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This chapter covers some of the fundamental characteristics, functions and properties of ionic polymer metal composites (IPMCs) as smart multi-functional biomimetic soft robotic actuators, sensors, energy harvesters and artificial muscles. Strips of these composites can undergo large bending, twisting, rolling and flapping displacement if an electric field is imposed across their thickness by network pairs of electrodes. Thus, in this sense they are large motion actuators. Conversely, by bending the IPMC strip, either quasi-statically or dynamically, a voltage is produced across the thickness of the strip very much in harmony with the kind of motion or deformation imposed on the IPMC strip. Thus, they are also large deformation sensors. The output voltage can be calibrated for a standard size sensor and correlated to the applied loads or stresses. They can be manufactured and cut into any size and shape. In this chapter, first the sensing capability of these materials is reported. The preliminary results show the existence of an almost linear relationship between the output voltage and the imposed displacement for almost all cases. Furthermore, the ability of these IPMCs to function as large motion actuators and soft robotic manipulators is presented. Several IPMC muscle configurations are constructed to demonstrate the capabilities of the IPMC actuators. A data acquisition system was used to measure the vibrational parameters involved and record the results in real time. Also, the load characterization of the IPMCs has been measured and it shows that these actuators exhibit very good force to weight characteristics or force density in the presence of low applied electric fields and voltages. In a cantilever form, a typical IPMC strip of 5 mm×20 mm×0.2 mm exhibits a force density of about 40, which is the ratio of the tip blocking force to the weight of the IPMC cantilever. Reported are also the cryogenic properties of these muscles for potential utilization in an outer space environment of a few Torrs and temperatures of the order of −140 °C. These muscles are shown to work quite well in such harsh cryogenic environments and thus present a great potential as actuators, energy harvesters and sensors that can operate at cryogenic temperatures and in particular in outer space. Furthermore, the phenomenological modeling of the underlying sensing and actuation mechanisms in IPMCs is presented based on linear irreversible thermodynamics with two driving forces—an electric field and a solvent pressure gradient—and two fluxes—electric current and solvent flux. Also presented are some quantitative experimental results on the Onsager coefficients. Charge dynamics modeling of IPMCs based on the Poisson–Nernst–Planck formulation is also briefly described. Finally, some recent development on novel design of IPMCs including the integration of graphene as electrodes, IPMCs with ZnO and ionic liquids, as well as extension to biopolymers such as chitosan and cellulose, are also briefly discussed.

Ionic polymers such as polyelectrolytes in a nano-composite form with a conductive phase such as a metal, a synthetic metal or a conductive polymer of carbon, graphite or graphene are active actuators, sensors and energy harvesters that show large deformation in the presence of a low applied voltage and yet generate a transient voltage signal if subjected to mechanical deformations, as sensors and energy harvesters. In particular, ionic polymer metal composites (IPMCs) have been shown to be excellent candidates for low-voltage biomimetic robotic soft actuation and self-powered biomimetic robotic sensing and energy harvesting. They have been modeled as both capacitive and resistive element actuators that behave like biological muscles and provide an attractive means of actuation as artificial muscles for biomechanics and biomimetics applications. Grodzinsky,1  Grodzinsky and Melcher2,3  and Yannas and Grodzinsky4  were the first to present a plausible continuum model for the electrochemistry of deformation of charged polyelectrolyte membranes such as collagen or fibrous protein and were among the first to perform the same type of experiments on animal collagen fibers essentially made of charged natural ionic polymers, and were able to describe the results through the electro-osmosis phenomenon. Kuhn,5  Katchalsky,6  Kuhn, Kunzle and Katchalsky,7  Kuhn, Hargitay and Katchalsky8  and Kuhn and Hargitay,9  however, should be credited as the first investigators to report the ionic chemomechanical deformation of polyelectrolytes such as polyacrylic acid (PAA)–polyvinyl chloride (PVA) systems. Kent, Hamlen and Shafer10  were also the first to report the electrochemical transduction of the PVA–PAA polyelectrolyte system. Recently, revived interest in this area concentrates on biomimetic artificial muscles, which can be traced to Shahinpoor and co-workers,11–14,22–37,40–61,62–111  Adolf et al.,15  Oguro, Takenaka and Kawami,16  Oguro et al.,17  Asaka et al.,18  Guo et al.,19  De Rossi et al.20,21  and Osada et al.38,62,63  and Brock, et al.64  Essentially, polyelectrolytes possess ionizable groups on their molecular backbone. These ionizable groups have the property of dissociating and attaining a net charge in a variety of solvent media. According to Alexanderowicz and Katchalsky,39  these net charge groups that are attached to networks of macromolecules are called polyions and give rise to intense electric fields of the order of 1010 V m−1. Thus, the essence of electromechanical deformation of such polyelectrolyte systems is their susceptibility to interactions with externally applied fields as well as their own internal field structure. In particular, if the interstitial space of a polyelectrolyte network is filled with liquid containing ions, then the electrophoretic migration of such ions inside the structure due to an imposed electric field can also cause the macromolecular network to deform accordingly. IPMC researchers62–111  have recently presented a number of plausible models for the micro-electro-mechanics of ionic polymeric gels as electrically controllable artificial muscles in different dynamic environments. The reader is referred to the references of this chapter for the theoretical and experimental results on dynamics of ion-exchange membrane–platinum composite artificial muscles.

IPMCs as multi-functional smart materials with actuation, energy harvesting and sensing capabilities were first introduced in 1997–1998 by Shahinpoor-Bar-Cohen–and co-workers as a member of the electroactive polymer (EAP) family based on research work supported by NASA–Jet Propulsion Laboratory (JPL) and under the leadership of Dr Yousef Bar-Cohen at JPL and Mohsen Shahinpoor, director of the University of New Mexico's Artificial Muscles Research Institute. However, the original idea of ionic polymer and polymer gel actuators goes back to the 1991–1993 time period of Osada et al., as depicted in the references at the end of the chapter. The two original patents on IPMCs were awarded in 1993 to Adolf et al.15  and Oguro, Takenaka and Kawami.16  These patents were followed by additional related patents on both the sensing and actuation of IPMCs (Shahinpoor and Mojarrad,22,23  Shahinpoor92  and Shahinpoor and Kim93 ). It should also be mentioned that Tanaka, Nishio and Sun112  introduced the phenomenon of ionic gel collapse or phase transition in an electric field, which led to a large number of publication by Tanaka and co-workers out of MIT. It should also be mentioned that Hamlen, Kent and Shafer10  introduced the electrochemical contraction of ionic polymer fibers. Credit should also be extended to Caldwell and Taylor113  for their early work on chemically stimulated gels as artificial muscles. Research on polyacrylonitrile gels in the form of contractile muscles that are either pH activated or electrochemically activated has also been reported by Shahinpoor and co-workers.114–120 

The IPMC actuators, sensors and artificial muscles used in our investigation are composed of a perfluorinated ion-exchange membrane, which is chemically composited with a noble metal such as gold, palladium, platinum and silver. A typical chemical structure of one of the ionic polymers (Nafion®) used in our research is shown below in Figure 1.1.

Figure 1.1

Chemical molecular structure of Nafion®.

Figure 1.1

Chemical molecular structure of Nafion®.

Close modal

Note that in Figure 1.1, n is such that 5<n<11 and m is ∼1, and M+ is the counter ion (H+, Li+, Na+, etc.). One of the interesting properties of this material is its ability to absorb large amounts of polar solvents (i.e. water). A typical perfluorinated ionic polymer is the well-known Nafion®, discovered in late 1960s and patented in early 1970s (US patent 3 784 399) by Dr Walther G. Grot of IBM with a chemical formula of C7HF13O5S·C3F7 per pendant group. Figure 1.2 depicts the Nafion® basic chemical formula.

Figure 1.2

Molecular structure of a protonated Nafion® monomeric pendant branch.

Figure 1.2

Molecular structure of a protonated Nafion® monomeric pendant branch.

Close modal

Nafion® is essentially a perfluorosulfonated proton conductor (H+) and incorporates perfluorovinyl ether groups attached to pendant sulfonate SO3H+ groups over a tetrafluoroethylene (Teflon) backbone. Nafion® is heavily used as a proton conductor for proton exchange membranes in fuel cells, water filtration and caustic soda production, among others. Protons on the sulfonic acid groups are capable of “hopping” from one acid site to another. Nafion® pores allow movement of cations but do not allow movement of anions or electrons. Polymeric actuation and sensing technology has advanced in the past decade primarily due to the unique properties of EAPs’ large strain, soft actuation, easy manufacturing and built-in sensing capabilities.

Another similar ion exchange material by the name of Flemion® has also been studied by a number of authors (Nemat-Nasser121  and Wang et al.122,123 ). Flemion® is a carboxylic acidic ionomer with a chemical backbone similar to Nafion® except for the carboxylic versus sulfonic charged pendant groups, as shown in Figure 1.3.

Figure 1.3

Chemical molecular structure of Flemion®.

Figure 1.3

Chemical molecular structure of Flemion®.

Close modal

Flemion®-based IPMC performance has been observed to be inferior by not being capable to work in air and dynamically slower than Nafion®-based IPMCs and thus have not enjoyed as much attention as perfluorosulfonated IPMCs.

