A Review on Knitted Flexible Strain Sensors for Human Activity Monitoring

The recent surge in using wearable personalized devices has made it increasingly important to flexible textile-based sensors that can be worn comfortably and can sense various body strains. Among them, the knit-based flexible strain sensors (KFSS) show growing promise for human activity monitoring applications due to its processing flexibility, low cost, wearing comfortability, large strain performance, and high elastic recovery rate. Herein, five sensing mechanisms of knitted flexible strain sensors containing contact resistance, length resistance, tunneling effect, micro-crack propagation, and disconnection mechanism are summarized, which can be used on different human activity strain conditions, e.g., physiological activities and limb activities. The main fabrication approaches, including knitting techniques, post-processing techniques, embroidery, and sewing techniques are discussed. Six crucial parameters with corresponding calculation formulas for evaluating sensing performance and review specific applications of KFSS for human limb activity and physiological activity monitoring in healthcare, motion tracking, safety protection, and human-machine interaction are also described. Finally, the tough challenges and prospects of KFSS with the aim of providing meaningful guidance and direction for future research are critically analyzed. In this study, knitted flexible strain sensors for human activity monitoring is reviewed. Human activities are divided into physiological activities and limbs activities according to its strain range. And the five sensing mechanisms, three fabrication approaches, six crucial parameters, and four specific applications in each field for the two human activities are discussed, respectively.image