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RESEARCH PROFILES Building a ship inside a bottle monoliths. Hasselbrink originally set out with Shepodd, a polymer chemist, to make stationary monolithic columns of porous polymers for microchannels— essentially, long strands of porous “spaghetti” that stuck to the insides of the capillary (or microchannel) walls, explains Hasselbrink. However, the two encountered problems. “When we applied a lot of pressure to them, they kept squirting out of the capillaries,” he says. It was then that Rehm and Hasselbrink, both mechanical engineers, brainstormed about other possibilities. “We knew there was a need for flow-control microdevices that could hold off HPLC pressures, were inexpensive, and were solvent-resistant,” he says. The “Aha!” moment came, he adds, when they asked, “What if, instead of making a long piece of ‘spaghetti’ that sticks to the wall we make a really short one that doesn’t stick to the wall and use it as a movable rubber stopper inside of a microchannel?” The two presented their idea to Shepodd, who then created a formula for making nonporous Teflon-like polymer monoliths that slid easily through the channels. One of the first in situ microsystems they built was a check valve. Valve channels are typically made by bonding an etched glass wafer to a flat cover to produce interconnecting channels. However, the flat cover led to a small gap between the top of the piston and the cover, which allowed leakage. (a) Schematic of a passive check valve. (b) The flow (dashed To solve the problem, line) bypasses the piston. (c) A piston, seated against the stop, Rehm etched out of prevents the flow.
In this issue of Analytical Chemistry (pp 4913–4918), Ernest Hasselbrink, Timothy Shepodd, and Jason Rehm at Sandia National Laboratories–Livermore describe how to build miniature passive valves, check valves, and accumulators by in situ polymerization within microchannels—similar to building a ship inside a bottle. And unlike the time required for micromachining parts made from silicon, these microparts only take minutes to construct and can hold off pressures >30 Mpa (4500 psi), opening the door for the integration of on-chip HPLC. Photopolymerization has been used before to construct lab-on-a-chip devices. Researchers such as David Beebe’s group at the University of Wisconsin have used this approach to make hydrogel valves and entire walls of channels; Jean Fréchet’s group at the University of California–Berkeley makes chromatographic media with porous polymer
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glass wafers two “half-pipes”, which were mirror images of each other, to make better, tighter-sealing stops. The pistons are molded in place against the stops, so they conform to the cross-sectional shape of the microchannel. Their valves reduced the flow rate by a factor of ~1 million, going from 160 µL/s with the channel open to a leak rate of around 160 pL/s with 1 MPa holding the valve shut. Most lab-on-a-chip companies use a standard of ~1500:1 for leakage, say the researchers. The group also created diverter valves that directed fluid into a device from either one of two input lines in a T-junction, while restricting unwanted flow into the closed channel. A more complex device was a 10-nL pipette made up of a piston/cylinder reservoir and two check valves. The 10-nL reservoir is connected to an inlet/outlet port on the chip, along with a syringe connected via capillary tubing. “It was the moving part that was new to us,” says Hasselbrink. The pistons inside the channel move by applying manual pressure on the syringe, thus moving the fluid from the sample reservoir through the inlet check valve. These moving pistons are often referred to as “soft”, because by controlling the degree of polymerization, they have the elasticity of a common rubber stopper, thus allowing a tighter seal. Some early problems encountered with the new polymers were swelling and shrinkage. Initial formulations had polymers shrinking as much as 40% when placed in water, only to swell back up when put in an organic solvent. Hasselbrink acknowledges that more work needs to be done to improve the bidirectional sealing of the pistons, as well. This new valve system looks to bring parallel HPLC another step closer to reality. “If you can do 48 HPLC systems on something the size of a compact disk, now you’ve got something that is very powerful,” says Hasselbrink. a —Wilder D. Smith
