Advanced carbon materials for flexible and wearable sensors

Abstract

Flexible and wearable sensors have drawn extensive concern due to their wide potential applications in wearable electronics and intelligent robots. Flexible sensors with high sensitivity, good flexibility, and excellent stability are highly desirable for monitoring human biomedical signals, movements and the environment. The active materials and the device structures are the keys to achieve high performance. Carbon nanomaterials, including carbon nanotubes (CNTs), graphene, carbon black and carbon nanofibers, are one of the most commonly used active materials for the fabrication of high-performance flexible sensors due to their superior properties. Especially, CNTs and graphene can be assembled into various multi-scaled macroscopic structures, including one dimensional fibers, two dimensional films and three dimensional architectures, endowing the facile design of flexible sensors for wide practical applications. In addition, the hybrid structured carbon materials derived from natural bio-materials also showed a bright prospect for applications in flexible sensors. This review provides a comprehensive presentation of flexible and wearable sensors based on the above various carbon materials. Following a brief introduction of flexible sensors and carbon materials, the fundamentals of typical flexible sensors, such as strain sensors, pressure sensors, temperature sensors and humidity sensors, are presented. Then, the latest progress of flexible sensors based on carbon materials, including the fabrication processes, performance and applications, are summarized. Finally, the remaining major challenges of carbon-based flexible electronics are discussed and the future research directions are proposed. [56,57], have been used as the active components for the fabrication of flexible sensors. Among these materials, metal NPs can be used to fabricate flexible sensors with high sensitivity, but the sensing range and stretchability of these sensors are limited [58]. Besides, due to the limited chemical stability and reproducibility of metal nanowires, it is challenging to fabricate stable sensors using metal nanowires [10]. Similarly, conductive polymers with poor stability and conductivity are also difficult to be used for the fabrication of high-performance sensors [59]. In contrast, carbon materials are one of the most commonly investigated materials, especially CNTs and graphene (including graphene oxide (GO) and reduced graphene oxide (rGO)), due to their remarkable mechanical, electrical and thermal properties. To implement macroscopic applications, CNTs and graphene with superior properties in microscopic level should be converted into macroscopic functional assemblies, such as one dimensional (1D) fibers or yarns [60,61], two dimensional (2D) films or sheets [62,63], three dimensional (3D) architectures [64][65][66] (Fig. 1). The multi-scaled macroscopic carbon nanomaterials endow the flexible sensors with high sensitivity, excellent flexibility and good stability as well as desired configurations. Also, lo...

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