Someday you may be able to s-t-r-e-t-c-h that headphone cable or other wired electronic devices without having to get some sort of extension cord.
Researchers at N.C. State have developed a way to make wires able to be stretched up to eight times their original length yet maintaining their conductivity.
The wires are called “ultrastretchable.”
The secret? Using liquid metal inside the elastic sheathing material.
“The ability of the liquid metal to flow during the elongation of the fibers results in electrical continuity up to 1000% strain and metallic conductivity … up to 700% strain,” the researchers wrote in a research paper. “As a demonstration of their utility, the ultrastretchable fibers were used as the wires for stretchable earphones and a stretchable battery charger.”
NCSU’s efforts have succeeded where others have failed, the scientists say.
“Previous efforts to create stretchable wires focus on embedding metals or other electrical conductors in elastic polymers, but that creates a trade-off,” Michael Dickey, an assistant professor of chemical and biomolecular engineering at N.C. State who is co-author of a paper on the research, said in announcing the findings..
“Increasing the amount of metal improves the conductivity of the composite, but diminishes its elasticity,” Dickey says. “Our approach keeps the materials separate, so you have maximum conductivity without impairing elasticity. In short, our wires are orders of magnitude more stretchable than the most conductive wires, and at least an order of magnitude more conductive than the most stretchable wires currently in the literature.”
The metal – a liquid allow of gallium and indium which conducts electricity well – is inserted in a thin tube of elastic polymer to form the wire.
However, there is a problem that is yet unsolved.
If the wire is cut or breaks, the metal leaks out.
The paper was published in Advanced Functional Materials.
The abstract:
“Ultrastretchable Fibers with Metallic Conductivity Using a Liquid Metal Alloy Core”
Authors: Shu Zhu, Ju-Hee So, Robin Mays, Sharvil Desai, William R. Barnes, Behnam Pourdeyhimi, Michael D. Dickey
Published: online Dec. 13, Advanced Functional Materials
Abstract: This paper describes the fabrication and characterization of fibers that are ultra-stretchable and have metallic electrical conductivity. The fibers consist of a liquid metal alloy, eutectic gallium indium (EGaIn), injected into the core of stretchable hollow fibers composed of a triblock copolymer, poly[styrene-b-(ethylene-co-butylene)-b-styrene] (SEBS) resin. The hollow fibers are easy to mass-produce with controlled size by using commercially available melt processing methods. The fibers are similar to conventional metallic wires (metal core, surrounded by polymeric insulation), but can be stretched orders of magnitude further while retaining electrical conductivity. Mechanical measurements with and without the liquid metal inside the fibers show the liquid core has a negligible impact on the mechanical properties of the fibers, which is in contrast to most conductive composite fibers. The fibers also maintain the same tactile properties with and without the metal because the conductive elements are confined to the core of the fiber. As expected, electrical measurements show that the fibers increase resistance as the fiber elongates and the cross sectional area narrows. Fibers with large diameters (~600 [micrometers]) change from a triangular to a more circular cross-section during stretching, which has the appeal of lowering the resistance below that predicted by theory. The ability of the liquid metal to flow during the elongation of the fibers results in electrical continuity up to 1000% strain and metallic conductivity (~3×10-5 [Omega] cm) up to 700% strain. As a demonstration of their utility, the ultrastretchable fibers were used as the wires for stretchable earphones and a stretchable battery charger.