Based on Nafion®-based IPMCs, a number of materials that could provide new applications for industrial, biomedical, defense and space applications have emerged. Obviously, there is a great potential for IPMCs to be adopted as soft biomimetic robotic actuators, artificial muscles, dynamic sensors and energy harvesters in nano-to-micro-to-macro size ranges. The base polymeric materials are typically ion-exchange materials that are designed to selectively pass ions of a single charge (either cation or anion). They are often manufactured from polymers that consist of fixed covalent ionic groups—perfluorinated alkenes with short side chains terminated by ionic groups or styrene/divinylbenzene-based polymers in which the ionic groups are substituted from the phenyl rings where the nitrogen atom fixes an ionic group. These polymers are highly cross-linked. Under an imposed electric potential across the material, ions are usually transported through the material, termed “migration”, and the direction of ions migration is determined by the polarity of the electrodes and the vectorial direction of the imposed electric field. The ion migration rate is determined by the applied potential and the properties of the materials. In practice there are two types of ion-exchange materials: homogeneous and heterogeneous. Homogenous materials are coherent ion-exchange materials having the form of thin films or sheets. Heterogeneous materials are typically fabricated by embedding fine resin particles in inert thermoplastic binders, thereby forming thin sheets or films. Improving the mechanical properties of resulting membranes is of interest. However, they have some disadvantages, showing high electric resistance and reduced long-term integrity due to repeated swelling and de-swelling.

Manufacturing an IPMC begins with selection of an appropriate ionic polymeric material. Often, ionic polymeric materials are manufactured from polymers that consist of fixed covalent ionic groups. The currently available ionic polymeric materials that are convenient to be used as IPMCs are:

  1. Perfluorinated alkenes with short side chains terminated by ionic groups (typically sulfonate or carboxylate [SO3 or COO] for cation exchange or ammonium cations for anion exchange [see Figures 1.1 and 1.3]). Large polymer backbones determine their mechanical strength. Short side chains provide ionic groups that interact with polar liquids such as water and the passage of appropriate ions.

  2. Styrene/divinylbenzene-based polymers in which the ionic groups have been substituted from the phenyl rings where the nitrogen atom is fixed to an ionic group. These polymers are highly cross-linked and are rigid.

The current state-of-the-art IPMC manufacturing technique124–131  incorporates two distinct preparation processes: an initial redox operation to embed a conductive medium within the material and an eventual surface electroding process. Due to different preparation processes, morphologies of precipitated metals are significantly different. The initial compositing process requires an appropriate metallic salt such as Pd (NH3)4HCl or other salts such as AuCl2(phenonthroline)Cl in the context of chemical oxidation and reduction processes similar to the processes evaluated by a number of investigators including Takenaka et al.132  and Millet et al.133  Noble metals such as gold (Au) or platinum (Pt), in the form of charged (oxidized) metal ions, which are dispersed throughout the hydrophilic regions of the polymer, are subsequently reduced to the corresponding metal atoms. This results in the formation of dendritic type electrodes within the molecular network of the polymer. The principle of the electroplating process is to metalize the inner surface of the material by a chemical reduction means such as LiBH4 or NaBH4. The ion-exchange polymer is soaked in a salt solution to allow metal-containing cations to diffuse through via the ion-exchange process. Later, a proper reducing agent such as LiBH4 or NaBH4 is introduced to metalize the polymeric materials by molecular plating. The metallic particles are not homogeneously formed across the material but concentrate predominantly near the interface boundaries. It has been experimentally observed that the metallic particulate layer is buried few microns deep within the IPMC boundary surface and is highly dispersed. The range of average particle sizes has been found to be around 40–60 nm due to reduction around micellar nanoclusters, as shown in Figure 1.4.

Figure 1.4

Nano-clusters within perfluorinated sulfonic ionic polymers.

Figure 1.4

Nano-clusters within perfluorinated sulfonic ionic polymers.

Close modal

These micellar-type nanoclusters generate fractal formations of reduced metallic particles, as shown in Figure 1.5.

Figure 1.5

Dendritic and fractal nature of reduced metals within the IPMC network.

Figure 1.5

Dendritic and fractal nature of reduced metals within the IPMC network.

Close modal

An effective recipe for the manufacturing of IPMCs is:

  • (i) Surface roughening and bead blasting to enhance molecular diffusion of a metallic salt during oxidation;

  • (ii) Ion-exchange processes by oxidation caused by exchanging the H+ cations with positively charged metallic cations such as Pt+ (oxidation);

  • (iii) Metallic molecular deposition by a reduction process, using a strong reducer such as sodium borohydride (NaBH4) or lithium borohyride (LiBH4), which converts the oxidized Pt+ to Pt and deposits them on macromolecules around the nanoclusters and exchanges the H+ cations with Na+ or Li+;

  • (iv) Surface plating and placement of electrodes.

A typical 200 micron ionic polymeric membrane after the above chemical plating will look like what is depicted in Figure 1.6. In the presence of chemically plated electrodes shown in Figure 1.6, an imposed electric field enables the cations such as Na+ or Li+ to migrate towards the cathode, causing the cathode side to expand due to injection of cations and thus create a pressure gradient across the thickness of the membrane to cause it to bend towards the anode electrode, as depicted in Figure 1.7.

Figure 1.6

A 200 micron thick ionic membrane after chemical plating showing fractal formation of reduced metal near the boundary of the membrane acting as a distributed electrode.

Figure 1.6

A 200 micron thick ionic membrane after chemical plating showing fractal formation of reduced metal near the boundary of the membrane acting as a distributed electrode.

Close modal
Figure 1.7

The mechanism of actuation and sensing/energy harvesting in IPMCs due to migration of cations towards the cathode electrode by an imposed electric field or deformation-induced mechanical migration of cations towards boundary surfaces, thus generating a voltage due to Poisson–Nernst–Planck phenomena.

Figure 1.7

The mechanism of actuation and sensing/energy harvesting in IPMCs due to migration of cations towards the cathode electrode by an imposed electric field or deformation-induced mechanical migration of cations towards boundary surfaces, thus generating a voltage due to Poisson–Nernst–Planck phenomena.

Close modal

On the other hand, mechanically deforming the IPMC forces the cations to migrate from compressed regions of the material to the expanding regions of the materials, thus generating an electrical signal due to Poisson–Nernst–Planck phenomena (Cardenas et al.,134  Bolintineanu et al.,135  Porfiri,136  Davidson and Goulbourne,137  Bahramzadeh and Shahinpoor138  and Bahramzadeh139 ). Sadeghipour, et al.65  was the first to establish that flat Nafion® sheets sandwiched flatly between two solid metallic electrodes and in a hydrogen environment can act as vibration damper by generating a sensing signal.

As described before, IPMCs are synthetic composite nanomaterials that display artificial muscle behavior under an applied voltage or electric field. IPMCs are composed of an ionic polymer like Nafion® or Flemion® whose surfaces are chemically plated or physically coated with conductors such as platinum or gold. Under an applied voltage (1–5 V for typical 10 mm×40 mm×0.2 mm samples), ion migration and redistribution due to the imposed voltage across a strip of IPMCs result in a bending deformation. If the plated electrodes are arranged in a non-symmetric configuration, the imposed voltage can induce all kinds of deformations such as twisting, rolling, torsioning, turning, twirling, whirling and non-symmetric bending. On the other hand, if such deformations are physically applied to an IPMC strip, they generate an output voltage signal (a few millivolts for typical small samples) as sensors and energy harvesters. IPMCs are a type of EAP. They work very well in a liquid environment as well as in air. They have a force density of about 40 in a cantilever configuration, meaning that they can generate a tip force of almost 40 times their own weight in a cantilever mode. IPMCs in actuation, sensing and energy harvesting have a very broad bandwidth to kHz and higher.

Ionic polymeric material in a composite form with a conductive medium such as a metal (IPMCs) can exhibit large dynamic deformation if suitably electroded and placed in a time-varying electric field (see Figures 1.8 through 1.12). Conversely, dynamic deformation of such polyelectrolytes can produce dynamic electric fields across their electrodes as shown in Figures 1.13 and 1.14. A recently presented model by de Gennes et al.140  describes the underlying principle of electro-thermodynamics in such ionic polymeric material based upon internal transport phenomena and electrophoresis. It should be pointed out that IPMCs show great potential as soft robotic actuators, artificial muscles and dynamic sensors in the micro-to-macro size range. In this section, the generalities of IPMCs with regard to their manufacturing techniques and phenomenological laws are presented. Later, we present the electronic and electromechanical performance characteristics of IPMCs.

Figure 1.8

Successive photographs of an ionic polymer metal nano-composite strip showing very large deformation (sample is 1 cm×8 cm×0.34 mm under 4 V).

Figure 1.8

Successive photographs of an ionic polymer metal nano-composite strip showing very large deformation (sample is 1 cm×8 cm×0.34 mm under 4 V).

Close modal
Figure 1.9

Typical actuation responses of IPMCs with back relaxation under static electricity (step voltage).

Figure 1.9

Typical actuation responses of IPMCs with back relaxation under static electricity (step voltage).

Close modal
Figure 1.10

Typical actuation response of IPMCs without back relaxation under static electricity (step voltage).

Figure 1.10

Typical actuation response of IPMCs without back relaxation under static electricity (step voltage).

Close modal
Figure 1.11

Non-dynamic tip deformation of a strip of IPMCs (5 mm×1.5 cm×0.2 mm) in cantilever configuration to dynamic voltage and various frequencies.

Figure 1.11

Non-dynamic tip deformation of a strip of IPMCs (5 mm×1.5 cm×0.2 mm) in cantilever configuration to dynamic voltage and various frequencies.

