Stretchy, High-Quality Conductors

Materials made from nanotubes could lead to conformable computers that stretch around any shape.

Malleable matrix: A researcher stretches a mesh of transistors connected by elastic conductors that were made at the University of Tokyo.

By adding carbon nanotubes to a stretchy polymer, researchers at the University of Tokyo made a conductive material that they used to connect organic transistors in a stretchable electronic circuit. The new material could be used to make displays, actuators, and simple computers that wrap around furniture, says Takao Someya, a professor of engineering at the University of Tokyo. The material could also lead to electronic skin for robots, he says, which could use pressure sensors to detect touch while accommodating the strain at the robots' joints. Importantly, the process that the researchers developed for making long carbon nanotubes could work on the industrial scale.

"The measured conductivity records the world's highest value among soft materials," says Someya. In a paper published last week in Science, Someya and his colleagues claim a conductivity of 57 siemens per centimeter, which is lower than that of copper, the metal normally used to connect transistors, but two orders of magnitude higher than that of previously reported polymer-carbon-nanotube composites. Someya says that the material is able to stretch up to about 134 percent of its original shape without significant damage.

Electronics that can bend and flex are already used in some applications, but they can't be wrapped around irregular shapes, such as the human body or complex surfaces, says John Rogers, a professor of materials science and engineering at the University of Illinois at Urbana-Champaign. Rogers, who recently demonstrated a spherical camera sensor using his own version of an elastic circuit, says that Someya's approach is a creative addition to the science of stretchable electronic materials. "It's a valuable contribution to an important, emerging field of technology," he says.

To make the stretchable polymer conductive, Someya's group combined a batch of millimeter-long, single-walled carbon nanotubes with an ionic liquid--a liquid containing charged molecules. The resulting black, paste-like substance was then slowly added to a liquid polymer mixture. This produced a gel-like substance that was poured into a cast and air-dried for 24 hours.

The benefit of adding the nanotubes to a polymer before it is cast, says Someya, is that the nanotubes, which make up about 20 percent of the weight of the total mixture, are more evenly distributed. And because each nanotube is about a millimeter in length, there's a high likelihood that in aggregate they will form an extensive network that allows electrical charge to propagate reliably throughout the polymer.

Previously, researchers have added micrometer-length carbon nanotubes to polymers, says Ray Baughman, a professor of materials science at the University of Texas. Most often, they would simply coat the polymer with nanotubes. Baughman says that Someya's work is exciting, but he notes that he would have expected that adding higher percentages of carbon nanotubes to polymers reduces their stretchiness.

According to Someya, the initial air-dried nanotube-polymer film is flexible but not that stretchable. In order to improve its stretchiness, a machine perforates it into a net-shaped structure that is then coated with a silicone-based material. This enables the material to stretch much farther without compromising its conductivity.

Baughman says that one of the main contributions of the University of Tokyo team's work is to demonstrate a way to make this sort of elastic conductor material in bulk. "This and so many other applications depend on the landmark advance of a team scaling up their production of ultralong carbon nanotubes," he says. The University of Tokyo group claims that from one furnace, it can make 10 tons of nanotubes per year. "It's nice work," Baughman says.