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Rapid Prototyping of Thermoset Polyester Microfluidic Devices Gina S. Fiorini, Robert M. Lorenz, Jason S. Kuo, and Daniel T. Chiu*
Department of Chemistry, University of Washington, Seattle, Washington 98195-1700
This paper presents a simple procedure for the fabrication of thermoset polyester (TPE) microfluidic systems and discusses the properties of the final devices. TPE chips are fabricated in less than 3 h by casting TPE resin directly on a lithographically patterned (SU-8) silicon master. Thorough curing of the devices is obtained through the combined use of ultraviolet light and heat, as both an ultraviolet and a thermal initiator are employed in the resin mixture. Features on the order of micrometers and greater are routinely reproduced using the presented procedure, including complex designs and multilayer features. The surface of TPE was characterized using contact angle measurements and X-ray photoelectron spectroscopy (XPS). Following oxygen plasma treatment, the hydrophilicity of the surface of TPE increases (determined by contact angle measurements) and the proportion of oxygen-containing functional groups also increases (determined by XPS), which indicates a correlated increase in the charge density on the surface. Native TPE microchannels support electroosmotic flow (EOF) toward the cathode, with an average electroosmotic mobility of 1.3 × 10-4 cm2 V-1 s-1 for a 50-µm square channel (20 mM borate at pH 9); following plasma treatment (5 min at 30 W and 0.3 mbar), EOF is enhanced by a factor of 2. This enhancement of the EOF from plasma treatment is stable for days, with no significant decrease noted during the 5-day period that we monitored. Using plasmatreated TPE microchannels, we demonstrate the separation of a mixture of fluorescein-tagged amino acids (glycine, glutamic acid, aspartic acid). TPE devices are up to 90% transparent (for ∼2-mm-thick sample) to visible light (400-800 nm). The compatibility of TPE with a wide range of solvents was tested over a 24-h period, and the material performed well with acids, bases, alcohols, cyclohexane, n-heptane, and toluene but not with chlorinated solvents (dichloromethane, chloroform). Although microfabricated systems in glass are robust, reusable, and well-established in microfluidics research,1-10 recent advances * To whom correspondence should be addressed. E-mail: chiu@ chem.washington.edu. (1) Harrison, D. J.; Manz, A.; Fan, Z.; Ludi, H.; Widmer, H. M. Anal. Chem. 1992, 64, 1926-1932. (2) Manz, A.; Harrison, D. J.; Verpoorte, E. M. J.; Fettinger, J. C.; Paulus, A.; Ludi, H.; Widmer, H. M. J. Chromatogr. 1992, 593, 253-258. (3) Culbertson, C. T.; Jacobson, S. C.; Ramsey, J. M. Anal. Chem. 1998, 70, 3781-3789. 10.1021/ac0498922 CCC: $27.50 Published on Web 07/10/2004
© 2004 American Chemical Society
