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

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.