Highly sensitive detection of subtle movement using a flexible strain sensor from helically wrapped carbon yarns

Flexible strain sensors have attracted much attention owing to the potential applications in monitoring body movement and health conditions. Despite the research in this field, it is still a challenge to develop flexible strain sensors for detection of subtle movement (e.g. strain <1%). In this study, we demonstrate a simple method to prepare a flexible strain sensor capable of detecting subtle movement with strain as small as 0.1% and larger strains up to 30% with high sensitivity. Such a strain sensor was prepared using a carbon fiber yarn with a novel sheath-core helical wrapping structure, which was fabricated by carbonization of a yarn precursor consisting of cotton yarn as the core and an electrospun polyacrylonitrile nanofiber yarn as the helical wrapping sheath. The sensor showed a gauge factor of 37.3 when detecting the strain of 0.1%, and a gauge factor of 11.5 for strains in the range of 0.3-30.0%. The sensor was still working after 1000 cycles of stretching. The helical wrapping structure was found to play a key role in achieving such a remarkable sensor performance. The coil pitch and yarn diameter showed effects on strain range and sensitivity. The sensor can be used to detect small bending, stretching and various facial expression changes. It may be useful for various wearable electronic applications.