have been facilitated by the ability to rapid prototype devices in poly(dimethylsiloxane) (PDMS).11-19 Simple planar systems are easily replicated and produced in PDMS, and by using multilayer rapid prototyping approaches, it is also straightforward to fabricate complex three-dimensional structures and networks of microchannels.20-24 Multilayer PDMS devices have been used for a variety of applications, including cell sorting and patterning,24,25 chemical reactors,26 solving computational problems,27,28 chaotic mixers,29 gradient generators,30,31 and large-scale microfluidic networks.32 The elastomeric nature of PDMS lends well to the (4) Culbertson, C. T.; Jacobson, S. C.; Ramsey, J. M. Anal. Chem. 2000, 72, 5814-5819. (5) Auroux, P. A.; Iossifidis, D.; Reyes, D. R.; Manz, A. Anal. Chem. 2002, 74, 2637-2652. (6) Reyes, D. R.; Iossifidis, D.; Auroux, P. A.; Manz, A. Anal. Chem. 2002, 74, 2623-2636. (7) Li, J. J.; LeRiche, T.; Tremblay, T. L.; Wang, C.; Bonneil, E.; Harrison, D. J.; Thibault, P. Mol. Cell. Proteomics 2002, 1, 157-168. (8) Paegel, B. M.; Emrich, C. A.; Weyemayer, G. J.; Scherer, J. R.; Mathies, R. A. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 574-579. (9) Ramsey, J. D.; Jacobson, S. C.; Culbertson, C. T.; Ramsey, J. M. Anal. Chem. 2003, 75, 3758-3764. (10) Grover, W. H.; Skelley, A. M.; Liu, C. N.; Lagally, E. T.; Mathies, R. A. Sens. Actuators, B 2003, 89, 315-323. (11) Duffy, D. C.; McDonald, J. C.; Schueller, O. J. A.; Whitesides, G. M. Anal. Chem. 1998, 70, 4974-4984. (12) Effenhauser, C. S.; Bruin, G. J. M.; Paulus, A.; Ehrat, M. Anal. Chem. 1997, 69, 3451-3457. (13) McDonald, J. C.; Duffy, D. C.; Anderson, J. R.; Chiu, D. T.; Wu, H. K.; Schueller, O. J. A.; Whitesides, G. M. Electrophoresis 2000, 21, 27-40. (14) Quake, S. R.; Scherer, A. Science 2000, 290, 1536-1540. (15) Beebe, D. J.; Moore, J. S.; Yu, Q.; Liu, R. H.; Kraft, M. L.; Jo, B. H.; Devadoss, C. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 13488-13493. (16) McDonald, J. C.; Whitesides, G. M. Acc. Chem. Res. 2002, 35, 491-499. (17) Beebe, D. J.; Mensing, G. A.; Walker, G. M. Annu. Rev. Biomed. Eng. 2002, 4, 261-286. (18) Hansen, C.; Quake, S. R. Curr. Opin. Struct. Biol. 2003, 13, 538-544. (19) Sia, S. K.; Whitesides, G. M. Electrophoresis 2003, 24, 3563-3576. (20) Anderson, J. R.; Chiu, D. T.; Jackman, R. J.; Cherniavskaya, O.; McDonald, J. C.; Wu, H.; Whitesides, S. H.; Whitesides, G. M. Anal. Chem. 2000, 72, 3158-3164. (21) Jo, B. H.; Van Lerberghe, L. M.; Motsegood, K. M.; Beebe, D. J. J. Microelectromech. Syst. 2000, 9, 76-81. (22) McDonald, J. C.; Chabinyc, M. L.; Metallo, S. J.; Anderson, J. R.; Stroock, A. D.; Whitesides, G. M. Anal. Chem. 2002, 74, 1537-1545. (23) Wu, H.; Odom, T. W.; Chiu, D. T.; Whitesides, G. M. J. Am. Chem. Soc. 2003, 125, 554-559. (24) Fu, A. Y.; Chou, H. P.; Spence, C.; Arnold, F. H.; Quake, S. R. Anal. Chem. 2002, 74, 2451-2457. (25) Chiu, D. T.; Jeon, N. L.; Huang, S.; Kane, R. S.; Wargo, C. J.; Choi, I. S.; Ingber, D. E.; Whitesides, G. M. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 2408-2413. (26) Ismagilov, R. F.; Ng, J. M. K.; Kenis, P. J. A.; Whitesides, G. M. Anal. Chem. 2001, 73, 5207-5213. (27) Chiu, D. T.; Pezzoli, E.; Wu, H. K.; Stroock, A. D.; Whitesides, G. M. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 2961-2966. (28) Groisman, A.; Enzelberger, M.; Quake, S. R. Science 2003, 300, 955-958.
