Bio-inspired gel is tough when it counts

Japanese researchers have developed a gel that goes hard when exposed to heat then softens again when cooled.

That’s at odds with what polymer-based materials such as plastic bottles normally do, they say, suggesting the potential to create unique protective clothing. 

Takayuki Nonoyama, Jian Ping Gong and colleagues from Hokkaido University were inspired by how proteins remain stable inside organisms that survive and thrive in extreme-heat environments such as hot springs and deep-sea thermal vents. 

Normally, heat alters the structure of proteins and breaks their bonds, but the proteins in these thermophile organisms remain stable with heat thanks to enhanced electrostatic interactions such as ionic bonds, the researchers say.

So they developed a polyacrylic gel based on this concept, as they describe in a paper in the journal Advanced Materials.

They started with a polyelectrolyte poly (acrylic acid) which, on its own, acts like any other polymer-based material and softens when heated. However, when immersed in a calcium acetate aqueous solution its side residues interact with the calcium acetate molecules, in a way similar to what happens inside thermophile proteins.

The team found that their originally uniform gel separated into a polymer dense “phase” and a polymer sparse one as the temperature rose. When it reached around 60 degrees Celsius, the dense phase underwent significant dehydration that strengthened ionic bonds and hydrophobic interactions between polymer molecules. 

This caused the material to rapidly transform from a soft, transparent hydrogel to a rigid, opaque plastic, the researchers say. The heated material was 1800 times stiffer, 80 times stronger and 20 times tougher than the original hydrogel.  

Moreover, the soft-to-rigid switching was completely reversible, and the researchers could fine-tune the switching temperature by adjusting the concentration of the ingredients.

They demonstrated a possible application of the material by combining it with a woven glass fabric. 

This new fabric was soft at room temperature, but when it was pulled against an asphalt surface for five seconds at 80 kilometres an hour, the heat generated by the friction hardened the material with only minor abrasions forming on the contact surface.

 “This polymer gel can be easily made from versatile, inexpensive and non-toxic raw materials that are commonly found in daily life,” says Jian Ping Gong. “Specifically, the polyacrylic acids are used in disposable diapers and calcium acetates are used in food additives.”