An aerosol jet printer puts down layers of carbon-based electronic inks to create transistors that can be fully recycled using only water, rather than requiring harsh, toxic chemicals. (Image: Jason Arthurs Photography)

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

Electronics manufacturers are faced with the challenge of successfully securing several layers of components on top of each other — imperative to making complex devices. Getting the layers to stick is the tricky part, particularly for printed electronics.

“If you’re making a peanut butter and jelly sandwich, one layer on either slice of bread is easy,” said professor and study lead Aaron Franklin. “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.”

Previously, Franklin’s team demonstrated the first fully recyclable printed electronics. The devices used three carbon-based inks: semiconducting carbon nanotubes, conductive graphene, and insulating nanocellulose. In trying to adapt the original process to only use water, the carbon nanotubes presented the largest challenge.

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

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“You want the carbon nanotubes to look like al dente spaghetti strewn down on a flat surface,” said Franklin. “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.”

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

Franklin explains that, by demonstrating a transistor first, he hopes to signal to the rest of the field that there is a viable path toward making some electronics manufacturing processes much more environmentally friendly. And while the process does use a lot of water, it’s not nearly as much as what is required to deal with the toxic chemicals used in traditional fabrication methods.

While more work remains, Franklin said the approach could be implemented in the manufacturing of other electronic components — e.g., the now-ubiquitous screens and displays.

“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.

Here is a Tech Briefs interview, edited for clarity and length, with Franklin.

Tech Briefs: What were some of the biggest technical challenges you faced throughout your work?

Franklin: Simply getting rid of the use of water may not seem that significant, especially since several of the inks we used were already water-based. The challenge is that any ink containing nanomaterials will have other additives included to functionalize the nanomaterials, keeping them from clumping together and ensuring they print into uniform films. Getting rid of these residual additives in the final printed films is not easily achieved, typically requiring more harsh solvents and high temperatures. The process we developed in this work uses only water and a maximum temperature of 70 °C — the lowest temperature to date for realizing fully printed thin-film transistors.

Tech Briefs: Can you explain in simple terms how the process works?

Franklin: We have been working for several years on developing printed recyclable electronics — where all materials can be reclaimed and reused. Throughout that work, we became increasingly aware of how reliant our processes were on hazardous chemicals, which have their own significant impact on the environment. Since some of the inks we used were water-based, we decided to try to develop a printed transistor process that used nothing but water. So, this process works by printing layers of semiconducting, conducting, and insulating nanomaterials to form the electronic switching devices known as transistors.

Tech Briefs: Do you have any set plans for further research/testing?

Franklin: To get closer to utility in applications, the performance of the transistors we demonstrated in this work needs to improve. What we have shown is the possibility, but now there is need for optimization. With further research into the space, I’m confident there will be more materials, processes, and ultimately applications made possible for recyclable, water-only printed electronics.

Tech Briefs: What are your next steps? Next goal(s)?

Franklin: The backplane electronics for displays are composed primarily of thin-film transistors (LTPS or LTPO) that rely on high greenhouse-gas-emitting manufacturing; if those transistors were able to be printed from a water-only process with recyclable materials it would be completely transformative. My group is continuing to seek research funding to pursue this path, with the next goal being demonstration of printed, water-only transistors that meet the performance targets for driving displays in the backplane.

Tech Briefs: Do you have any advice for engineers aiming to bring their ideas to fruition?

Franklin: Always be aware of discoveries you did not necessarily set out to make; it’s often the unexpected findings that are the most impactful and able to carry ideas to fruition.

Tech Briefs: Is there anything else you’d like to add?

Franklin: We are just scratching the surface for exploring printed, recyclable electronics and hope to see a much greater effort fueled by increased research funding in the years ahead.