Michael J. Felton
icromechanical valves were supposed to be so inexpensive and ubiquitous that entire analytical processes were foreseen to take place on single chips. With cheap microscale valves and pumps, researchers would be able to build devices that would replicate macroscale chemical processes or tests in such a small format that even the most complex analyses could be done in the field. So what went wrong? One major problem was that the material most early developers used—silicon or metal—is not the best choice for valves. “You’ve got metal on metal or metal on silicon, and it doesn’t take a big error to lead to a pretty poor leakage ratio,” says Charlie Hasselbrink of the University of Michigan. The complex fabrication steps proved to be another problem. “There are some [techniques with] 14 production steps to get a chip that does some valving,” says Andreas Manz at Imperial College (U.K.). The more steps there are, the more time and equipment it takes to manufacture the valves. More steps also equal a greater likelihood for errors, increasing the prevalence of faulty valves and adding to the cost.
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In addition, early valves seemed to be providing solutions to the wrong problems. Manz suggests that the engineering for these systems was particularly ingenious, but it gave the wrong answers: “You wanted a car and got a bicycle, or the other way round.” David Beebe, at the University of Wisconsin, Madison, suggests that it was a natural stage that microfluidics had to go through. “And now,” he says, “I think you are starting to see a new generation of valves coming out, sometimes with specific applications in mind, and sometimes with different techniques that didn’t come from the traditional microfab realm.” He adds that the new generation of valves has followed one chief rule: “It’s got to be really simple.”
Applications for microvalves Some applications for which microvalves were originally seen as perfect are now handled in other ways. “I always think about microfluidics in terms of the end user,” says Manz. “The end user, first of all, doesn’t care whether there is a chip in the box or something else, and so the end user even cares less if there O C T O B E R 1 , 2 0 0 3 / A N A LY T I C A L C H E M I S T R Y
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deform or be destroyed by the pressure. Researchers are exploring several valve techniques for use as check valves, including pressure restriction and miniature versions of ball valves.
Pressure restrictors In the late 1990s, two researchers at Sandia National Laboratories, Phillip Paul and David Rakestraw, formed a check valve by generating electroosmotic force (EOF) through porous materials. The EOF pushes the fluid through the pores, but the fluid on the other side does not flow back, producing a pressure restrictor. In effect, the pump and the check valve are constructed together. Another incarnation of this technique, described by FIGURE 1. Using hydraulic resistance as a valve. Iulia M. Lazar and Barry Karger at Northeastern Uni(a) Sample is delivered to an LC system using electroosmotic force (EOF) along a series versity, uses numerous very small diameter channels inof very narrow channels. (b) Once the sample is delivered, the voltage on the valve is turned off and the EOF pump is turned on. The pressure generated by the pump does not stead of a porous material for both pumping and valvforce the sample back into the valve because the difference in channel diameter is suffiing. The relative ratio of the channel diameters between ciently large. the pumping channels and the rest of the microfluidic is a valve on the chip or not.” Existing technology, like peri- channels is called the relative hydraulic resistance. If this resiststaltic pumps and motor-driven syringes, can be used in many ance is strong enough, according to Lazar, now at Virginia situations to control fluid flow on a chip surface. “For $10 [per Bioinformatics Institute/Virginia Tech, then fluid will not flow valve], I would throw away my motor-driven syringes,” says back through the small channels even at high pressure, thus Manz. “But if I have to pay $300 or $3000 each, then I’ll keep forming a check valve. “I started looking at it as a pump, but I wouldn’t be surmy motor-driven syringes because they do work quite nicely.” Although microvalves may not be necessary or economical prised if it was used more as a valve,” says Lazar. The multiin certain situations, according to some researchers, three ap- channel structure prevents the pressure-driven flow of materiplications warrant them: LC on a chip, large-scale integrated al, but fluid can be driven through it electrically. Lazar calls the technique electroosmotic valving. As a valve, the structures systems, and on-chip actuated systems. could be connected to a reservoir containing a sample while the main EOF pump or other type of pump forced carrier fluid LC on a chip On-chip LC is attractive for many reasons, from faster analysis through the LC column. The main flow would not enter the to extremely small sample and reagent volumes. But one of the sample reservoir, but the sample could be injected by applying most important advantages is not immediately obvious. The extremely small surface