Stretchy material that becomes stronger when struck has exciting potential

Much of the work Yue (Jessica) Wang does at UC Merced sounds like science fiction: She creates flexible material that becomes stronger the more you hit it. And it conducts electricity.

Science, yes. Fiction, no.

This work happens. It was featured in a presentation that materials scientist Di Wu of the Wang lab gave this spring at the American Chemical Society meeting in New Orleans.

The material Wang’s team developed has “adaptive durability,” meaning it remains soft when used or moved through consistent movements, then becomes stronger when abruptly hit or stretched. The material also conducts electricity, making it ideal for next-generation wearables or personalized medical sensors.

The inspiration for the new material came from a method often used to thicken mixtures during cooking: a cornstarch slurry.

“When I stir cornstarch and water slowly, the spoon moves easily,” Wang said. “But if I take the spoon out and then stick it in the mixture, the spoon won’t go back in. It’s like stabbing into a hard surface.” This slurry has adaptive durability, which varies from malleable to strong depending on the force applied. Wang’s team wanted to mimic this property in a sturdy conductive material.

Many materials, such as metals, that conduct electricity are hard, stiff or brittle. But researchers have developed ways to make soft and bendable versions using conjugated polymers: long, spaghetti-like molecules that are conductive. Yet most flexible polymers disintegrate when subjected to repeated, rapid, or large impacts. Wang’s team set out to find the right combination of conjugated polymers to create a durable material that would mimic the adaptive behavior of corn starch particles in water.

“Polymer-based electronics are promising,” says Di Wu, a postdoctoral researcher. “We want to make polymer electronics lighter, cheaper and smarter.”

Initially, the researchers made an aqueous solution of four polymers. After spreading a thin layer of the mixture and drying it to make a film, the team tested the mechanical properties of the stretchy material.

They found that, instead of breaking loose with very fast impacts, it deformed or stretched. The faster the impact, the more stretchy and tough the film became.

The four polymers, two with positive charges and two with negative charges, get mixed up like a big bowl of spaghetti and meatballs, said Wu, who presented the lab’s work at the ACS meeting. “Because the positively charged molecules don’t like water, they aggregate into meatball-like microstructures.”

The team’s hypothesis is that the adaptive behavior comes from the fact that the meatballs absorb the energy of an impact and flatten when hit, but do not completely disintegrate.

“They can become flexible and stronger if you make a sudden movement, but it is flexible if you only do your daily routine movement,” Wu said. “They’re not constantly stiff or constantly flexible. They just respond to your body movement.”