Hydrated Deep Eutectic Solvent Conductive Gels with Freeze-Resistance and Water-Free Conductivity for Flexible Sensors

Conductive hydrogels used in flexible sensors commonly suffer from poor freeze resistance, susceptibility to water loss, and low sensitivity. To address these issues, this study employed a cyclic freeze–thaw method to incorporate a urea/choline chloride deep eutectic solvent into a poly(vinyl alcohol) (PVA)/sodium alginate (SA) double-network structure, thereby developing an ionic gel based on a hydrated low-eutectic solvent. This U-DES forms strong hydrogen bonds with water molecules (binding energy: −20.2 kcal/mol), converting free water into nonfreezing bound water. Consequently, it completely suppresses ice crystal formation (no ice-melting peak) at temperatures above −60 °C and endows the material with excellent water-retention capacity. Simultaneously, the DES provides abundant ion carriers (Cl^–, choline⁺), endowing the hydrogel with stable electrical conductivity. The resulting PSU-4 hydrogel exhibits balanced mechanical properties: tensile strength of approximately 0.13 MPa, elongation at break of approximately 500%, and maintains good fatigue resistance even after 200 cycles at 50% strain. As a strain sensor, the PSU-4 hydrogel exhibits linear response within a strain range of 0.5–200% (GF = 0.01925, R² = 0.98998), with rapid and repeatable signal response. Demonstrated applications include real-time monitoring of wrist and finger movements, as well as encoding gestures into Morse code (e.g., “SOS” and “YES”). This work provides a simple and effective strategy for overcoming key limitations of conventional hydrogels in the field of flexible wearable electronics.