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RESEARCH PROFILES Microfluidics takes a page from the trees (a)
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A unique capillary valve was developed to protect the (b) Capillary Service Capillary port Access retention Reaction reaction chamber from drying. channel valve chamber Via pump The valve is the narrowest part of the CS; as the liquid passes CS PDMS substrate through the narrow opening, (c) 0 the rear meniscus—which is Wetting formed as the liquid is drained –2 from the reservoir and re–4 Dewetting placed by air—does not make –6 it through the narrow open(d) 1.0 ing. This is important because 0.8 this retention valve stops the 0.6 flow of liquid until the next 0.4 0.2 liquid dispensed into the sys0.0 tem reaches the valve and de0 1 2 3 4 5 6 7 stroys the meniscus. A CS Position in the flow path (mm) with such a valve allows many solutions to be flushed in seThe CS as viewed from (a) above and (b) the side, with quence while protecting the (c) the capillary pressure for wetting (solid line) and reaction chamber located imdewetting (dashed line) and (d) flow-rate resistance vermediately behind the valve sus position in the flow path. from being drained. The authors constructed their system face; then a solution containing CRP so that sample loading and the evaporaproteins was flushed over the immobition capillaries are located on the upper lized antibodies, followed by a solution face of the wafer exposed to air, but recontaining fluorescently labeled monoactions could occur in a small capillary clonal anti-CRP antibodies, which atsection that is not exposed to air. Initial- tached themselves to the CRP proteins. ly, this reaction chamber is open on the The usual rinsing and washing steps for lower surface of the wafer, but the wafer this type of assay were done in the reacis bound to another material suitable for tion chamber before the CS was separatthe desired reaction. ed from the PDMS surface. To illustrate their concept, Juncker “For quantitative analyses, each step and colleagues attached the CS to a must be well controlled,” says Juncker. poly(dimethylsiloxane) (PDMS) slab, “A prerequisite for this is to have a which closed the reaction chamber, and known concentration of biomolecules the device arrayed a 50 50-µm2 spot immobilized on the surface.” If the CS device is further reduced in size, the of fluorescently labeled proteins onto protein spots can be placed as dense arthe PDMS surface. The 150-nL sample did not dry out because of the retention rays on a surface. Juncker foresees many applications for valve, allowing the system to produce such microfluidic systems, adding, “One a highly homogeneous pattern of procould imagine using many CSs in parallel teins with a root mean square variation or performing many types of water-based of fluorescence of only 2.5% in the cenchemical reactions that necessitate the setral 80% of the spot. quential delivery of reagents. The real In another experiment, a full sanddifficulty with microfluidic systems is not wich immunoassay was created. Antibodies against C-reactive protein (CRP), making them, but making them convenient and simple to use.” a a protein associated with heart attack, —Michael J. Felton were immobilized on the PDMS sur-
A N A LY T I C A L C H E M I S T R Y / J A N U A R Y 1 , 2 0 0 3
Normalized total Capillary pressure flow-rate resistance × 103 (Pa)
Microfluidics is something nature has been working on for eons. Trees, for example, use capillary networks to transport water and nutrients from their roots to their leaves, sometimes hundreds of feet high, without the advantage of mechanical pumps, actuators, or electroosmotic forces. In the December 15 issue of Analytical Chemistry (pp 6139–6144), David Juncker, Emmanuel Delamarche, and colleagues at IBM’s Zurich Research Laboratory introduce a microfluidic capillary system (CS) similar to those found in trees and use it as an arrayer to lay down homogeneous spots of proteins on surfaces with dimensions that are suitable for high-density arrays. “There are almost no integrated microfluidic systems to date that can control the flow of liquid without any outside connections for either actuation or control,” explains Juncker. The researchers achieved flow control and actuation by tailoring the geometry of the channels and their surface chemistry to construct valves and pumps that are powered by only capillary phenomena. A capillary produces a pressure on a liquid on the basis of the capillary’s cross section and wetting properties. This means that the forces on the liquid can be calculated, allowing a system to be designed to produce a particular flow rate from a reservoir through a series of capillaries. “Coding” the flow, however, is not enough. Once the fluid reaches the end of the capillary network, it will stop flowing, and the system will reach equilibrium. Trees overcome this by using their leaves to evaporate water, providing additional negative pressure at the ends of the capillaries. Juncker and colleagues, along with other researchers, have mimicked this effect by splitting the flow into numerous small capillaries and using air or heat to evaporate the fluid. Alternatively, they have used additional networks of capillaries to remove the liquid at the end of the system.
