Printing Polymers Combinatorially

For a look at the technology behind one of the hottest new tools in combinatorial polymer research, one need only crack open an ordinary desktop ink-jet printer. It's a rudimentary view, of course, but scientists are taking the same mechanisms that drive this bit of inexpensive office equipment and transforming them into fast, flexible instruments for economically making libraries of polymer thin films.

Using these devices for something other than ordinary printouts certainly isn't a new idea. Within the past decade, researchers working in several areas have capitalized on ink-jet printers' ability to precisely and reproducibly place minuscule drops of liquid. The technology has been used to screen catalysts for carbon nanotube synthesis and to make arrays of DNA and other bioactive molecules.

Ink-jet technology also has caught the eye of display makers who think the printers will be key to manufacturing polymeric light-emitting diode (PLED) displays in the future. The electroluminescent polymers that make up PLED displays need to be exactingly printed pixel by pixel--a job that seems well suited to ink-jet's high-precision capabilities.

"There are tremendous efforts ongoing to fabricate PLEDs, solar cells, or electronic circuits using ink-jet printing," says Ulrich S. Schubert, a chemistry and nanoscience professor at Eindhoven University of Technology, in the Netherlands. However, he adds that "the efficiency and lifetime of all these devices critically depends on the quality of the printed layers."

THAT'S WHY Schubert, who also works with the Dutch Polymer Institute as program manager for high-throughput experimentation, has spent the past two years working in earnest to make ink-jet printing an effective tool for combinatorial polymer research. "Ink-jet printing opens the way to the automatic preparation of libraries of polymers, polymer blends, and composites, with a systematic variation of parameters such as chemical composition or thickness," he says.

A pioneer in the area, Schubert presented his work at a number of meetings around the world in 2003 and 2004. His group, which includes project leader Berend-Jan de Gans, postdoc Elisabeth Holder, graduate students Emine Tekin and Veronica Marin, and technician Antje van den Berg, has also published a flurry of papers on the topic, most recently in the Journal of Materials Chemistry [14, 2627 (2004)] and Langmuir [20, 7789 (2004)]. Schubert also spearheaded a special issue of Macromolecular Rapid Communications that is devoted to ink-jet printing of polymers and should be published early next year.

Schubert says his group turned to ink-jet printing as a simple way to study polymer libraries as thin films on surfaces because that's how the materials would be used in polymeric electronic devices. "We evaluated all potential methods to look at thin films combinatorially," Schubert explains, "and then we looked at what the graphics industry did."

The researchers also took a cue from Philips, their neighbor in Eindhoven. The electronics giant has been using ink-jet technology to print organic light-emitting diode (OLED) displays (C&EN, May 24, page 13). Schubert figured that if he could modify the technology for combinatorial use, he could synthesize a variety of different polymers, print a library of them on a substrate of choice, and then directly compare the thin films' properties.

While standard desktop color printers have been modified to print polymeric solutions, Schubert says his group never seriously considered using those machines. "Desktop printers are meant to work with water-based inks," he explains. Their cartridges and print heads are usually made of plastic, so, Schubert says, "if you start to use organic solvents, then it makes a big mess." Scientists may use them for preliminary work, he adds, but specialized machines are necessary for serious combinatorial polymer research.

The group started asking around at exhibitions where specialized ink-jet instruments were displayed. "None of the companies had any idea what 'combinatorial' meant," Schubert recalls. Still, the researchers knew what parameters would be most important and compared the available printers' capabilities in terms of work-space dimensions, accuracy, drop volume, number of nozzles, and maximum heating temperature [Macromol. Rapid Commun., 24, 659 (2003)].

Then they took the plunge. Schubert acknowledges that buying one of the specialized high-tech printers is a serious investment. According to Björn Fischer, a sales representative at the ink-jet manufacturer Microdrop, Hamburg, Germany, a basic setup with only one dispenser head costs roughly $92,000. Add all the bells and whistles, and the system will set you back by a little more than $104,000.

RETROSPECTIVELY, Schubert recognizes that trying to apply ink-jet technology to combinatorial polymer discovery was a risky endeavor. "You really can play the whole combi game with ink-jet printing," he says, "but that was not clear when we started."

