Design and evaluation of flexible sensors with enhanced sensitivity based on piezoresistive polymer composites

Flexible sensors with high sensitivity have shown great potentials in applications such as human motion detection and wearable electronic devices. In this paper, we propose a design of flexible piezoresistive sensors with hole structures that exhibit enhanced sensitivity. The hole structures re-arrange the strain distribution under tensile loadings, and the resulting non-uniform strain distribution amplifies the overall resistive response. The underlying mechanism of the enhanced sensitivity is based on the nonlinear piezoresistive behavior of conductive polymer composites under large strains. Simulation studies accounting for the nonlinear piezoresistivity are performed to investigate the influence of the aspect ratio of an elliptical hole structure on the sensor’s sensitivity. Results show that the design of hole structures increases the piezoresistive response, and a higher sensitivity can be achieved at a smaller aspect ratio. To experimentally validate the design, sensors that are made of exfoliated graphite polymer composites with an elliptical hole of different aspect ratios are prepared and characterized. Results show that the strain sensitivity reaches 7.65 when the aspect ratio decreases to 0.1, improved by two folds compared with the control sample. To demonstrate the performance in monitoring human motions, sensors with and without hole structures are attached on the finger and the wrist to detect the bending and relaxing motion, and enhanced sensitivity is also obtained from the structure with a low aspect ratio. This work provides an effective approach to improve the sensitivity of flexible sensors that are made of piezoresistive polymer composites.