Biomimetic Dual Sensing-Actuators Based on Conducting Polymers. Galvanostatic Theoretical Model for Actuators Sensing Temperature Toribio F. Otero,* ,† Juan J. Sanchez, † and Jose G. Martinez † † Universidad Polite ́ cnica de Cartagena, ETSII, Center for Electrochemistry and Intelligent Materials (CEMI), Paseo Alfonso XIII, Aulario II, 30203 Cartagena, Spain * S Supporting Information ABSTRACT: A theoretical model is proposed for the quantitative description of the chronopotentiometric (E-t) responses, under galvanostatic control, of either conducting polymer films or dual sensing-actuating devices. Assuming that the reaction occurs by extraction, or injection, of n consecutive electrons from, or to, a polymer chain the material moves through n consecutive oxidation or reduction states. Stair functions are obtained describing either potential or consumed electrical energy evolutions as a function of both, driving (current) and environmental (temperature, electro- lyte concentration...) variables. The current quantifies the actuation of any electrochemical device (charge/discharge of bat- teries, movement rate, and position of muscles): the stair functions are dual actuating-sensing functions. A good agreement exists between theoretical and experimental results from either polypyrrole films or artificial muscles at different temperatures. Only two connecting wires include, at any time, sensing (potential) and working (current) information of any dual device. ■ INTRODUCTION Conducting polymers (CPs), when considered as reactive materials (they can be oxidized and reduced in a reversible way), provide electrochemical properties as electro-chemo- mechanical, electro-chromic, charge storage, electro-chemo- porosity, electron-ion transduction, and so on. 1,2 The material composition mimics that of natural organs: reactive macro- molecules, solvent, and ions. Based on those reactive properties, reactive biomimetic devices and products such as artificial muscles, 3-11 smart windows, 12-15 smart membranes 16-21 or batteries and supercapacitors, 22-25 smart chemical dosage; 26,27 electron/ion transduction at very low overpotential and nervous 28,29 interfaces; wettability 30-32 and so on are being developed. Most of those devices may act, while working (moving, changing its color, etc.), as sensors of the surrounding conditions. Artificial muscles sensing working temperature, 33,34 electrolyte concentration, 33,35 or attached and shifted weights, 35 and tactile muscles sensing obstacles 4 and indicating the mechanical resistance of the obstacle to be shifted have been developed. Three layer artificial muscles also can be considered as mobile batteries (charging during movement in one direc- tion and discharging-a fraction of the working energy can be recovered- while moving in the opposite direction) sensing working conditions. 3,4,34-36 All those electrochemical devices constitute unique actuator/sensor systems only preceded by natural organs in mammals. When we touch and catch an object in darkness, our brain knows the exact energy that our muscles need to produce to move the obstacle. Muscles in arms are electro-chemo-mechanical motors that sense the mechanical energy required to shift the obstacle. Mimetic sensing and tactile electrochemical artificial muscles are several (one actuator and several sensors: temperature, elec- trolyte concentration, obstacles) in one device working simulta- neously. The actuator here is a soft electrical motor which movement rate and position are, under current and charge control, described by faradic equations. 3,11,37 The evolution of the device potential or that of the consumed electrical energy while working are the empirical sensing magnitudes. 4,33-35 At the moment this biomimetic, dual and simultaneous actuating- sensing property is outside any theoretical description. The above-described electrochemical (reactive) devices, developed from conducting polymers, can work under flow of constant currents. The material adjusts its potential to the oxidation state attained at every oxidation time by the polymer film giving a chronopotentiometric response (Figure 1). The continuous linear increase of the potential with time used to be considered as evidence of the capacitive nature of electrochem- ical responses from conducting polymers. 38-47 For redox processes, like batteries, one or several plateaus should be expected at increasing potentials. An unexpected result for a capacitor is that, after consumption of a constant charge, the potential steps to very high values (Figure 1), like in batteries at the end of the charge process. Received: January 10, 2012 Revised: March 23, 2012 Published: March 28, 2012 Article pubs.acs.org/JPCB © 2012 American Chemical Society 5279 dx.doi.org/10.1021/jp300290s | J. Phys. Chem. B 2012, 116, 5279-5290