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creation of active fluid manipulation components, such as pneumatically driven valves and pumps,10,33-35 or actuators based on thin PDMS membranes in conjunction with pH-sensitive hydrogels15,36,37 and magnetic components.38,39 The surface free energy of PDMS permits the easy establishment of conformal seals with itself and the smooth surfaces of a variety of materials, including glass, silicon, and plastics. This ability to form good conformal seals has been employed in patterning substrates with proteins and cells,25 in chip-based immunoassays,40 and in two-dimensional gel-based protein separations.41 PDMS is gas permeable, and this trait is especially important in applications that involve living cells, such as in microfluidic devices for cell-based biosensors,42 cellular manipulations,24,43-45 and cell culture.46-48 The excellent optical transparency of PDMS, which is from 240 to 1100 nm, has also been exploited to implement different schemes in optical detection49,50 and for the fabrication of PDMS-based optical elements.51-53 Despite these advantageous attributes of PDMS, there are significant drawbacks with PDMS that hinder its use in certain applications. PDMS is a hydrophobic material when cured and its surface can be oxidized in oxygen plasma to become hydrophilic, but this hydrophilic surface is not stable and can revert to its original hydrophobic form sometimes within hours. This reversion may be caused by the rearrangements occurring at the surface, in which surface groups or molecules are exchanged for those in the bulk.54 Because PDMS is hydrophobic, it tends to (29) Stroock, A. D.; Dertinger, S. K. W.; Ajdari, A.; Mezic, I.; Stone, H. A.; Whitesides, G. M. Science 2002, 295, 647-651. (30) Dertinger, S. K. W.; Chiu, D. T.; Jeon, N. L.; Whitesides, G. M. Anal. Chem. 2001, 73, 1240-1246. (31) Jiang, X. Y.; Ng, J. M. K.; Stroock, A. D.; Dertinger, S. K. W.; Whitesides, G. M. J. Am. Chem. Soc. 2003, 125, 5294-5295. (32) Thorsen, T.; Maerkl, S. J.; Quake, S. R. Science 2002, 298, 580-584. (33) Unger, M. A.; Chou, H. P.; Thorsen, T.; Scherer, A.; Quake, S. R. Science 2000, 288, 113-116. (34) Ismagilov, R. F.; Rosmarin, D.; Kenis, P. J. A.; Chiu, D. T.; Zhang, W.; Stone, H. A.; Whitesides, G. M. Anal. Chem. 2001, 73, 4682-4687. (35) Jeon, N. L.; Chiu, D. T.; Wargo, C. J.; Wu, H. K.; Choi, I. S.; Anderson, J. R.; Whitesides, G. M. Biomed. Microdevices 2002, 4, 117-121. (36) Beebe, D. J.; Moore, J. S.; Bauer, J. M.; Yu, Q.; Liu, R. H.; Devadoss, C.; Jo, B. H. Nature 2000, 404, 588-590. (37) Liu, R. H.; Yu, Q.; Beebe, D. J. J. Microelectromech. Syst. 2002, 11, 45-53. (38) Khoo, M.; Liu, C. Sens. Actuators, A 2001, 89, 259-266. (39) Jackson, W. C.; Tran, H. D.; O’Brien, M. J.; Rabinovich, E.; Lopez, G. P. J. Vac. Sci. Technol., B 2001, 19, 596-599. (40) Bernard, A.; Michel, B.; Delamarche, E. Anal. Chem. 2001, 73, 8-12. (41) Chen, X. X.; Wu, H. K.; Mao, C. D.; Whitesides, G. M. Anal. Chem. 2002, 74, 1772-1778. (42) DeBusschere, B. D.; Kovacs, G. T. A. Biosens. Bioelectron. 2001, 16, 543556. (43) Fu, A. Y.; Spence, C.; Scherer, A.; Arnold, F. H.; Quake, S. R. Nat. Biotechnol. 1999, 17, 1109-1111. (44) Clark, S. G.; Walters, E. M.; Beebe, D. J.; Wheeler, M. B. Biol. Reprod. 2002, 66, 312-312. (45) Cho, B. S.; Schuster, T. G.; Zhu, X. Y.; Chang, D.; Smith, G. D.; Takayama, S. Anal. Chem. 2003, 75, 1671-1675. (46) Folch, A.; Toner, M. Annu. Rev. Biomed. Eng. 2000, 2, 227-256. (47) Takayama, S.; McDonald, J. C.; Ostuni, E.; Liang, M. N.; Kenis, P. J. A.; Ismagilov, R. F.; Whitesides, G. M. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 5545-5548. (48) Whitesides, G. M.; Ostuni, E.; Takayama, S.; Jiang, X. Y.; Ingber, D. E. Annu. Rev. Biomed. Eng. 2001, 3, 335-373. (49) Chabinyc, M. L.; Chiu, D. T.; McDonald, J. C.; Stroock, A. D.; Christian, J. F.; Karger, A. M.; Whitesides, G. M. Anal. Chem. 2001, 73, 4491-4498. (50) Costin, C. D.; Synovec, R. E. Anal. Chem. 2002, 74, 4558-4565. (51) Schueller, O. J. A.; Zhao, X. M.; Whitesides, G. M.; Smith, S. P.; Prentiss, M. Adv. Mater. 1999, 11, 37-41. (52) Schueller, O. J. A.; Duffy, D. C.; Rogers, J. A.; Brittain, S. T.; Whitesides, G. M. Sens. Actuators, A 1999, 78, 149-159. (53) Camou, S.; Fujita, H.; Fujii, T. Lab Chip 2003, 3, 40-45.