Close modal
Figure 1.12

Variation of tip blocking force (gram force) and the associated deflection (mm) if allowed to move versus the applied step voltage for a 1 cm×5 cm×0.3 mm IPMC Pt–Pd sample in a cantilever configuration.

Figure 1.12

Variation of tip blocking force (gram force) and the associated deflection (mm) if allowed to move versus the applied step voltage for a 1 cm×5 cm×0.3 mm IPMC Pt–Pd sample in a cantilever configuration.

Close modal
Figure 1.13

A typical sensing response of an IPMNC strip of 0.5 cm×2 cm×0.2 mm manually flipped off by about 1 cm tip deflection in a cantilever form and then released.

Figure 1.13

A typical sensing response of an IPMNC strip of 0.5 cm×2 cm×0.2 mm manually flipped off by about 1 cm tip deflection in a cantilever form and then released.

Close modal
Figure 1.14

General response of an IPMC sensor to high-frequency excitations followed by slow bending accompanied by high-frequency noise.

Figure 1.14

General response of an IPMC sensor to high-frequency excitations followed by slow bending accompanied by high-frequency noise.

Close modal

In perfluorinated sulfonic acid polymers, there are relatively few fixed ionic groups. They are located at the ends of side chains so as to position themselves in their preferred orientation to some extent. Therefore, they can create hydrophilic nano-channels, so called cluster networks (Gierke et al.141 ). These configurations are drastically different in other polymers such as styrene/divinylbenzene families that limit, primarily by cross-linking, the ability of the ionic polymers to expand (due to their hydrophilic nature).

The preparation of IPMCs requires extensive laboratory work including an extensive chemical redox operation and electroless chemical plating by means of chemical reduction. The next chapter in this volume presents a detailed state-of-the-art IPMC manufacturing techniques description of this chemical plating. Different preparation processes result in morphologies of precipitated platinum that are significantly different.

Ionic polymeric materials suitably made into a functionally graded composite with a conductor (ionic polymer conductor nano-composites [IPCCs] or IPMCs) such as a metal or synthetic metal, such as conductive polymers, graphite or graphene, which act as a distributed electrode, can exhibit large dynamic deformation if placed in a time-varying electric field (see Figure 1.8) (Shahinpoor,11,13  Shahinpoor and Kim,142–148  2001c).

Typical experimental deflection curves are depicted in Figures 1.9 through 1.12. “Typical” refers to experimenting with smaller samples of the order of a few centimeters in length, about a centimeter in width and about 200 micron thick membranes.

Many investigators have reported a back relaxation phenomenon in IPMCs. Back relaxation in IPMCs is due to the presence of non-hydrated loose water in IPMCs that is carried by the hydrated cations towards the cathode electrode, and once they are settled close to the cathode electrode, they allow the loose water, which had been carried as added mass towards the cathode electrode, to migrate back and thus cause back relaxation or bending of the IPMC strip in the opposite direction. Back relaxation can be avoided by creating a semi-dry IPMC with no loose water, but only hydrated water bonded to cations. Thus, no back relaxation occurs. Another approach to prevent back relaxation is to reduce the IPMCs in the reduction phase at a higher temperature, say 80 °C. This causes a very rapid reduction of metallic nanoparticles around the nanoclusters and traps any loose water in closed cavities. Thus, if the IPMCs are just moist enough not to have loose water molecules but only hydrated water molecules bonded to cations, then no back relaxation occurs, as depicted in Figure 1.10.

Typical frequency-dependent dynamic deformation characteristics of IPMCs are depicted in Figure 1.11.

Once an electric field is imposed on an IPMC cantilever, in their polymeric network the conjugated and hydrated cations rearrange to accommodate the local electric field and thus the network is deformed, which in the simplest of cases, such as in thin membrane sheets, spectacular bending is observed under small electric fields such as tens of volts per millimeter.

Figure 1.12 depicts typical force and deflection characteristics of cantilever samples of IPMC artificial muscles.

Conversely, dynamic deformation of such polyelectrolytes can produce dynamic electric fields across their electrodes, as shown in Figure 1.13 (Shahinpoor143,144 ) and Figure 1.14 (Bahramzadeh139 ). A recently presented model by de Gennes et al.140  describes the underlying principle of electro-thermodynamics in such ionic polymeric materials based upon internal transport phenomena and electrophoresis. It should be pointed out that ionic polymer metal nano-composites (IPMNCs) show great potential as soft robotic actuators, artificial muscles and dynamic sensors in the micro-to-macro size range. In this section, the generalities of IPMCs with regard to their manufacturing techniques and phenomenological laws are presented. Later, we present the electronic and electromechanical performance characteristics of IPMCs.

Figure 1.14 shows the dynamic sensing or transduction response of an IPMC strip in a cantilever form subject to dynamic impact loading followed by slow movement in a cantilever configuration.

A dampened electric response is observed due to impact loading or flipping followed by a user-friendly response perfectly matching the movement of the external mover, which is highly repeatable with a high bandwidth of up to tens of kHz. Such direct mechanoelectric behaviors are related to the endo-ionic mobility due to imposed stresses.

In perfluorinated sulfonic acid polymers, there are relatively few fixed ionic groups. They are located at the ends of side chains so as to position themselves in their preferred orientation to some extent. Therefore, they can create hydrophilic nano-channels, so called cluster networks (Gierke et al.141 ). Such configurations are drastically different in other polymers such as styrene/divinylbenzene families that limit, primarily by cross-linking, the ability of the ionic polymers to expand (due to their hydrophilic nature).

A study by de Gennes et al.140  has presented the standard Onsager formulation on the fundamental principles of IPMC actuation/sensing phenomena using linear irreversible thermodynamics: when static conditions are imposed, a simple description of mechanoelectric effects is possible based upon two forms of transport: ion transport (with a current density, J, normal to the material) and electrophoretic solvent transport (with a flux, Q, we can assume that this term is hydrated cation water flux).

The conjugate forces include the electric field, E⃑, and the pressure gradient, −∇p. The resulting equations have the following concise forms:

Equation 1.1
Equation 1.2

where σ and K are the material conductance and the Darcy permeability, respectively. A cross coefficient is usually L=L12=L21, experimentally measured to be of the order of 10−8 ([m s−1]/[V m−1]). The simplicity of the above equations provides a compact view of fundamental principles of both actuation and sensing of IPMCs, as depicted in Figure 1.7.

The IPMC is composed of a perfluorinated ionic polymer, which is chemically surface composited with a conductive medium such as platinum. A platinum layer is formed a few microns deep within the perfluorinated ionic polymer.

Typically, the strip of perfluorinated ionic polymer membrane bends toward the anode (in case of cation exchange membranes) under the influence of an electric potential. As reported earlier, de Gennes et al.140  presented the first phenomenological theory for sensing and actuation in IPMCs. Asaka and coworkers18  discussed the bending of polyelectrolyte membrane–platinum composites by electric stimuli and presented a theory on actuation mechanisms in IPMC by considering the electro-osmotic drag term in transport equations. Let us now summarize the underlying principle of the ionic polymeric nano-composites’ actuation and sensing capabilities, which can be described by the standard Onsager formulation using linear irreversible thermodynamics.

When static conditions are imposed, a simple description of mechanoelectric effects is possible based upon two forms of transport: cation transport (with a current density, , normal to the material) and solvent transport (with a flux, Q̰, we can assume that this term is hydrated cation water flux). The conjugate forces include the electric field, , and the pressure gradient, −∇̰p. A cross coefficient is usually L=L12=L21. The simplicity of the above equations provides a compact view of the underlying principles of actuation, transduction and sensing of the ionic polymer nanocomposites. When one measures the direct effect (actuation mode), one works (ideally) with electrodes that are impermeable to cation species flux and the cation flux has come to static equilibrium under the imposed electric field and thus Q̰=0. This then yields from eqn (1.2):

Equation 1.3

This ∇̰p(x,y,z,t) will, in turn, induce a curvature κ̰E proportional to ∇̰p(x,y,z,t) according to simple beam theory. The relationships between the curvature κ̰E and pressure gradient ∇̰p(x,y,z,t) are fully derived and described in de Gennes et al.140  Let us just mention that (1/ρc)=M(E)/YI, where M(E) is the local induced bending moment and is a function of the imposed electric field E, Y is the Young's modulus (elastic stiffness) of the strip, which is a function of the hydration of the IPMCs, and I is the moment of inertia of the strip. Note that, locally, M(E) is related to the pressure gradient such that, in a simplified scalar format:

Equation 1.4

Note from eqn (1.4) that if the thickness of the IPMC bending strip is t*, then due to symmetry, σt=M(E)t*/2I and σc=−M(E)t*/2I are, respectively, the tensile and compressive stresses on the cathode and the anode sides of the IPMC bending strip. Thus, one can easily show that:

Equation 1.5

Now from eqn (1.3) through (1.5), it is clear that the vectorial form of curvature κ̰E is related to the imposed electric field E by:

Equation 1.6

Based on this simplified model, the tip bending deflection δmax of an IPMC strip of length lg should be almost linearly related to the imposed electric field due to the fact that:

Equation 1.7

It is also clear that in the sensing or energy harvesting mode, a mechanical moment is externally applied to the IPMC strip, which causes it to generate a pressure gradient ∇̰p(x,y,z,t) and curvature κ̰E. The deformation characteristics observed experimentally (Shahinpoor et al.120 ) are clearly consistent with the above predictions obtained by the above linear irreversible thermodynamics formulation, which is also consistent with eqn (1.3) and (1.4) in the steady-state conditions and has been used to estimate the value of the Onsager coefficient L to be of the order of 10−8 m2 V-s−1.

