The rapid development of wearable smart devices has contributed to the enormous demands for smart flexible strain sensors. However, to date, the poor stretchability and sensitivity of conventional metals or inorganic semiconductor-based strain sensors have restricted their application in this field to some extent, and hence many efforts have been devoted to find suitable candidates to overcome these limitations. Recently, novel resistive-type electrically conductive polymer composites (ECPCs)-based strain sensors have attracted attention based on their merits of light weight, flexibility, stretchability, and easy processing, thus showing great potential applications in the fields of human movement detection, artificial muscles, human–machine interfaces, soft robotic skin, etc. For ECPCs-based strain sensors, the conductive filler type and the phase morphology design have important influences on the sensing property. Meanwhile, to achieve a successful application toward wearable devices, several imperative features, including a self-healing capability, superhydrophobicity, and good light transmission, need to be considered. The aim of the present review is to critically review the progress of ECPCs-based strain sensors and to foresee their future development.

An electrically conductive ultralow percolation threshold of 0.1 wt% graphene was observed in the thermoplastic polyurethane (TPU) nanocomposites. The homogeneously dispersed graphene effectively enhanced the mechanical properties of TPU significantly at a low graphene loading of 0.2 wt%. These nanocomposites were subjected to cyclic loading to investigate the influences of graphene loading, strain amplitude and strain rate on the strain sensing performances. The two dimensional graphene and the flexible TPU matrix were found to endow these nanocomposites with a wide range of strain sensitivity (gauge factor ranging from 0.78 for TPU with 0.6 wt% graphene at the strain rate of 0.1 min⁻¹ to 17.7 for TPU with 0.2 wt% graphene at the strain rate of 0.3 min⁻¹) and good sensing stability for different strain patterns. In addition, these nanocomposites demonstrated good recoverability and reproducibility after stabilization by cyclic loading. An analytical model based on tunneling theory was used to simulate the resistance response to strain under different strain rates. The change in the number of conductive pathways and tunneling distance under strain was responsible for the observed resistance-strain behaviors. This study provides guidelines for the fabrication of graphene based polymer strain sensors.

Both aminopropylisobutyl polyhedral oligomeric silsesquioxane (NH₂-POSS) and hydrazine monohydrate were utilized to functionalize graphene oxide (GO), and to obtain chemically modified graphene (CMG), which was then used for preparing thermally conductive CMG/polyimide (CMG/PI) nanocomposites via a sequential in situ polymerization and electrospinning-hot press technology. NH₂-POSS molecules were grafted on the GO surface, and CMG was obtained by the reaction between NH₂-POSS and GO. The thermal conductivity coefficient (λ), glass transition temperature (T_g) and heat resistance index (T_HRI) of the prepared CMG/PI nanocomposites were all increased with increasing the CMG loading. The λ value of the CMG/PI nanocomposites with 5 wt% CMG was significantly improved to 1.05 W m⁻¹ K⁻¹, about 4 times higher than that of the pristine PI matrix (0.28 W m⁻¹ K⁻¹). The corresponding T_g and T_HRI values were also increased to 213.0 and 282.3 °C, respectively. Moreover, an improved thermal conductivity model was proposed and predicted the λ values of the nanocomposites more precisely than those obtained from the typical Maxwell, Russell and Bruggemen classical models.