Highly sensitive and self-healing conductive hydrogels fabricated from cationic cellulose nanofiber-dispersed liquid metal for strain sensors

The emergence of liquid metal (LM) emulsion as a soft multifunctional filler brings a new opportunity for fabricating hydrogel-based strain sensors with multifunctional properties. However, the extremely large surface tension and high density of LMs inhibit emulsification. Herein, we demonstrated a strategy for stabilizing LM emulsions using cationic cellulose nanofibers (CCNFs) to encapsulate LM droplets through strong electrostatic attraction with LM. By inducing acrylic acid (AA) polymerization in the presence of a CCNF-stabilized LM emulsion, a conductive hydrogel was prepared with the formation of reversible hydrogen bonds, ionic coordination, and electrostatic interactions among CCNFs, LM droplets, and poly(acrylic acid) (PAA). The hydrogel obtained, named the CCNF-LM-PAA hydrogel, shows good conductivity (1.54 S m−1), remarkable tensile strength and elongation at break, self-adhesiveness, and quick self-healing capability. As a strain-sensing material, the CCNF-LM-PAA hydrogel exhibits a very high sensing sensitivity (gauge factor = 16.2), a low strain detection limit (less than 1%), a short response/recovery time (107/91 ms), and good durability (300 cycles). These results enable the CCNF-LM-PAA hydrogel-based strain sensor to be an excellent wearable device for monitoring various human activities. Therefore, introducing additional electrostatic interactions by using CCNFs to stabilize LM emulsions provides a practical way to enhance the strain-sensing performance of LM emulsion-based hydrogels for assembling self-attached wearable devices.