"Virtual Walls" Confine Liquids In Microfluidic Devices

Controlling the flow of gas and liquids within microchannel networks is critical in the design and fabrication of microfluidic devices for applications in bioassays, microreactors and chemical sensing. As reported in the Feb. 9 issue of the journal Science, researchers at the University of Illinois and the University of Wisconsin have devised a simple way to accomplish this with surface chemistry.

“The conventional technique of etching a channel into a substrate and then adding a top confines the fluid on all sides by physical walls, which doesn’t permit gas-liquid reactions to easily occur,” said Jeffrey Moore, a UI professor of chemistry and a researcher at the university’s Beckman Institute for Advanced Science and Technology. “With our surface-directed flow technique, the wall is created by an attraction between the liquid and the top and bottom surfaces. Therefore, we don’t need any physical barriers on the sides.”

To create their virtual walls, Moore and his colleagues – Bin Zhao at the Beckman Institute and David Beebe at the University of Wisconsin at Madison – begin with a “generic cartridge” of two microscope slides sandwiched together with slivers of cover slip only 180 microns thick. The cavity’s top and bottom are then coated with a hydrophobic (not having an affinity for water), self-assembled monolayer. A mask with the desired pattern is placed on top of the sandwich and irradiated with light.

Where light strikes the monolayer, the molecules break apart, leaving behind a hydrophilic (having an affinity for water) region. An aqueous solution injected into the finished cartridge will span the gap from top to bottom, but confine itself to the hydrophilic pathway and not spill into adjacent areas.

“Using these patterned surfaces, we can transport liquids from one reservoir to another along a specified pathway, or flow two streams side by side separated by a gas membrane,” Moore said. “The photopatterning method provides greater flexibility in the design and generation of complex flow networks and could facilitate mass manufacturing of surface-directed flow devices.”

The researchers also used their surface-directed flow technique to create pressure-sensitive switches inside the channel networks. “Depending upon the chemicals used to modify the surface, we can control where the fluid flows by varying the input pressure,” Moore said.

The ability to confine liquid flow inside microchannels with only two physical walls should prove useful in many applications where a large gas-liquid interface is required. For example, virtual walls could mimic the function of lungs – readily exchanging components between liquid and gas phases.

“By taking advantage of the large interface of virtual walls, we can produce functionality on microchips that is difficult to obtain by other methods,” Moore said. “This technique can greatly expand the toolbox for the design and fabrication of microfluidic systems.”