Self-Assembled Zwitterionic Lignin-Induced Joint Enhancement of Mechanics and Adhesion in High-Performance Hydrogels for Flexible Strain Sensors

The urgent demand for stable and accurate sensors emphasizes the importance of high mechanical and adhesion properties in conductive hydrogels. However, it remains challenging to concurrently enhance these two properties due to the inherent contradiction between internal cohesion and external adhesion. Here, lignin, an underutilized natural resource, is proposed to conquer the design difficulty described above by zwitterionization. Upon π-π aggregation, electronegative lignin readily self-assembles into zwitterionic lignin with a benzyltrimethylammonium cation. The zwitterionic lignin can simultaneously promote supramolecular interactions among lignin, polymer chains (poly(acrylic acid) and gelatin), and adhesive substrates. Hence, the internal cross-linking and external adhesion are enhanced synchronously. Notably, the hydrogel demonstrates a remarkable true stress of 2.34 MPa at a strain of 1597.1%, and the peak adhesion strength of the hydrogel is achieved at 40.87 kPa. By valorizing waste lignin, this work opens up a new path to overcome the inherent design contradiction for high-performance hydrogels in the future.