Flexible printed electronics and solar-cell arrays promise to be cheaper and more versatile than their rigid counterparts. But their components still need to be linked by tiny metal electrodes in order to get electrons flowing through a device. A new silver-nanoparticle ink could be just the thing for printing high-performance electrical connections for flexible devices.

The ink is the first that can be printed out of plane--that is, it can be printed from a stylus that moves in three dimensions rather than just two. Other nanoparticle inks are slow setting and compatible only with ink-jet or screen printing methods, which require several passes in order to make even a thin electrode. The new fast-setting ink, made by Jennifer A. Lewis, a professor of materials science and engineering at the University of Illinois at Urbana-Champaign, is described online in the journal Science.

Lewis's ink consists of a water solution that's 75 percent silver nanoparticles by weight. Because of the high proportion of silver, the ink flows readily through a nozzle and sets quickly once it's exposed to a flash of heat. The ink can be used to make silver wires that are only about two micrometers in diameter. These microelectrodes can be repeatedly stretched and flexed without any degradation of their electrical performance. The Illinois researchers printed a wire on a stretched-out spring, then repeatedly stressed the spring, but the wire still conducted electricity just as well as before. They also used the ink to print microelectrodes for solar-cell and LED arrays.

Lewis says that her research team is now working on improving two of the inks' properties. First, they want to design silver inks that set at lower temperatures. Today, the ink must be heated to between 200 and 250 °C, an amount of heat that would melt the polymers used in many flexible electronic devices. Lewis would like to bring this temperature range down by about 100 degrees. Second, the current technique is simply too slow for large-scale manufacturing: the new inks can be printed at a speed of one millimeter per second. So Lewis says that she'll need to develop inks that can be printed two orders of magnitude faster.

Below is a video of the printing process sped up to twice the actual speed.

Video credit: AAAS/Science