Here, we have used a low-frequency electric field in order to minimize the effect of loose water back diffusion under a step voltage or a DC electric field. Other parameters have been experimentally measured to be K=∼10−18 m2 CP−1 (Centipoise) and σ=∼1A mV−1 or S (Siemens) m−1. Figure 1.15 depicts a more detailed set of data pertaining to Onsager coefficient L.

Figure 1.15

Experimental determination of Onsager coefficient L using three different samples.

Figure 1.15

Experimental determination of Onsager coefficient L using three different samples.

Close modal

On the other hand, one may consider charge transport modeling of actuation and sensing, as described in the next section following the work of Porfiri,136  Davidson and Goulbourne137  Bahramzadeh and Shahinpoor138  and Bahramzadeh.139 

The actuation and sensing behavior of IPMCs is due to the ionic behavior of material in which cations are able to freely move across the Nafion® membrane while anions are fixed in the polymer network. Unlike the ordinary mass transport in which diffusion is caused merely because of uneven mass concentration in a media, ion transport is governed by both mass concentration and electrical field, which respectively cause the diffusion and migration of ions across the membrane.

  1. Constitutive equation of Nernst–Planck

  2. Continuity equation of ions

  3. Equilibrium equation of Nernst–Planck

  4. Poisson's equation

Assumptions

  1. The diffusion coefficient D is constant over the thickness.

  2. The ion diffusion is more dominant over the thickness of the membrane than in two other dimensions.

  3. Mobile ions contributing for diffusion are cations.

  4. In this configuration, we define our coordinate axis (i.e. x=0) to coincide with the neutral axis.

The constitutive equation that correlates the ion diffusion to local ion concentration as well as ion migration due to potential gradient was given by the Nernst–Planck equations as follows:

Equation 1.8

In eqn (1.8), J is the flux of ionic species in mol (m2 s)−1, C is the concentration of ionic species in mol m−3, V is the electric potential field in volts, D is the diffusion coefficient in m2 s−1, z is the valence of ionic species, F is Faraday's constant, R is the universal gas constant and T is the temperature in degrees Kelvin. The first term on the right-hand side of the eqn (1.8) is the diffusion effect and the second term is the migration term due to electrophoresis potential. Parameters J, C and V are functions of position and time. The Nernst–Planck equation is used for modeling of other phenomena such as swelling of hydrogels or induced local electric charge at the cellular level since ion transport plays the major role in their kinetics. Using the Nernst–Planck equation, the sensing and actuation mechanism can be described as described below.

As an external voltage is applied at both sides of membrane, an electric field gradient across the membrane is induced. According to Nernst–Planck equation 1, the second term on the right-hand side acts as an external force that excites the movement of ions and results in differences in ion concentrations across the membrane. Differences in ion concentrations result in expansion and contraction of polymers, which consequently applies a mechanical pressure due to ion diffusion at two sides of membrane, which results in bending of the membrane.

Another important characteristic of IPMCs is their step response, during which bending relaxation occurs after a fast bending. We will investigate this phenomenon after ion charge modeling and solving the ion kinetic equations for IPMCs.

Applying a mechanical pressure induces the ion diffusion, which results in an electric current in short periods of time and also an electric potential at the electrodes of both sides of the IPMC. The exact mechanism that causes ion diffusion due to mechanical stimuli should be investigated by considering the micro-mechanics of ion diffusion in ion channels of porous Nafion® membranes. Refer to Park and Kim,149  Henderson et al.150  Shahinpoor et al.151  and Leo et al.152 

The electric potential between two electrodes lasts for a few seconds. Again, by using the Nernst–Planck equation 1, the phenomena can be described as follows: after moving cations to one side of the membrane and generating an electric signal, a difference in ion concentration is created that causes ion diffusion at the reverse side and ions tend to distribute evenly across the membrane to maintain a more stable condition. This causes the induced electric potential to disappear after a few seconds. It should be noted that the charge dynamic coupling with mechanical stimulation is modeled using an equation that relates the osmosis pressure to the ion concentration; however, this relation gives a macroscopic model of this coupling and it is not able to model different mechanical stimulation mode effects of current outputs of IPMC sensors. Exact microscopic models are required to take into account the effects of induced strain mode shape (bending, twisting and tension) on the porous media, their effect on the expansion and contraction of micro-channels of membrane and also ion species diffusion.

The continuum equation for ion flow has been driven using two of the Maxwell's equation (i.e. Ampère's law and Gauss’ law). The equation states that the divergence of the current density is equal to the negative rate of change of the charge density or:

Equation 1.9

The general form of the continuity equation can be derived by combining eqn (1.8) and (1.9), which gives us the general partial differential equation for ion transport in one dimension:

Equation 1.10

Here, we assumed that the diffusion constant D is constant over thickness.

Eqn (1.10) includes two unknown variables, cation concentration c+ and electric potential V. In order to solve this equation, a relation between these two unknowns is required. Poisson's equation (which is derived from Maxwell's equation) gives the relation between charge density and electric potential V. It states that ion concentration induces an electric field according to the following relation:

Equation 1.11

where ρ is the charge density in (Coulomb m−3), ε is the permittivity of the medium and:

Equation 1.12

where εr is the relative permittivity of the material, ε0 is the vacuum permittivity or ε0=8.85×10−12 F m−1. On the other hand, charge density is proportionally related to ion concentration by the following equation:

Equation 1.13

Ion species include cations and anions and valences of these ions are 1 for cations and −1 for anions, so eqn (1.13) can be written as:

Equation 1.14

Eqn (1.9) through (1.11) are the most general governing equations for charge kinetics of ionic polymers. The equations have been rewritten here:

Equation 1.15
Equation 1.16
Equation 1.17

This concludes a very brief coverage of charge dynamics in IPCCs as multi-functional intelligent materials with distributed nano-sensing, nano-actuation and nano-transduction capabilities. The reader is referred to a comprehensive treatment of charge dynamics in IPMCs by Porfiri.136  For microelectromechanical modeling of IPMCs, the reader is referred to Nemat-Nasser et al.153 

Figure 1.16 shows tensile testing results, in terms of normal stress versus normal strain, on a typical IPMC (H+ form) relative to Nafion®-117 (H+ form). Recognizing that Nafion®-117 is the adopted starting material for this IPMC, this comparison is useful.

Figure 1.16

Tensile testing results. These show normal stress, σN, vs. normal strain, εN; IPMC and Nafion®-117™. Note that both samples were fully hydrated when they were tested.

Figure 1.16

Tensile testing results. These show normal stress, σN, vs. normal strain, εN; IPMC and Nafion®-117™. Note that both samples were fully hydrated when they were tested.

Close modal

There is a little increase in mechanical strength of IPMC (both stiffness and the modulus of elasticity), but it still follows the intrinsic nature of Nafion® itself. This means that, in the tensile (positive) strain, the stress/strain behavior is predominated by the polymer material rather than the metallic coatings.

Although the tensile testing results show the intrinsic nature of the IPMC, a problem arises when the IPMC operates in a bending mode. Dissimilar mechanical properties of the metal particles (the electrode) and polymer network seem to affect each other. Therefore, in order to construct the effective stress/strain curves for IPMCs, strips of IPMCs are suitably cut and tested in a cantilever configuration. In a cantilever configuration, the end deflection δ due to a distributed load w(s,t), where s is the arc length of a beam of length L and t is the time, can be related approximately to the radius of curvature r of the bent cantilever beam, i.e.,

Equation 1.18

The stress σ can be related to the strain ε by an appropriate constitutive equation. If deflections are very small, then simply Hooke's law can be used, assuming linear elasticity. On the other hand, one may assume rubber elasticity for IPMCs such that stress σ is related to the stretch λ in a nonlinear fashion, as depicted below:

Equation 1.19

At any rate, it is generally accepted that the stress is related to bending moment M such that σ=Mt*/2I, where σ is the stress tensor, M is the maximum moment at the built-in end, t* is the thickness of the strip and I is the moment of inertia of the cross-section of the beam. Thus, the moment M can be calculated based on the distributed load on the beam or the applied electrical activation of the IPMC beam. Having also calculated the area moment of inertia I, which for a rectangular cross-section of width b will be I=bh3/12, the stress σ can be related to the strain ε. According to the Euler–Bernoulli beam theory, the bending moment is proportional to the change in the beam curvature. Thus we have:

Equation 1.20

where r is the radius of beam curvature and EI is the equivalent flexural rigidity of the actuator, which can be defined based on the elastic modulus and thickness of each layer. The relation between the bending displacement and the curvature is shown in Figure 1.17, where L is the actuator length and r, x and y are given by eqn (1.21).

Equation 1.21
Figure 1.17

Relation between the bending displacement and the curvature of IPMC cantilever beams.

Figure 1.17

Relation between the bending displacement and the curvature of IPMC cantilever beams.

Close modal

Here, electric activation refers to the IPMC in the electromechanical mode exhibiting increased stiffness due to redistributed hydrated ions or the nonlinear characteristics of the electromechanical properties of the IPMC.

In order to assess the electrical properties of IPMCs and their equivalent circuits, the standard AC impedance method that can reveal the equivalent electric circuits has been adopted. A typical measured impedance plot, provided in Figure 1.18, shows the frequency dependency of impedance of the IPMC. It is interesting to note that the IPMC is nearly resistive (>50 Ω) in the high-frequency range and fairly capacitive (>100 μF) in the low-frequency range (Leary et al.,109  Henderson et al.150  and Shahinpoor et al.151 ).