area in the channels on a chip allows the use of extremely high pressures—up to many thousands “For $10 [per valve], I would throw away my motorof atmospheres. However, high pressures introduce three driven syringes,” says Manz. “But if I have to pay $300 main problems. First, it is advantageous to have valves and pumps reside inside the chip. “If the or $3000 each, then I’ll keep my motor-driven syringes valves are on chip, the high pressure is only inside the chip, and all the chip-to-lab interfaces because they do work quite nicely.” can be low pressure (equals easy),” says Manz. Otherwise, the interface between the chip and external valves and pumps would have to deal with the pressure a voltage to the valve, making it a small pump. A schematic of drop, which “is a real nightmare,” he says. The interface mate- the device can be seen in Figure 1. “One of the greatest advantages is that the concept is so rials would also have to handle much greater forces because, at a given pressure, the force increases as the surface area increas- simple that you have a fully integrated valving component with es. Therefore, some microscale devices made of glass or silicon no need to do anything once you etch the channels,” says can handle extremely high pressures, possibly up to 10,000 Lazar. The technique does have drawbacks. “The major limitation atm. Second, check valves are needed to generate high pressures on a chip; they allow liquid to flow in only one direction is that you need to have EOF if you want the valve to be open,” through a pump. Third, not many valve technologies can han- says Lazar. This requirement should not be a problem with dle high pressures. For example, poly(dimethylsiloxane) many LC applications, but it may limit the valve’s usefulness in (PDMS), hydrogels, and other soft polymers would most likely other areas. 430 A
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Microball valves Ball valves are often used in macroscale pumps as check valves. As liquid is pumped, it flows around a ball, but when fluid begins to travel back into the pump, the ball is carried along and plugs the opening. Researchers at Sandia have developed a technique to produce monolithic polymer structures inside microscale channels that act like balls in a ball valve. A schematic and image of the valve can be seen in Figure 2. “I jokingly refer to it as the ship in the bottle method,” says Hasselbrink, who was at Sandia during the technique’s development. “You fill the whole [chip] up with a monomer solution and say, ‘Well, where do I want a piece of polymer in here?’ and you can use a mask or a projection to define the shape of it,” he says. “It’s the interplay between the shape of the polymer that you put in and the shape of the channel that gives you the functionality.” The advantage of the technique is that the classes of polymers that can be used can have a wide array of properties. The polymers can be porous to relatively nonporous, charged or uncharged, and rigid to soft. In addition, the technique could be used on any substrate that is UV-transparent enough to allow photopolymerization and can handle the solvents used for the monomers. But can the valves handle high pressures? Hasselbrink answers, “The first piston we tested could hold off 3400 psi [231 atm] on the first try.” However, work is still needed. Hasselbrink says, “If you can do it 10,000 times and get every single valve to work, that would obviously be an improvement. Yield is the big word.” The researchers are also looking at actuating the valves from off the chip, either electrically or by another chip layer that contains control lines.
Large-scale integration The use of massive amounts of transistors has become the basis of modern computer processors, but can similar massively parallel structures be used in microfluidics? The answer to this question appears to be yes when one considers what Stephen Quake and colleagues at California Institute of Technology have done with PDMS valves. In developing their valves, the researchers hit upon a design that could be used for many applications. “Other actuation techniques all have merit for particular applications,” says Quake. “However, they tend to either be slower or have other restrictions that make them a less general solution for all-purpose use.” The valves are created by an ingenious manufacturing system called multilayer soft lithography, which uses at least two different levels of PDMS that are patterned by means of a mold that is typically made out of silicon. The first layer forms the fluid channels. Two or more monomers are used in the polymer, but an excess of one monomer is included in this first layer. The second layer forms the control channels but has an excess of the second monomer. When the two layers are joined and cured, the excess of the two monomers cross-link, fusing the layers together. Where the control channels pass above the fluid channels, a valve is formed. This can be seen in Figure 3. If air pressure is applied to the control channel, the small amount of polymer in between the two channels deforms, cutting off the fluid flow.
FIGURE 2. Monolithic “ball valve”. (a) Schematic of a passive check valve showing the valve (left) open and (right) closed. An image of the valve (b) open and (c) closed.
