Contents lists available at ScienceDirect European Polymer Journal journal homepage: www.elsevier.com/locate/europolj Soft and highly responsive multi-walled carbon nanotube/pullulan hydrogel composites as electroactive materials Kochakorn Saeaeh a , Natlita Thummarungsan a , Nophawan Paradee a,b , Pongpol Choeichom a , Katesara Phasuksom a , Wanchai Lerdwijitjarud a,c , Anuvat Sirivat a, ⁎ a Conductive and Electroactive Polymer Research Unit, The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok 10330, Thailand b Department of Chemistry, Faculty of Science, King Mongkut's University of Technology Thonburi, Bangkok 10140, Thailand c Department of Materials Science and Engineering, Faculty of Engineering and Industrial Technology, Silpakorn University, Nakorn Pathom 73000, Thailand ARTICLE INFO Keywords: Pullulan Multi-walled carbon nanotube Hydrogel Electromechanical properties ABSTRACT Pullulan is a polysaccharide as obtained from the black yeast fermentation. Soft and flexible pullulan hydrogels and multi-walled carbon nanotube/pullulan hydrogel composites were fabricated by a solvent casting method. The effects of crosslinking agent, MWCNT content, and electric field strength on the electromechanical beha- viours were investigated. The storage modulus (G′) or the elasticity monotonically increased with increasing crosslinking agent and electric field strength. The MWCNT/pullulan hydrogel composite with 0.01%v/v MWCNT provided the highest storage modulus sensitivity of 71.2 at the applied electric field strength of 800 V/mm, relative to other hydrogel composites previously reported. The storage and loss moduli were further transformed to the creep compliance through the relaxation spectrum and the retardation spectrum, respectively. The creep compliance of the pristine hydrogels and composites decreased with increasing crosslinking agent amount and electric field strength. The 01MWCNT/Pullulan_5STMP hydrogel composite yielded the lowest creep compliance function, illustrating the ability to resist deformation through electric field. The dielectrophoresis force density of the MWCNT/pullulan hydrogel composite with 0.01%v/v MWCNT was the highest obtained at 0.2258 mN/ mm 3 , suggesting as the most suitable for practical actuator applications. 1. Introduction Many bio-based polymeric materials have been investigated and developed for various applications such as artificial muscles, sensors, imaging devices, and microsurgical devices, owing to their advantages of low cost, flexibility, light weight, and biocompatibility. The bio- based polymeric materials previously reported were the capsaicin loaded chitosan used as an intelligent antibiofouling nanocomposites in marine applications [1], bamboo (D. sinicus) lignin [2], milled wood lignin from dendrocalamus sinicus [3], prunus cerasifera gum poly- saccharide [4], and carboxymethyl chitosan [5]. In particular, bio- based polymeric electroactive actuators fabricated are potential to be utilized as artificial muscles and soft robot parts [6]. Pullulan, one of many non-ionic polysaccharides as derived from the black yeast (Aureobasidium pullulan) fermentation, is non-toxic, non- immunogenic, non-mutagenic, non-carcinogenic, odorless, tasteless, and edible; it has been explored in various biomedical applications namely tissue engineering, targeted drug delivery and therapy, and wound healing [7,8]. Pullulan hydrogels are promising bio-based materials as they can be expected to provide a response to external stimuli, thus eligible as candidate artificial muscles [9]. In addition, if chemicals used in fabricating pullulan hydrogels are required to be non- toxic, then sodium trimetaphosphate (STMP) is one suitable choice to crosslink pullulan hydrogels as it is acceptable in food packaging and non-toxic towards human in contrast to epichlorohydrin [10,11]. A carbon based hydrogel nanocomposite, consisting of a hydrogel and a carbon-based nanomaterial, is a potential material which can be tailored towards improved electromechanical and mechanical proper- ties relative to a pristine hydrogel. Recently, there has been an in- creasing interest to fabricate and develop various polymer nano- composites to improve their performances towards various applications: sensors, electromagnetic interference shielding, wound healing, and anti-corrosion [12–16]. Carbon based nanomaterials such as graphene, carbon nanotube, and buckminsterfullerene (C60) have been explored and utilized in various applications namely sensors and actuators, electrical insulators, electromagnetic interference (EMI) shielding, tissue engineering scaffolds, drug delivery systems, and bio- medical devices [17–21]. Existing carbon nanotubes are the single- https://doi.org/10.1016/j.eurpolymj.2019.109231 Received 30 April 2019; Received in revised form 28 August 2019; Accepted 2 September 2019 ⁎ Corresponding author. E-mail address: anuvat.s@chula.ac.th (A. Sirivat). European Polymer Journal 120 (2019) 109231 Available online 03 September 2019 0014-3057/ © 2019 Published by Elsevier Ltd. T