ELSEVIER Synthetic Metals 78 (1996) 339-353 Conducting polymer artificial muscles R.H. Baughman Allied Signal, Research and Technology, Morristown, NJ 07962, USA Received 24 October 1995; accepted 22 November 1995 Abstract The application of conducting polymers for the direct conversion of electrical energy to mechanical energy in electromechanical actuators is analyzed using theoretical and experimental results. Basic principles of operation, predicted performance advantages and disadvantages, comparisons with natural muscle, evaluations of initial device demonstrations, and methods for improving device performance are provided. The very high predicted work densities per cycle, force generation capabilities, and power densities provide major advantages compared with piezoelectric polymers - as do the low operation voltages. These advantages are countered by cycle life and energy conversion efficiency limitations, as well as the need to use microelectrodes in order to achieve very high rates. Hydrostatic devices and extensional devices that provide either in-phase or out-of-phase electrode deformations are considered. Special types of conducting polymer actuators are also proposed, including photo-powered, chemically powered and self-powered actuators, which provide novel methods for assembling complex micros• tructures. Novel methods are described for actuator fabrication, such as 'redox poling', wherein anode, cathode and separating electrolyte layers are generated from a film in a single redox step. New actuator compositions are also proposed for obtaining improved performance, such as conjugated carbon phases having conjugation in either two or three dimensions. Finally, conducting polymer actuators based on double-layer charging are proposed which are predicted to provide increased energy efficiency and cycle life compared with the faradaic actuators. Keywords: Artificial muscles; Actuators 1. Introduction Piezoelectric and electrostrictive materials have major technological importance for the direct conversion of electri• cal energy to mechanical energy in actuators. As an alterna• tive, we previously proposed that conducting polymers could provide attractive actuator materials that function more anal• ogously to natural muscle [1-3], and demonstrated a simple cantilever actuator based on the electrochemical doping and dedoping of a conducting polymer. This work has been fol• lowed by research efforts here and abroad focused on making practical conducting polymer actuator devices [4-18]. This overview updates the status of this embryonic area. The emphasis is on ( I) the principles of conducting polymer actuator operation, (2) predicted performance advantages and disadvantages, (3) comparisons with natural muscles, ( 4) initial device demonstrations and (5) proposed methods for improving performance. The various applications consid• ered include actuators for micromachining, actuator micro• flaps for aircraft wings, microscopic valves and pumps for 'chemical laboratories on a chip', actuators for adaptive optics and steerable catheters, artificial muscles for robotic and prosthetic devices, and adjustable structures for vibration 0379·6779/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSDI0379·6779(95)03600-8 suppression and failure avoidance. While the focus is on conducting polymer actuators that utilize intercalation• induced dimensional changes to accomplish work, a new type of conducting polymer actuator is also described. This type of proposed conducting polymer actuator operates via dou• ble-layer charging. The solid conducting polymers that are the subject of the present work contrast with the gel polymers that have been of interest for actuator devices for many decades [19-25]. The low elastic modulus and the low yield strength of these gel polymers provide important limitations for actuator per• formance. Nevertheless, conceptually interesting types of actuator devices have been made using gel polymers, and the gel polymers might find utility for actuator applications where the low mechanical properties are compensated by the extraordinarily large achievable volume changes (which can exceed a factor of a thousand). 2. Principles of device operation Conducting polymer electromechanical actuators can be based on the large dimensional changes that result from the