Electrical engineers at Duke University have demonstrated the ability to print fully functional and recyclable electronics at sub-micrometer scales. The technique could impact the more than $150 billion electronic display industry and its environmental impact while providing a toehold for U.S. manufacturing to gain traction in a vital and quickly growing industry.

The research appears in the journal Nature Electronics.

“If we want to seriously increase U.S.-based manufacturing in areas dominated by global competitors, we need transformational technologies,” said Aaron Franklin, the Edmund T. Pratt, Jr. Distinguished Professor of Electrical & Computer Engineering and Chemistry at Duke.

“Our process prints carbon-based transistors that can be fully recycled and provide comparable performance to industry standards. It’s too promising of a result not to be given further attention.”

Electronic displays play a key role across just about every industry: think TVs, computer screens, watch faces and car displays. Nearly all of them are made overseas, mostly in South Korea, China and Taiwan.

The manufacturing process has a significant environmental impact due to the greenhouse gas emissions and enormous energy footprint required by vacuum-based processing. And to top it off, according to a United Nations estimate, less than a quarter of the millions of pounds of electronics thrown away each year are recycled.

Several years ago, Franklin’s laboratory developed the world’s first fully recyclable printed electronics. That demonstration, however, used aerosol jet printing that can’t form features smaller than 10 micrometers, greatly limiting their potential applications in the world of consumer electronics.

In the new research, Franklin and his colleagues worked with Hummink Technologies to break through this size barrier. Their “high precision capillary printing” machines use natural competing surface energies to pull tiny amounts of ink out of an equally tiny pipette. This is the same phenomenon that makes paper towels so absorbent, as liquid is drawn into the narrow spaces between their fibers.

“We sent Hummink some of our inks and had some promising results,” said Franklin. “But it wasn’t until we got one of their printers here at Duke that my group could harness its real potential.”

The researchers used three carbon-based inks made from carbon nanotubes, graphene and nanocellulose that can be easily printed onto rigid substrates like glass and silicon or flexible substrates like paper or other environmentally friendly surfaces. These are essentially the same inks that were originally demonstrated in Franklin’s previous research, but with tweaked fluid properties that allow them to work with the Hummink printers.

In the demonstration, they show this combination of novel ink and hardware can print features tens of micrometers long with small, submicrometer-sized gaps between them.

These small, consistently formed gaps form the channel length of the carbon-based thin-film transistors (TFTs), with smaller channel dimensions translating to strong electrical performance. And it’s these kinds of transistors that form the backplane control of all flat-panel displays.

“These types of fabrication approaches will never replace silicon-based, high-performance computer chips, but there are other markets where we think they could be competitive—and even transformative,” said Franklin.

Behind every digital display in the world is a huge array of microscopic thin-film transistors that control each pixel. While OLED displays require more current and need at least two transistors for each pixel, LCD displays require only one.

In a previous study, the researchers were able to demonstrate their printed, recyclable transistors driving a few pixels of an LCD display. And Franklin believes the new submicrometer printed TFTs are close to having the performance needed for demonstrating the same for OLED displays.

While there are other potential use cases for this technology, such as squeezing more sensors into a chip’s footprint to increase its accuracy, Franklin believes digital displays are the most promising. Besides being fully recyclable, the printing process requires much less energy and produces many fewer greenhouse gas emissions than traditional TFT manufacturing methods.

“Displays being fabricated with something similar to this technique is the most feasible large-scale application I’ve ever had come out of my lab,” said Franklin.

“The only real obstacle, to me, is getting sufficient investment and interest in addressing the remaining obstacles to realizing the considerable potential.”

“Unfortunately, the National Science Foundation program that we were pursuing funding from to continue working on this, called the Future Manufacturing program, was cut earlier this year. But we’re hoping to find a fit in a different program in the near future.”