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Ionic (wet) electroactive polymers

This section gives a short overview of various ionic EAPs.

Ionic polymer-metal composites (IPMCs)

Ionic polymer-metal composites (IPMCs) consist of a polyelectrolyte membrane sandwiched between two high-surface-area compliant electrodes. The polyelectrolyte membrane is typically a perfluorinated or sulfonated polymer membrane (Nafion from Du Pont and Flemion from Asahi) permeable to cations but impermeable to anions. The negatively charged polymer backbone is neutralized with mobile positive ions, which are dissolved in water or other solvents. The compliant electrodes consist of chemically or physically (mixing) loaded nanoparticles (Pt or Au, 3 – 60 nm) penetrating from both sides 0 – 25 µm into the polyelectrolyte membrane and a 1 – 5 µm thin flexible metal coating (e.g. Pt, Pd, Ag, Au, Cu, carbon, graphite, or carbon nanotubes). The metal plating increases the surface conductivity of the electrodes and reduces solvent loss when the IPMC is actuated, while the nanoparticles enhance the charging of the polymer boundary layers.

When a low voltage (∼ 1 – 4 V) is applied between the two electrodes, the solvated mobile cations drift toward the negatively charged electrode, resulting in a swelling of the electrolyte on the cathode side and a shrinking on the anode side. This electrically controlled ion/liquid transport causes a fast bending (up to 100 Hz) of the trilayer actuator.

IPMCs achieve actuation strains of more than 3 % and very large bending at moderate drive frequencies of up to 100 Hz. However, the electromechanical coupling efficiency is low and encapsulation is required for operation in air. Furthermore, slow relaxation requires a steady current to hold position.

IPMCs have been applied to a wide range of mechanical engineering problems including metering valves, diaphragm pumps, sensors, fins for robotic fish, artificial flies, an eye ball compression band , tactile output devices , and mechanical grippers .

(a) Schematic of the principle operation of IPMC actuators when (top) no voltage and (bottom) a low voltage is applied. (b) Illustration of the cross-section of a typical Pt/Au-plated Nafion 117 actuator. Inset A shows the morphology of the electrode zone and inset B depicts the polyelectrolyte membrane. (c) Active eye ball compression band changing the axial length of the eye globe. (d) Tactile output device based on self-switching bistable IPMC actuators. (e) Mechanical gripper.

Carbon nanotubes

Carbon nanotubes (CNTs) were first discovered by Radushkevich and Lukyanovich in 1952. Single-walled CNTs (SWNTs) and multi-walled CNTs consist of one and multiple layers of rolled graphene sheets, respectively. The diameter of SWNTs depends on the direction in which the graphene sheet is rolled up and is typically a few nanometers. In solution, bundles of CNTs form due to Van der Waals attraction. Furthermore, scientists have produced sheets and even spun yarns of CNTs. The exceptional electrical and mechanical properties of individual SWNTs, including large current densities (1000 times higher than in copper), huge tensile modulus of 640 GPa and tensile strength of 20 – 40 GPa make them a promising engineering material for nano- and macroscale devices.

In 1999, Baughman et. al. found that sheets of nanotubes actuate when used as electrodes in electrochemical cells. Since then, various actuation mechanisms have been reported for CNTs, including carbon nanotube-nematic liquid crystal elastomers, electrostatic, light-driven and pneumatic actuation. The latter is an electrochemically driven actuation mechanism in which the high positive potential, applied to a porous multilayer CNT sheet, immersed in an aqueous NaCl electrolyte, results in a gas production. This pneumatic mechanism generates, after a few seconds, a thickness strain of up to 300 % normal to the direction of the carbon nanotube sheets. Nevertheless, the most promising actuation process remains the non-Faradaic actuation by double-layer charge injection. In this actuation mechanism, nanotubes are immersed in an electrolyte. When a voltage is applied between the nanotubes and a counter electrode, ions are attracted to the nanotubes. This accumulation of ionic charges on the surface of the CNTs is balanced by an electronic charge removal or injection within the CNTs. This change of the charge on the carbon atoms results in a variation of the carbon-carbon bond lengths. At low levels of charge injection, it is believed that quantum mechanical effects dominate the elongation and expansion of the nanotubes. At large charge injection levels, Coulomb forces dominate. The repulsive interaction between like charges results in a parabolic relationship between strain and applied potential. The maximum achievable strain for this actuation process is limited by the discharging of the double layer at high potentials as electrolyte (ions and solvent) and CNTs begin to exchange electrons.