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absorb and dissolve hydrophobic molecules present in the microchannel,55 and nonpolar solvents will swell PDMS.56 Consequently, PDMS is most suitable for aqueous applications. The gas-permeable nature of PDMS makes it an ideal material for cellbased applications but is detrimental in other instances. In particular, we have difficulty cross-linking polyacrylamide gels, which are commonly used in protein separations, within PDMS microchannels since the polymerization reaction is inhibited by oxygen. The ease by which PDMS microfluidic systems can be fabricated provides a strong motivation to develop strategies to overcome some of these limitations, typically by focusing on surface modifications of PDMS for improved surface properties,56,57 through polymer grafting,55,58 polyelectrolyte multilayers,59 surfactant coatings,60 and phospholipid bilayers.61 In addition, the extraction of unreacted oligomers from the bulk PDMS was shown to improve the stability of oxidized hydrophilic surfaces from hours to days.56 The solvent compatibility of PDMS was also thoroughly investigated and catalogued.56 In contrast to these efforts to modify PDMS and to improve its performance for certain applications, we, and others,62 have worked to identify and implement a material complementary to PDMS. A key requirement here is the ability to retain similar procedures in fabrication (e.g., molding and facile bonding) that are characteristic of rapid prototyping in PDMS. Although a number of rapid replication techniques (e.g., injection molding, imprinting, and embossing)63-69 exist, we find molding to be especially versatile because of the mild processing conditions, the ability to replicate multilayer structures, and the ability to create three-dimensional networks of microchannels. Thermoset polyester (TPE), or unsaturated polyester, is a resin that can be processed using molding techniques in a manner similar to PDMS.70,71 This paper describes our procedure for fabricating TPE microfluidic systems in less than 3 h by direct casting of TPE on (54) Kima, J.; Chaudhury, M. K.; Owen, M. J. J. Colloid Interface Sci. 2000, 226, 231-236. (55) Hu, S. W.; Ren, X. Q.; Bachman, M.; Sims, C. E.; Li, G. P.; Allbritton, N. Anal. Chem. 2002, 74, 4117-4123. (56) Lee, J. N.; Park, C.; Whitesides, G. M. Anal. Chem. 2003, 75, 6544-6554. (57) Makamba, H.; Kim, J. H.; Lim, K.; Park, N.; Hahn, J. H. Electrophoresis 2003, 24, 3607-3619. (58) Hu, S. W.; Ren, X. Q.; Bachman, M.; Sims, C. E.; Li, G. P.; Allbritton, N. Electrophoresis 2003, 24, 3679-3688. (59) Liu, Y.; Fanguy, J. C.; Bledsoe, J. M.; Henry, C. S. Anal. Chem. 2000, 72, 5939-5944. (60) Dou, Y. H.; Bao, N.; Xu, J. J.; Chen, H. Y. Electrophoresis 2002, 23, 35583566. (61) Yang, T. L.; Jung, S. Y.; Mao, H. B.; Cremer, P. S. Anal. Chem. 2001, 73, 165-169. (62) Rolland, J. P.; Van Dam, R. M.; Schorzman, D. A.; Quake, S. R.; DeSimone, J. M. J. Am. Chem. Soc. 2004, 126, 2322-2323. (63) Soper, S. A.; Ford, S. M.; Qi, S.; McCarley, R. L.; Kelly, K.; Murphy, M. C. Anal. Chem. 2000, 72, 642A-651A. (64) Becker, H.; Gartner, C. Electrophoresis 2000, 21, 12-26. (65) Becker, H.; Locascio, L. E. Talanta 2002, 56, 267-287. (66) McCormick, R. M.; Nelson, R. J.; AlonsoAmigo, M. G.; Benvegnu, J.; Hooper, H. H. Anal. Chem. 1997, 69, 2626-2630. (67) Martynova, L.; Locascio, L. E.; Gaitan, M.; Kramer, G. W.; Christensen, R. G.; MacCrehan, W. A. Anal. Chem. 1997, 69, 4783-4789. (68) Becker, H.; Heim, U. Sens. Mater. 1999, 11, 297-304. (69) Galloway, M.; Stryjewski, W.; Henry, A.; Ford, S. M.; Llopis, S.; McCarley, R. L.; Soper, S. A. Anal. Chem. 2002, 74, 2407-2415. (70) Xu, W.; Uchiyama, K.; Shimosaka, T.; Hobo, T. J. Chromatogr., A 2001, 907, 279-289. (71) Fiorini, G. S.; Jeffries, G. D. M.; Lim, D. S. W.; Kuyper, C. L.; Chiu, D. T. Lab Chip 2003, 3, 158-163.
