Investigation of bi-ionic contribution for the enhancement of bending actuation in polypyrrole ®lm Wataru Takashima * , Shyam S. Pandey, Keiichi Kaneto Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, Iizuka, Fukuoka 820-8502, Japan Received 22 May 2002; received in revised form 9 August 2002; accepted 5 November 2002 Abstract Electrochemomechanicaldeformation(ECMD)intheelectrodepositedpolypyrrole(PPy)®lmswasexaminedbyjudiciousselectionofthe supporting electrolytes during electropolymerization and cyclic voltammetry (CV). A critical perusal of the ECMD characteristics corroborates the cation-driven and/or anion-driven deformation in the PPy ®lms. It was found that a freestanding PPy ®lm successively electrodeposited as a cation-driven layer followed by an anion-driven layer works as a bimorph actuator. The large bending motions were observedin1MNaClsolutioninwhichbothanion-drivenandcation-drivenlayersareelectroactive.Incontrast,thebendingmotionswere foundtobesuppressedinboth1Methylbenzenesulfonicacidsodiumsaltand1Mtetrabutylammoniumchloride,inwhichonlyoneofthe cation-drivenortheanion-drivenlayeriselectroactive.Theresultsindicatethatthebimorphactuatordrivenbythesynchronizedinsertionand deinsertion of anion and cation, which can be expressed as the bi-ionic actuator, clearly enhances its bending motion compared to that observed in the conventional bimorph actuators driven by mono-ionic ECMD behaviors. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrochemomechanical deformation; Conducting polymer; Polypyrrole; Anion-driven actuation; Cation-driven actuation; Bimorph actuator; Bi-ionic actuator 1. Introduction Reversible dimensional changes during electrochemical reactions in intrinsically electro-conducting polymers have advocated the potentiality of these novel materials in the areaofsoftactuatorsintherecentpast [1,2].Thisdeforma- tion,whichcanbeexpressedastheelectrochemomechanical deformation (ECMD), is regarded as one of the most appropriate applications of the conducting polymers in the area of biomimetic devices like human robotics [3]. Some practical applications such as microactuator [4], encapsulated actuators [5,6] and human robotics [7] were also demonstrated. In the area of development of soft actuators, polypyrrole (PPy) is one of the potential candidates among conducting polymersbecauseof(1)thehighconductivityinbothofthe oxidized and reduced states [8], (2) large electrochemical activityathigherpH [9] and (3) the environmental stability anddurability.PPyfreestanding®lmcanbeobtainedbythe electrodeposition method in which anion is incorporated as the initial dopant. The initial dopant anions and the ®lm morphologies strongly in¯uence the electrochemical and ECMDcharacteristics.Adetailedinvestigationofindividual constituent ions participating in cyclic voltammetry (CV) and ECMD behaviors is important for fundamental and appliedaspectsand,therefore,tuningsofthemareexpected to enhance the performance of the thus designed soft actuators. We have precisely investigated the ECMD beha- viors of freestanding conducting polymer ®lms using a handmade electrochemical cell with a laser displacement meter,resultinginquantitativeanalysis [10],transientmea- surement [11] andmechanochemoelectricaleffect [12].The results indicate that at present, the ECMD ratios in con- ducting polymers stay in the range of 2±4% only, which is not enough for their application as liner actuator like a muscle. Under such situation, bimorph actuator can be regarded as one of the practical designs for the device applications. Bimorph actuator generally consists of an active layer as stretcher along with a passive layer as supporter. The dimensional difference between both layers isthedrivingforceforbendingmotion.Thus,theincreaseof the dimensional difference enlarges the actuation. Mean- while,cyclicECMDbehaviorgeneratesandreleasesalarge Sensors and Actuators B 89 (2003) 48±52 * Correspondingauthor.Tel.: 81-948-29-7684; fax: 81-948-29-7651. E-mail address: takashima@life.kyutech.ac.jp (W. Takashima). 0925-4005/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0925-4005(02)00426-4