Review of flexible strain sensors based on cellulose composites for multi-faceted applications

Flexible strain sensors have attracted much attention due to their wide applications in human health monitoring, motion detection, human–computer interaction, and smart robots in recent years. Cellulose-based materials include cellulose fibers, cellulose nanofibers, cellulose nanocrystals, and cellulose derivatives such as methylcellulose, carboxymethyl cellulose, and cellulose acetate. They exhibit excellent properties and diverse functions that play key roles in the preparation of flexible strain sensors. The conductive networks of the cellulose-based composite materials are established by mixing or coating conductive materials or directly carbonizing the cellulose materials. This paper was divided into five parts according to the macroscopic forms of sensors, including fibers and yarns, films, papers, fabrics and gels. The materials, preparation methods, and structures of the flexible strain sensors are detailly compared and discussed. The solutions to the difficulties met in the preparation process are proposed. The sensing performance of the flexible strain sensors based on cellulose composite and their applications in physiological signals and human motion detection are summarized. The potential applications and challenges during future development are analyzed. This review provides some suggestions and strategies for further development of flexible strain sensors based on cellulose composite for physiological signals and human motion detection.