Flexible conductive materials have attracted considerable attention recently due to their potential applications in wearable displays, electronic sensors for human motion and electrically driven heaters [1
]. The most common flexible sensors are generally fabricated using electrically conductive metal nanoparticles or nanowires, metal thin films, carbon nanotubes and graphene [8
]. Although these sensors are electrically conductive and have high sensitivity, they have a small range of workable strain, which limits their practical applications. Some key factors need to be considered in designing strain sensors, such as a large strain range to monitor the human motion, rapid recoverable deformation, high sensitivity (high gauge factor (GF)) and fast response [16
]. It is still a challenge to prepare strain sensors with a large workable strain range and high sensitivity.
Conductive textile materials (CTMs) have been widely used as flexible wearable devices because of their light weight, good flexibility, high stretchability and recoverable deformation [17
]. Moreover, the CTMs possess high strength, good tear resistance, and excellent flexibility and comfort attributes. The response mechanism of CTMs-based strain sensors is that the resistance changes with stretching, which can be explained by changes in the fabric’s geometric structure and associated contact points between fibers and yarns within the structure. A number of strategies have been developed to prepare CTMs with sensing performance [21
]. For example, reduced graphene oxide was combined with cotton to fabricate flexible sensing fabrics [17
]. Nevertheless, these sensors from ordinary cotton fabrics had a relatively small strain range. Flexible strain sensors were also prepared by combining elastic polymers with carbonized cotton or silk fabrics [33
]. The obtained sensors exhibited excellent sensitivity and a large strain sensing range. However, the carbonization process destroyed the structure of fabrics, which led to the loss of their original mechanical properties. Novel approaches are desirable to realize the preparation of flexible sensors with a wide strain range and intrinsic mechanical properties of fabrics. Carbon nanotubes (CNTs) have been widely used as electrical conductive materials owing to their high surface area, low electrical resistance, low mass density and high stability. Modification of textiles with CNTs can give conductivity to the substrate materials [4
]. Some research works have reported on the CNT-coated cotton yarn and fabrics [42
]. Although these CNT-coated cotton yarns and fabrics had good electrical conductivity, their elasticity and deformability were limited due to the weaving structure and material type, which limited the applications of the conductive fabrics in wearable items. Herein, we used cotton elastic fabrics to fabricate wearable fabrics that have a large strain range and good recoverable deformation. CNTs have been coated on yarns of polyurethane (PU) and cotton to develop flexible wearable devices [46
]. However, yarns are one-dimensional textile materials, which limits their application in wearable electronic devices.
In this study, we fabricated flexible fabric strain sensors by treating elastic knitted cotton fabrics in a CNT suspension using the “dip-and-dry” method. The surface morphology, chemical structures and electrical conductivity of the CNT modified fabrics were characterized. The electromechanical performance and strain sensing properties of the CNT-cotton fabric were investigated. Human motion was monitored by the obtained flexible strain sensor. In addition, the CNT-cotton fabric displayed excellent electric heating effect.