Check valve incorporating a mobile polymer monolith into a multiple-level silica microchannel.

Integration of microscale fluidic elements, whether for incorporating several elements to make a device or for massively parallelizing a microfluidic analysis, is essential for reaching microfluidics' potential for better, faster, and cheaper chemical/biochemical analysis. Complex fluid flow control at microscales requires microvalves. Many microfabricated valves have been proposed; however, few can hold off pressures more than a few bars. High-pressure liquid chromatography typically uses pressures above 200 bar, requiring that fluid routing devices be designed for high pressures.

Mobile polymer monolith are in use at Sandia for a variety of fluid control elements. These polymer monoliths are closed-shell, porous polymer structures; a typical modulus of elasticity is .01-.1 GN/m². The mobile monoliths are used in conjunction with multiple-level etched silica microchannels for fluid control. An example is shown at right, in which a check valve is constructed by constructing a mobile polymer piston within a contained space. This piston moves in response to pressure differences >1 psi. When pressure is applied at left, the piston moves to the right but allows flow to proceed along the bypass. Thus flow can move from left to right. When pressure is applied at right, the piston moves to the left and seals at the interface between the deeply etched channel (60 um) and the shallowly-etched channel (20 um) to which it is connected.

Schematic of laser-polymerization.

We laser-fabricate these microvalves by photoinitiating phase-separated polymer monoliths in-situ. The microchannel and the lithographic mask define the shape the monolith takes upon polymerization. Monomers, solvents, and polymerization time define the bulk properties of the monolith.

Actuation pressure vs. polymerization energy for a mobile monolith element.

In capillary, these elements, have been shown to seal against 340 bar with leak rates below 50 pl/min. On chip; pressures over 200 bar have been sealed, with worst-case leak rates on the order of 1 nl/min. Pressure required to actuate these mobile elements is shown at right, where the pressure has been normalized to a nominally 100 micron diameter microchannel. At low crosslinking levels, the pressure required to actuate these elements can be under 1 psi.

Upcoming Presentations
		B.J. Kirby, T.J. Shepodd, D.S. Reichmuth "Metered Microchip Pressure Injections Using Mobile Polymer Monolith Microvalves, " oral presentation, HPCE 2003, January 2003, San Diego, CA.
References
		B.J. Kirby, T.J. Shepodd "Microvalve architectures for high-pressure hydraulic and electrokinetic fluid control in microchips," in Micro Total Analysis Systems 2002, Kluwer Academic Publishers, p. 338, 2002.
		B.J. Kirby, T.J. Shepodd, E.F. Hasselbrink "Voltage-Addressable on/off microvalves for high-pressure microchip separations," J. Chrom. A , v. 979, p. 147-154, 2002.
		E.F. Hasselbrink, T.J. Shepodd, J.E. Rehm "High Pressure Microfluidic Control in Lab-on-a-Chip Devices using Mobile Polymer Monoliths," Analytical Chemistry , v74, p4913-4918, 2002.
		J.E. Rehm, T.J. Shepodd, E.F. Hasselbrink "Mobile Flow Control Elements For High-Pressure Micro-Analytical Systems Fabricated Using In-Situ Polymerization," in Micro Total Analysis Systems 2001, Kluwer Academic Publishers, 2001.