Highly Stretchable and Tough Hydrogels below Water Freezing Temperature

Abstract

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 . 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.

Effect of solvent–matrix interactions on structures and mechanical properties of micelle‐crosslinked gels

“…[49][50][51] The introduction of organic solvents or salt into hydrogels has been widely shown to modulate the properties including mechanical properties, antifreezing, and responsiveness. For example, with high salt concentration, hydrogels become highly stretchable, tough, and antifreezing to~−57 C. 23 However, the toughness of hydrogels was lower (10-2 kJ m −2 ) than that of the current gels. Besides, the present gels crosslinked by micelles are reversible, while the alginate network crosslinked by Ca 2+ is rigid and nearly irreversible.…”

“…The viscoelastic behavior of the gels was tunable by changing the solvent ratio (IPA/water), with IPA to mediate the hydrophobic interactions. However, those works are focusing on the destructive effect of nonaqueous solvents on the gel's networks, 23 and yet few studies have investigated the transformation process from organogels to hydrogels or vice versa. It is very important to unveil the correlations among polymer matrix, solvent quality, and hydrophobic association during the transformation between hydrogels and organogels to broaden the application fields of such soft matter and develop the cognition about nonaqueous solvent gels.…”

Inorganic Salts Induce Thermally Reversible and Anti‐Freezing Cellulose Hydrogels

“…The resulting hydrogels remain flexible and strain‐sensitive at −55 °C. Very recently, the Suo group prepared a polyacrylamide–alginate double‐network hydrogel and then equilibrated the hydrogel with CaCl 2 solution to depress the freezing point . At −57 °C, the twofold physically cross‐linked hydrogels still have a fracture toughness of 5000 J m −2 .…”

Biomimetic Extreme‐Temperature‐ and Environment‐Adaptable Hydrogels

“…Elastomer inhibits water diffusion n.a. 2 [72] -6 [75] days at room temperature TENG, living materials and devices, flexible conductor, stretchable microfluidic chip Salt addition Salt enables freezing point depression, slurry gel generation À 57°C [79] (slurry gel À 80°C [82] ) � 5 days [79] (20°C, 20 % rel. humidity) durable at 75°C [82] Flexible conductor, capacitance touch sensor…”