Anal. Chem. 2001, 73, 4045-4049
Technical Notes
Electroosmotically Induced Hydraulic Pumping with Integrated Electrodes on Microfluidic Devices Timothy E. McKnight, Christopher T. Culbertson, Stephen C. Jacobson, and J. Michael Ramsey*
Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6142
Electroosmotic manipulation of fluids was demonstrated using thin metal electrodes integrated within microfluidic channels at the substrate and cover plate interface. Devices were fabricated by photolithographically patterning electrodes on glass cover plates that were then bonded to polymeric substrates into which the channels were cast. Polymeric substrates were used to provide a permeable membrane for the transport and removal of gaseous electrolysis products generated at the electrodes. Electroosmotic flow between interdigitated electrodes was demonstrated and provided electric field-free pumping of fluids in sections of the channel outside of the electrode pairs. The resultant pumping velocities were shown to be dependent on the applied voltage, not on the applied field strength, and independent of the length of the electroosmotically pumped region. Presently, the electrokinetic manipulation of fluids in microchannel structures represents the state-of-the-art in controlled, high-precision, small-volume handling. In recent years, electrokinetic manipulations have been demonstrated for mixing and reacting reagents,1-6 the injection or dispensing of samples,2,7-11 and chemical separations.7,8,12-20 In contrast to the use of hydraulic forces for fluid transport, electrokinetic forces are generally more (1) Jacobson, S. C.; Koutny, L. B.; Hergenro ¨der, R.; Moore, A. W., Jr.; Ramsey, J. M. Anal. Chem. 1994, 66, 3472-3476. (2) Jacobson, S. C.; Hergenro¨der, R.; Moore, A. W., Jr.; Ramsey, J. M. Anal. Chem. 1994, 66, 4127-4132. (3) Mangru, S. D.; Harrison, D. J. Electrophoresis 1998, 19, 2301-2307. (4) Chiem, N. H.; Harrison, D. J. Clin. Chem. 1998, 44, 591-598. (5) Kutter, J. P.; Jacobson, S. C.; Ramsey, J. M. Anal. Chem. 1997, 69, 51655171. (6) Kutter, J. P.; Jacobson, S. C.; Matsubara, N.; Ramsey, J. M. Anal. Chem. 1998, 70, 3291-3297. (7) Jacobson, S. C.; Hergenro ¨der, R.; Koutny, L. B.; Ramsey, J. M. Anal. Chem. 1994, 66, 1114-1118. (8) Jacobson, S. C.; Hergenro ¨der, R.; Koutny, L. B.; Warmack, R. J.; Ramsey, J. M. Anal. Chem. 1994, 66, 1107-1113. (9) Seiler, K.; Harrison, D. J.; Manz, A. Anal. Chem. 1993, 65, 1481-1488. (10) Seiler, K.; Fan, Z. H.; Fluri, K.; Harrison, D. J. Anal. Chem. 1994, 66, 34853491. (11) Fan, Z. H.; Harrison, D. J. Anal. Chem. 1994, 66, 177-184. (12) Effenhauser, C. S.; Paulus, A.; Manz, A.; Widmer, H. M. Anal. Chem. 1994, 66, 2949-2953. (13) Effenhauser, C. S.; Manz, A.; Widmer, H. M. Anal. Chem. 1993, 65, 26372642. (14) Harrison, D. J.; Manz, A.; Fan, Z.; Luedi, H.; Widmer, H. M. Anal. Chem. 1992, 64, 1926-1932. 10.1021/ac010048a CCC: $20.00 Published on Web 07/10/2001
© 2001 American Chemical Society
convenient to implement and can generate higher effective pressures than is practical with hydraulic flows. Electrokinetically driven forces also follow the flow of electrical current and, thus, allow greater control over fluid transport within subregions of a microchannel manifold versus the application of pressure or vacuum to the termini of such a manifold. To date, most applications of electrokinetically driven transport have been demonstrated by applying voltages to the termini, or reservoirs, of microfluidic manifolds. The ability to provide voltage potentials to more limited regions of the microfluidic manifold, as opposed to entire channel lengths, could enable much more robust control