tough hydrogels
subzero temperatures
non-freezing ionic devices
ice-salt slurry phase
freezing point depression

Xavier P. Morelle, Widusha R. Illeperuma, Kevin Tian, Ruobing Bai, Zhigang Suo, Joost J. Vlassak

Hydrogels consist of hydrophilic polymer networks dispersed in water. Many applications of hydrogels rely on their unique combination of solid-like mechanical behavior and water-like transport properties. If the temperature is lowered below 0°C, however, hydrogels freeze and become rigid, brittle, and non-conductive. Here, we demonstrate a general class of hydrogels that do not freeze at temperatures far below 0°C, while retaining high stretchability and fracture toughness. These hydrogels are synthetized by adding a suitable amount of an ionic compound to the hydrogel. The present study focuses on tough polyacrylamide-alginate double network hydrogels equilibrated with aqueous solutions of calcium chloride. The resulting hydrogels cab be cooled to temperatures as low as -57°C without freezing. In this temperature range, the hydrogels can still be stretched more than four times their initial length adn have a fracture toughness of 5000 J m-2. It is anticipated that this new class of hydrogels will prove useful in developing new applications operating under a broad range of environmental and atmospheric conditions.