(a)–(b) Side-view of a pneumatically actuated carbon nanotube sheet before (left) and after (right) Faradaic actuation in 5 M NaCl. (c) Schematic of a double-layer charge injection nanotube actuator. (d) Charge injection at the surface of a nanotube bundle. (e) SEM image of a twist-spun MWNT yarn.

Because the surface area is important for non-Faradaic actuation, SWNT films or yarns are preferred over MWNTs. So far, actuation strains of up to 1 %, response times of < 10 ms, effective strain rates of 19 %, work densities of ∼ 1 MJ/m3, effective power-to-mass rations of 270 W/kg, and operation at 1000 °C (when sealed from oxygen) were reported.

The carbon nanotube actuator technology is at an early stage of development and several problems such as creep, low electromechanical coupling of presently < 1 %, low strain rates due to a relatively large internal resistance of the electrolyte, and the poor mechanical properties of macroscopic structures (like yarns and sheets) compared to single CNTs need to be addressed.

CNT actuators are envisioned for aerospace applications in which weight and temperature stability are critical.

Conducting polymers

Conducting polymers (e.g. polypyrrole and polyaniline) are conjugated organic semiconductors which conduct when doped with donor or acceptor ions. The actuation process in conjugated polymers is controlled by electrochemical reduction or oxidation. Typically, conducting polymer actuators consist of an electrolyte sandwiched between two conducting polymer layers. When a sufficiently high potential (usually positive) is applied to the intrinsically conducting polymer, charges are removed from or added to the polymer backbone. To maintain charge neutrality, mobile solvated ions flow in or out of the polymer backbone into the surrounding electrolyte, causing an expansion or contraction of the polymer. This dominating mass transport mechanism is illustrated in the figure (a) below. Assuming that the backbone is p-doped and filled with charge compensating anions, two counteracting effects can happen. If the anions are small, they leave the polymer when a negative potential is applied, causing contraction. On the other hand, if the anions are large and immobile, solvated cations are incorporated into the conducting polymer causing a swelling of the material. Besides the dominating mass transport mechanism, several other side processes occur when the oxidation state of the polymer is varied, including the change of the C–C bonds on the polymer backbone causing a variation of the angle between adjacent monomer units, cis-trans isomerization, changes in the interaction between polymer chains and solvent, backbone folding, and interchain interactions .

The main advantages of conducting polymers are their low operating voltage (∼ 2 V), high tensile strength (up to 100 MPa), large stress (up to 34 MPa), large linear strain of typically 20 % and high stiffness (∼ 1 GPa modulus), making them especially attractive for biomedical applications. Similar to CNTs, the elctromechanical coupling is low (typically < 1 %). Furthermore, the need for high currents, encapsulation and moderate strain rates (∼ 13.8 %/s), limited by the ion diffusion rate inside the polymer and by the internal resistance of the polymers and electrolytes, are the major constraints in the development of large-scale devices.

Recently, researchers reported on large improvements of conducting polymers, including new air working conducting semi-IPN/ionic liquid based actuators, achieving 7·10⁶ cycles at 10 Hz without degeneration and gel-like polypyrrole based conducting polymer actuators with extremely large linear strains of 40 % .

Conducting polymers have been applied to several microsystems including hinges in “cell clinics” for single-cell studies, as depicted in figure (b) and microrobots. Other applications such as actuated catheters, circulation pumps, biorobotic fins, Braille displays, and valves have been demonstrated.

(a) Illustration of the dominant mass transport mechanism in conjugated polymers. A reduction of the p-doped polymer can result in (I) a contraction or (II) swelling of the material depending on the size of the anions (I: small and II: large). (b) Schematic illustration of the “cell clinic”. (Adapted from [88].) (c) Circulation pump (Courtesy of EAMEX Inc., Japan). (d) MIT’s biorobotic fin-based on conducting polymers.

Other ionic EAPs not discussed in detail include:

Molecular actuators use chemically or electrochemically driven molecular conformational changes to generate macroscopic material deformations. The current state of these relatively new actuators is discussed in .

Mechano-chemical polymers/gels achieve large strains but have a long response time (> 1 min). Ionic polymer gels are normally pH activated but electrical shape control is possible, too.

Electrorhelogical fluids (ERFs) have been discovered in 1994 by Okada et al.. In their work they discussed the actuation principle and demonstrated the applicability of ERFs to tactile arrays for virtual environments

It can be seen that in the last decade, the EAP technology has developed at an incredible speed. Many new actuation mechanisms like carbon nanotubes have emerged and new applications have been explored. Nevertheless, many actuation mechanisms are not yet fully understood and require further investigations. The main properties of the discussed actuator technologies are summarized in an overview table.