Actuator based on doping/undoping-induced volume change in anisotropic polypyrrole film

We propose an actuator using the cylindrical polypyrrole (PPy) film, with an anisotropy of morphology and conductivity, can be polymerized electrochemically in a Teflon slender pipe. The PPy actuator bends and reverts in a regular direction upon electrochemical undoping/doping. The bending speed becomes faster as the undoping potential is increased, but the magnitude of displacement depends not on the undoping potential but on the amount of charge or dopant concentration. The bending behavior strongly depends on the type of supporting salt in the electrolyte solutions, suggesting that cation transport into and out of the PPy plays a key role in the bending behavior and the size of the cation is a critical factor. It was also found that, the shape of the transient reduction current strongly depends on the type of cation and that the current does not follow the diffusion law. A bending mechanism based on the cation insertion and extraction, as well as on the anisotropic volume change upon doping/undoping, is proposed.