Mechanical tough and stretchable quaternized cellulose nanofibrils/MXene conductive hydrogel for flexible strain sensor with multi-scale monitoring.

• The resultant hydrogel possesses an admirable feature of mechanical robustness and excellent electrical performance. • The intriguing overall properties are attributed to the synergy of multiple physical interactions and nano-reinforcement. • The hydrogel delivers sensitive strain-induced resistance change under variational deformations. • A strain sensor based on such hydrogel can accurately monitor multi-scale deformations from large mechanical deformation to tiny physiological motions. For advanced conductive hydrogels, adaptable mechanical properties and high conductivity are essential requirements for practical application, e.g., soft electronic devices. Here, a straightforward strategy to develop a mechanically robust hydrogel with high conductivity by constructing complicated 3D structures composed of covalently cross-linked polymer network and two nanofillers with distinguishing dimensions is reported. The combination of one-dimensional quaternized cellulose nanofibrils (QACNF) and two-dimensional MXene nanosheets not only provides prominent and tunable mechanical properties modulated by materials composition, but results in electronically conductive path with high conductivity (1281 mS m^–1). Owing to the uniform interconnectivity of network structure attributed to the strong macromolecular interaction and nano-reinforced effect, the resultant hydrogel exhibits a balanced mechanical feature, i.e., high tensile strength (449 kPa), remarkable stretchability (> 1700 %), and ultra-high toughness (5.46 MJ m^–3), outperforming those of virgin one. Additionally, the enhanced conductive characteristic with the aid of QACNF enables hydrogels with impressive electromechanical behavior, containing high sensitivity (maximum gauge factor: 2.24), wide working range (0–1465 %), and fast response performance (response time: 141 ms, recover time: 140 ms). Benefiting from the excellent mechanical performance, a flexible strain sensor based on such conductive hydrogel can deliver an appealing sensing performance of monitoring multi-scale deformations, from large and monotonous mechanical deformation to tiny and complex physiological motions (e.g., joint movement and signature/vocal recognition). Together, the hydrogel material in this work opens up opportunities in the design and fabrication of advanced gel-based materials for emerging wearable electronics. [Display omitted] [ABSTRACT FROM AUTHOR]