Recyclable electronics ditch chemicals for water

Engineers at Duke University have produced the world’s first fully recyclable printed electronics that replace the use of chemicals with water in the fabrication process. By bypassing the need for hazardous chemicals, the demonstration points down a path industry could follow to reduce its environmental footprint and human health risks.

Engineers at Duke University have produced the world’s first fully recyclable printed electronics that replace chemicals with water in the fabrication process. By bypassing the need for hazardous chemicals, the demonstration points down a path industry could follow to reduce its environmental footprint and human health risks.

The research appeared online on February 28 in the journal Nano Letters titled All-Carbon Thin-Film Transistors Using Water-Only Printing.

One of the dominant challenges facing any electronics manufacturer is successfully securing several layers of components on top of each other, which is crucial to making complex devices. Getting these layers to stick together can be frustrating, particularly for printed electronics.

“If you’re making a peanut butter and jelly sandwich, one layer on either slice of bread is easy,” explained Aaron Franklin, the Addy Professor of Electrical and Computer Engineering at Duke, who led the study. “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.”

Water-based printed electronics not easy

Franklin and his group demonstrated the first fully recyclable printed electronics in previous work. The devices used three carbon-based inks: semiconducting carbon nanotubes, conductive graphene and insulating nanocellulose. The carbon nanotubes presented the largest challenge in adapting the original process only to water use.

A surfactant similar to detergent is added to make a water-based ink in which the carbon nanotubes don’t clump together and spread evenly on a surface. The resulting ink, however, does not create a layer of carbon nanotubes dense enough for a high current of electrons to travel across.

“You want the carbon nanotubes to look like al dente spaghetti strewn down on a flat surface,” said Franklin. “But with water-based ink, they look more like they’ve been taken individually and tossed on a wall to check for doneness. Using chemicals, we could print multiple passes repeatedly 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.”

This is because the surfactant that keeps the carbon nanotubes from clumping also prevents additional layers from adhering to the first. In a traditional manufacturing process, these surfactants would be removed using either very high temperatures, which takes a lot of energy, or harsh chemicals, which can pose human and environmental health risks. Franklin and his group wanted to avoid both.

What the team did to develop water-based printed electronics

In the paper, Franklin and his group developed a cyclical process. The device is rinsed with water, dried in relatively low heat and printed on again. When the amount of surfactant used in ink is also tuned down, the researchers show that their inks and processes can create fully functional, fully recyclable, fully water-based transistors.

Compared to a resistor or capacitor, a transistor is a relatively complex computer component in devices such as power control, logic circuits, and sensors. Franklin explained that, by demonstrating a transistor first, he hoped to signal to the rest of the field that there was a viable path toward making some electronics manufacturing processes much more environmentally friendly.

Franklin has already proven that nearly 100 per cent of the carbon nanotubes and graphene used in printing can be recovered and reused in the same process, losing very little of the substances or their performance viability. Because nanocellulose is made from wood, it can be recycled or biodegraded like paper. While the process uses a lot of water, it’s not nearly as much as required to deal with the toxic chemicals used in traditional fabrication methods.

The future of recycling and water-based printed electronics

According to a United Nations estimate, less than a quarter of the millions of pounds of electronics thrown away each year are recycled. The problem will only worsen as the world eventually upgrades to 6G devices and the Internet of Things (IoT) continues to expand. So any dent that could be made in this growing mountain of electronic trash is vital to pursue.

While more work needs to be done, Franklin says the approach could be used to manufacture other electronic components like the screens and displays that are now ubiquitous to society. Every electronic display has a backplane of thin-film transistors similar to what is demonstrated in the paper. The current high-energy fabrication technology relies on hazardous chemicals and toxic gases. The US Environmental Protection Agency has flagged the entire industry for immediate attention.

“The performance of our thin-film transistors doesn’t match the best currently being manufactured, 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,” Franklin said.

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