Electro-driven polypyrrole actuators working in air

Abstract

The combination of electrical and hygroscopic nature of conducting polymers provided an insight into the development of a new class of electro-driven actuators or artificial muscle systems that worked in ambient air. The electrochemically synthesized polypyrrole films underwent quick and intensive bending in air as a result of a dimensional change due to the sorption of water vapor from one side of the film. Furthermore, an application of electric field caused contraction of the film in air. The dimensional change of the polypyrrole film under the electric field was expressed by two processes: one was the contraction due to the desorption of water vapor and the other was the thermal expansion of polymer chains both caused by Joule heating. The degree of contraction attained 1.2% under 2 V, where the initial speeds of contraction and elongation of the film were 4.4 and 1.8%/min, respectively. Under loading conditions, the power density increased with increasing load and the value attained 0.78 W/kg (6 μW) under the load of 60 g (4 MPa). Under isometric conditions, when dc 2 V is applied to the film under the thermostatic conditions (25°C, 50% RH), the film generated contractile stress repeatedly in response to the applied voltage. The stress reached 6.1 MPa, which was 4 orders of magnitude larger than its own weight and nearly 20 times that of skeletal muscle in animals. The generated stress under 2 V increased to 8.9 MPa upon stretching the film by 1%, which could be associated with the Young's modulus of the film rose due to the desorption of water vapor that plasticized polymer chains. The work capacity of the film increased as the applied voltage became higher and reached 48.2 kJ/m³ at 3 V, while the energy efficiency, defined as the ratio of work capacity to the electric energy, was the order of 10^-3%.