Highly Stretchable and Tough Hydrogels below Water Freezing Temperature

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, a general class of hydrogels that do not freeze at temperatures far below 0 °C, while retaining high stretchability and fracture toughness, is demonstrated. These hydrogels are synthesized 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 can 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 and 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.