Recyclable, anti-freezing and anti-drying silk fibroin-based hydrogels for ultrasensitive strain sensors and all-hydrogel-state super-capacitors

Multifunctional conductive hydrogels have triggered extensive research interest owing to their potential in artificial intelligence, human motion detection, soft electronic skins, and energy storage. However, unattainable reusable properties and unsatisfactory environmental tolerance seriously hinder potential applications in these emerging fields. Herein, an anti-freezing, anti-drying, and recyclable conductive hydrogel has been successfully prepared by integrating silk fibroin (SF) into polyvinyl alcohol (PVA)/glycerin (Gly)/lithium chloride (LiCl) hydrogel. Silk fibroin provides a variety of physical interactions for the construction of polymer hydrogels due to its abundant binding sites (–COO–, –NH2, and –OH) on its peptide chain. The obtained SPGL (SF/PVA/Gly/LiCl) hydrogels demonstrate outstanding anti-freezing properties (below −70 °C) and excellent anti-drying properties (17.4% weight loss after 30 days). Meanwhile, the prominent remolding property allows the SPGL hydrogel to be recycled, and the ionic conductivity of the remolded hydrogel remains at 84.7%. The important roles of Gly and LiCl in inhibiting the freezing and dehydration of hydrogels have been further revealed by density functional theory (DFT) simulations at the molecular scale. The SPGL hydrogel-based strain sensors possess relatively high sensitivity with a gauge factor (GF) of 2.18, enabling them to monitor human motions precisely, including macro movements and micro movements. Furthermore, we designed a circuit diagram with the sensor to detect human motion in real-time. In addition, the SPGL hydrogel-based supercapacitors display favorable cycle stability (88.5% capacitance retention after 10,000 cycles) and excellent energy density of 15.4 μWh cm−2 at a power density of 50 μW cm−2. The strain sensors and supercapacitors also have anti-freezing and anti-drying properties, maintaining excellent sensing and electrochemical properties even in extreme environments. Therefore, these SPGL hydrogel-based soft electronics have promising applications in health monitoring, human motion detection, human-machine interactions, and energy storage.