The high-tech instruments are actually quite different from their desktop cousins. A fully automated robotic system dispenses picoliter drops of fluid through its printheads. Two cameras record drop formation and impact onto the substrate.

Schubert likens the system to the sputtering techniques that have been used to create libraries of inorganic materials and catalysts. "With ink-jet, you can do the same thing with solutions. You can mix whatever you want and you can print it," he says. "It really opens a new door in combi science."

For example, Schubert's group has used the technique to print libraries of thin films onto transparent substrates. On one substrate plate, they can vary a polymer's chemical composition and film thickness. They then analyze the plate using high-throughput analytical instruments, such as ultraviolet-visible, fluorescence, or Raman spectrometers. Schubert says it's also possible to compare the effects of different additives or print parameters. A library can be heated up to see how well the components of the polymer library hold up to high temperatures.

So far, the group has managed to use 40 different solvents in their combinatorial experiments. They've also been able to work with high-molecular-weight polymers--materials that are notoriously difficult to print because of their viscosity and elasticity.

"I have been impressed with the systematic approach that Schubert's group has taken to working through the range of problems in application of ink-jet printing for deposition of complex fluids, such as polymers and multiphase mixtures," remarks Eric J. Amis, chief of the Polymers Division at the National Institute of Standards & Technology, Gaithersburg, Md. "Problems of flow control, droplet breakup, dosing, mixing, surface deposition, stability, et cetera, all enter into the technology needed to prepare a combinatorial library. All of these issues are amplified when you want the tool to be flexible enough to cope with diverse materials as could be present in a screening library."

A few weeks ago at an international plastic electronics conference in Eindhoven, Schubert demonstrated how his group created the first test library of 16 polymer formulations that could be used in a PLED display. The group will use the library to see which formulation has the longest lifetime. Next, they plan to expand the device so that it screens 384 different test pixels.

Also, by using an ink-jet printer with a print stage that can be adjusted for height--called an XYZ-stage--it's possible to create libraries on a range of substrates, such as glass, alumina, plastic, or silicon.

Schubert thinks this flexibility will make ink-jet printing a very useful tool for developing, among other things, the next generation of PLED and OLED displays. Currently, most of these displays are made on glass, but there's a push in the display industry to put PLEDs and OLEDs onto plastic. Plastic displays, it's thought, would be cheaper, lighter, and less fragile than glass. And plastic substrates will be necessary to make flexible displays.

Just because a polymer works on a glass substrate, however, doesn't necessarily mean it will perform on plastic. "You really have to develop things from scratch again," Schubert explains. That's where combinatorial ink-jet printing comes in.

While much of the published work on using ink-jet printing for combinatorial polymer research has come out of Schubert's lab, a growing number of researchers are also using the technique. Microdrop's Fischer says he's sold a couple of ink-jet systems for combinatorial work recently, and he's had roughly 100 inquiries about the equipment.

Ghassan E. Jabbour, a chemical and materials engineering professor at Arizona State University, has been using an ink-jet system for combinatorial research at the university's Army Flexible Display Center. The center was founded earlier this year with a $43.7 million grant from the Army to develop flexible displays.

"What we want to do is to put a polymer conductor on a plastic circuit," Jabbour says. By using ink-jet combinatorial techniques, he and postdoc Yuka Yushoika are trying to map the sheet resistance of polymers that could be used in solar cells or flexible displays. "With other techniques, it would take forever to get where we want to go," he says.

Jabbour adds that for liquid-based materials, he's found ink-jet printing to be the most effective combinatorial tool for his research. "It's fast and it's flexible," he explains. "It's also environmentally friendly because, unlike spin-coating, you're dealing with solvents in the picoliter range. These small amounts of solvent also make ink-jet printing cost-effective because you don't need to make a lot of your solution."

Putting the research in perspective with other techniques for combinatorial polymer research, NIST's Amis comments: "The ink-jet methods sit between the gradient approaches we have taken and the more traditional discrete sample methods. All three approaches have limitations that can be difficult to overcome, but they each also have their strengths. Ink-jet methods should allow one to make enormous, systematic, and diverse multicomponent libraries.

"The fundamental creativity in combinatorial research is in the design of the experiment," Amis continues. Ink-jet methods "hold the promise of a versatile fabrication tool that will let creative scientists design more beautiful experiments."