Radio-frequency identification (RFID) tags have made paying toll fees and public transit fares a breeze. But the tags, which are made of silicon, are still too expensive to replace ubiquitous barcodes to similarly speed up grocery store checkout lines by remotely scanning a product while it’s still in the basket.
Cheap plastic RFID tags could soon change that. Researchers in Sunchon, South Korea, have printed RFID circuits on plastic films using a combination of industrial methods: roll-to-roll printing, ink-jet printing, and silicone rubber-stamping. They use inks containing various materials–silver, carbon nanotubes, and a nanoparticle-polymer hybrid–to deposit the circuit’s components, such as capacitors and transistors, layer by layer.
Gyoujin Cho, a professor of printed electronics engineering at Sunchon National University, who led the work, estimates that the tags cost three cents apiece. To replace barcodes, RFID tags will need to cost a penny or less. But Cho says this should be achievable if all the layers on a tag can be deposited with a roll-to-roll process. A version of the current prototype that is capable of holding useful amounts of data should be on the market later this year, he says.
The new RFID tags will be the first product to use printed transistors made from carbon nanotubes. Researchers have been developing nanotube inks for a decade, but the only nanotube electronic product on the market so far is a film for display electrodes. Rick Jansen at carbon nanotube ink maker SouthWest NanoTechnologies says that good quality nanotube inks that are uniform and viscous enough to print have been costly to produce.
Making transistors using nanotube ink is also hard because mixtures are typically two-thirds semiconducting and one-third metallic, and the metallic component makes the mixture conducting overall. Cho and researchers at Paru Corporation in Sunchon have patented a simple process to make nanotube inks semiconducting. They coat the metallic tubes in the solution with a polymer. “You shake them with certain polymers and wrap them up and you just leave them in,” says Rice University chemistry professor James Tour, who was also involved in the new work.
The resulting transistors are large and don’t perform on par with silicon devices. But, says Tour, “RFID tags are a perfect application for them because you only need a handful of bits.”
Making transistor arrays that control the pixels in a flexible display with nanotube ink would be more challenging. “With displays you need better transistors,” he says. “We can print small transistors with carbon nanotube inks, but printing a large number of them with good alignment is hard.” Nevertheless, Cho says, the Korean team is working on making display control circuits with their nanotube transistors.