Film could provide bendiness for wearables

Researchers at Korea University and the University of Illinois at Chicago (UIC) have developed an ultrathin film that is transparent and highly conductive to electric current, making it suitable for use in wearable devices.

The film, produced by a cheap and simple method, is actually a mat of tangled nanofibre, electroplated to form a self-junctioned copper nano-chicken wire. It is also bendable and stretchable, offering potential applications in roll-up touchscreen displays, wearable electronics, flexible solar cells and electronic skin.

“It’s important, but difficult, to make materials that are both transparent and conductive,” said Alexander Yarin, UIC distinguished professor of mechanical engineering.

The film establishes a “world-record combination of high transparency and low electrical resistance”, the latter at least tenfold greater than the previous existing record, said Sam Yoon, professor of mechanical engineering at Korea University.

The film also retained its properties after repeated cycles of severe stretching or bending, Yarin said, an important property for touchscreens or wearables.

Manufacture begins by electro-spinning a nanofibre mat of polyacrylonitrile, or PAN, whose fibres are about one-hundredth the diameter of a human hair. The fibre shoots out like a rapidly coiling noodle, which when deposited onto a surface intersects itself a million times.

“The nanofibre spins out in a spiral cone, but forms fractal loops in flight,” Yarin said. “The loops have loops, so it gets very long and very thin.”

The naked PAN polymer doesn’t conduct, so it must first be spatter-coated with a metal to attract metal ions. The fibre is then electroplated with copper, silver, nickel or gold.

The electro-spinning and electroplating are both relatively high-throughput, commercially viable processes that take only a few seconds each, according to the researchers.

“We can then take the metal-plated fibres and transfer to any surface – the skin of the hand, a leaf or glass,” Yarin said. An additional application may be as a nano-textured surface that dramatically increases cooling efficiency.

Yoon said the “self-fusion” by electroplating at the fibre junctions “dramatically reduced the contact resistance”. Yarin noted that the metal-plated junctions facilitated percolation of the electric current and account for the nanomaterial’s physical resiliency.

“But most of it is holes,” he said, which makes it 92 per cent transparent. “You don’t see it.”