Polypyrrole artificial muscles: a new rhombic element. Construction and electrochemomechanical characterization

A novel rhombus-shaped electrochemomechanical unit constituted by 4 polypyrrole bilayer (conducting polymer/tape) muscles and two plastic hinges, able to transform reversible angular movements from the basic bilayers into longitudinal movements, has been successfully constructed and electrochemically characterized. During operation two of the bilayers act as anode and the other two as cathode. Thus, all the electrical energy is used, avoiding an additional metallic counterelectrode and the subsequent generation of products able to degrade the muscle. The reference electrode is short-circuited to the counterelectrode in order to monitor the muscle potential along the galvanostatic experiments and the sensing abilities of the device. The devices were checked by repetitive galvanostatic contraction/extension of up to 20% of the original length. About 50% of the devices produced irregular movements, due to different ohmic resistances in the electrical contacts between the wires and polymeric films. Once the contacts were improved, the new devices showed good reproducibility. The influence of electrolyte concentration, experimental current and weight trailed by the device indicates that the complex device maintains most of the sensing properties of the basic bilayer muscles. Then checking the life-time of the device, contact failures and fissures around the metal/polymer joint were immediately detected from the chronopotentiometric noises which appeared after several cycles. Such failures must be solved before the device can be miniaturized and in order to construct different shapes, and three-dimensional sensing muscles for robotics from combinations of basic units.