Stretchable electronics promise video displays that could be rolled up and tucked into a shirt pocket, or cell phones that could swell or shrink. Electronic sheets that could be draped like cloth would be a boon for robotic skin and embedded medical devices.
Now engineers at the University of California, Los Angeles, have taken a step toward these handy electronics by creating the first fully stretchable organic light-emitting diode (OLED). Previously, researchers had only been able to create devices that are bendable but can't stretch, or stretchable pieces that connect smaller, rigid LEDs.
One challenge in creating stretchable electronics is to develop an electrode that maintains its conductivity when deformed. To achieve this property, some researchers have turned to carbon nanotubes because they are stretchable, conductive, and appear transparent in thin layers, letting light shine through. However, for carbon nanotubes to hold their shape, they must be attached to some surface. Coating carbon nanotubes onto a plastic backing has not worked well, because the nanotubes slide off or past each other instead of stretching with the plastic. While some researchers have gotten around this problem, they still were not able to make a completely stretchable OLED.
To make their device entirely pliable, the UCLA researchers devised a novel way of creating a carbon nanotube and polymer electrode and layering it onto a stretchable, light-emitting plastic. To make the blended electrode, the team coated carbon nanotubes onto a glass backing and added a liquid polymer that becomes solid yet stretchable when exposed to ultraviolet light. The polymer diffuses throughout the carbon nanotube network and dries to a flexible plastic that completely surrounds the network rather than just resting alongside it. Peeling the polymer-and-carbon-nanotube mix off of the glass yields a smooth, stretchable, transparent electrode.
"The infusion of the polymer into the carbon nanotube coatings preserved the original network and its high conductance," says Qibing Pei, professor of materials science and engineering and principal investigator of the project.
"The approach we used is very simple and can be easily scaled up for real production," says Zhibin Yu, previously a researcher in Pei's group and now a researcher at University of California, Berkeley, and first author of the work, which was published online last month in Advanced Materials.
To create the stretchable display, the team sandwiched two layers of the carbon nanotube electrode around a plastic that emits light when a current runs through it. The team used an office laminating device to press the final, layered device together tightly, pushing out any air bubbles and ensuring that the circuit would be complete when electricity was applied. The resulting device can be stretched by as much as 45 percent while emitting a colored light.
"The fact that the fabricated OLED can work under stretched conditions is quite impressive," says Jay Guo, a professor of electrical engineering at the University of Michigan who works on manufacturing plastic electronics.
The proof-of-concept device is a two-centimeter square with a one-centimeter square area that emits a sky-blue light. This week, the group published an additional paper showing that swapping in more-conductive silver nanowires for carbon nanotubes in a similar process made a more efficient light-emitting diode.
This work is interesting and significantly different from past work, according to John Rogers, a professor of materials science at the University of Illinois who develops stretchable, deformable electronics.
Another benefit of the electrode is that it is less likely to short out. "Typically, carbon nanotube film is rough, so that can cause shorting in electronic devices," says Zhenan Bao, a Stanford professor of chemical engineering who works on stretchable solar cells. "Using this method, they ended up with a relatively flat surface that can be used for an electrode."
She adds that the stretchable electronics demonstrated thus far lose conductivity after being stretched too far or too many times, so more research is needed in this area.
"We are still some ways off from having high-performance, really robust, intrinsically stretchable devices," says Bao, but "with this work and those from others, we are getting closer and closer to realizing this kind of sophisticated and multifunctional electronic skin."
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