Recent Advances of Carbon-Based Flexible Strain Sensors in Physiological Signal Monitoring

Flexible strain sensors have attracted much attention due to their good flexibility, high sensitivity, superior repeatability, and great potentials for application in physiological signal detection. Carbon materials, including carbon nanotubes, graphene, carbon black, graphite, and natural-bioderived carbon materials are often used as active materials for the fabrication of flexible strain sensors because of their superior electrical conductivity and flexibility. Among them, carbon nanotubes and graphene can be prepared into flexible sensors in various forms, such as fibers, films, or textiles. Therefore, carbon material flexible sensors used for physiological signal detection have been sufficiently studied. Herein, the sensing mechanism of flexible strain sensors and the recent advances are reviewed. Sensor characteristics and functions of fibers/films with carbon nanotubes, graphene, and other carbon materials are described in terms of materials, preparation, and properties. From the aspect of sensor application, the sensors with different materials in large- and small-amplitude physiological signals are introduced in detail. Eventually, the superiorities and disadvantages of various carbon-based flexible strain sensors are summarized, and the challenges and opportunities of them in the future are also presented.