Three-dimensional printed cellulose nanofibers/carbon nanotubes/silicone rubber flexible strain sensor for wearable body monitoring

Flexible pressure sensors have a wide range of applications in motion monitoring, human-computer interaction, and other fields. In this study, we designed and prepared a 3D printable flexible electronic composite called modified cellulose nanofibers/carbon nanotube/silicone rubber (MCNF/CNT/SR). This composite was created by adsorbing cellulose nanofibers onto carbon nanotubes in a solution, which offered a new approach for uniformly incorporating nano-compounds into the SR matrix. To fabricate the flexible strain sensor, we used this composite as ink by a commercial direct writing 3D printer for layer deposition. By surface silanization of cellulose nanofibers (CNFs), we improved the interface compatibility between modified cellulose nanofibers (MCNFs), carbon nanotubes (CNTs), and the silicone rubber (SR) matrix, thereby enhancing the mechanical properties of the nanohybrid polymer network. This synergistic effect enabled MCNF/CNT/SR elastomers to exhibit remarkable tensile strength (2.97 MPa) and elongation (>250%) while maintaining high conductivity (0.086 S m(-1)). The incorporation of MCNF-CNT hybrid significantly improved the rheological properties of SR composites and endowed them with excellent thixotropic properties. Furthermore, the presence of CNTs provided MCNF/CNT/SR sensing materials with high sensitivity (maximum gauge factor (GF) = 64.32), a wide strain range (similar to 300%), and exceptional stability. These flexible sensors have been successfully utilized for monitoring human movements.