PerspectiveMaterials Science

A Clear Advance in Soft Actuators

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Science  30 Aug 2013:
Vol. 341, Issue 6149, pp. 968-969
DOI: 10.1126/science.1243314

Development of actuator technologies with capabilities that can match or exceed those found in biology represents a topic of long-standing interest within the advanced robotics community. One promising and remarkably simple class of such an “artificial muscle” exploits a dielectric elastomer (an electrical insulator) sandwiched between a pair of mechanically compliant electrodes (1, 2). Electrostatic force generated by an applied voltage deforms the dielectric and causes rapid, controlled displacements with large amplitudes. On page 984 of this issue, Keplinger et al. (3) describe an important advance in this dielectric elastomer actuator (DEA) technology, in which the authors replace the electrodes with soft, ionic hydrogels. The result provides a clever solution to a daunting materials challenge; it enables delivery of high voltages for fast, effective operation without any mechanical constraint on the motions of the dielectric, in a form that also provides almost perfect optical transparency.

Films of carbon powder or grease loaded with carbon black served as electrodes for the earliest DEAs (1, 2). Although valuable for initial prototypes, such materials have poor reliability and are not readily compatible with established manufacturing techniques. Improved characteristics can be achieved with alternatives based on sheets of graphene (4), coatings of carbon nanotubes (5), surface-implanted layers of metallic nanoclusters (5), and corrugated or patterned films of metals (5). These options yield working DEAs, but with limited mechanical properties, sheet resistances, switching times, and capacity to integrate into advanced actuator designs. The authors show that a different class of material (soft, transparent hydrogels) and a different mode of charge transport (ionic, rather than electronic) can yield electrodes with characteristics that are remarkably well suited for use in DEAs. A key but nonobvious realization is that even aqueous ionic hydrogels can deliver potentials of several kilovolts, despite the onset of water electrolysis at less than 1.5 V.

Thin, stretchy transparent actuators.

The hydrogel electrodes developed by Keplinger et al. carry current through ionic flow for use in soft, electrostatic actuators. In this artificial-muscle technology, sheets of hydrogels deliver large voltages to a dielectric elastomer. The applied potential creates electrical double layers that induce electrostatic forces to compress the elastomer. These deformations are well controlled, reversible, and capable of high-frequency operation. The resulting devices can be perfectly transparent, with potential for use in applications such as noise-canceling windows and display-mounted tactile interfaces.

The physics is relatively simple. A potential applied to a conductor in contact with a hydrogel induces ionic transport that yields a net charge at the interface together with an adjacent screening charge, known collectively as an electric double layer. A corresponding charge appears at the interface with the dielectric elastomer (see the figure). The enormous difference between the capacitance of the double layer and the dielectric leads to a potential across the dielectric that can be millions of times greater than that across the double layer. As a result, potentials in the kilovolt range can be realized in the DEA without electrochemically degrading the hydrogel.

High-frequency actuation is also possible. Careful analysis shows that switching speeds in practical systems are limited only by mechanical inertia. Furthermore, because the stiffness of the hydrogel can be thousands of times smaller than that of the dielectric, actuation can occur freely, without mechanical constraint. These attractive characteristics are complemented by an additional, interesting feature: Hydrogels can have exceptionally high optical clarity across the visible range, thereby opening up a range of application possibilities enabled by transparent actuators.

The authors formed DEAs with this design simply by laminating films of polyacrylamide hydrogels formed with salt water onto the surfaces of dielectric elastomers. Such actuators can change their dimensions by nearly a factor of 2 and switch with millisecond speeds. As a demonstration, the authors built loudspeakers that produce high-fidelity sound throughout the audible range. The thin, planar geometries of these devices, taken together with their nearly complete optical transparency, foreshadow interesting applications such as active noise-canceling windows and display-mounted tactile interfaces. Adaptive optics represents another potential field of use. These and other prospects motivate the development of further refinements in the materials, including schemes to prevent drying of the hydrogels and methods to eliminate ionic build-up, hysteresis, and electrical shorting.

The success of ionic hydrogel conductors in DEAs hints at possibilities for their use in other unusual electrical systems, such as new classes of circuits and sensors that have elastic properties and shapes precisely matched to biological tissues for implants, surgical tools, and diagnostic systems that intimately integrate with the curved, dynamic external or internal surfaces of the body (610). Ionic hydrogels can offer favorable mechanics, and they can be biocompatible. Also, their operation exploits transport of ions, much like the intrinsic mode of electrical function in biological systems. Ionics, therefore, provides a natural type of biotic-abiotic interface.

Although the relatively slow speeds and the physical mass transport associated with ionic conduction preclude the general use of hydrogels as alternatives to metals in electronics, many possibilities can be considered. In fact, seminal experiments in the earliest days of semiconductor device research relied critically on ionic conductors to investigate field modulation of contact potentials in silicon and to enable the first solid-state amplifiers, as summarized in Bardeen's Nobel lecture in 1956 (11). Work in just the past 10 years has established the utility of similar electrolyte gate electrodes in printed and organic electronics (12). More recently, demonstration experiments showed that deformable ionic gels can serve as elements of high-performance, stretchable graphene transistors (13).

In the context of biomedical devices, related types of gels are already in widespread use for low-impedance interfaces between metal electrodes and the surface of the skin. One vision for system design might strategically combine both electronic and ionic modes of operation, in which the latter enables conformal electrical interfaces to biological tissues and provides soft mechanical actuation and sensing, whereas the former affords signal processing, control, acquisition, data storage, and transmission. Developing an associated base of fundamental knowledge in materials and device designs represents a promising direction for future work.

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