Engineers at the University of Michigan, Ann Arbor, have made extremely flexible electrical conductors by embedding networks of spherical gold nanoparticles in polyurethane. To find out what happened as the material stretched, the team took 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.
The team made two versions of the 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 design 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 a par with mercury, while five layers of the filtered material came in at 1,800 S/cm, more akin to good plastic conductors.
Flexible electronics have a wide variety of possibilities, from bendable displays and batteries to medical implants that move with the body. Nicholas Kotov, the Joseph B. and Florence V. Cejka Professor of Engineering, chiefly sees stretchable conductors as implantable electrodes. “Rigid electrodes create scar tissue that prevents the electrode from working over time, but electrodes that move like brain tissue could avoid damaging cells,” he says. Brain implants are of particular interest. “They can alleviate a lot of diseases-for instance, severe depression, Alzheimer’s diseas,e and Parkinson’s disease. They can also serve as a part of artificial limbs and other prosthetic devices controlled by the brain.”
Whether in the brain, heart or other organs-or used for measurements on the skin-these electrodes could be as pliable as the surrounding tissue.
