Chemistry in Microfluidic Channels - Journal of Chemical Education

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Chemistry in Microfluidic Channels Matthew C. Chia and Christina M. Sweeney Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, United States Teri W. Odom* Department of Chemistry and Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208-3113, United States *[email protected]

Microfluidics;the manipulation of fluid streams in microscale dimensions;is of growing technological interest because of its diverse and modern applications, such as medical diagnostics and the environmental monitoring of contaminants (1, 2). Microfluidic devices are becoming increasingly important because (i) they require only small reaction (and hence reagent) volumes (nano- to picoliters) and (ii) their narrow channel diameters (10-500 μm) facilitate efficient analytical systems (3, 4). For example, microfluidic channels have been used to deliver samples for gas and liquid chromatography (2-4), immunoassays (5, 6), DNA manipulation (7, 8), and proteomic analysis (1, 9, 10). Lab-on-a-chip or micro-total-analysis-system (μTAS) devices based on microfluidic networks can be viewed as high-throughput, miniaturized, mobile laboratories (2-4). At these smaller length scales, some physical properties are different from the analogous bulk properties. Fluid flow within microfluidic channels, for example, is laminar, which means that parallel fluid streams in the same channel will flow independently of each other with little or no mixing at the interface (11). Practically, however, diffusion across adjacent streams allows reagents from each stream to react. A microfluidic channel therefore provides a useful platform for performing confined chemical reactions and demonstrating the dynamics of fluid flow in small volumes. In this laboratory exercise, students can visualize chemical reactions under laminar flow in a microfluidic device and compare the results to those in bulk solutions. The main barrier to introducing microfluidics concepts into the classroom has been the fabrication of the microfluidic device. Typical methods rely on photolithography (12) or chemical etching of glass (13, 14), but the cost, time, and safety concerns of a teaching laboratory can prohibit their use. We present a simple bench-top method that uses readily accessible materials and soft lithography techniques (15, 16) to fabricate microfluidic channels. The activity was devised and tested by an undergraduate student over 3 years and can be performed in two, 2-h laboratory sessions. The first session is used to fabricate microfluidic channels, and the second session is used to demonstrate laminar flow and confined chemistry using three different chemical systems. The laboratory has been incorporated into a nonmajors chemistry elective course, and the channels have been used as a demonstration platform for nanoscience-based workshops. This approach offers an effective teaching platform to explore the intersection among chemistry, biology, and materials science principles. Fabrication of Microfluidic Device The limitations of conventional microfluidic device fabrication can be overcome by (i) using available materials and equipment

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Figure 1. Scheme of the fabrication of the master mold: (A) manual arrangement of In wire to form the Y-shaped channel master; (B) preparation of the wire structure by heating and flattening; (C) PDMS molding of the stamp to generate the master mold; (D) removal of the master mold from the channel master; and (E) optical image of the PDMS master mold.

to make the microchannel master, such as In wire, glass slides, and a hot plate; (ii) using inexpensive polymers, poly(dimethylsiloxane) (PDMS), polyurethane (PU), and photothermally cured epoxy, to replicate the device and form the inlets and outlet of the channels; and (iii) molding techniques to transfer the master into stable and reusable templates. Successive replications can be used to generate robust “daughter masters” and enables scalability to a large number (over 20) of students. Microfluidic Channel Master A suitable channel master can be made with In wire and replica molded in PDMS to generate a reusable “master mold” (Figure 1). An In master is fabricated by manually placing pieces of 250 μm thick In wire onto a glass slide in a Y-shape (Figure 1A). Adhesion between the In and the glass can be increased by placing the substrate on a hot plate at ∼150 °C, which is close to the melting point of In. To create channels with a rectangular cross-section, pressure is applied during heating by placing another glass slide with a 1.4 kg Al block (15 cm 9 cm

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r 2011 American Chemical Society and Division of Chemical Education, Inc. pubs.acs.org/jchemeduc Vol. 88 No. 4 April 2011 10.1021/ed1008624 Published on Web 02/14/2011

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Journal of Chemical Education

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In the Laboratory

Figure 3. Mixing of food coloring. Visualization of laminar flow using water dyed with red and blue food coloring. Images were captured with a standard digital camera through the eyepiece of a compound microscope.

placed into conformal contact with a glass slide to complete the microfluidic channel system. Chemical Reactions at the Fluid-Fluid Interface We selected reactions that are common in general chemistry curricula so that students could easily recognize how the microfluidic channel manipulates fluid flow. These systems include (i) a visualization of laminar flow at the interface between two dye-colored fluid streams; (ii) an acid-base reaction at the fluidfluid interface; and (iii) a precipitation reaction of calcium carbonate (calcite) crystals at the fluid-fluid interface. Figure 2. Scheme of microfluidic device generation: (A) placement of polyurethane (PU) prepolymer on the glass slide; (B) replica molding of the master mold in PU; (C) formation of the daughter master following UVcure of the PU; (D) generation of the inlets and outlet by application of epoxy to form the microfluidic channel master; (E) PDMS mold of the channel after removal from the master to form the final microfluidic channel; and (F) optical image of the assembled microfluidic device, a glass slide below the PDMS mold completes the device.

