Innovative ink draws national, UI interest

Imagine an ink that was particle-free, fast to make and could print through 100-nanometer nozzles. These are just a few of the innovative features of a new reactive silver ink that has been created by the University’s Lewis Research Group led by Jennifer Lewis, the Hans Thurnauer professor of materials science and engineering.

“I am fortunate to work with a very talented group of students and postdoctoral researchers,” Lewis said.

These students and researchers include Brett Walker, graduate student in Engineering, who was one of the co-authors with Lewis in the recent Journal of the American Chemical Society (JACS) publication about the reactive silver ink.

While the reactive silver ink is not yet commercially available, the Lewis Research Group has filed a patent for it through the Office of Technology Management on campus, and their JACS publication has generated a lot of interest from companies.

The reactive silver ink is used for printing high-performance electronics on materials such as plastic and paper. The ink’s low viscosity makes it suitable for inkjet printing, direct ink writing or airbrush spraying over large areas.

The idea to create the reactive silver ink originated when the Lewis Research Group was discussing what would be the ideal silver ink design, said Walker. The optimal ink design would have low viscosity, high conductivity, a simple and high-yielding ink synthesis procedure, and would remain stable at room temperature.

To create the silver ink, silver acetate is dissolved in ammonium hydroxide, which is ammonium dissolved in water. Then, formic acid is titrated into the solution and mixed thoroughly, which forms the silver ink. In weight, the main ingredients of the reactive silver ink are water and silver acetate.

The silver reactive ink has multiple advantages compared to traditional particle-based inks. One advantage is that is much faster to make. While particle-based inks take multiple steps to prepare and several hours to make, a batch of the reactive silver ink only takes minutes to mix.

Another advantage of the silver ink is that it can print through 100-nanometer nozzles, which is much smaller than particle-based inks. There is no real limit to nozzle size, compared to particle-based inks, which have some finite limit on nozzle size.

“Even with direct ink write, our particle-based inks can’t reliably print under five microns,” Walker said. “We can print down to one micron with particle-based inks, but they clog really easily.”

Unlike other inks, the silver reactive ink can be left out for hours and one can still successfully print with it — even if it dries on the surface.

“All you have to do is re-wet the tip, and then you can continue to write with very small nozzles. So that could help in the creation of fine features,” he said.

Another advantage of the reactive silver ink is that it remains stable for months.

The last advantage of the silver is that it has a low processing temperature. Metallic inks generally need to be heated through a process called annealing. For many particle-based inks, the annealing temperatures are too high for the plastics or paper. The reactive silver ink can be annealed at a temperature of 90 degrees Celsius to achieve bulk conductivity. The ink is also conductive when allowed to dry at room temperature.

Currently, the Lewis Research Group is working on many applications with the reactive silver ink.

“You can easily create sensors that require just basic conductors to be printed down,” Walker said.

One application they are currently working on is transparent electrodes. Transparent conductive surfaces are used in many modern electronics, such as cell phone screens and TVs.

In this application, they take a metallic grid, and make the grids about 5 to 10 microns in width. They then space out those 5 to 10 micron width features to be only every 100 to 400 microns. This creates a high conducting structure that is transparent to the naked eye, Walker said.

“There’s a lot of demand to replace indium tin oxide (ITO), which is currently the major transparent conducting oxide on the market, and so silver and other metallic microgrids are very promising candidates as opposed to indium, since indium is very rare and costly,” Walker said.

The Lewis Research Group is now discussing with several companies to identify options for commercializing this ink design in printed electronic applications.

Not only has the new reactive silver ink attracted the attention of many companies, but it has also interested many University students.

Dario Aranguiz, freshman in Engineering, said he was proud to be a part of the University when learning about innovations such as the reactive silver ink. In a sense, the reactive silver ink feels revolutionary.