Imagine a wire that can stretch eight to 10 times its original length and still send crystal clear audio from your music player to your earphones, or imagine accidentally cutting a cable to a tactical radio and repairing the cut just by physically putting the wires back together.
Those are just two of the many possible products that could result from materials science research now underway at North Carolina State University under the direction of Dr. Michael Dickey, assistant professor of chemical and biomolecular engineering at the university.
Both scientific developments are the result of separate but related avenues of scientific research into advanced materials, explains Dickey, who says much of the work has been conducted by graduate and undergraduate students. “They’re related in the sense that we’ve used some common materials, but they are two different projects,” he says.
“Both ideas are almost embarassingly simple,” Dickey goes on to relate. “What we’ve done is taken the architecture of a conventional wire, which is a metal core surrounded by a plastic casing, and we’ve done two things. We’ve replaced the plastic casing with an elastomeric polymer that’s more like a rubber band, so it's stretchable, and then for the core of the wire, we use a special liquid metal alloy.”
That alloy, made up of gallium and indium, is a liquid at room temperature but has a unique characteristic. “We can shape it because there’s an oxide ‘skin’ that forms on the metal. The best analogy I can use is a waterbed, which, in the absence of a plastic casing, would be a big puddle.”
Dickey says the stretchable wire technique, outlined in a research paper entitled “Ultrastretchable Fibers with Metallic Conductivity Using a Liquid Metal Alloy Core” and published in the December 2012 edition of the journal Advanced Functional Materials, differs substantially from traditional methods for creating stretchable wires that included adding conductive materials to a polymer. “The more of the conductive material you add to the polymer, the more you deviate from the stretchable mechanical properties of the polymer,” he says. The new technique yields a wire that has metallic conductivity, while retaining the stretchable mechanic properties of the polymer.
“If you make these things out of a rubber-band-like material, you’re going to have a rubber-band-like wire,” he concludes. Current research has yielded a wire that is not as electrically conductive as traditional wires of the same gauge, “but it still has metallic conductivity, and it is still very conductive. One of the things we did just for fun is to make some iPod headphone wires, and there’s no degradation of the sound quality when you stretch it.”
Dickey adds that in theory, the stretchable wire technique is scalable, in that wires that are both larger and smaller in diameter should be possible, but he and his team were limited by the fabrication equipment available to them.
In a similar vein, Dickey and his research team also have developed a wire that is entirely self-healing—capable of restoring its electrical conductivity after being cut.
“We’ve got a wire that is also made of this liquid metal alloy (indium and gallium). Instead of using a fiber that is extremely stretchable, the emphasis here was on making it out of a special self-healing polymer that can be cut, with a razor blaze or scissors, and put back together within, say, 10 minutes, and it will regain its original mechanical properties.” As an added bonus, the self-healing wire is also stretchable, but only by a factor of two-and-a-half times its original length.
“When you cut the wire, the metal oxidizes almost instantaneously, forming a skin,” at the point of the cut. This allows the rest of the wire to retain its liquid metal state. “When you bring it back together, you’re able to bring the (plastic) polymer back together, and the metal is right there at the interface. They’re both soft, and because you’re bringing two liquids together, they link right back up, and if you give it a few minutes, it self-heals.” Using a small electric light, Dickey and his research team demonstrated how the self-healing wire works.
The self-healing wire, detailed in the paper entitled “Self-Healing Stretchable Wires for Reconfigurable Circuit Wiring and 3-D Microfluidics” and published in the January 2013 edition of Advanced Functional Materials, has potential for solving problems with wires used in high-stress environments, including the possibility of creating circuits that can be easily reconfigured as needed with simple tools such as razor blades or scissors.
Both projects were conducted with funding from the National Science Foundation.