Robots, bionic limbs, and wearable sensors rely on large and stretchable electronics to receive and transmit information. But making conducting materials that are elastic, strong, and conform to different shapes is a challenge. In a recent paper, Matsuhisa et al.1 find that silver flakes' ability to form nanoparticles in situ can be exploited in the printing of high-performance, flexible electronics.

There are two general types of elastic, conducting materials: those that are intrinsically stretchable, such as liquid metals or conducting polymers; and those that are engineered as conducting pathways. One way to fabricate conducting pathways is by generating microstructures (e.g., accordion and serpentine patterns), which can be used for small flat areas. Another approach, which can enable printability of large materials, is to engineer them by self-assembly of conducting pathways using carbon nanotubes and metal nanowires, flakes, and nanoparticles. Of these, metal nanoparticles have shown the highest conductivity at the highest strain, but nanoparticles cannot be easily patterned to make materials or films.

The recent discovery by Matsuhisa et al.1 addresses this limitation by showing that silver nanoparticles can be formed in situ from silver flakes. Although the authors had previously used silver flakes in self-assembly of conductors, silver nanoparticles had not been observed, probably due to differences in the selection of materials and processing conditions. Mixing silver flakes with fluorine rubbers and surfactant, the scientists create an ink that is compatible with printing. Repeated heating of the ink during material preparation is critical to the formation of nanoparticles, with a temperature of 120 °C yielding the best conductivity and stretchability.

Exploring the importance of each constituent in their ink, Matsuhisa et al.1 demonstrate that its composition is critical to the formation of silver nanoparticles. They also provide evidence that the nanoparticles mediate the desirable characteristics of their conductors, particularly conductivity, stretchability, and cyclic durability. Testing the conductor in a printed elastic pressure and temperature sensor that can be laminated onto textile substrate, the scientists show the device is functional, even when stretched by 250%.

“Nanoparticles endow better conductivity because they lower the threshold for percolation,” says George Malliaras, a professor in the Department of Bioelectronics at Ècole des Mines de Saint-Ètienne in France. Malliaras explains that “if you were trying to cross a river, the percolation threshold would be the concentration of logs you would need to create a continuous path from one bank to the other.” Similarly, for an electron traveling through silver flakes, the nanoparticles lower this threshold, increasing conductivity. “There is also improved resistance to wear,” he says, “because tear will be arrested by the nanoparticles.”

Prostheses that can 'feel', on-skin health-monitoring sensors, and robots designed to surpass human stretchability could all benefit from the improved conductors. “The approach is advantageous whenever you need to make large electronics that are not flat,” says Malliaras. And although silver provides a rich seam of new possibilities for flexible electronics, substituting it for cheaper metals could reduce costs, extending the applicability of the approach.