Highly Stretchable and Tough Hydrogels
below Water-freezing Temperature

Hydrogel behavior in subzero environment illustrated for three different CaCl<sup>2</sup>-containing gels.
Hydrogel behavior in subzero environment illustrated for three different CaCl2-containing gels: (left) the initial state at room temperature, (center) the undeformed and deformed state at -15°C, and (right) the corresponding molecular pictures. This illustrates the three physical states for a hydrogel below the freezing point of water: the frozen state forming an opaque aggregate of ice crystals and polymer chains, the slurry state with a mixture of ice crystals and salt solution in a polymer network, and the regular hydrogel state with an aqueous phase in the liquid state.

Hydrogels exhibit the unique combination of solid-like mechanical behavior and water-like transport properties in the room temperature. If the temperature is lowered below 0°C, however, hydrogels lose their functions: they freeze and become rigid, brittle, and non-conductive.

A team at the Harvard MRSEC led by Suo and Vlassak recently reported in Advanced Materials a new general class of hydrogels that do not freeze at temperatures far below 0°C, while retaining their functional properties. These hydrogels were synthesized by adding a suitable amount of an ionic compound (e.g., CaCl2) to the hydrogel. This freezing point depression strategy is also used by insects and frogs to survive extremely cold temperatures. The MRSEC team found these new hydrogels can be cooled to temperatures as low as -57°C without freezing, and remaining stretchable and tough. This class of hydrogels will prove useful in developing new applications for ionic conducting skin and sensors operating under a broad range of environmental and atmospheric conditions.

Zhigang Suo (MechEng) and
Joost J. Vlassak (MatSci)
2018-2019 Harvard MRSEC (DMR-1420570)