"Water-Based" Transistors Are Eco-Friendly, Recyclable, and "Competitive Enough," Researchers Say

Harsh chemicals are replaced with water in this multi-pass printing approach to fully-recyclable printed electronics.

Gareth Halfacree
12 months agoSustainability / HW101

Engineers from Duke University have come up with what they claim to be the "world's first fully-recyclable printed electronics" which can be fabricated with water in place of toxic chemicals — making them a giant leap more environmentally friendly than alternatives.

"If you’re making a peanut butter and jelly sandwich, one layer on either slice of bread is easy,” says Aaron Franklin, professor and study lead, by way of background to the team's work. "But if you put the jelly down first and then try to spread peanut butter on top of it, forget it, the jelly won't stay put and will intermix with the peanut butter. Putting layers on top of each other is not as easy as putting them down on their own — but that’s what you have to do if you want to build electronic devices with printing."

Traditionally, printed electronics — in contrast to etched electronics — use a carbon-based ink which is applied in the required pattern to a substrate. Previous attempts to use water and carbon nanotubes, mixed with a detergent-like surfactant, have resulted in too low a density of carbon to work as electronic components. It's here the team has made the breakthrough.

"You want the carbon nanotubes to look like al dente spaghetti strewn down on a flat surface,” Franklin explains. “But with a water-based ink, they look more like they've been taken one-by-one and tossed on a wall to check for doneness. If we were using chemicals, we could just print multiple passes again and again until there were enough nanotubes. But water doesn't work that way. We could do it 100 times and there'd still be the same density as the first time."

The solution is a new multi-stage process in which the printed device is rinsed with water then dried before another printing pass — then the process repeated until an adequate density of carbon is achieved. Unlike prior approaches, which required high temperatures or harsh chemicals, the print-wash-print technique allows for the amount of surfactant used to be reduced or removed entirely — resulting in a fully-functional and fully-recyclable transistor printed using an eco-friendly water-based ink.

Devices printed using the technique can, the researchers claim, be recycled to retrieve nearly all of the carbon nanotubes and graphene used in the printing process, while the nanocellulose material can be recycled like paper — or left to harmlessly biodegrade. The multiple rinsing processes do consume water, the team admits, but less than is currently required to handle the chemicals used in traditional electronic fabrication processes.

"The performance of our thin-film transistors doesn't match the best currently being manufactured," Franklin admits, "but they’re competitive enough to show the research community that we should all be doing more work to make these processes more environmentally friendly."

The team's work has been published in the journal Nano Letters under closed-access terms.

Gareth Halfacree
Freelance journalist, technical author, hacker, tinkerer, erstwhile sysadmin. For hire: freelance@halfacree.co.uk.
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