Gold fingers: University of Michigan scientists invented a gold and polyurethane film that stays conductive even when stretched. Photo: Joseph Xu/Michigan Engineering
A team of researchers at the University of Michigan, in Ann Arbor, recently found that a material made of gold spherical nanoparticles nested in polyurethane is conductive even when stretched up to several times its original length.
Nicholas Kotov, one of the Michigan researchers, hopes these flexible conductors can help resolve some of the problems associated with brain implants. “The brain tissue and the implant need to behave just like one object,” he explains. If not, the implant may become surrounded by scar tissue and thus no longer function properly. The implant must also survive the surgery itself, during which it may be stretched to well more than double its original length.
By combining gold nanoparticles and the polymer often used as foam insulation, “we thought that we would get a reasonable conductor, but we actually got a great conductor,” Kotov says. The conductivity was “higher than pretty much every stretchable conductor that had been [made] from carbon nanotubes. The reason is that there was an unexpected process occurring there: the self-organization of nanoparticles upon strain.” Pulling the material creates conditions under which the nanoparticles migrate closer to one another. The nanoparticles are then able to form chains of conductivity. When the tension is released, the chains disband.
The team developed two versions of the material: They constructed one type in five alternating layers of positively charged polyurethane and negatively charged gold nanoparticles. This form has a conductivity of 11 000 siemens per centimeter before stretching and 2400 S/cm when pulled to twice its original length (close to the material’s breaking point). They created the other form, a more bendable mixture, by filtering a liquid containing polyurethane and gold nanoparticles. This composite has a conductivity of 1800 S/cm in its unstretched form and 35 S/cm when pulled to 5.8 times its original length. By comparison, pure copper has a conductivity of 596 000 S/cm.
According to Kotov, one reason for the team’s success is that they chose to work with thin stabilizing shells around the nanoparticles. Other attempts at making conductors from gold nanoparticles and gold materials have not been as successful, he claims, because researchers usually used thick insulating shells. The shell is necessary to stabilize the particle in the solution, but it can inhibit conductance.
“The next steps will be to determine routes for integrating these materials into functional systems and assessing their performance compared to alternatives,” says John Rogers, a professor of materials science and engineering at the University of Illinois at Urbana-Champaign who has worked extensively to create flexible conductors. Indeed, there have been several other approaches to flexible conductors. These include a graphene-based ink that can be sprayed to make extrathin conductors, Rogers’s own arching metal ribbons, and carbon nanotubes. “We have spherical nanoparticles. They’re much easier to organize upon strain. That’s a great advantage for stretchability,” says Kotov of his team’s success.
“This is really a clever way of making new types of composites that are conductive,” according to Ali Javey, an electrical engineering and computer sciences professor at the University of California, Berkeley, who is developing his own user-interactive electronic skin. “This will again enable us to move forward and make new types of devices.”