A Model for Mechanical Force Sensing in Conducting Polymers

Abstract

Conducting polymers have been shown to work as force sensors in an electrolyte by generating a voltage or current when the force applied to them is changed. One possible sensing mechanism is the change in the polymer capacitance as a result of an induced tension. This change in capacitance can be related to perturbation of Donnan equilibrium potential by external load. In this paper we present a model using this proposition to predict conducting polymer sensing behaviour. The model also takes the dependence of the sensing voltage on the polymer oxidation state into account. The predicted model results are compared with experimental measurements on a free-standing film of polypyrrole in an aqueous electrolyte of NaPF6. Sensing voltages ranging from ~ 80-110 μV were detected in response to 1.72 MPa change in the polymer stress, when the polymer was at oxidation states of -0.2 V to 0.4 V vs. Ag/AgCl reference electrode. The load-dependent capacitance necessary to induce the observed sensing currents and voltages is determined. The cycle life of the polypyrrole mechanical sensor was also studied and it was shown that after an initial transient period, the sensing current amplitude stays constant for at least 5400 cycles.