electric dipoles associated with these sulfonate groups, thus resulting in a fast and large actuation. Over

time the cations get redistributed and the dipoles reorient themselves and the IPMC relaxes. The weakly-polar carboxylate groups in Flemion do not strongly interact with the cations and therefore Flemion does not show the relaxation in the opposite direction 60.

Figure 11: Actuation model for IPMCs (adapted from reference 59) (a) before the voltage is applied and (b) after the application of the voltage.

Actuation strains of above 3 % have been reported for IPMCs under applied voltages of < 7 V 57, 61. Substantial bending in these cantilever-type actuators is caused as one side of the material contracts and the other expands (Figure 12). Reported stresses of actuation are as high as 30 MPa 56, 60. It is not uncommon that IPMCs drift in position after initial displacement and require some steady current to maintain a fixed position. This, along with the gradual leakage of water (the solvating species) from the surface of the device reduces efficiency, as well as degrading performance 2, 62. One method of greatly reducing leakage and relaxation is to use highly inert, non-volatile ionic liquid electrolytes 63. IPMCs have been actuated at up to 100 Hz 62, with the fast transport being achieved due to the ease of transport though the highly porous polyelectrolyte.

Figure 12: Actuation of a Flemion-based IPMC strip as a function of time in seconds 64 (© 2003 American Institute of Physics).

Applications suggested for IPMC actuators include mechanical grippers, metering valves, diaphragm pumps, sensors, fins for robotic fish and an IPMC-operated artificial eye, devised by the Japanese company Eamex Inc. 4, 57, 65, 66.

4. Molecular Mechanisms Muscles are elegant machines in which protein filaments interact repeatedly in order to slide past each other 67. It is exciting to contemplate designing similar mechanisms using organic chemistry, perhaps guided by engineering considerations. Such molecular actuators are beginning to be designed and investigated, and include mechanisms that employ both voltage and light-driven conformational changes. Materials that have been investigated include shape changing conducting polymers mentioned under conducting polymer actuators 2, 68 and those using light-driven shape changes. An interesting feature of the light-activated artificial muscles (such as azobenzenes) is that there is no need to contact the actuator in order to deliver energy. This could be useful, for example, for enabling drug delivery using infrared radiation at wavelengths that are transparent for the body. Optically-excited cis-to-trans isomerization in azobenzene actuators results in an expansion in the polymer length, producing deformations that have been measured both macroscopically 69 and at the single molecule level 70. The mechanism is depicted in

Figure 13. For aligned polymers it is predicted that strains of 7 % and work densities potentially exceeding 1 MJ/m3 are achievable, with measured strains in a single molecule being ~ 1 %. The large height of the energy barrier between conformations (relative to the energy difference between states) suggests that efficiency will be moderate, with Hugel et al. estimating 10 % energy conversion efficiency when the quantum efficiency is 100% 70. A number of alternative photo-chemical actuation mechanisms exist, including use of cinnamic acid polymers that reversibly cross-link when UV irradiated 71. Challenges for such molecular actuators include obtaining high electrical conductivity for electrically or electrochemically driven actuators and high quantum efficiency for photon driven actuators. If individually addressed molecular actuators are sought, the problems of achieving addressing capabilities are especially challenging.

(a)

(b)

Figure 13: (a) The potential energy landscape of the reversible azobenzene cis-trans transition. The relevant conformational coordinate is the bond angle, NNC, which changes from about -60° to +60° for a transition from the cis to the trans. The transition can be induced by optical excitation from the electronic ground state S0 to the first excited state S1. Transition can be done from cis to trans using

1λ = 420 nm or vice versa using 2λ = 365 nm. (b) polyazobenzene peptides 70.

5. Discussion A variety of electronic, ionic, and photoactivated artificial muscle technologies are emerging. Ultimately these may replace traditional actuator technologies such as electric motors and piezoelectrics in many applications, and make new applications possible that have been beyond the abilities of the currently-used actuators. Some of these novel artificial muscles feature a combination of high work density, large actuator stroke, and high stress generation, with which only shape memory alloys actuators can compete. Some of the described electric-field-driven EAP technologies demonstrate particularly high work densities, very good electromechanical coupling and high bandwidth. High voltages may prove to be an obstacle to application of electronic-field-driven EAPs. A number of electrochemical EAPs can achieve high work density, high actuator stroke, and large stress generation, and can universally provide the additional benefit of operating at low voltages. Ionic EAPs often suffer from slow actuation rates and low electromechanical coupling. None of the artificial muscle technologies has yet demonstrated exceptional cycle life, and a degree of irreversible deformation during high load actuation is a problem for the electrochemical types of artificial muscles in particular. Some advantages of natural muscle that are rarely even partially exploited in artificial muscles are use of high energy density fuel (carbohydrates + oxygen: an important advantage for autonomous devices), the ability to modulate stiffness over a wide range, and the ability to regenerate, enabling high cycle life. Recently artificial muscles employing high-energy-density fuels have been reported 27, 28. New materials are being generated using molecular design concepts, with the goal of optimizing all aspects of actuator performance – though incorporating self-repair is still a distant dream. The exciting challenge in developing materials for actuation is that careful consideration needs to be given to the balance between electrical, mechanical and chemical properties. Input from the materials community will help overcome problems with many of the technologies and enable commercial applications to be exploited, much as has been done for piezoelectrics. References 1. Madden, J. D. W., et al., Oceanic Engineering, IEEE Journal of (2004), 29, 738 2. Madden, J. D. W., et al., Oceanic Engineering, IEEE Journal of (2004), 29, 706 3. Bar-Cohen, Y., World-Wide EAP: EAP Materials and Product Manufacturers.

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OPTICAL ENGINEERING (2003), ISSU 5051 281 12. Pelrine, R., et al., Applications of dielectric elastomer actuators. In B.-C. Yoseph,

(eds.), SPIE, (2001) 4329, 335 13. Kofod, G., Dielectric Elastomer Actuators, Ph.D., The Technical University of

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