Design and development of nanocomposites by carbon-based 2D materials for piezoresistive sensing application

Abstract

In the realm of wearable electronic devices, considerable attention has been directed toward flexible compressive piezoresistive sensors, given their potential applications in real-time human health monitoring and gesture recognition for robotics. Development of such sensors using electrically conductive polymer nanocomposites (E-CPnC) holds great promise due to their remarkable characteristics such as lightweightness and cost effectiveness. Despite notable advancements in this field, the fabrication techniques for compressive polymer-based piezoresistive sensors still tend to rely on complex and multi-step processes. This highlights the urgent need for developing facile and cost-effective manufacturing methods to propel advancements in the electronic industry. In this research we address the growing demand for facile and cost-effective manufacturing methods in the development of compressive piezoresistive sensors for wearable electronic devices. This study introduces a novel one-pot soft-templating method for fabricating PDMS-based conductive compressive sensors, benefiting the unique physical properties of cyclohexane. By controlling temperature and pressure conditions, a microcellular structure was prepared through the solvent's crystals sublimation, effectively serving as a soft template. Graphene, chosen for its conductive properties, was dispersed in varying proportions of cyclohexane, with the addition of PDMS and its curing agent. The solution freeze-dried, and the rapid evaporation of cyclohexane solid crystals within the structure coupled with the crosslinking of PDMS facilitated the creation of pores. Consequently, diverse microcellular structures with varying levels of porosity were successfully fabricated. For investigating the morphology of the resulting microcellular nanocomposites scanning electron microscopy (SEM) was used. The electrical conductivity, dielectric properties, and sensitivity of the nanocomposites could be analyzed through electrochemical impedance spectroscopy (EIS) and EIS coupled with universal testing machine (UTM). The mechanical performance, considering the stress-strain curves, mechanical strength, Young’s modules and the fatigue testing were analyzed by UTM. The fabricated sensors exhibit remarkable electrical conductivity, mechanical strength and sensitivity that can be achieved through the optimization of graphene concentration in the microcellular structure. Optimization of solvent concentration led to the attainment of varied pore sizes and void fractions. When 0.43 vol% graphene was introduced, the microcellular nanocomposite displayed an electrical conductivity of 2.5×10-2 S/m and a mechanical strength of 780 kPa at 85% compression strain. This remarkable compressibility was attributed to the robust 3D interconnected structure featuring a high void fraction of 82%. With an increase in graphene concentration to 0.87 vol%, the electrical conductivity rose to 35×10-2 S/m, and the compressive strength reached 1500 kPa at 70% strain, accompanied by a void fraction of 64.8%. Moreover, the electromechanical performance analysis reveals two linear resistance responses along the compressive strain range, demonstrating the versatility of the sensors in capturing different levels of compression. For instance, the foam loaded with 0.43 vol% exhibited a notable change in resistivity up to 10% strain, resulting in a high gauge factor of 0.5 kPa-1. From 10 to 85% strain, the foam displayed a second linear detection region with a gauge factor of 1.2 MPa-1. The compressive sensors demonstrated a rapid response time of 20 ms and exceptional cyclic stability of 500 cycles, owing to their resilient 3D interconnected structure, indicating their suitability for mid and high-pressure (10 kPa- 1 MPa) sensing applications, as well as real-time monitoring of human joint movements.