Engineers at the University of Michigan, Ann Arbor, report that networks of spherical gold nanoparticles embedded in polyurethane elastic materials may make the best stretchy conductors yet. The team discovered that the gold nanoparticles align into chain form, making excellent conductive pathways, when the polyurethane is stretched. Flexible electronics have a wide variety of possibilities, from bendable displays and batteries to medical implants that move with the body.
“Essentially the new nanoparticle materials behave like elastic metals,” said Nicholas Kotov, the Joseph B. and Florence V. Cejka Professor of Engineering. “It’s just the start of a new family of materials that can be made from a large variety of nanoparticles for a wide range of applications.”
To find out what happens as the material stretched, the team made state-of-the-art electron microscope images of the materials at various tensions. The nanoparticles started out dispersed, but under strain, they could filter through the minuscule gaps in the polyurethane, connecting in chains as they would in a solution.
“As we stretch the polyurethane, the nanoparticles rearrange themselves to maintain conductivity, and this is the reason that the material develops the amazing combination of stretchability and electrical conductivity,” Prof. Kotov said.
The team made two versions of their material: by building it in alternating layers, and by filtering a liquid containing polyurethane and nanoparticle clumps to leave behind a mixed layer. Overall, the layer-by-layer material is more conductive, while the filtered method makes for extremely supple materials. Without stretching, the layer-by-layer material with five gold layers has a conductance of 11,000 Siemens per centimeter (S/cm), on par with mercury, while five layers of the filtered material came in at 1,800 S/cm, more similar to good plastic conductors
