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leaves behind a net positive charge, which is delocalized (shared) in the π-orbital structure.17,19 In this region, there is a “defect” in the bond alternation (Structure 1). Upon application of a voltage, this

Actuators

Elisabeth Smela Abstract Conjugated polymer artificial muscles fill a unique niche in the electroactive polymer portfolio. They combine high strength, low voltage, and reasonable speed with versatile fabrication and design. This article reviews the actuation mechanism in these materials and presents some of the designs that have been developed for applications such as Braille displays, catheters, and bioMEMS devices.

Introduction There is a growing interest in actuators based on biomimetic materials that resemble mammalian skeletal muscle. Artificial muscle-like actuators used in devices such as robot legs, hand-like manipulators, surgical instruments, and animatronic faces are anticipated to benefit numerous fields, including medicine, defense, and entertainment. These actuators should be compact, lightweight, accurately positionable, reasonably fast, silent, and strong. Ideally, they would also be efficient and inexpensive. Conjugated polymers are attractive candidates for artificial muscles, because they have many of these desired characteristics and because of their operational similarity to biological muscles.1,2 The basic structure of conjugated polymers consists of carbon atoms connected by alternating single and double bonds (Chart 1). This pattern repeats across the ring structures of polymers such as polypyrrole and polythiophene, and with some variations in other structures, such as polyaniline. The alternating single and double bonds (i.e., conjugation) are like those found in benzene, and lead to sp2 hybridization, with the π-orbitals perpendicular to the plane of the molecule. Such a structure allows net charge to be shared among the carbon atoms. Conjugated polymers have a wide range of applications, of which actuators are just one, including light-emitting diodes, photovoltaic cells, chemical sensing, drug delivery, electrochromic windows, electronic paper, electrical contact to neurons, charge storage devices, and organic electronics. They are also scientifically interesting materials in their own right, having a unique chemical structure that makes these

materials semiconductors and thus the organic analogues of materials like silicon and gallium arsenide. (They typically have bandgaps from 1 eV to a few eV.3) For further general information on conjugated polymers, see References 4–14, which are reviews written at the layperson level, and References 15–18, which are books covering these topics.

Actuation Mechanism The actuation mechanism is intimately related to the electronic structure of these polymers, which allows electrons to be removed relatively easily either by chemical or electrochemical oxidation. This a

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Chart 1. Structures of some conjugated polymers in their neutral states: (a) polyacetylene, (b) polypyrrole (PPy), (c) polyaniline, and (d) poly(3,4ethyle