Researchers at the University of California, Merced, have developed a flexible, conductive material that could one day improve the durability of wearable devices such as smartwatches. The new material has adaptive durability, meaning it becomes stronger when stretched or impacted. Oddly enough, this material was inspired by the kitchen.
Jessica Wang, the lead researcher on the project, noted that when cornstarch and water are slowly stirred together, the stirring spoon moves easily through the mixture. When you remove the spoon and try to force it back in, you get a different result. This is the behavior of non-Newtonian fluids. "It's like stabbing a hard surface and the spoon can't go in anymore," Wang said.
The team's goal was to simulate this peculiar property in solid conducting materials. To achieve their goal, the team had to identify the right combination of conjugated polymers, long strips of conducting molecules shaped like strands of spaghetti. Most flexible polymers break when subjected to repeated, rapid or severe impacts.
The researchers first used an aqueous solution of four polymers: spaghetti-like poly(2-acrylamide-2-methylpropanesulfonic acid), shorter polyaniline molecules, and a conductive combination called poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS).
They tweaked the formula to improve conductivity and adaptive durability. For example, adding 10% PEDOT:PSS to the mix improves conductivity and adaptive durability.
The team also tried adding small molecules to the mixture, noting how each additive changed the polymer's properties. Ultimately, positively charged nanoparticle additives most improved adaptive function.
"Adding positively charged molecules to our material makes it stronger at higher stretching rates," said Di Wu, a postdoctoral researcher in the lab.
Practical applications may include integrated straps and back sensors for smartwatches that can easily withstand the harsh environment of daily life on the human wrist. The flexible material could also have medical applications, perhaps integrating into wearable devices such as cardiovascular sensors or blood glucose monitors.
Wu and his team even modified an early version of the material suitable for 3D printing and created a replica of a human hand to demonstrate its potential for use as a prosthetic limb.
"It has many potential application areas, and we're excited to see where this new, unconventional feature will take us," Wang said.