3.7 cm) on the In wire (Figure 1B). PDMS is then poured over the In master and cured to form a microfluidic channel mold, referred to as the “master mold” (Figure 1C). The master mold is then removed from the substrate using a razor blade (Figure 1D). The optical image of the finished master mold is shown in Figure 1E. Step-by-step details and pictures of these procedures can be found in the supporting information. Microfluidic Device Generation of the final microfluidic channel and assembly of the device are illustrated in Figure 2. Although the In wire provides a convenient template material to create microfluidic channels, the metal is extremely fragile. Hence, structurally robust replicas or daughter masters of this Y-channel template can be created out of PU by replica molding using the PDMS master mold in Figure 1E (Figure 2A-C). Next, two inlets and one outlet are formed by placing 5 minute, thermally cured epoxy at the ends of the channels (Figure 2D). The combined structure of the PU channel and the epoxy inlets and outlets is referred to as the “microfluidic channel master”. PDMS is molded against this master and the resulting PDMS mold is removed from the substrate (Figure 2E). Holes for the inlets and outlet are punched into the PDMS with a needle, and tubing is inserted and fixed into place with 5 minute epoxy (Figure 2F). This structure is then 462


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Visualization of Laminar Flow The first experiment is analogous to food coloring added dropwise into a beaker of water, where a three-dimensional mixing pattern is observed. Here, mixing in a microfluidic channel can be visualized with two colored solutions of water. The red solution is introduced into inlet 1 and the blue solution into inlet 2. These solutions are pressure-driven through the microfluidic device by a syringe pump at 10 mL/h. A single syringe pump is used to drive both solutions to ensure equal flow rates of each solution in the channel. The image in Figure 3 shows that the solutions remain separate upon injection and through the entire length of the channel (30 mm). Fluid streams in a microfluidic channel exhibit laminar flow and do not display turbulent mixing. Mixing only occurs at the outlet, where the two fluid streams combine to form a purple solution. Diffusion across Fluid-Fluid Interface Although laminar flow results in physically separated fluid streams, some diffusion occurs at the interface between the streams. The interaction of reactants at this interface can be demonstrated with an acid-base reaction. Students expect a colorimetric change throughout the solution when an acid reacts with a base in the presence of an indicator. Here, a colorless acid solution (HCl, 0.05 M) with phenolphthalein (C20H14O4) indicator is introduced into inlet 1, and a colorless basic solution (NaOH, 0.1 M) into inlet 2. A pink line down the middle of the channel indicates interaction of the reagent solutions at the laminar flow interface (Figure 4). The width of the pink region is wider toward the outlet because of increased mixing across the two fluid streams. In a microfluidic channel, the colorimetric change is limited to the interface between the laminar flow streams.

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r 2011 American Chemical Society and Division of Chemical Education, Inc.

In the Laboratory

Figure 4. Acid-base reaction. Neutralization of an acid (0.05 M HCl, phenolphthalein) with a base (0.1 M NaOH) is localized only to the interface between the two fluid streams. Images were captured with a standard digital camera through the eyepiece of a compound microscope.

bicarbonate (NaHCO3, 100 mM) into inlet 2. Upon mixing, calcite crystals (8 μm ( 3 μm) precipitate out of solution but are localized only to the fluid-fluid interface (Figure 5A). Because solutions in a microfluidic channel only mix as a result of diffusion across the fluid-fluid interface, the rate of reaction is significantly slower. With this system, it is possible to see the process of crystal nucleation on the order of seconds instead of the perceived instantaneous reaction under bulk conditions. Different flow rates can be investigated to demonstrate control over ion diffusion and crystal nucleation. At 10 mL/h, only a small degree of mixing between the fluid streams can occur, and only a narrow region of calcite forms (Figure 5B). The region of calcite deposition has a similar width to the tightly confined pink line from the second experiment. At a slower flow rate of 5 mL/h, there is more time for the ionic species to diffuse across the fluid-fluid interface. This effect can be seen by the increased width of the imaged calcite crystals (Figure 5B). At the slowest flow rate of 1 mL/h, the degree of diffusion between the fluid streams increases dramatically to result in a band (80 μm) of calcite deposition. Hazards Students must be careful when using the razor blades, scalpel blades, and needles. Eye protection should be worn when the UV source is active. Adequate ventilation is needed because poly(dimethylsiloxane) and epoxy give off fumes while curing. Poly(dimethylsiloxane), polyurethane, and epoxy may cause irritation to skin, eyes, and respiratory tract and may be harmful if swallowed or inhaled. Hydrochloric acid and sodium hydroxide are corrosive and can cause burns to any area of contact. Summary

