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Langmuir 2002, 18, 948-951
Surface-Tension-Confined Microfluidics Philippe Lam,† Kenneth J. Wynne,* and Gary E. Wnek Department of Chemical Engineering, Virginia Commonwealth University, Richmond, Virginia 23284 Received April 23, 2001. In Final Form: November 13, 2001
strates permit economical fabrication compared with conventional microfluidic devices. There are a number of methods for producing patterned surfaces of differential wettability, including masking, stamping, and direct writing, all of which are currently available and frequently used. In the following sections we present several examples of STCM devices generated by patterning with a Hewlett-Packard plotter and modified pen. A theoretical description of STCM is also provided.
Introduction
Experimental Section
Traditionally, microfluidic devices are constructed by photolithographic methods using silicon or glass substrates,1 by rapid prototyping of poly(dimethylsiloxane) elastomers,2 or by conventional machining.3 All these techniques produce three-dimensional channels for confining and guiding fluids in the device. Also reported is the use of hydrophobic patterning for controlling fluid motion inside flow channels4 or to confine liquids inside microwells.5 The recent work of Oh describes an alternative method for generating microfluidic devices by depositing hydrophobic “curbs” on flat glass substrates.6 Examples described by Oh focus on capillary electrophoresis. In addition, a range of patterning processes that include photolithographic methods is described by Zhao et al.7 This work anticipates a broader span of applications. We have independently developed a similar approach for guiding fluids on micropaths. We have demonstrated that this new and simple microfluidics fabrication method, which we term surface-tension-confined microfluidics (STCM), can be used to produce useful devices. STCM exploits the principle of wetting behavior and capillarity in order to guide liquids along 2-D paths with no sidewalls. By application of a hydrophilic pattern on a hydrophobic substrate, or conversely, by application of a hydrophobic pattern on a hydrophilic substrate, selective wetting by an aqueous solution is achieved. If two surfaces presenting mirror image patterns are stacked one atop the other, separated by a thin gap, specific flow paths are obtained. The aqueous liquid moves between the surfaces by capillary forces, confined to the hydrophilic areas by surface tension. With water and other high surface tension liquids, the hydrophobic regions act as “curbs”, achieving liquid confinement without physical sidewalls. Very intricate flow patterns, dead-ended flow paths, bubblefree filling of large chambers, and multilayer configurations are easily obtained with no necessity for air vents. Furthermore, the two-dimensional nature of STCM devices and the ability to use inexpensive polymeric sub-
Isopropyl alcohol and dibutyltin diacetate were obtained from Aldrich (Milwaukee, WI). Poly(ethoxysiloxane) “ES-50” and (tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane were from Gelest (Tullytown, PA). All reagents were used as received. Optically transparent polypropylene plaques (resin PD 9505E1, 2 mm thickness) were obtained from Exxon Mobil Chemical Co. Hydrophilic Patterns on Hydrophobic Surfaces. Patterning hydrophilic regions on 2 mm thick polypropylene substrates of various sizes was accomplished by modifying the pen on a HP 7475A plotter. A standard plotter pen was retrofitted with a new felt tip nib (spare nibs for TRIA pens, available in art stores) and loaded with “reactive ink”, RI. The ink was comprised of 9% w/w of poly(diethoxysiloxane) (ES-50) in 2-propanol with 2 µL/g dibutyltin diacetate catalyst. ES50 is an oligo-ethoxysiloxane mixture with the approximate formula (SiO(OEt)2)n8,9,10 and may be used as a reagent for cure in “sol-gel” processes. ES50 is a