Electromechanical response of silk fibroin hydrogel and conductive polycarbazole/silk fibroin hydrogel composites as actuator material Thanida Srisawasdi a , Karat Petcharoen a , Anuvat Sirivat a, ⁎, Alexander M. Jamieson b a The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok 10330, Thailand b Department of Macromolecular Science and Engineering, Case Western Reserve University, OH 44106, USA abstract article info Article history: Received 19 February 2015 Received in revised form 22 April 2015 Accepted 9 June 2015 Available online 12 June 2015 Keywords: Silk fibroin Hydrogels Polycarbazole Actuator Biopolymer Pure silk fibroin (SF) hydrogel and polycarbazole/silk fibroin (SF/PCZ) hydrogels were fabricated by solvent casting technique to evaluate electromechanical responses, dielectric properties, and cantilever deflection properties as functions of electric field strength, SF concentration, glutaraldehyde concentration, and PCZ concentration in the blends. Electromechanical properties were characterized in oscillatory shear mode at electric field strengths ranging from 0 to 600 V/mm and at a temperature of 27 °C. For both the pristine SF and SF/PCZ hydrogels, the storage modulus response (ΔG′) and the storage modulus sensitivity (ΔG′/G′ 0 ) increased dramatically with increasing electric field strength. The pristine hydrogel possessed the highest storage modulus sensitivity value of 5.87, a relatively high value when compared with other previously studied electroactive polymers. With the addition of conductive PCZ in SF hydrogel, the storage modulus sensitivity and the relative dielectric constant decreased; the conductive polymer thus provided the softening effect under electric field. In the deflection response, the dielectrophoresis force and deflection distance increased monotonically with electric field strength, where the pure SF hydrogel showed the highest deflection distance and dielectrophoresis force. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Development of artificial muscle has received interests based on biologically inspired actuators or devices which possess electromechan- ical responses. The applications of artificial muscles are presently intended for animals and human-like robots. Electroactive polymer is one type of electroactive material [1]. Electroactive polymers offer novel and promising characteristics such as light weight, high flexibility, and high energy density. The examples of electroactive materials are dielectric elastomers [2], electrostrictive papers [3], conductive poly- mers [4], electrorheological fluids [5], ionic polymer gels [6], and ionic polymer metal composites [7]. Silk fibroin (SF) is a protein biopolymer derived from silkworm (Bombyx mori), consisting of light (~25 kDa) and heavy (~350 kDa) chains of polypeptides and linked by disulfide bond. The amino acid composition of SF primarily consists of glycine (43%), alanine (30%), and serine (12%) [8,9]. Due to a wealth of merits, such as biocompatibil- ity, biodegradability, mechanically superior, amenable to aqueous or organic solvent processing, SF has been widely used in the medical and pharmaceutical fields [9]. The most popular morphology is to fab- ricate SF into a film type, because it is of a relatively easy preparation technique and the processing conditions can be controlled. However, SF films have shown low mechanical properties because of the brittle- ness in the dry state, which limits the actuator applications [10,11]. Recently, composites of conductive polymers and biopolymers have been of keen interest due to better thermal stability and mechanical properties, variable conductivity [12]. Polycarbazole (PCZ) is one of the conductive polymers that has been interested in many applications, such as electrochromic displays, rechargeable batteries, light-emitting diodes, and organic transistors [13]. However, toxicity of polycarbazole has not been fully investigated. It has been reported that polycarbazole was a stable substance, but it was incompatible with the strong oxidiz- ing agent such as hydrogen peroxide, sulfuric acid, and silver oxide [14]. In the present study, the objective was in fabricating an electroactive material from conductive PCZ embedded in SF hydrogel. Also, it is of interest to study and test SF hydrogels and SF/PCZ hydrogel under elec- tric field for actuator applications. The electromechanical properties, electrical properties, and actuator performances were investigated and examined along with the effects of SF concentration, glutaraldehyde concentration, PCZ concentration, and electric field strength. 2. Experimental 2.1. Materials Carbazole (AR grade, Merck) was used as the monomer. Ammonium persulfate (AR grade, Sigma-Aldrich) was used as the oxidant. Hydro- chloric acid 37% and dichloromethane (AR grade, RCI Labscan) were Materials Science and Engineering C 56 (2015) 1–8 ⁎ Corresponding author. E-mail address: anuvat.s@chula.ac.th (A. Sirivat). http://dx.doi.org/10.1016/j.msec.2015.06.005 0928-4931/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec