In situ synthesis of highly stretchable, freeze-tolerant silk-polyelectrolyte double-network hydrogels for multifunctional flexible sensing

Conducting hydrogels have a broad application prospect in the manufacture of wearable electronics. Nevertheless, it still remains a huge challenge to concurrently achieve high stretchability, ionic conductivity, and freezing resistence via a simple and efficient method. Herein, a one-pot in situ synthesis is presented for developing enzymatically integrated dual anti-freezing silk-crosslinked polyelectrolyte hydrogels (SCPEHs). It is worth noting that this strategy aims to conveniently and efficiently synthesize double-network (DN) ionic-conducting hydrogel matrices by polymerizing acrylic acid monomers and simultaneously crosslinking silk fibroins (CSF), which generates freezing-tolerant SCPEHs with a novel protein-polyelectrolyte DN anti-freezing strategy. The prepared SCPEHs displays brilliant mechanical properties including high mechanical strength (0.4 MPa) and outstanding stretchability of 1450%. Molecular dynamics simulation (MDS) shows Poly(lithium acrylate/N-methylol acrylamide)-crosslinking of silk fibroin (P(LiAA/N-MAM)-CSF) system has lower potential energy, which increases interactions between H2O molecules and the DN hydrogel network, endowing the hydrogel with freezing tolerance of −80 °C. In addition, the SCPEHs-based sensor shows excellent ionic conductivity (∼2.58 S m−1) and can monitor multiple stimuli like stretching/compression/bending with high sensitivity (gauge factor of 1.25) and durability at low temperature. It can be utilized to sense weak signals from the human body (vocal cord, breath, or heartbeat). This work has a certain guiding significance for the simple design and development of silk-polyelectrolyte DN hydrogel and lays a foundation for sensing applications of silk-based flexible electronics in extreme environments.