Fundamentals of Ionic Polymer Metal Composites (IPMCs)

This chapter covers some of the fundamental characteristics, functions and properties of ionic polymer metal composites (IPMCs) as smart multi-functional biomimetic soft robotic actuators, sensors, energy harvesters and artificial muscles. Strips of these composites can undergo large bending, twisting, rolling and flapping displacement if an electric field is imposed across their thickness by network pairs of electrodes. Thus, in this sense they are large motion actuators. Conversely, by bending the IPMC strip, either quasi-statically or dynamically, a voltage is produced across the thickness of the strip very much in harmony with the kind of motion or deformation imposed on the IPMC strip. Thus, they are also large deformation sensors. The output voltage can be calibrated for a standard size sensor and correlated to the applied loads or stresses. They can be manufactured and cut into any size and shape. In this chapter, first the sensing capability of these materials is reported. The preliminary results show the existence of an almost linear relationship between the output voltage and the imposed displacement for almost all cases. Furthermore, the ability of these IPMCs to function as large motion actuators and soft robotic manipulators is presented. Several IPMC muscle configurations are constructed to demonstrate the capabilities of the IPMC actuators. A data acquisition system was used to measure the vibrational parameters involved and record the results in real time. Also, the load characterization of the IPMCs has been measured and it shows that these actuators exhibit very good force to weight characteristics or force density in the presence of low applied electric fields and voltages. In a cantilever form, a typical IPMC strip of 5 mm×20 mm×0.2 mm exhibits a force density of about 40, which is the ratio of the tip blocking force to the weight of the IPMC cantilever. Reported are also the cryogenic properties of these muscles for potential utilization in an outer space environment of a few Torrs and temperatures of the order of −140 °C. These muscles are shown to work quite well in such harsh cryogenic environments and thus present a great potential as actuators, energy harvesters and sensors that can operate at cryogenic temperatures and in particular in outer space. Furthermore, the phenomenological modeling of the underlying sensing and actuation mechanisms in IPMCs is presented based on linear irreversible thermodynamics with two driving forces—an electric field and a solvent pressure gradient—and two fluxes—electric current and solvent flux. Also presented are some quantitative experimental results on the Onsager coefficients. Charge dynamics modeling of IPMCs based on the Poisson–Nernst–Planck formulation is also briefly described. Finally, some recent development on novel design of IPMCs including the integration of graphene as electrodes, IPMCs with ZnO and ionic liquids, as well as extension to biopolymers such as chitosan and cellulose, are also briefly discussed.