Stretchable electronics could lead to developments in video displays that could be rolled up, or cell phones that could swell or shrink, or electronic sheets that could be draped and aid in development of robotic skin and embedded medical devices. 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. “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. 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% while emitting a colored light. Another benefit of the electrode is that it is less likely to short out. Using this method, they ended up with a relatively flat surface that can be used for an electrode. 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.”
The first polymer organic light-emitting diodes (PLEDs) that can be stretched while lit have been produced by researchers in Europe and Japan. Matthew White from the Linz Institute for Organic Solar Cells in Austria and his teammates call the 2µm thick PLEDs ‘the thinnest and most flexible electroluminescent devices’ yet. Thinner PLEDs can be more flexible. And though polymer materials just a few hundred nanometers thick can emit light, existing commercial PLEDs surround them with comparatively bulky layers. They must be assembled on a robust base substrate and use a comparatively stiff indium-tin oxide (ITO) electrode. Also, today’s commercial materials are air- and water-sensitive and therefore need a protective encapsulant layer on top. That makes final devices over 100µm thick. The team produced the PLEDs on a 1.4µm thick PET foil substrate, akin the foil used in helium balloons, only a lot thinner. It is hard to spin-coat PLED materials onto such thin films, but working together with researchers from the University of Tokyo, the team achieved this. They stuck films onto silicone-coated rigid glass, which holds them in place using van der Waals forces alone, allowing easy post-fabrication removal, as easy as just peeling it off. The scientists also swap ITO for a polymer electrode, and synthesised an air-stable red light-emitting polymer so they could dispense with the protective layer. However, as the devices’ electrode materials weren’t air stable, they only worked for a few hours. Being so thin gives their devices uniquely small bending radii, which allows them to be crumpled. Exploiting this, the team made stretchable PLEDs by attaching their films to extended elastomeric tape. When the tape contracts, the PLEDs fold randomly, ready to be pulled flat when stretched. ‘This can add functionality without adding weight,’ White says. ‘Obvious applications would be police tape at a traffic accident that glows bright red, or balloons where the wrinkles would break a non-flexible chip.’ The team hopes to make foils combining entire PLED displays with a power source.