Facile Strategy to In Situ Synthesize Gallic Acid-Modified Lignin and Its Utilization for Fabricating Conductive and Self-Adhesive Hydrogels as Strain Sensors

As lignin is one of the most abundant renewable resources, utilization to fabricate functional hydrogels has gained increasing attention. However, the heterogeneous structure and the few available functional groups of lignin limited its applications. Herein, we presented a simple strategy to synthesize gallic acid (GA)-modified lignin as a key component to prepare conductive hydrogels. The addition of GA during the organic solvent pretreatment of poplar changed the structure of lignin and increased the total content of phenolic hydroxyl groups from 3.20 to 3.89 mmol/g (EOL-G30). Moreover, the enzymatic hydrolysis of pretreated poplar was enhanced from 49.1 to 55.2% (OP-G30) at a low enzyme loading of 5 FPU/g. When using GA-modified lignin to prepare hydrogels, the resulting hydrogel exhibited excellent self-adhesion to various substrates and demonstrated effective free radical scavenging capabilities. When incorporating sodium chloride (NaCl) as conductive ions, the hydrogels exhibit reliable strain-sensing capability. Moreover, they also displayed robust functionality in a low-temperature environment (−20 °C). This study has presented valuable insights to realize in situ modification of lignin and fabricate lignin-based functional materials, offering new perspectives for the biorefinery industry.