PerspectiveMaterials Science

Stretching Dielectric Elastomer Performance

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Science  24 Dec 2010:
Vol. 330, Issue 6012, pp. 1759-1761
DOI: 10.1126/science.1194773

The idea that a solid material can deform when stimulated by electricity originated in the late-18th century with observations of ruptures in overcharged Leyden jars, the first electrical capacitors. In 1776, Italian scientist Alessandro Volta mentioned in a letter that Italian experimenter Felice Fontana had noted volume changes in the Leyden jar upon electrification (1), an observation that launched a new field of investigation—“deformable” materials affected by electricity. More than two centuries later, the concept of “electrically stretchable materials” is at the forefront of devising bioinspired robots, tactile and haptic interfaces, and adaptive optical systems (2, 3).

This diversity of applications took a great leap 10 years ago in a landmark study by Pelrine and colleagues (4). They reported high-speed, giant-strain, electrically actuated elastomers with unprecedented electromechanical transduction performance. These materials were demonstrated for so-called dielectric elastomer actuators, deformable capacitors made of a film of a soft insulator (such as acrylic, polyurethane, or silicone elastomer), with compliant electrodes. Upon electrical charging, purely electrostatic forces caused the elastomer film to undergo substantial thickness compression and surface expansion (4). The exceptional performance of these dielectric elastomer actuators gave rise to a scientific and technological revolution in the field of electroactive polymers, materials that can undergo electrically induced deformations. Since that milestone study, elastomers with improved electromechanical properties have been developed, such as interpenetrating networks of acrylic polymers (3).

The most widely recognized potential of dielectric elastomer actuators is for creating artificial muscles (2, 3). Indeed, electrically driven elastomers have already exceeded the performance of natural muscles in terms of strain (up to 380% in area), stress (up to 7.2 MPa), and elastic energy density (up to 3.4 J cm−3) (3). Moreover, they show fast response and long lifetime; have high resilience; are light weight, scalable, shock-tolerant, and noise- and heat-free; and are inexpensive (25).

New frontiers.

Future applications of electrically charged dielectric elastomers include replacing traditional Braille printed pages with refreshable Braille displays, using millimeter-sized actuators that control the tactile pattern of dots. Touch screens that are used in smart devices and laptop computers that lack tactile feedback could have this new dimension. Dielectric elastomer actuators could produce effects that enhance the user's experience with such devices.

CREDIT: (LEFT) COMSTOCK; (RIGHT) NICOLAS LORAN/ISTOCKPHOTO

Possible future applications of dielectric elastomer actuators that have been under development over the past decade, and seem to be promising, deal with haptics and optics. For example, they are expected to be used in electronic smart devices such as mobile phones to provide users with vibro-tactile feedback, transmitting clicks and vibrations through the sense of touch. Smooth touch screens do not provide key sensorial experience (see the photo, above). Dielectric elastomer actuators may deliver feedback that integrates visual, audio, and tactile responses. Adaptive optical devices, such as lenses and diffraction gratings, are another area in an advanced stage of development (6). Lenses with electrically tunable focal length are potentially useful in autofocus cameras. Miniaturization of dielectric elastomer actuators is a new frontier of research (2, 3, 7) in which millimeter-sized actuators could, for example, control the tactile pattern of Braille interfaces, allowing for simple and compact electrically refreshable tactile displays (2, 3).

Key early investigations on electrically induced deformations of insulators include studies in 1775 by the French scientist Nicolas-Phillipe Ledru (also known as Comus), who observed the rise and fall of mercury within glass tubes upon electrification (8). In 1776, Volta explained Fontana's observations on the Leyden jar: “The glass is strongly compressed…by the two armatures, i.e., exterior metallic leaf, and interior water, which…armatures weight, I will say so, one against the other, because they are oppositely electric…So…it behaves alike, both when it is interiorly charged by excess, and when it is charged by defect” (1). Volta was the first to interpret the observations as electrostatically induced deformations of the solid dielectric, independent of the voltage polarity. Today, this appears as a sort of unaware anticipation of the physical principle that underpins dielectric elastomer actuation, introduced two centuries later. Moreover, electromechanical effects experienced in those early capacitors that were filled with conductive liquids have recently inspired contractile “hydrostatically coupled” dielectric elastomer actuators (9), which exactly follow Volta's explanation. Here, an incompressible liquid confined between elastomer membranes allows for electrically safe transmission of forces to loads.

In 1880 German physicist Wilhelm Conrad Röntgen reported that a “caoutchouc stripe…[is] electrified by…an isolated comb of needles…connected… to…a strong Holtz influence machine…As…the caoutchouc becomes electrified…one observes a continuous increase of the length of the band” (10). Today, this experiment is a milestone in the historical background of dielectric elastomer actuation, as the first example of a charge-controlled electrode-free actuator. Indeed, this result was reproduced in a study this year (11) using the acrylic elastomer tested by Pelrine et al. A process called corona charging was used, in which a piece of elastomer is electrified by ionizing the surrounding medium with high voltages. Corona-charged electrified actuators performed better than those controlled directly by electrodes. This is because the former overcome the elastomer “pull-in” instability—that is, the mechanical collapse of the elastomer when the electrostatic force exceeds the elastic force.

At present, the greatest challenge for dielectric elastomer actuation is reducing the driving voltages (on the order of 1 kV for electrode separations of 10 to 100 um). To this end, developing high–dielectric constant elastomers and processing them as thin films is strategic (2, 3). There are promising advances in boosting the dielectric constant of a material while preserving low mechanical stiffness and high dielectric strength (12). Reaching voltages comparable to those of piezoelectrics (on the order of 100 V) may be feasible through thin-film processing in a few years. This would allow the dielectric elastomer actuator technology to permeate an enormous variety of products in haptic, automation, robotic, fluidic, biomedical, optical, and acoustic systems (2, 3).

Fault tolerance is increased with “self-healing” electrodes; on this front, conductive polyaniline nanofiber or carbon nanotube electrodes, wherein dielectric breakdown results in isolated local burns, show promise (3). Ion-implanted electrodes (7) are paving the way to dielectric elastomer-based microelectromechanical systems and microfluidic devices, such as electrically controlled valves (3).

The use of dielectric elastomer transduction technology in a reverse mode to convert mechanical energy into electrical energy (especially from renewable sources, such as offshore ocean waves) is one of the latest frontiers. The voltage is increased when the force of a stretched and charged elastomer is reduced (2). Here, high-voltage operation is of utmost advantage for energy distribution. While experimental assessments are in progress (13), theoretical estimates anticipate energy densities around 6 J/g (14). Current challenges include the development of optimized cycles to maximize electromechanical efficiency at any strain.

The field of dielectric elastomer transducers is rapidly maturing and broadening, and the limits of their applications surely will be stretched. The question is whether future applications will be enabled by the two key factors that have thus far prompted their vast and diverse impacts: a simple and reliable physical principle, and the possibility of effective implementation with inexpensive and off-the-shelf materials.

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