Figure 1.18

The measured AC impedance spectra (magnitude) of an IPMC sample. (The IPMC sample has a dimension of 5 mm width, 20 mm length and 0.2 mm thickness.)

Figure 1.18

The measured AC impedance spectra (magnitude) of an IPMC sample. (The IPMC sample has a dimension of 5 mm width, 20 mm length and 0.2 mm thickness.)

Close modal

Based upon the above findings, we consider a simplified equivalent electric circuit of the typical IPMC, such as the one shown in Figure 1.19. In this approach, each single-unit circuit (i) is assumed to be connected in a series of arbitrary surface resistance (Rss) in the surface. This approach is based upon the experimental observation of the surface electrode resistance. It is assumed that there are four components to each single-unit circuit: the surface electrode resistance (Rs), the polymer resistance (Rp), the capacitance related to the ionic polymer and the double layer at the surface electrode/electrolyte interface (Cd) and impedance (Zw) due to a charge transfer resistance near the surface electrode.

Figure 1.19

A possible equivalent electric circuit of typical IPMCs (top) and measured surface resistance, Rs, as a function of platinum penetration depth (bottom). Note that scanning electron microscope (SEM) was used to estimate the penetration depth of platinum in the membrane. The four-probe method was used to measure the surface resistance, Rs, of the IPMCs. Clearly, the deeper the penetration, the lower the surface resistance.

Figure 1.19

A possible equivalent electric circuit of typical IPMCs (top) and measured surface resistance, Rs, as a function of platinum penetration depth (bottom). Note that scanning electron microscope (SEM) was used to estimate the penetration depth of platinum in the membrane. The four-probe method was used to measure the surface resistance, Rs, of the IPMCs. Clearly, the deeper the penetration, the lower the surface resistance.

Close modal

For the typical IPMC, the importance of Rss relative to Rs may be interpreted from ΣRss/RsL/t≫1, where notations L and t are the length and thickness of the electrode, respectively. In order to increase the surface conductivity, a thin layer of a highly conductive metal (such as gold) is deposited on top of the platinum surface electrode (Shahinpoor and Kim149,150 ).

Realizing that water contained in the perfluorinated IPMC network is the sole solvent that can create useful strains in the actuation mode, another issue to deal with is the so-called “decomposition voltage”. As can be clearly seen in Figure 1.20, the decomposition voltage is the minimum voltage above which electrolysis occurs.

Figure 1.20

Steady-state current, I, versus, applied voltages, Eapp, on typical IPMCs. ERI-K1100 stands for a proprietary IPMC fabricated by Environmental Robots, Inc.—it has a thickness of 1.9 mm and is suitably plated with platinum/gold.

Figure 1.20

Steady-state current, I, versus, applied voltages, Eapp, on typical IPMCs. ERI-K1100 stands for a proprietary IPMC fabricated by Environmental Robots, Inc.—it has a thickness of 1.9 mm and is suitably plated with platinum/gold.

Close modal

This figure contains the graph of steady-state current, I, versus applied DC voltage, Eapp, showing that as the voltage increases, there is little change in current (obeying Faraday's law). However, a remarkable increase in DC current is observed with a small change of voltage. Even though the intrinsic voltage causing water electrolysis is about 1.23 V, a small over-potential (approximately 0.3–0.5 V) can be observed.

Figure 1.21 depicts measured cyclic current/voltage responses of a typical IPMC (the scan rate of 100 mV s−1 is used). As can be seen, a rather simple behavior with a small hysteresis is obtained. Note that the reactivity of the IPMC is mild such that it does not show any distinct reduction or re-oxidation peaks within ±4 V, except for a decomposition behavior at ∼±1.5 V, where the extra current consumption is apparently due to electrolysis.

Figure 1.21

Current/voltage curves for a typical IPMC (Nafion®-117-based IPMC).

Figure 1.21

Current/voltage curves for a typical IPMC (Nafion®-117-based IPMC).

Close modal

Clearly, the behavior of the IPMC shows a simple trend of ionic motions caused under an imposed electric field. Note that the scan rate is equal to 100 mV s−1.

In Figure 1.22, frequency responses of the IPMC are expressed in terms of the normal stress versus the normal strain. Its frequency dependency shows that as frequency increases, the beam displacement decreases. However, it must be realized that, at low frequencies (0.1–1 Hz), the effective elastic modulus of the IPMC cantilever strip under an imposed voltage is also rather small. On the other hand, at high frequencies (5–20 Hz), such moduli are larger and displacements are smaller.

Figure 1.22

Frequency dependency of the normal stress, σN, vs. the normal strain, εN, under an imposed step voltage of 1 V (this Nafion®-117 IPMC has a cation of Li+ and a size of 5 mm×20 mm). Note that the scan rate is 100 mV s−1.

Figure 1.22

Frequency dependency of the normal stress, σN, vs. the normal strain, εN, under an imposed step voltage of 1 V (this Nafion®-117 IPMC has a cation of Li+ and a size of 5 mm×20 mm). Note that the scan rate is 100 mV s−1.

Close modal

This is due to the fact that at low frequencies water and hydrated ions have time to appear on the expanded surface by gushing out of the surface electrodes. In fact, under these circumstances, one can observe oscillatory color changes on the surface as, for example, greenish–bluish hydrated Li cations migrate from one surface to another. At high frequencies, the hydrated cations are rather contained and trapped inside the polymer. Therefore, the nature of water and hydrated ion transport within the IPMC can affect the moduli at different frequencies.

A simple behavior with a small hysteresis can be seen. It does not show any distinct reduction or re-oxidation peaks within ±4 V, except for a decomposition behavior at ∼±1.5 V, where the extra current consumption is apparently due to electrolysis.

The back relaxation phenomenon observed in IPMC cantilever strips under an imposed DC voltage is due to the presence of non-hydrated loose water molecules within the network. Thus, as hydrated cations migrate from the anode electrode side towards the cathode electrode side, they invariably carry some loose water molecules, as added mass, to the cathode side, and if the actuation voltage is DC, then the IPMC should in fact stay at the end point of activation and not sway back (back relaxation), but it does because upon static equilibrium, the loose water then flows back to the anode side and thus causes the IPMC strip to bend backwards and thus show some back relaxation. If the IPMCs are just moist and with no loose water molecules, then no back relaxation occurs. Note that various cations have various hydration numbers, which are the numbers of water molecules that bind and stick to cations and are somehow in solid crystalline form (Shahinpoor et al.120 ). For example, the hydration number for Na+ is 3–4 and for Li+ is 5–6 water molecules. Thus, in order to prevent back relaxation in air, the IPMCs should be just moist with no loose water content. Of course, no back relaxation occurs when IPMCs are dynamically oscillating. Further, experiments have shown that addition of polyvinyl pyrrolidone (PVP) and some temperature manipulation during the redox operation totally eliminates back relaxation and allows IPMCs to operate in air elegantly and spectacularly.120  Note that the nature of water and hydrated ion transport within the IPMCs can also affect the moduli at different frequencies and presents potential applications to smart materials with a circulatory system. This obviously is a biomimetic phenomenon in the sense that all living systems have some kind of circulatory fluid to keep them smart and surviving. This is also of interest in a similar way to ionic hydraulic actuators (Shahinpoor and Kim144,145 ).

IPMCs or IPCCs are basically water-loving living muscles. However, water can be replaced with ionic liquids or other polar liquids as reported in a number of publications recently (Wang et al.123  and Bennet and Leo146 ). Ionic liquids are salt-like ionic materials that are in a liquid state below 100 °C. They are heavily used in chemical processes, for example as solvents, separation media and performance chemicals, such as electrolytes and lubricants. Typical ionic liquids used are ammonium, choline, imidazolium, basionics, phosphonium, pyrazolium, pyridinium, pyrrolidinium and sulfonium, among others. Imidazolium has been used by IPMC researchers more than other, and in particular, one of its derivatives (C9H11F6N3O4S2 or 1-allyl-3-methylimidazoliumbis [trifluoromethylsulfonyl] imide) has been used more than others. Use of ionic liquids has not become popular because of their toxicity as well as the fact that they are very expensive.

Encapsulation by highly elastic thin membranes such as Saran® F-310 (Dow Chemicals) or liquid latex has been effective in maintaining a fairly constant polar medium for cation mobility and consistent performance. For example, if an IPMC sample is just moist enough to not show any back relaxation, then it can be immediately encapsulated in an elastically flexible plastic encapsulate. Thus, the IPMC sample maintains the same moisture or humidity and renders consistent and robust actuation and sensing capabilities. The only disadvantage is that part of the electrical forces causing, say, bending in IPMCs is in fact used to elastically stretch the encapsulate as it deforms with the IPMC inside.

As discussed earlier, a key engineering problem in achieving high-force density IPMCs is reducing or eliminating the water leakage out of the surface electrode (made of finely dispersed platinum particles within or near the boundary region) so that water transport within the IPMC can be more effectively utilized for actuation. As reported by Shahinpoor et al.,120  the average size of platinum particles in the IPMC near boundary is in the order of 40–60 nm, much larger than that of incipient particles associated with ion clusters (∼5 nm). Thus, the incipient particles coagulate during the chemical reduction process and eventually grow large, as schematically illustrated in Figures 1.23 and 1.24.

Figure 1.23

A schematic illustration of platinum coagulation during the chemical reduction process.

