Tuning Surface Terminations and Hydration Interactions in MXene Nanosheet-Based Hydrogel Composites for Self-Healable Strain Sensors

MXenes have emerged as promising candidates for enhancing the properties of self-healable flexible strain sensors by serving as conductive fillers. Leveraging the rich functional surface of MXene nanosheets, this study focuses on the transformation of F-rich MXene to O-rich MXene and explores its impact on the mechanical and healing properties of partially crystallized polymeric hydrogels. Through a facile hot-alkali process with KOH, the F terminations are replaced by O and OH groups, dominated by the earlier. Incorporating KOH-treated O-rich MXene (k-MXene) into a partially crystallized polymer hydrogel matrix significantly improves mechanical properties even with a mere 1 wt % addition. The resulting tensile strength reaches 0.93 MPa, with a remarkable healing efficiency of 88.3%. Notably, k-MXene exhibits a higher healing efficiency compared to fluorine-terminated MXene (f-MXene) when incorporated in the polymer matrix. Furthermore, the dry annealing and swelling process alters the hydration interactions between the host and matrix allowing for a 4-fold increase in the electrical response to the same elongation compared to conventional hydrogels. These findings underscore the critical role of tuning surface chemistry of MXene nanosheets and tailoring hydration interactions between the nanofiller and matrix to enhance healing capabilities and electromechanical response. This work opens avenues for designing advanced self-healable flexible strain sensors with superior performance and functionality.