Fully recyclable printed electronics dispense toxic chemicals to water
(Nanowerk News) Engineers at Duke University have produced the world’s first fully recyclable printed electronics that replaces the use of chemicals with water in the fabrication process. By ignoring the need for hazardous chemicals, a demonstration point on the paths the industry can follow to reduce its environmental footprint and human health risks.
The research appears in the journal Nano Letters (“All Carbon Thin Film Transistors Use Water Only Printing”).
One of the dominant challenges any electronics manufacturer faces is successfully securing multiple layers of components on top of one another, which is critical for building complex devices. Putting these layers together can be a frustrating process, especially for printed electronics.
“If you’re making a peanut butter and jelly sandwich, one layer on both slices of bread is easy,” explains Aaron Franklin, the Addy Professor of Electrical and Computer Engineering at Duke, who led the study. “But if you put the jelly on first and then try to spread the peanut butter on top, forget it, the jelly won’t hold and will mix with the peanut butter. Putting layers on top of each other isn’t as easy as putting them on your own — but it’s what you have to do if you want to build electronic devices with printing.”
In previous work (Natural Electronics, “Printable and Recyclable Carbon Electronics Using Nanocellulose Crystals”), Franklin and his group demonstrated the first fully recyclable printed electronics. The device uses three carbon-based inks: semiconducting carbon nanotubes, conductive graphene, and insulating nanocellulose. In trying to adapt the original process to use only water, carbon nanotubes presented the biggest challenge.
To make water-based inks where the carbon nanotubes don’t clump and spread evenly on the surface, a surfactant similar to a detergent is added. The resulting ink, however, does not create layers of carbon nanotubes that are dense enough to allow the high electron currents to pass.
“You want the carbon nanotubes to look like al dente spaghetti spread out on a flat surface,” says Franklin. “But with water-based inks, they look more like they’ve been picked up one at a time and thrown at the wall to check for doneness. If we use chemicals, we can print several passes again and again until there are enough nanotubes. But water doesn’t work like that. We can do it 100 times and there is still the same density as the first time.”
This is because the surfactant used to keep the carbon nanotubes from agglomerating also prevents the additional layer from sticking to the first layer. In traditional manufacturing processes, these surfactants are removed using extremely high temperatures, which consume a lot of energy, or harsh chemicals, which can pose risks to human health and the environment. Franklin and his group wanted to avoid both of them.
At the paper, Franklin and his group developed a cyclical process in which the device was rinsed in water, dried over relatively low heat and reprinted. When the amount of surfactant used in the ink was also tuned down, the researchers demonstrated that their inks and processes could make fully functional, recyclable, completely water-based transistors.
Compared to resistors or capacitors, transistors are relatively complex computer components used in devices such as power control or logic circuits and sensors. Franklin explained that, by demonstrating the transistor first, he hoped to signal to the rest of the field that there were feasible ways to make some electronics manufacturing processes much more environmentally friendly.
Franklin has proven that nearly 100% of the carbon and graphene nanotubes used in printing can be recovered and reused in the same process, losing very little of the substance or continuity of performance. Since nanocellulose is made of wood, it can be easily recycled or biodegradable like paper. And while the process does use a lot of water, it’s not as much as is needed to handle the toxic chemicals used in traditional fabrication methods.
According to United Nations estimates, less than a quarter of the millions of pounds of electronics thrown away each year can be recycled. And the problem will only get worse as the world finally upgrades to 6G devices and the Internet of Things (IoT) continues to grow. So any dents that can occur in this growing pile of e-waste are important to catch up on.
While more work needs to be done, Franklin said the approach could be used in manufacturing other electronic components such as screens and displays that are now ubiquitous in society. Every electronic display has a thin-film transistor backside similar to that shown on paper. Today’s fabrication technologies are high-energy and rely on hazardous chemicals and toxic gases. The entire industry has been flagged for immediate attention by the US Environmental Protection Agency.
“The performance of our thin-film transistors doesn’t match the best currently produced, but they are competitive enough to show the research community that we all have to do more work to make this process more environmentally friendly,” said Franklin.