The gel is soft and transparent at 25° C and cannot support a 10 kg weight (top) but becomes rigid and opaque when heated to 60° C, becoming strong enough to support the weight (bottom). Source: Nonoyama T. et al.Researchers from Japan’s Hokkaido University have created a hydrogel that hardens when hot and softens when cold — an unusual feat for polymer-based materials.

To develop the material, the team created a non-toxic polyacrylic gel composed of polyelectrolyte poly (acrylic acid) (PAAc), which was submerged in an aqueous solution of calcium acetate. Independently, PAAc behaves like a polymer-based material — softening when heated, hardening when cooled. Yet, with the addition of calcium acetate, PAAc’s side residues responded to the calcium acetate molecules in a manner much like thermophile proteins wherein the proteins within thermophile organisms survive and stabilize in excessively hot environments like hot springs due to enhanced electrostatic interactions.

The team discovered that the gel, which is typically unified, divides into a polymer dense phase and eventually a polymer sparse phase in step with increases in temperature. When the temperature reaches around 60° C, the dense phase experiences a type of dehydration, thereby strengthening the ionic bonds as well as the hydrophobic interactions among the polymer molecules, which results in the transformation of the hydrogel from soft and transparent to an opaque plastic that is 80 times stronger and 1,800 times stiffer than the initial hydrogel.

The research team envisions that the material might prove useful as protective gear worn for sports and traffic-related applications. Likewise, the team also believes it could be used as a heat-absorbing window coating to cool indoor environments.

The research appears in the journal Advanced Materials.