A new, ultrathin film that is both transparent and highly conductive to electric current has been produced by a cheap and simple method devised by an international team of nanomaterials researchers from the University of Illinois at Chicago and Korea University.
The film is also bendable and stretchable, offering potential applications in roll-up touchscreen displays, wearable electronics, flexible solar cells and electronic skin. The results are reported in Advanced Functional Materials.
The new film is made of fused silver nanowires, and is produced by spraying the nanowire particles through a tiny jet nozzle at supersonic speed. The result is a film with nearly the electrical conductivity of silver-plate — and the transparency of glass, says senior author Alexander Yarin, UIC Distinguished Professor of Mechanical Engineering.
“The silver nanowire is a particle, but very long and thin,” Yarin said. The nanowires measure about 20 microns long, so four laid end-to-end would span the width of a human hair. But their diameter is a thousand times smaller — and significantly smaller than the wavelength of visible light, which minimizes light scattering.
The researchers suspended the nanowire particles in water and propelled them by air through a de Laval nozzle, which has the same geometry as a jet engine, but is only a few millimeters in diameter.
“The liquid needs to be atomized so it evaporates in flight,” Yarin said. When the nanowires strike the surface they are being applied to at supersonic speed, they fuse together, as their kinetic energy is converted to heat.
“The ideal speed is 400 meters per second,” Yarin said. “If the energy is too high, say 600 meters per second, it cuts the wires. If too low, as at 200 meters per second, there’s not enough heat to fuse the wires.”
The researchers applied the nanowires to flexible plastic films and to three-dimensional objects. “The surface shape doesn’t matter,” Yarin said.
The transparent flexible film can be bent repeatedly and stretched to seven times its original length and still work, said Sam Yoon, the corresponding author of the study and a professor of mechanical engineering at Korea University.
Earlier this year, Yarin and Yoon and their colleagues produced a transparent conducting film by electroplating a mat of tangled nanofiber with copper. Compared to that film, the self-fused silver nanowire film offers better scalability and production rate, Yoon said.
“It should be easier and cheaper to fabricate, as it’s a one-step versus a two-step process,” said Yarin. “You can do it roll-to-roll on an industrial line, continuously.”
A team of researchers from the University of Chicago, Northwestern University, the University of Illinois at Chicago and the U.S. Department of Energy’s (DOE’s) Argonne National Laboratory have engineered silicon particles one-fiftieth the width of a human hair, which could lead to “biointerface” systems designed to make nerve cells fire and heart cells beat.
Bozhi Tian, who led one of the University of Chicago research groups, said the particles can establish unique biointerfaces on cell membranes, because they are deformable but can still yield a local electrical effect.
“Biological systems are soft, and if you want to design a device that can target those tissues or organs, you should match their mechanical interface as well,” Tian said. “Most of the current implants are rigid, and that’s one of the reasons they can cause inflammation.”
Over time biointerfaces made out of these particles will also degrade, unlike alternative materials like gold and carbon, said study co-author Yuanwen Jiang, a graduate student in the Tian group. This means that for future applications patients wouldn’t have to undergo a second procedure to have the particles removed.
Jiang and Tian said they believe the material has many potential applications in biomedicine, because the particles and light can be used to excite many types of cells.
The mesostructured silicon, named for its complex internal structures of nanoscopic wires, was created using a process called nano-casting.
To make the particles, each between one and five micrometers in size, researchers filled the beehive structure of synthetic silicon dioxide with semiconductive silicon the same way a blacksmith would pour molten metal into a cast iron mold. The outer mold was then etched away with acid, leaving behind a bundle of wires connected by thin bridges.
In order to test whether the particles could change the behavior of cells, the team injected a sample onto cultured rat dorsal root ganglia neurons, which are found in the peripheral nervous system.
The team was able to activate the neurons using pulses of light to heat up the silicon particles, causing current to flow through the cells.
In conventional biointerfaces, materials must be hooked up to a source of energy, but because researchers need only apply light to use the silicon particles, the new system is entirely wireless. Researchers can simply inject the particles in the right area and activate them through the skin.
“Neuromodulation could take full advantage of this material, including its optical, mechanical and thermal properties,” Jiang said.
Along with the implications that controlling neurons might have with neurodegenerative disorders, researchers in Tian’s lab have used similar materials to control the beating of heart cells, he said.
The University of Illinois at Chicago, or UIC, is a state-funded public research university located in Chicago, Illinois, United States.
Its campus is in the Near West Side community area, adjacent to the Chicago Loop. The second campus established under the University of Illinois system, UIC is also the largest university in the Chicago area, having approximately 28,000 students enrolled in 15 colleges.
UIC operates the largest medical school in the United States, and serves as the principal educator for Illinois’ physicians, dentists, pharmacists, nurses and other healthcare professionals. UIC’s medical school has research expenditures exceeding $412 million and consistently ranks in the top 50 U.S. institutions for research expenditures.
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