of electrokinetic manipulations. Although methods for integration of electrodes directly into channels have been reported, the use of these devices has proven problematic mostly because of the generation and accumulation of gaseous electrolysis products in the channels at the electrodes during electrokinetic manipulations.21 Although most microchip devices that have been fabricated to date use glass or silicon substrates, there are also several reports of devices fabricated from a variety of polymeric substrates22 including poly(dimethylsiloxane) (PDMS),23-27 poly(methyl methacrylate) (PMMA),28,29 acrylic30 and polycarbonate (15) Harrison, D. J.; Fluri, K.; Seiler, K.; Fan, Z.; Effenhauser, C. S.; Manz, A. Science 1993, 261, 895-897. (16) Jacobson, S. C.; Hergenro ¨der, R.; Koutny, L. B.; Ramsey, J. M. Anal. Chem. 1994, 66, 2369-2373. (17) Woolley, A. T.; Mathies, R. A. Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 11348-11352. (18) Woolley, A. T.; Mathies, R. A. Anal. Chem. 1995, 67, 3676-3680. (19) von Heeren, F.; Verpoorte, E.; Manz, A.; Thormann, W. Anal. Chem. 1996, 68, 2044-2053. (20) Moore, A. W., Jr.; Jacobson, S. C.; Ramsey, J. M. Anal. Chem. 1995, 67, 4184-4189. (21) Macounova, K.; Cabera, C. R.; Holl, M. R.; Yager, P. Anal. Chem. 2000, 72, 3745-3751. (22) Soper, S. A.; Ford, S. M.; McCarley, R. L.; Kelly, K.; Murphy, M. C. Anal. Chem. 2000, 72, 643A-651A. (23) Hosokawa, K.; Fujii, T.; Endo, I. Anal. Chem. 1999, 71, 4781-4785. (24) Martin, R. S.; Gawron, A. J.; Lunte, S. M. Anal. Chem. 2000, 72, 31963202. (25) Ocvirk, G.; Munroe, M.; Tang, T.; Oleschuk, R.; Westra, K.; Harrison, D. J. Electrophoresis 2000, 21, 107-115. (26) Effenhauser, C. S.; Bruin, G. J. M.; Paulus, A.; Ehrat, M. Anal. Chem. 1997, 69, 3451-3457. (27) Duffy, D. C.; McDonald, J. C.; Schueller, O. J. A.; Whitesides, G. M. Anal. Chem. 1998, 70, 4974-4984. (28) Martynova, L.; Locascio, L. E.; Gaitan, M.; Kramer, G. W.; Christensen, R. G.; MacCrehan, W. A. Anal. Chem. 1997, 69, 4783-4789. (29) Ford, S. M.; McWhorter, S.; Davies, J.; Soper, S. A.; Klopf, M.; Calderon, G.; Saile, V. J. Microcolumn Sep. 1998, 10, 413-422.
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(PC).31 The interest in polymeric microfluidic devices stems primarily from the fact that plastic chips may be less expensive to produce. Lower production costs are expected because microfeatures are easier to form in polymeric materials than in either glass or silicon, because these materials lend themselves readily to laser ablation, injection molding, extrusion, casting, and machining.28 Some polymeric substrates may also have an additional advantage not discussed previously, high gas permeability rates. In this paper, we describe the performance of microfluidic devices that incorporated thin metal electrodes that were in direct contact with the microfluidic channel at multiple points. This design enabled the electroosmotic flow of the fluid between individual electrode pairs and provided hydraulic pumping of the fluid in field-free segments. To minimize the accumulation of electrolytically generated gas bubbles, gas permeable polymeric channel substrates were employed that allowed the rapid transport of gas-phase products away from the fluidic channels during electrokinetic operations. EXPERIMENTAL SECTION Microchips consisting of a poly(dimethylsiloxane) (PDMS, Sylgard 184, Dow Corning Corp., Midland, MI) substrate and a glass cover plate (1 mm, soda lime, Telic, Santa Monica, CA) were fabricated as described previously27 with the addition of photolithographic electrode patterning on the mating surface of the glass cover plate. Linear channels were cast into