Figure 1. Schematic showing the procedure used for the fabrication of TPE microfluidic devices. (1) TPE resin containing UV photoinitiator and thermal initiator is poured onto a SnO2-coated, SU-8 patterned silicon master. (2) The resin is exposed to UV radiation. (3) The semicured TPE replicas are brought into contact. (4) Additional UV exposure and heat are used to completely cure the TPE chip.
a SU-8 patterned silicon master. We present our characterizations of TPE as a substrate for microfluidics and discuss the performance of the final device in several key areas, including surface properties as related to capillary electrophoresis, optical characteristics, and solvent compatibilities. EXPERIMENTAL SECTION Fabrication Procedure. Figure 1 schematically illustrates the TPE fabrication procedure. Silicon masters were fabricated using photolithography as described in detail elsewhere.11 Briefly, SU-8 50 (Microchem, Newton, MA), a negative photoresist, was spincoated onto 3-in. silicon wafers (Montco Silicon Technologies, Royersford, PA) to the desired thickness. A transparency with the printed design was used as a mask for photolithography. Following exposure, the wafers were baked and developed with propylene glycol methyl ether acetate (Sigma-Aldrich, Milwaukee, WI) yielding a patterned silicon master. Rather than silanizing the surface of the wafer, which is required for the easy release of PDMS molds, the surface of the master was instead coated with a thin layer of transparent tin oxide in a sputter coater (Technics Hummer II Sputter Coater, Anatech Ltd., Denver, NC). To deposit tin oxide, tin foil (Goodfellow, Berwyn, PA) was sputtered for 3 min in residual air (0.3 mbar) and at a sputter current of 20 mA. TPE was prepared by mixing the resin (Polylite 32030-10, Reichhold, Inc., Research Triangle, NC) with additional crosslinker (styrene) (Sigma-Aldrich), ultraviolet (UV) photoinitiator (2,2-dimethoxyphenylacetophenone, Irgacure 651) (SigmaAldrich), and methyl ethyl ketone peroxide (MEKP) catalyst (Crompton Corp., Greenwich, CT). Approximately 0.10 g of the photoinitiator was dissolved in 0.25 g of styrene monomer and
then added to 10 g of resin. Three drops of MEKP catalyst (∼0.09 g) were added to the resin/styrene mix, and then the mix was stirred and degassed to remove air bubbles. Following degassing, the TPE resin was poured onto the patterned master. Short pieces of stainless steel rod or PDMS posts were placed on the master to define access holes or reservoirs; alternatively holes can be drilled, after the chip is cured, and sealed with adhesive tabs (Grace Bio-Labs, Bend, OR). A piece of PDMS (Sylgard 184, Dow Corning, Midland, MI) was cut to form a mold surround, which was conformally sealed to the master to contain the resin to a particular area of the master. A piece of transparency film (3M), cut to an appropriate size, was used as a top cover over the resin to ensure a flat surface during the UV curing procedure. The cast TPE resin was exposed