Figure 5. Calcite precipitation: (A) precipitation of calcium carbonate (calcite) crystals at the interface of CaCl2 (25 mM) and NaHCO3 (100 mM). Flow rate of 5 mL/h. Images were captured with a standard digital camera through the eyepiece of a compound microscope. (B) Scanning electron microscopy (SEM) images of crystals resulting from three different flow rates: 10, 5, and 1 mL/h.

Controlled Precipitation of Calcite Crystals In general chemistry, nonsoluble salts are precipitated from aqueous solutions to (i) demonstrate reactions between ionic species; (ii) show how nucleation events in solution can induce product formation; and (iii) illustrate how products can have solubilities different from the reagents. Bulk reactions, however, do not easily provide size control over the precipitates that form, and the precipitates cannot be confined to specific locations within a reaction vessel. The third experiment is based on the insolubility of calcium carbonate crystals in an aqueous solution, where the ionic components of the crystals are solubilized in separate flow streams. An aqueous solution of calcium chloride (CaCl2, 25 mM) is introduced into inlet 1, and an aqueous solution of sodium


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We have presented a method to perform chemical reactions under laminar flow in a microfluidic channel. The laboratory allows students to contrast the behavior of the reactions in the channel with bulk behavior. The fabrication procedure is simple and inexpensive, which allows students to make a microfluidic device without the complexity or cost of traditional photolithographic methods. In the future, instructors could use the device or the fabrication technique in more advanced classes for lowvolume sample handling or lab-on-a-chip detection systems. Acknowledgment We thank Christopher L. Stender for help in optimizing the chemical reactions and characterizing the calcite crystals. This work was supported by the NSF REU Site for nanoscale science and engineering (EEC-0755375) and the NSF National Center for Learning and Teaching (NCLT) (ESI-0426328) at the Materials Research Institute of Northwestern University. This work used the NUANCE and IMSERC Center facilities, which are supported by NSF-MRSEC, NSF-NSEC, and the Keck Foundation. Literature Cited


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3. Reyes, D. R.; Iossifidis, D.; Auroux, P.-A.; Manz, A. Micro Total Analysis Systems. 1. Introduction, Theory, and Technology. Anal. Chem. 2002, 74 (12), 2623–2636. 4. Terry, S. C.; Jerman, J. H.; Angell, J. B. A Gas Chromatographic Air Analyzer Fabricated on a Silicon Wafer. IEEE Trans. Electron Devices 1979, 26 (12), 1880–1886. 5. Yang, T.; Jung, S.-y.; Mao, H.; Cremer, P. S. Fabrication of Phospholipid Bilayer-Coated Microchannels for On-Chip Immunoassays. Anal. Chem. 2000, 73 (2), 165–169. 6. Hatch, A.; Kamholz, A. E.; Hawkins, K. R.; Munson, M. S.; Schilling, E. A.; Weigl, B. H.; Yager, P. A Rapid Diffusion Immunoassay in a T-Sensor. Nat. Biotechnol. 2001, 19 (5), 461– 465. 7. Liu, J.; Enzelberger, M.; Quake, S. A Nanoliter Rotary Device for Polymerase Chain Reaction. Electrophoresis 2002, 23 (10), 1531– 1536. 8. Fujii, T. PDMS-Based Microfluidic Devices for Biomedical Applications. Microelectron. Eng. 2002, 61-62, 907–914. 9. Vilkner, T.; Janasek, D.; Manz, A. Micro Total Analysis Systems. Recent Developments. Anal. Chem. 2004, 76 (12), 3373–3386. 10. Auroux, P.-A.; Iossifidis, D.; Reyes, D. R.; Manz, A. Micro Total Analysis Systems. 2. Analytical Standard Operations and Applications. Anal. Chem. 2002, 74 (12), 2637–2652.

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Supporting Information Available A laboratory worksheet with a detailed step-by-step procedure and images. This material is available via the Internet at http://pubs.acs.org.


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Chemistry in Microfluidic Channels - Journal of Chemical Education

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