Lignin-induced rapid polymerization of asymmetrical adhesion Janus gel for strain sensor

Functional hydrogel sensors have shown explosive growth in the health and medical fields. However, the uniform adhesion and the complicated polymerization process of hydrogels seriously hinder their further development. Herein, inspired by the layered structure of human skin, we prepare a Janus gel using in-situ polymerization. Based on the lignin-Fe³⁺ dual catalytic system, the rapid polymerization of the gel was achieved at room temperature. By tailoring the mass ratio of lignin and Fe³⁺ in the precursor, the adhesion of the upper and bottom layers can be easily adjusted. In addition, hydrophobic association is introduced into the upper layer to improve the gel's mechanical properties. The obtained asymmetric bilayer gel has a significant difference in adhesion (7 times), and exhibits excellent mechanical properties in the elongation at break (1437 %) and the breaking strength (463.2 kPa). Moreover, the bilayer gel also has good freezing and UV resistance. We use the bilayer gel as a wearable strain sensor, which shows a wide strain detection range of 0-800 % (maximum gauge factor = 5.3). The proposed simple strategy avoids UV irradiation and heating processes, which provides a new idea for the rapid polymerization of multifunctional Janus hydrogels with adjustable performances.