to UV radiation using a custom-built UV exposure box, which contained two long-wave UV bulbs with peak intensity at 365 nm (TLK 40W/10R, Philips). Samples were placed ∼15 cm from the bulbs. TPE pieces (a patterned piece and a flat piece) were exposed for 2 min and then peeled away from the masters; water was applied at the interface of the TPE pieces and the masters during peeling, keeping water in contact with the surfaces of the masters. A patterned piece was brought into contact with the flat piece to form an enclosed chip. This enclosed chip was then exposed to UV light for an additional 2 min, using four periods of 30-s exposures with 1.5-min intervals between exposures. Following UV exposure, the TPE pieces were heated to 60 °C for 30 min and 120 °C for 1.5 h and then allowed to cool to room temperature. To increase the surface charge of TPE, the cured and sealed chips were oxidized for 5 min (∼0.3 mbar and 30 W) in oxygen plasma (Harrick Scientific Corp., model PDC-001, Ossining, NY). Characterization. (A) Scanning Electron Microscopy. Samples were sputter coated with gold (SPI Supplies, Westchester, PA) prior to imaging on a FEI Sirion scanning electron microscope (Hillsboro, OR). (B) Surface Properties. X-ray Photoelectron Spectroscopy (XPS) and Contact Angle Measurements. XPS analyses were obtained using a Surface Sciences Instruments (Mountain View, CA) X-Probe system equipped with an Al KR X-ray source (hv ) 1486.6 eV) and a hemispherical analyzer (with 0.1-eV resolution). The elemental composition and high-resolution C1s scans were acquired at pass energies of 150 and 50 eV, respectively. All analyses were obtained at a photoelectron takeoff angle of 55°. The binding energy scales for the samples were calibrated by referencing the CHx peak maximums in the C1s spectra to 285.0 eV. Samples were prepared in triplicate for data averaging, and spectra were obtained before and after plasma treatment. Static contact angles for water were measured using the sessile drop method with a CCD camera. Measurements were taken at ambient temperature using 1-µL water drops. All reported contact angles are the average of four data points. (C) Electroosmotic Flow (EOF). Electroosmotic flow in TPE channels was determined by the current monitoring method,72 with the exception that 20 and 10 mM buffer concentrations were used instead of the recommended 20 and 19 mM concentrations.73 Channels were 50 µm by 50 µm and were ∼3 cm in length. (72) Huang, X.; Gordon, M. J.; Zare, R. N. Anal. Chem. 1988, 60, 1837-1838. (73) Locascio, L. E.; Perso, C. E.; Lee, C. S. J. Chromatogr., A 1999, 857, 275284.
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Figure 2. SEM images of thermoset polyester replicas fabricated by direct casting from SU-8 patterned silicon masters. Scale bars for (A) and (B) are 5 µm. Scale bars for (C-F) are 200 µm. Insets in the upper-right corners (C-F) show the patterned masters; the circular inset (F) is a magnification of a portion of the image.
