Electro-Conductive Double-Network Hydrogels Ryoichi Kishi, 1 Kazuaki Hiroki, 2 Taiki Tominaga, 2 Ken-Ichi Sano, 2 Hidenori Okuzaki, 3 Jose G. Martinez, 4 Toribio F. Otero, 4 Yoshihito Osada 2 1 Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan 2 Advanced Science Institute, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan 3 Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, 4-4-37 Takeda, Kofu, Yamanashi 400-8511, Japan 4 Universidad Polit ecnica de Cartagena (UPCT), ETSII, Center for Electrochemistry and Intelligent Materials (CEMI), Paseo Alfonso XIII, Aulario II, 30203 Cartagena, Spain Correspondence to: R. Kishi (E-mail: r-kishi@aist.go.jp) Received 4 January 2011; revised 27 February 2012; accepted 5 March 2012; published online 23 March 2012 DOI: 10.1002/polb.23066 ABSTRACT: Novel electro-conductive and mechanically-tough double network polymer hydrogels (E-DN gels) were synthe- sized by polymerization of 3, 4-ethylenedioxythiophene in the presence of a double network hydrogel (DN gel) matrix. The E- DN gels showed not only excellent mechanical performance, having a fracture stress of 1.4–2.1 MPa, but also electrical con- ductivity as high as 10 3 S cm 1 , both under dry and water- swollen states. The fracture stress and fracture energy of the E- DN gel was increased by 1.7 and 3.4 times, respectively, as compared with the DN gel. From scanning electron microscope and AFM observations, it was found that electro-conductive poly(3,4-ethylenedioxythiophene) (PEDOT) was incorporated into DN gel matrix, apparently due to the formation of a poly- ion complex with sulfonic acid group of the DN gel network. Thus, PEDOT incorporated into the DN gel matrix greatly improves not only electronic conductivity, but also mechanical properties, reinforcing the double network gel matrix. V C 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: 790– 796, 2012 KEYWORDS: conductive polymers; double-network; high me- chanical strength; polymer hydorgels; poly-ion complex INTRODUCTION Polymer hydrogels 1–4 are soft, wet materials which show reversible size and shape changes under appli- cation of an electric field. 5–7 This electro-driving phenom- enon has been attracting the attention of many researchers for possible application in a variety of fields such as soft actuators, artificial muscles, robotics, biomimetic devices, flexible and stretchable electronics, medical appliances, so forth. However, to utilize polymer hydrogels in these fields, some serious technical problems must be overcome. The first is lack of mechanical strength. Usually, hydrogels prepared by radical polymerization of vinyl monomers with crosslinkable monomers are mechanically very weak and brittle, and are easily broken even by applying a small stress (as low as 20– 30 kPa). However, this problem was recently solved by Gong et al. 8,9 who introduced a second flexible network into the first rigid network. This polymer hydrogel is known as a ‘‘double network hydrogel’’ (DN gel) consisting of a rigid, ionic first polymer network (such as poly(2-acrylamido-2- methylpropane sulfonic acid, PAMPS) and a flexible, neutral second polymer network (such as polyacrylamide, PAAm). These DN gels are a new type of soft, wet matter containing 60–90% water, and capable of exhibiting excellent mechani- cal performance: they show a fracture strength of more than 17 MPa, and a compressive strength of 0.3–1 MPa of Young’s modulus. The second problem hydrogels have is lack of electro-con- ductivity. Although some ionic polymer gels have ionic con- ductivity as high as 10 2 S cm 1 depending on the ionization degree, they cannot respond at higher frequencies (>10 2 Hz) due to extremely low ion mobility. Decomposition of water, generating hydrogen and oxygen gases, is also a serious problem for ion-conductive gels. Thus, ionized hydrogels do not show a response under high frequency and low voltage. In contrast, a variety of conductive polymers 10–12 exhibiting quick, reversible shape changes under high frequency and relatively low voltage have been developed, since they are driven by electrochemical redox reactions. 13–15 However, they also have some technical problems: most have limited solubility. Polypyrrole and polythiophene, for example, are insoluble in water as well as in most common organic sol- vents. In addition, they are brittle due to a rigid, branched, V C 2012 Wiley Periodicals, Inc. 790 JOURNAL OF POLYMER SCIENCE PART B: POLYMER PHYSICS 2012, 50, 790–796 FULL PAPER WWW.POLYMERPHYSICS.ORG JOURNAL OF POLYMER SCIENCE