Kenneth Wynne's new polymer attracts water one moment, repels it the next. This schizo coating could have potential as a micro-fluidic switching device.

by James A. Bacon

Umit Makal, a Turkish graduate student at the Virginia Commonwealth University School of Engineering, was working on a project to develop a polymer-based coating that killed microbes. Under the direction of Dr. Kenneth J. Wynne, he was incorporating a well-known antimicrobial molecule called hydantoin into fluorine-containing polymer chains.

When he tested the behavior of water on one of these coatings, Makal noticed something unusual: The water drops spread, wetting the surface. But when he took the drop off and then put it back again, it beaded up. Says Makal: “The surface became water repellant where the original drop of water had been.”

To the uninitiated, the behavior of the water droplet might sound like a curiosity--an interesting quirk of nature--but of questionable significance. But to Makal and Wynne, it was an unprecedented phenomenon. Says Wynne: “I’ve never seen anything like this in my career. … Most people work an entire lifetime in the polymer materials without making a discovery this important.”

Writing in the chemical journal Langmuir, Wynne and Makal proposed that when water touches the material, the polymer side chain undergoes a thermodynamically driven rearrangement, exposing the hydrophobic fluorine-containing groups to the surface and causing them to repel water. The effect is completely reversible, Wynne notes, by drying the surface.

By harnessing this process, Wynne suggests, it may be possible to create micro-fluidic “intelligent switching” devices for a variety of applications. … “Sometimes an engineer wants to guide the flow, or turn off tiny streams of fluid, such as blood, in a test tube, and this kind of phenomenon could be useful in creating channels for that purpose.”

The discovery has created a stir in the chemical engineering community. Wynne says he’s gotten inquiries from seven or eight companies, including one of the top polymer development companies in the world, about the new polymer. At least one company has been interested enough to pursue discussions with VCU’s technology transfer office.

Wynne has taught at VCU since 2000, where he arrived after a career managing an outsourcing program in advanced polymeric materials for the Office of Naval Research. Makal joined Wynne’s VCU’s research team from Koç University in Turkey, where he had been studying polymer chemistry. “He’s been an extraordinary student,” says the professor. “He’s made several important discoveries.”

The schizoid polymer may be Makal's most important finding yet. If you've ever watched water bead up on a freshly waxed car, you've seen how hydrophobic surfaces act. Likewise, when water drops onto a windshield, spreads out and runs in rivulets, it's displaying hydrophilic behavior. Makal draws an analogy to a drop of water in a coated frying pan. “On our surface, when the pan is dry, water just loves the surface … it tries to stick to it," he says. "But then, when it is wet, the water rolls off.

Some researchers have made polymer coatings that become more wettable in the presence of water and more water-repellant in the presence of organic solvents. But no one has produced a compound that reverses itself like Wynne's "contraphilic" coating.

The behavior was so unexpected, says Wynne, that he thought there must be some malfunction of the instrument used for testing. Makal repeated the experiment several times before they believed the result.

In recent work, Wynne and Makal have found that processing details are important in maximizing the spontaneous switch between water-loving and water-repelling states. They are continuing research under an ongoing National Science Foundation grant. Meanwhile, the VCU Technology Transfer Office has supported a patent application covering the new polymeric material.

“Additional coverage will have to be sought, as we have found some new surprises,” Wynne says. “It would be nice to speculate on further applications, but I would prefer to get protection first for the VCU intellectual property.”

-- May 25, 2005