Lignin sulfonate induced ultrafast polymerization of double network hydrogels with anti-freezing, high strength and conductivity and their sensing applications at extremely cold conditions

Conductive hydrogels (CHs) have attracted immense research interest in wearable devices. However, current CHs will not only lose their mechanical properties, conductivity, and sensitivity at subzero temperatures, but also need a long polymerization time and external stimuli in the fabricating process. Herein, a double network hydrogel was synthesized by incorporating lithium chloride into the composite hydrogel consisting of poly(acrylic acid), poly(vinyl alcohol), and lignosulfonate (LS). Additionally, the DN hydrogels form rapidly (in few minutes) without external heating/UV radiation in a dynamic redox system composed of LS, Fe3+, and ammonium persulfate. The addition of LiCl enhances the mechanical properties (mechanical strength of 1.04 MPa, elongation at break of 758%) even at −30 °C, thanks to the LiCl-induced hydrophobic interactions and LiCl-filler effect; furthermore, the LiCl loading imparts the hydrogel with an outstanding conductivity, even at extremely low temperatures (9.81 S/m at −30 °C). The hydrogel-based sensor exhibits fast, accurate and reliable electric signal changes to precisely monitor various physiological activities (e.g., limb activity, and breathe) even at a very low temperatures (−30 °C). In addition, the hydrogels can be applied as a bio-electrode to collect ECG and EMG signals, and as a T-pen for smartphones at −30 °C.