Heat-loving hydrogel hardens as temperature rises

Inspired by thermophile organisms that rely on the heat of hot springs and thermal vents for survival, the material was created by immersing a polyacrylic acid gel in a calcium acetate aqueous solution. At room temperature, the resulting substance has the consistency of a regular hydrogel, but as the heat rises it separates into a polymer dense phase and a polymer sparse phase.

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When the temperature hits around 60°C, the dense phase undergoes significant dehydration which strengthens ionic bonds and hydrophobic interactions between polymer molecules, causing the material to rapidly transform from a soft, transparent hydrogel to a rigid, opaque plastic. According to the research, published in Advanced Materials, this rigid plastic is 1,800 times stiffer, 80 times stronger, and 20 times tougher than the original hydrogel.

“This polymer gel can be easily made from versatile, inexpensive and non-toxic raw materials that are commonly found in daily life,” said one of the research leads Jian Ping Gong, from Hokkaido University.

“Specifically, the polyacrylic acids are used in disposable diapers and calcium acetates are used in food additives. Our study contributes to basic research on new temperature-responsive polymers, and to applied research on temperature-responsive smart materials.”

To demonstrate the material’s practical applications, the hydrogel was combined with a woven glass fabric, creating a new material. At room temperature this material was soft to the touch, but when pulled against an asphalt surface for five seconds at 80km/h, the heat from the friction caused it to harden, with just minor scratches forming on the contact surface. Protective clothing made from the material appears a logical next step.

“Clothing made from similar fabric could be used to protect people during traffic or sports-related accidents, for example,” said Hokkaido University’s Takayuki Nonoyama. “Our material could also be used as a heat-absorbent window coating to keep indoor environments cooler.”