Novel conductive material gets tougher when hit or stretched

If you drop your smart watch, or it gets hit really hard, the device probably won’t work anymore. But now, researchers have developed a soft, flexible material with so-called ‘adaptive durability’, meaning it gets stronger when hit or stretched. Inspiration for the new material came from a mixture commonly used in cooking — a corn starch slurry.

“When I stir corn starch and water slowly, the spoon moves easily,” explained Yue (Jessica) Wang, a materials scientist at the University of California, Merced. “But if I lift the spoon out and then stab the mixture, the spoon doesn’t go back in. It’s like stabbing a hard surface.”

This slurry, which helps thicken stews and sauces, has adaptive durability, shifting from malleable to strong, depending on the force applied. Wang’s team set out to mimic this property in a solid conductive material.

Many materials that conduct electricity are hard, stiff or brittle. Researchers have developed ways to make soft and bendable versions using conjugated polymers — long, spaghetti-like molecules that are conductive — but most flexible polymers break apart if they undergo repeated, rapid or large impacts. So Wang’s team set out to select the right combination of conjugated polymers to create a durable material that would mimic the adaptive behaviour of corn starch particles in water.

Initially, the researchers made an aqueous solution of four polymers: long, spaghetti-like poly(2-acrylamido-2-methylpropanesulfonic acid), shorter polyaniline molecules and a highly conductive combination known as poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). After spreading a thin layer of the mixture and drying it to make a film, the team tested the stretchy material’s mechanical properties.

They found that rather than breaking apart from very rapid impacts, it deformed or stretched out. The faster the impact, the more stretchy and tough the film became. And surprisingly, just a 10% addition of PEDOT:PSS improved both the material’s conductivity and adaptive durability. Wang said this result was unexpected because, on their own, PEDOT and PSS don’t get tougher with rapid or high impacts.

Di Wu, a postdoctoral researcher in Wang’s lab, added that the four polymers — two with positive charges and two with negative charges — tangle up like a big bowl of spaghetti and meatballs, stating, “Because the positively charged molecules don’t like water, they aggregate into meatball-like microstructures.” The team’s hypothesis is that the adaptive behaviour comes from the ‘meatballs’ absorbing the energy of an impact and flattening when hit, but not completely splitting apart.

However, Wu wanted to see how adding small molecules could create a composite material that was even tougher when stretched or dropped quickly. Because all the polymers had charges, the team chose molecules with positive, negative or neutral charges to test. Then they assessed how the additives modified the polymers’ interactions and impacted each material’s adaptive durability.

Preliminary results have indicated that the positively charged nanoparticles made of 1,3-propanediamine were the best additive, imparting the most adaptive functionality. Wu said this additive weakened the interactions of the polymers that form the meatballs, making them easier to push apart and deform when hit, and strengthened the tightly entangled ‘spaghetti strings’.

“Adding the positively charged molecules to our material made it even stronger at higher stretch rates,” he said.

With their results having been presented at the spring meeting of the American Chemical Society (ACS Spring 2024), Wang said the team will shift in future towards demonstrating the applicability of their lightweight conductive material. The possibilities include soft wearables, such as integrated bands and backside sensors for smart watches, and flexible electronics for health monitoring, such as cardiovascular sensors or continuous glucose monitors.

Additionally, the team has formulated a previous version of the adaptive material for 3D printing and produced a replica of a team member’s hand, demonstrating the potential incorporation into personalised electronic prosthetics. Wang thinks the new composite version should also be compatible with 3D printing to make whatever shape is desired.

According to Wang, the adaptive durability of the material means that future biosensor devices could be flexible enough for regular, human motion but resist damage if they’re accidentally bumped or hit hard. “There are a number of potential applications, and we’re excited to see where this new, unconventional property will take us,” she said.

Image caption: This flexible and conductive material has adaptive durability, meaning it gets stronger when hit. Image credit: Yue (Jessica) Wang.