Quake and colleagues have placed thousands and plan to place tens of thousands of valves on chips made with this technique. “Our challenge going forward is to stay on the ‘Moore’s law’ curve,” says Quake, mentioning the rule of thumb that the number of transistors per square inch on computer chips doubles every 18 months. “We are doubling valve densities four times as fast as he predicted,” he adds. Fluidigm is commercializing the process and currently offers the Topaz protein crystallization system. The device uses 3 mL of purified protein and reacts the protein with up to 48 different reagents, each of which can be supplied at one of several concentrations. According to the company, more crystals are formed more quickly than by using vapor diffusion or microbatch techniques. Like all valve technologies, this technique has limitations. The PDMS that is used to make the system cannot handle the high pressures needed for LC, may not work with strong solvents, and may have limits to how fast valves can respond, say researchers. However, there is widespread praise for Quake’s work: “I wish I could put 1000 of [my valves] on a chip like Quake can,” says Hasselbrink.
On-chip actuation Researchers have different opinions on whether valves and valve actuation are best located on- or off-chip; however, the key to O C T O B E R 1 , 2 0 0 3 / A N A LY T I C A L C H E M I S T R Y
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making the smallest complete microfluidic devices clearly lies in incorporating valves and actuation methods inside the chip. Some may want external valves or pumps, and almost all other on-chip valves are actuated externally; but in order to have very small microfluidic devices, actuation must occur within a device. “One of the problems with a lot of microsystems is that they aren’t micro because they require external power, external pumps, or external valves,” says Beebe. Hydrogel valves may serve this need because some hydrogel materials—dubbed “smart” or, more accurately, stimuli-responsive hydrogels—swell or shrink depending on surrounding conditions. “It actually gives you the ability to make microcomponents, which can both sense and actuate,” says Beebe, “and they get their power directly from chemical to mechanical conversions.” Such valves can be seen in Figure 4. In order to achieve the same sensing and actuation with other valve techniques, more parts are needed, like sensors, electronics to interpret them, and actuators. However, hydrogels do have limitations, mainly because of the properties of the hydrogel. Diffusion can occur through hydrogels because they are porous, and when they swell, they can adsorb sample, which may become stuck within the gel. To address these concerns, Beebe and other researchers cover the hydrogel with a thin layer of PDMS. This approach isolates the hydrogel from the sample and reagents; however, it also removes the “smart” actuation because the hydrogel doesn’t contact the sample fluid. Hydrogels, whether covered with PDMS or not, also cannot handle the high pressures needed for LC. Hydrogels are commonly thought to have slow response times; however, the rate
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FIGURE 4. (a) Automated response of a hydrogel. In this image, the hydrogel valve is open. (b) With a change in conditions such as pH or temperature, the valve will close.
at which hydrogels expand increases as they get smaller because expansion speed is based on diffusion. “We have demonstrated valves down in the second time response range, which is not lightning fast, but for many microfluidic applications, a second is just fine,” says Beebe.
Other answers Control channels
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Isolation of reagents
(b) Mixing by rotary pump
Mixed solution
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FIGURE 3. Microfluidic valve matrix. (a) The pneumatic control channels in this two-layer system have been dyed green. At each location where the green channels cross the undyed channels, a valve is formed. (b) Reagents can be loaded without cross-contamination by selectively closing some valves. (c and d) Reaction areas are separated from each other by closed valves, and the reagents are mixed.
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Many other valve technologies are being developed, and some are already in use. One of the simplest is the hydrophobic/hydrophilic valve, which is simply a surface treatment that prevents a liquid from crossing a threshold. If the pressure on the fluid is increased, it overcomes the surface repulsion and goes through the valve. However, once the liquid overcomes the surface repulsion, the valve remains open until it dries out. Several companies are using these valves, including Gyros (Sweden), Tecan, and iStat. An interesting new technology is freeze/thaw valves, which appear in various configurations. Some freeze water, which creates a plug, while others use paraffin, which is heated to open the valve. “Few if any valves are going to be the valve of all applications,” says Beebe. However, they are much more likely to enable the realization of microscale laboratories than the expensive, complex valves first developed. Michael J. Felton is an associate editor of Analytical Chemistry.