Figure 1.23

A schematic illustration of platinum coagulation during the chemical reduction process.

Close modal
Figure 1.24

TEM micrographs of two samples of IPMCs with (a) and without (b) polyvinyl pyrolidone (PVP) treatment. Note that the addition of PVP causes the nanoparticles of platinum not to coalesce and create a uniform and fairly homogeneous distribution of particles. This is believed to create more uniform internal electric fields and cause the increased force capability of IPMCs.

Figure 1.24

TEM micrographs of two samples of IPMCs with (a) and without (b) polyvinyl pyrolidone (PVP) treatment. Note that the addition of PVP causes the nanoparticles of platinum not to coalesce and create a uniform and fairly homogeneous distribution of particles. This is believed to create more uniform internal electric fields and cause the increased force capability of IPMCs.

Close modal

It is realized that there is significant potential for controlling the reduction process in terms of platinum particle penetration, size and distribution. This could be achieved by introducing effective dispersing agents (additives) during the chemical reduction process. It is observed that the additives would enhance the dispersion of platinum particles within the ionic polymer molecular network and thus reduce coagulation. As a result, a better platinum particle penetration in the polymer with a smaller average particle size and more uniform distribution could be obtained.

This uniform distribution makes it more difficult for water to pass through (granular damming effect). Thus, the water leakage out of the surface electrode could be significantly reduced by maintaining platinum nanoparticles and preventing them from coalescing. The use of effective dispersing agents during the platinum metallization process has recently resulted in dramatically improved force density characteristics. The results are shown in Figure 1.25, which reports the measured force of the improved IPMCs relative to conventional ones. As clearly seen, the additive-treated IPMC has shown:

  • (i) A much sharper response to the input electric field; and

  • (ii) A dramatically increased force density generation by as much as 100%.

One key observation is the virtual disappearance of the delayed response, which has been observed in the conventional IPMCs. Such an effect can be translated into a higher-power IPMC than any other IPMCs reported so far.

Figure 1.25

Force response characteristics of the improved IPMC versus the conventional IPMC. Note that the improved IPMC is treated with an effective dispersing agent.

Figure 1.25

Force response characteristics of the improved IPMC versus the conventional IPMC. Note that the improved IPMC is treated with an effective dispersing agent.

Close modal

In Figure 1.26, a scanning electron microscope (SEM) micrograph along with its X-ray line-scan is provided.

Figure 1.26

A SEM micrograph of an IPMC treated with a dispersing agent (top) and its X-ray line-scan (middle) and platinum penetration profiles (bottom). As can be seen, the Pt penetration is more and consistent.

Figure 1.26

A SEM micrograph of an IPMC treated with a dispersing agent (top) and its X-ray line-scan (middle) and platinum penetration profiles (bottom). As can be seen, the Pt penetration is more and consistent.

Close modal

As can be seen, good platinum penetration is achieved, meaning that an effective additive enhances platinum dispersion, leading to better penetration in the polymer. A convenient way to handle this situation (free diffusion into a finite porous slab or membrane) is to use an effective diffusivity, Deff, and then to consider it as one-dimensional. Assuming fast kinetics for the metal precipitation reaction of:

Equation 1.22

The precipitated platinum concentration, Nx, can be expressed as:

Equation 1.23

where notations CPt(δt), CPt,i and δt are the platinum concentration, the platinum concentration at the interface and the particle penetration depth, respectively. For a typical reduction time of t=15 minutes (Figure 1.26), eqn (1.23) is plotted for values of Deff=1×10−10, 1×10−9 and 1×10−8 cm2 s−1, respectively. The effective diffusivity, Deff, could be estimated to be of the order of 1×10−8 cm2 s−1 for the improved IPMC. Although this situation is somewhat complicated due to the simultaneous effect of a mass transfer and significant kinetics, nevertheless the estimated value of Deff of ∼1×10−8 cm2 s−1 would be a convenient value for the engineering design of the platinum metallization process described here for the improved IPMC.

In Figure 1.27, the results of a potentiostatic analysis are presented. The variation of currents following the application of an electric potential to the IPMCs (both the PVP-treated IPMC and the conventional IPMC) are shown.

Figure 1.27

Potentiostatic Coulombmetric analysis of the additive-treated IPMC and the conventional IPMC.

Figure 1.27

Potentiostatic Coulombmetric analysis of the additive-treated IPMC and the conventional IPMC.

Close modal

This graph shows that an increased current passage (Faraday approach) can contribute to the observed improvement in the force characteristics of IPMC strips (see Figure 1.25). The current decays exponentially. The charge transfer is . It is useful to make a direct comparison between Qt,PVP (for the PVP-treated IPMC) and Qt (for the conventional IPMC). The data shown in Figure 1.27 give Qt,PVP/Qt≅1.1. This means that the PVP-treated IPMC consumes approximately 10% more charge. This raises the question as to whether a 10% increased consumption of charge is not the only reason to increase the force density by as much as 100%.

An increase in force density of as much as 100% represents a very favorable gain for a 10% increase in consumed charge. Therefore, it can be concluded that the “granular damming effect” that minimizes the water leakage out of the porous surface electrode region, when the IPMC strip bends, is important.

In connection with the phenomenological laws and irreversible thermodynamics considerations previously discussed in Section 1.6, when one considers the actuation with ideal impermeable electrodes, which results in Q=0 from eqn (1.1) and (1.2), one has:

Equation 1.24

Also, the pressure gradient can be estimated from:

Equation 1.25

where σmax and h are the maximum stress generated under an imposed electric field and the thickness of the IPMC, respectively. The values of σmax can be obtained when the maximum force (=the blocking force) was measured at the tip of the IPMC per a given electric potential. In Figure 1.28, the maximum stresses generated, σmax, under an imposed electric potential, Eo, for both calculated values and experimental values of the conventional IPMC and the improved IPMC are presented. It should be noted that the improved IPMC (by the method of using additives) is superior to the conventional IPMC approaching the theoretically obtained values. For theoretical calculation, the following experimentally measured values were used: (i) L12=L21=2×10−8 (cross coefficient, [m s−1]/[V m−1]); (ii) k=1.8×10−18 (hydraulic permeability, m2 [Shahinpoor et al.120 ]); and (iii) E⃑=E0/h, where h=200 µm (membrane thickness).

Figure 1.28

Maximum stresses generated by the IPMCs at given voltages.

Figure 1.28

Maximum stresses generated by the IPMCs at given voltages.

Close modal

The bending force of the IPMC is generated by the effective redistribution of hydrated ions. Typically, such a bending force is electric field-dependently distributed along the length of the IPMC strip. Further, note that a surface voltage drop occurs, which can be minimized (Shahinpoor and Kim,144,145  and Shahinpoor.154–160  The IPMC strip bends due to this ion migration-induced hydraulic actuation and redistribution.

The bending force of the IPMC is exerted by the effectively strained IPMC due to hydrated ion transport. Typically, such force is field-dependently distributed along the length of the IPMC strip. The IPMC strip bends due to this force. The total bending force, Ft, can be approximated as:

Equation 1.26

where f is the force density per unit arc length S and L is the effective beam length of the IPMC strip. Assuming a uniformly distributed load over the length of the IPMC, then, the mechanical power produced by the IPMC strip can be obtained from:

Equation 1.27

where v is the local velocity of the IPMC in motion. Note that v is a function of S and can be assumed to linerarly vary, such that v=(vtip/L)S, 0≤SL. Finally, the thermodynamic efficiency, Eff,em, can be obtained as:

Equation 1.28

where Pin is the electrical power input to the IPMC, i.e. Pin=V(t)I(t), where V and I are the applied voltage and current, respectively. Based on eqn (1.28), one can construct a graph (see Figure 1.29) that depicts the thermodynamic effciency of the IPMC as a function of frequency.

Figure 1.29

Thermodynamic efficiency of actuation of the IPMC as a function of frequency.

Figure 1.29

Thermodynamic efficiency of actuation of the IPMC as a function of frequency.

Close modal

Note that this graph presents the experimental results for the conventional IPMC and the improved additive-treated IPMC. It is of note that the optimum efficiencies occur at near 5–10 Hz for these IPMCs. The optimum values of these IPMCs are approximately 1.5–3.0%.

At low frequencies, the water leakage out of the surface electrode seems to reduce the efficiency significantly. However, the additive-treated IPMC shows a dramatic improvement in efficiency due to reduction in water leakage out of the electrode surface.

The important sources of energy consumption for the IPMC actuation could be from:

  • (i) The necessary mechanical energy needed to cause the positive/negative strains for the IPMC strip;

  • (ii) The I/V hysteresis due to the diffusional water transport within the IPMC;

  • (iii) The thermal losses—Joule heating (see Figure 3.22);

  • (iv) The decomposition due to water electrolysis;

  • (v) The water leakage out of the electrode surfaces.

Despite our effort to improve the performance of the IPMC by blocking water leakage out of the porous surface electrode, the overall thermodynamic efficiencies of all IPMC samples tested in a frequency range of 0.1–10 Hz remain somewhat low. However, it should be noted that the obtained values are favorable compared to other types of bending actuators, i.e. conducting polymers and piezoelectric materials at similar conditions, which exhibit considerable lower efficiencies (Wang et al.122,123 ).

  • Nafion®-117 was used as a starting material.

  • The samples had a dimension of a 20 mm length, 5 mm width and 0.2 mm thickness.

  • The applied potential is 1 V step.

  • Lines are least square fits.