less-volatile siliceous precursor compared to tetraethoxysiloxane (TEOS).11 The cure of ES50 generates a thin glass film of approximate composition SiO2-x(OH)x, where x is about 0.5 for ambient temperature cure.9 The substrate was attached to a piece of paper at a predetermined position by double-sided tape and inserted into the plotter. The pattern was drawn on a computer and plotted on the substrate by means of the HP 7475A plotter. The pattern generated by this process was precise and reproducible. That is, there was no visual difference among multiple patterns. The deposited nanofilm was allowed to cure at 37 °C and 100% relative humidity (RH) for about 1 h. Surprisingly, the treated areas were not hydrophilic until the patterned plaques were dipped into aqueous 6 M hydrochloric acid, typically for less than 30 s. After immersion in HCl, the patterned plaque was rinsed a number of times with deionized water. It is evident visually during rinsing when differential wetting has been achieved, as water wets the pattern and avoids the substrate. The patterns were also observable by optical microscopy. Hydrophobic Patterns on Hydrophilic Surfaces. Parts of the surface that were intended to remain hydrophilic were protected by hand-cut segments of 3M Scotch Magic Tape. Hydrophobic areas on the microscope cover glass slide were formed by rubbing a laboratory tissue paper (Kimwipe) moistened with a 3% (w/w) solution of tridecafluoro-1,1,2,2-tetrahydrooctyl trichlorosilane in 2-propanol. After overnight cure at 40 °C and 100% RH, the tape “mask” was removed. The slide was rinsed with deionized water. Flow Visualization. Patches of adhesive tape (3M Magic Tape, 60 µm, 3M double-sided tape, 90 µm, or 3M black electrical tape, 180 µm) were attached as spacers. A micrometer was used to measure thickness. The patches, ca. 1 × 2 mm on edge, were positioned at multiple locations on the substrate to maintain the top and bottom surfaces at a fixed distance. The top surface was
* Corresponding author. † Current address: Genentech Inc., 1 DNA Way, South San Francisco, CA 94080. (1) Burns, M. A.; Johnson, B. N.; Brahmasandra, S. N.; Handique, K.; Webster, J. R.; Krishnan, M.; Sammarco, T. S.; Man, P. M.; Jones, D.; Heldsinger, D.; Mastrangelo, C. H.; Burke, D. T. Science 1998, 282, 484-487. (2) Duffy, D. C.; McDonald, J. C.; Shueller, O. J. A.; Whitesides, G. M.; Anal. Chem. 1998, 70, 4974-4984. (3) Duffy, D. C.; Gillis, H. L.; Lin, J.; Sheppard, N. F.; Kellogg, G. J. Anal. Chem. 1999, 71, 4669-4678. (4) Handique, K.; Burk, D. T.; Mastrangelo, C. H.; Burns, M. A. Anal. Chem. 2000, 72, 4100-4109. (5) Jackman, R. J.; Duffy, D. C.; Ostuni, E.; Wilmore, N. D.; Whitesides, G. M. Anal. Chem. 1998, 70, 2280-2287. (6) Oh, C. S. United States Patent 5,904,824; May 18, 1999. (7) Zhao, B.; Moore, J. S.; Beebe, D. J. Science 2001, 291, 1023-1026.
(8) Cihla´r, J. Colloids Surf., A 1993, 70, 253. (9) Ho T.; Honeychuck, R. V.; Wynne, K. J. ES40 manuscript in preparation. (10) The empirical formula is calculated from the Si analysis given in the Gelest catalog: Gelest Inc., 612 William Leigh Dr., Tullytown, PA 19007-6308. (11) Uilk, J.; Bullock, S.; Johnston, E.; Myers, S. A.; Merwin, L.; Wynne, K. J. Macromolecules 2000, 33, 8791-8801.
10.1021/la010589v CCC: $22.00 © 2002 American Chemical Society Published on Web 01/10/2002
Notes
Langmuir, Vol. 18, No. 3, 2002 949
Figure 1. (A) Schematic of a STCM device, showing mirror-image patterns. (B) Photograph of the actual device constructed from clarified polypropylene plates filled with food-dye-colored deionized water for visualization. Double-sided tape provided a 90 µm spacer between the two surfaces. placed on the spacers, slightly offset in relation to the bottom surface (see Figure 1). The inlet was located at the edge of the top plate. A few drops of food colorant dye were added to deionized water to facilitate observation of flow. To investigate flow, small drops (
