Composites Science and Technology 187 (2020) 107959 Available online 17 December 2019 0266-3538/© 2019 Elsevier Ltd. All rights reserved. Multi-modal strain and temperature sensor by hybridizing reduced graphene oxide and PEDOT:PSS Fan Zhang a , Hailong Hu a, b , Mohammad Islam a , Shuhua Peng a , Shuying Wu a, c , Sean Lim d , Yang Zhou a , Chun-Hui Wang a, * a School of Mechanical and Manufacturing Engineering, University of New South Wales, Sydney, New South Wales, 2052, Australia b School of Aeronautics & Astronautics, Central South University, Changsha City, 410083, China c School of Engineering, Macquarie University, Sydney, NSW, 2109, Australia d Electron Microscopy Unit, University of New South Wales, Sydney, 2052, Australia A R T I C L E INFO Keywords: Multi-modal sensor Piezoresistive performance Temperature-sensing properties PEDOT:PSS/reduced graphene oxide aerogels Dual-frequency impedance method ABSTRACT Mechanically stretchable sensors are responsive to multiple stimuli, such as strain and temperature, making it diffcult to identify individual stimulus using a single senor. Herein, we present a hybrid sensor featuring dis- similar impedance frequency-responses under different stimuli. The sensor is made by infusing poly- dimethylsiloxane (PDMS) into an aerogel containing a hybrid of conductive materials consisting of PEDOT:PSS and reduced graphene oxide. Exhibiting positive and negative sensitivity to strain and temperature, respectively, this sensor makes it possible to determine both stimuli using an impedance method. By measuring the sensor’s electrical impedance at two different frequencies, both strain and temperature can be readily determined using just one sensor. Test results under long-term cyclic loading confrm that the hybrid sensor also retains high sensitivity, stretchability, and cyclic stability. The new impedance method used in conjunction with the hybrid sensor enable simultaneous determination of multiple stimuli, such as strain and temperature. 1. Introduction Wearable sensors capable of high sensitivity and large strain defor- mation are attracting extensive interests due to their tremendous po- tentials in electronic skins for human-machine interface [1,2], fexible displays [3], health or sports monitoring [4,5], and soft robotics [6–8]. Soft sensors based on hyper-elastic materials reinforced with conductive nanomaterials (i.e., carbon nanoparticles, nanofbers, nanotubes, metal nanowires or nanosheets) are emerging as a key solution [9–13] to offer higher stretchability than the strain limits (i.e. <5%) of conventional strain gauges made of metal foils or semiconductors. The electrical resistance of soft sensors depends on both mechanical deformation and temperature change [14–17], making it diffcult to simultaneously determine multiple stimuli using one sensor. As a result, multiple sensors, either in the form of a planar array or vertically inte- grated sensors [18–21], are needed to identify different stimuli [22–24]. The number of sensors are equal to or greater than the number of stimuli. Wang et al. [22] reported a 3 � 3 sensor array based on silk-combo derived carbon fbre membrane for temperature and pressure detection, which showed a high temperature sensitivity of 0.81%/ � C. Ho et al. [23] fabricated a complete graphene multifunc- tional 6 � 6 sensor array with 36 sensors, simultaneously detecting three stimuli including pressure, temperature and humidity. Harada et al. [18] reported a multifunctional electronic whiskers integrated with strain and temperature sensors via ink-printing method, showing a high pressure sensitivity of ~59%/Pa. Tien et al. [19] described a device directly integrating both gate dielectric and semiconductor channel into feld-effect transistor (FET) platforms, which could simultaneously and disproportionally respond to strain and temperature. However, these types of sensor arrays and integrated sensors need multiple electrodes and interconnects, increasing the structural complexity and fabrication costs. The research presented herein aims to develop a multi-modality sensor that can identify both mechanical strain and temperature change using an impedance approach. The new sensor consists of an aerogel made of hybrid Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) and reduced graphene oxide (rGO). This hybrid design gives the sensor an ability to respond differently to strain and * Corresponding author. E-mail address: chun.h.wang@unsw.edu.au (C.-H. Wang). Contents lists available at ScienceDirect Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech https://doi.org/10.1016/j.compscitech.2019.107959 Received 19 September 2019; Received in revised form 5 December 2019; Accepted 12 December 2019