  • Resonant efficiencies are not included in this figure.

Figure 1.30 displays infrared thermographs taken for an IPMC in action (the sample size is 1.2×7.0 cm). They show spectacular multi-species mass/heat transfer in a sample of IPMCs under an oscillatory step voltage of 3 V and a frequency of 0.1 Hz. The temperature difference is more than 10 °C. In general, the hot spot starts from the electrode and propagates toward the tip of the IPMC strip (left to right). The thermal propagation is simultaneously conjugated with the mass transfer along with the possible electrochemical reactions. It clearly shows the significance of water transport within the IPMC. These coupled transport phenomena are currently under investigation.

Figure 1.30

Infrared thermographs of an IPMC in action.

Figure 1.30

Infrared thermographs of an IPMC in action.

Close modal

Note that in Figure 1.30 the hot spot starts from the electrode and propagates toward the tip of the IPMC strip. The electrode is positioned at the left side of the IPMC. The temperature difference is more than 10 °C when a DC voltage of 3 V was applied to the IPMC sample with a size of 1.2×7.0 cm.

In order to determine the cryogenic characteristics of IPMNC sensors and actuators for harsh space conditions, various samples of IPMNCs were tested in a cryochamber under very low pressures of down to 2 Torrs and temperatures of down to −150 °C. This was done to simulate the harsh, cold, low-pressure environments in space. The results are depicted in Figures 1.31–1.36.

Figure 1.31

Deflection characteristics of IPMCs as a function of time and temperature.

Figure 1.31

Deflection characteristics of IPMCs as a function of time and temperature.

Close modal
Figure 1.32

Power of the IPMC strip bending actuator versus voltage.

Figure 1.32

Power of the IPMC strip bending actuator versus voltage.

Close modal
Figure 1.33

Deflection of the bending IPMNC strip as a function of voltage.

Figure 1.33

Deflection of the bending IPMNC strip as a function of voltage.

Close modal
Figure 1.34

Deflection versus current drawn (top) and power input (bottom) at a high pressure of 850 Torrs and a low pressure of 0.4–1 Torrs.

Figure 1.34

Deflection versus current drawn (top) and power input (bottom) at a high pressure of 850 Torrs and a low pressure of 0.4–1 Torrs.

Close modal
Figure 1.35

IPMC strip static (V/I) and dynamic (V/I) resistances at various temperatures.

Figure 1.35

IPMC strip static (V/I) and dynamic (V/I) resistances at various temperatures.

Close modal
Figure 1.36

The relation between voltage and current for an IPMNC strip that was exposed to room temperature=20 °C and to −100 °C.

Figure 1.36

The relation between voltage and current for an IPMNC strip that was exposed to room temperature=20 °C and to −100 °C.

Close modal

IPCCs and IPMCs have been shown to be capable of inducing electrically controllable autonomous changes in material properties by an intrinsic distributed circulatory system.

Thus, they have the potential for creating a new class of structural nano-composites of ionic polymers and conductors such as metals, graphene, conductive polymers or carbon nano-tubes with an embedded circulatory system capable of producing electrically controlled localized internal pressure changes to hydraulically pump liquids containing hydrated ions and chemicals to various parts of the material to cause sensing, actuation, large changes in stiffness and conductivity and perform self-repair or healing. Ionic polymers equipped with a distributed network of electrodes created by a chemical plating procedure such as IPMCs are capable of creating an intrinsic distributed circulatory system of ions, chemicals, water and polar and ionic liquids whose fluid motion is generated by electrically induced migration and redistribution of conjugated ions within its polymeric network of nano-clusters. Every part of the material can be reached by electric field-induced migration and redistribution of conjugated ions on a nanoscale for robotic motion action and feedback, as well as embedded distributed sensing and transduction.

One of the most important characteristics of these nano-IPMCs as smart multi-functional polymeric nano-composites is their ability to allow ionic migration on a molecular and nano scale by means of an imposed local intrinsic electric field within the material, which then causes hydrated or otherwise loaded cations to move and create a local pressure or fluid motion while carrying additional water as hydrated water as well. Such fluid circulatory migration in ionic polymers had been observed (Shahinpoor et al.120 ) as early as in 2000 in the form of hydrated cations’ appearance and disappearance on the cathode side of IPMC strips under an imposed sinusoidal or square wave electric field for actuation. This area of research in IPMCs will be further explored in the near future.

The fact that one could put a pair of electrodes in the middle of a strip of IPMCs and make it bend and grab objects like a soft parallel jaw robotic gripper inspires one to think about the rapid nastic sensing and actuation of higher plants such as the carnivorous or insectivorous plants. The induced spectacular bending motion and the built-in sensing characteristics of the IPMCs leads one to consider that some higher plants such as the Venus flytrap (Dionaea muscipula) may use the same ionic migration and water circulation for sensing and rapid actuation.

Shahinpoor and Thompson42  had concluded that what was happening in ionic polymers in connection with ionic migration and induced bending and deformation was possibly the mechanism for the same amazing nastic movement in plants such as in the Venus flytrap (Figure 1.37), in which almost digital sensing (the trapped insect has to move or flip the ionoelastic trigger hairs more than two to three times before the lobes close rapidly to trap the insect or prey) and rapid actuation and deployment occurs with ionic migration using the plant's sensing and ionic circulatory system. Mechanical movement of the trigger hairs (Figure 1.37d) puts into motion ATP-driven changes in water pressure within the cells of trigger hairs. These trigger hairs are located on the flytrap leaves or lobes. If the trigger hairs are stimulated twice or more in rapid succession, an electrical signal is generated which causes embedded calcium cations (Ca+) to migrate across the lobes and change the osmotic pressure in the lobes and create a pressure gradient that would cause the lobes to rapidly close (Shahinpoor and Thompson42  and Shahinpoor154–160 ).

Figure 1.37

Venus flytrap (Dionaea muscipula). (a–c) Examples of a plant capable of rapid nastic deployment and movement based on its trigger hair (d) sensing and an IPMC gold strip (1 cm×6 cm×0.3 mm) performing similar rapid closure (e–g) under a dynamic voltage of 4 V.

Figure 1.37

Venus flytrap (Dionaea muscipula). (a–c) Examples of a plant capable of rapid nastic deployment and movement based on its trigger hair (d) sensing and an IPMC gold strip (1 cm×6 cm×0.3 mm) performing similar rapid closure (e–g) under a dynamic voltage of 4 V.

Close modal

The mechanisms responsible for such rapid deployment of lobes in the Venus flytrap are acid growth and turgor pressure leaf movement, which is an osmotic effect in which an ion (in the case of Dionaea, Ca+ or K+) is released into the leaf tissues and makes the cells swell on one surface of the leaf. It is remarkable how these changes are similar to what actually happens in ionic polymer nano-composites. One can even use Ca+ or K+ cations in IPMCs to cause sensing and actuation. It is clear that ionic polymer nano-composites have opened a door to the mysterious ion engineering world of nastic plant movements and rapid deployments and this new ionic world now needs to be further explored. The first observation on the circulatory migration of chemicals to boundary surfaces of IPMC samples occurred in 2000 in the Artificial Muscle Research Institute laboratories (Shahinpoor et al.120 ). These observations in connection with such circulatory systems enabling sensing and actuation by creating internal pressure change and causing internal hydraulic actuation were reported by Shahinpoor and Kim.145,146  One could consistently observe that the color of the surface on the cathode side of a cantilever sample of IPMC changed with the application of a step electric field. Figure 1.38 depicts one such experiment.

Figure 1.38

Migration of lithium cations to the surface on the cathode electrode side of a cantilever sample of IPMC. (a) Sample bent downward with greenish–bluish lithium ions appearing on the surface. (b) Sample bent upward with greenish–bluish lithium ions disappearing by migration to the other side.

Figure 1.38

Migration of lithium cations to the surface on the cathode electrode side of a cantilever sample of IPMC. (a) Sample bent downward with greenish–bluish lithium ions appearing on the surface. (b) Sample bent upward with greenish–bluish lithium ions disappearing by migration to the other side.

Close modal

If the imposed electric field was dynamic and oscillatory (i.e. sinusoidal), the color of the surfaces on the cathode side changed alternately with the frequency of the applied dynamic electric field. When we changed the cations to other cations, such as sodium or calcium, then the color of the migrated cations on the cathode side changed. The emergence of water on the cathode side was also always observable. We concluded that the mechanism of electrically induced bending was due to ionic polymer nanoscale energetics and ionic migration from one side of the cantilever film to the other side while carrying hydrated water or added mass water in such hydraulic type actuation. It was observed that the ionic migration and redistribution caused water, chemicals, polar fluids or ionic liquids contained within the macromolecular network to circulate within the materials and to transport ions and chemicals from one point to another to cause large changes in value of a number of properties such as stiffness, conductivity and material transport.

In fact, in the case of lithium cations, the color was greenish–blue, which then indicated that it was Li+ cations migrating under the influence of the imposed electric field and carrying loose water as well as hydrated water along with it. These observations have been reported (Shahinpoor and Kim144,145 ). Figure 1.39 depicts the essential mechanism in such electrically controllable ionic migration accompanied by water or ionic liquid movement.

Figure 1.39

The cation transport-induced actuation principle of IPMCs (left) before a voltage is applied and (right) after a voltage is applied.

Figure 1.39

The cation transport-induced actuation principle of IPMCs (left) before a voltage is applied and (right) after a voltage is applied.

Close modal

Furthermore, such ionic migration could also increase the local stiffness. It has been observed that stiffness changes more than one order of magnitude in IPMCs, as also depicted in Figure 1.16. The migration of loaded or hydrated cations by means of an imposed local electric field has been observed to cause deformation, stiffening, substantial changes in local elastic modulus, substantial material transport within the material and the ability to transport healing and repairing materials and chemicals to any location in the body or to the surfaces and skin of the body, and many more.

As discussed before, IPMNCs have excellent sensing capabilities both in flexing and compression (Henderson et al.150  and Shahinpoor et al.151 ). Further, as will be discussed in detail in Chapter 7 of this book, IPMNCs’ active elements not only are capable of sensing rather high frequencies, but also are capable of near-DC dynamic sensing and acceleration measurement, as shown in Figure 1.40.

Figure 1.40

Near-DC sensing data in terms of produced voltages, ΔE, versus displacements. Note that the displacements are shown in terms of the deformed angle relative to the standing position in degrees in a cantilever configuration. The dimensions of the sample sensor are 5×25×0.12 mm.

Figure 1.40

Near-DC sensing data in terms of produced voltages, ΔE, versus displacements. Note that the displacements are shown in terms of the deformed angle relative to the standing position in degrees in a cantilever configuration. The dimensions of the sample sensor are 5×25×0.12 mm.

Close modal

In this sense, they are far superior to piezoelectric materials, which are only suitable for high-frequency sensing, while for low-frequency or near-DC sensing, piezoresistors are generally used. Thus, they span the whole range of frequencies for dynamic sensing and thus have wide bandwidths. The power harvesting capabilities of IPMNCs are also related to the near-DC or even high-frequency sensing and transduction capabilities of IPMNCs. Figure 1.41 depicts a typical near-DC voltage and current production of IPMC cantilevers.

Figure 1.41

Typical voltage/current output of IPMC samples under flexing/bending (the IPMC sample has dimensions of 10 mm width, 30 mm length and 0.3 mm thickness).

Figure 1.41

Typical voltage/current output of IPMC samples under flexing/bending (the IPMC sample has dimensions of 10 mm width, 30 mm length and 0.3 mm thickness).

Close modal

The experimental results show that almost a linear relationship exists between the voltage output and the imposed displacement of the tip of the IMPC sensor (Figure 1.41).

IPMC sheets can also generate power under normal pressure. Thin sheets of IPMCs were stacked and subjected to normal pressure and normal impacts and were observed to generate large output voltages. Endo-ionic motion within IPMC thin sheet batteries produced an induced voltage across the thickness of these sheets when a normal or shear load was applied. A material testing system was used to apply consistent pure compressive loads of 200 N and 350 N across the surface of an IPMC 2×2 cm sheet. The output pressure response for the 200 N load (73 psi) was 80 mV in amplitude and for the 350 N (127 psi) was 108 mV.

This type of power generation may be useful in the heels of boots and shoes or places where there is a lot of foot or car traffic. Figure 1.42 depicts the output voltage of the thin sheet IPMC batteries under 200 N normal load. The output voltage is generally about 2 mV cm−1 length of the IPMC sheet.

Figure 1.42

Output voltage due to normal impact of a 200 N load on a 2 cm×2 cm×0.2 mm IPMC sample.

Figure 1.42

Output voltage due to normal impact of a 200 N load on a 2 cm×2 cm×0.2 mm IPMC sample.

Close modal

The fabricated IPMCs can be optimized to produce a maximum force density by changing multiple process parameters including bath temperature (TR), time-dependent concentrations of the metal containing salt, Cs(t), and the reducing agents, CR(t).

The Taguchi design of experiment technique was conducted to identify the optimum process parameters (Rashid124  and Rashid and Shahinpoor125 ). The analysis techniques for larger-the-better quality characteristics incorporate noise factors into an experiment involving larger-the-better characteristics, for the maximum force generated by the manufactured IPMCs in this case. Such an analysis allows us to determine the key factors and the possible best settings for consistently good performance. These experimental force optimization techniques are fully discussed in Chapter 3 and will not be repeated here. The force measurement configuration is depicted in Figure 1.43. The blocking force is measured at zero displacement.

Figure 1.43

A blocking force measurement configuration.

Figure 1.43

A blocking force measurement configuration.

Close modal

Furthermore, initial stretching of the ionic polymer samples prior to manufacturing also appears to increase the force capability of IPMCs. One approach is to stretch the base material prior to the platinum composition process. By doing so, we anticipate that the base materials are plastically deformed and, as a result, larger pores (higher permeability) could be created relative to the starting materials. One approach was to stretch the base material uniaxially and carry out the routine optimal IPMC manufacturing technique. It is found that, by using such a stretching technique, the particle penetration within the material is much more effective so as to form a much denser platinum particle phase and distribution. The basic morphology of particle formation appears different, as can be seen in Figure 1.44.

Figure 1.44

Two TEM micrographs showing the intrinsic platinum particles for an IPMC that have been mechanically stretched prior to making the metal–ionic polymer composite (left) and with no stretching (right). A 17% uniaxial stretching was performed.

Figure 1.44

Two TEM micrographs showing the intrinsic platinum particles for an IPMC that have been mechanically stretched prior to making the metal–ionic polymer composite (left) and with no stretching (right). A 17% uniaxial stretching was performed.

Close modal

In general, the effect of such a stretching method of manufacturing IPMCs seems to benefit the IPMC performance in terms of the blocking force in a cantilever configuration. The results are presented in Figures 1.45 and 1.46. Significantly improved generative forces were produced.

Figure 1.45

IPMC blocking force improvements by pre-stretching before chemical plating, showing force densities almost doubled.

Figure 1.45

IPMC blocking force improvements by pre-stretching before chemical plating, showing force densities almost doubled.

Close modal
Figure 1.46

Surface resistance of IPMC samples A, B and C prepared with either Pt or Pt/Au. The standard sample size is 5 mm×10 mm×0.2 mm.

Figure 1.46

Surface resistance of IPMC samples A, B and C prepared with either Pt or Pt/Au. The standard sample size is 5 mm×10 mm×0.2 mm.

Close modal

The development of ionic polymer metal composites (IBMCs) based on composites of chitosan and Nafion® has been successful, as can be seen in Shahinpoor.157–160  The results have been promising in the sense that both low-voltage actuation and self-powered sensing have been observed in these new composites. Related to this, a design methodology has been developed for the manufacturing of IBMCs from cationic chitosan membranes derived from chitin with actuation and sensing capabilities. However, the observed actuation and sensing of these chitosan/Nafion®-based IBMCs are inferior to IPMCs with comparable dimensions under the same activation voltages and currents. Commercial chitosan is derived from the shells of shrimp and other sea crustaceans such as crabs and lobsters. Chitosan is produced commercially by deacetylation of chitin, which is the structural element in the exoskeleton of crustaceans. The amino group in chitosan has an acid constant pKa value of ∼6.5, which leads to protonation in acidic to neutral solution with a charge density dependent on pH. Thus, cationic chitosan membranes can be used for designing electroactive multi-functional nanocomposites with a noble metal such as platinum, palladium or gold. Note that chitosan is essentially a cationic polysaccharide. On the other hand, cellulose can also be used as an ionic polymer (cellulose acetate) for manufacturing IBMCs. However, cellulose is not naturally cationic and needs to be hydrolyzed to become so. Once it is cationic, the same procedure of chemical plating can be applied to cellulose to convert it to IBMCs.

Some of the fundamental characteristics, functions and properties of IPMCs as smart multi-functional biomimetic soft robotic actuators, sensors, energy harvesters and artificial muscles were discussed in this chapter. It was established that IPMCs are multi-functional smart materials with tremendous potential for industrial and medical applications. Strips of these composites were shown to be capable of generating large bending, twisting, rolling and flapping dynamic deformations if an electric field of the order of a few tens of kV m−1 is imposed across their thickness by pairs of electrodes. Conversely, by bending the IPMC strip either quasi-statically or dynamically, a voltage is produced across the thickness of the strip very much in harmony with the kind of motion or deformation imposed on the IPMC strip. Thus, they are self-powered large deformation sensors. They can be manufactured and cut into any size and shape. It was further shown that an almost linear relationship exists between the output voltage and the imposed displacement of IPMCs. Several IPMC muscle configurations were constructed to demonstrate the capabilities of the IPMC actuators. A data acquisition system was used to measure the vibrational parameters involved and record the results in real time. Also, the load characterization of the IPMCs has been measured and it has been shown that these actuators exhibit very good force to weight characteristics or force densities in the presence of low applied electric fields and voltages. In a cantilever form, a typical IPMC strip of 5 mm×20 mm×0.2 mm exhibits a force density of about 40, which is the ratio of the tip blocking force to the weight of the IPMC cantilever. The cryogenic properties of IPMCs for potential utilization in an outer space environment of a few Torrs and temperatures in the order of −140 °C were discussed. Furthermore, the phenomenological modeling of the underlying sensing and actuation mechanisms in IPMCs was presented based on linear irreversible thermodynamics with two driving forces—an electric field and a solvent pressure gradient—and two fluxes—electric current and solvent flux. Also presented were some quantitative experimental results on the Onsager coefficients. Charge dynamics modeling of IPMCs based on the Poisson–Nernst–Planck formulation was also briefly described. Finally, some recent development in the novel design of IPMCs, including the integration of graphene as electrodes, IPMCs with ZnO and ionic liquids as well as extension to biopolymers such as chitosan and cellulose, were also briefly discussed.

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