Channels were filled with a 20 mM sodium borate solution (J. T. Baker, Phillipsburg, NJ), and then the solution was removed from one of the reservoirs and replaced with 10 mM sodium borate. Platinum electrodes were placed in both of the wells, and a highvoltage power supply (Stanford Research Systems PS350, Sunnyvale, CA) was used to produce electroosmotic flow in a direction that displaced the 20 mM buffer in the channel with the 10 mM buffer. The current was recorded as a function of time for each applied voltage; the time required to reach a steady current was taken to be the time necessary to displace the volume of 20 mM buffer over the length of the channel. The linear velocity of the electroosmotic flow is the length of the channel divided by the time needed to achieve steady current, vEOF ) l/t. To calculate the electroosmotic mobility, µEOF, the linear velocity was divided by the applied electric field, µEOF ) vEOF/E. (D) Electrophoretic Separation of Amino Acids in a Thermoset Polyester Chip. The chip design consisted of a double-T injection junction and a 3.5-cm-long separation channel. Amino acids (gylcine, glutamic acid, aspartic acid) (Sigma-Aldrich) were labeled with fluorescein isothiocyanate (FITC; Molecular Probes, Eugene, OR) by adding 100 µL of 10 mM FITC in dimethyl sulfoxide to 1 mL of 10 mM solutions of each amino acid in a 20 mM sodium borate buffer and allowing the solutions to react overnight in the dark. Microchip capillary electrophoresis was performed using a mixture of the amino acids (200 µM with respect to FITC) with 20 mM borate as the running buffer. Approximately 0.3 nL of labeled amino acids was injected for separation, as determined by the channel geometry at the intersection and taking into account the actual loading profile resulting from the applied potentials. The chip was illuminated with a focused light spot from a high-powered blue light-emitting diode (λmax ∼475 nm, ∼40 mW optical power). The filtered fluorescence emission was detected using a photomultiplier (Hamamatsu R928), with the output measured by a picoammeter (Keithley 6485). A computer program (LabView) was used to control four power supply outputs and record the electropherogram. (E) Immersion Tests To Determine Solvent Compatibility of Thermoset Polyester. TPE was exposed to various chemicals commonly used for microanalytical applications. For each chemical solution, three TPE pieces were tested. The pieces used were 4700
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cylindrical in shape, ∼7 mm in diameter and 2 mm in thickness. Each piece was imaged before and after a 1-day immersion in the solvents using a CCD camera. Image processing software (ImageJ) was used to measure the change in dimensions (circular area). (F) Optical Transmittance. A Hewlett-Packard model 8452A UV/visible spectrophotometer was used to characterize the optical transparency of TPE. The percent transmittance (T) of light through TPE was measured from 200 to 800 nm. Samples were TPE casts of standard thicknesses (2 mm), and TPE spin-coated on glass coverslips to a thickness of 10 µm. Each sample was prepared in triplicate, and five measurements were taken from each sample for averaging. To spin-coat TPE on cover glass, the fabrication procedure was modified by using an extra 0.75 g of styrene in the resin mixture. The spin parameters were 500 rpm for 15 s, immediately followed by 2000 rpm for 30 s. The spincoated glass was exposed to UV for 4 min and then heated at 60 °C overnight. RESULTS AND DISCUSSION Fabrication Using Thermoset Polyester. Figure 1 schematically shows the procedure for fabricating TPE devices through replica molding. A key step is the sputter coating of the patterned wafer with tin oxide, which is very hydrophilic. We found SnO2 greatly reduces the interaction of TPE resin with the silicon wafer and the SU-8 photoresist, thereby allowing the UV semicured resin to be peeled away from the patterned master without damage to the features in the resin. Early investigations with untreated masters or plasma oxidized masters yielded unsatisfactory or mediocre replication, often with loss of features on the chip and damage to the master, emphasizing the need for surface treatment of the masters. With SnO2-coated masters, water is used as a surface wetting or release agent during the peeling of the resin from the master to ensure easy removal and to prevent distortion of the features, and it most likely acts through preferred interfacial interactions with the SnO2 surface. Using this procedure (Figure 1), we replicated a variety of feature sizes and designs in TPE (Figure 2), demonstrating the ability of TPE to replicate a wide range of features that are relevant to applications in microfluidics. For small features, we replicated
a 5-µm tapered point (Figure 2A) and a 6-µm-wide constriction along a 15-µm-wide channel (Figure 2B). For complex designs, we fabricated patterns in TPE that contain sharp turns (Figure 2C), tight packing (Figure 2D), and multiple layers (Figure 2E). The insets in the panels are SEM images of the original SU-8 patterned silicon masters from which the TPE replicas were molded. The fidelity of replication in TPE is excellent, as can be seen in Figure 2F; the scratches on the SU-8 feature were accurately replicated in the TPE copy. The rough edges of the features in Figure 2C and D, caused by pixelation in the transparency mask, were also precisely duplicated. We used two types of initiators, a photoinitiator and a thermal initiator, to control the curing of the chips. The UV photoinitiator provides for quick replication of the patterned features by requiring only a few minutes (∼2 min) of UV exposure to produce a piece with established and “locked-in” features. The use of the thermal initiator, MEKP, in conjunction with a two-stage thermal cure produces a clear, nonyellowed chip, which is important for optical transparency, and also provides for increased solvent compatibility, most likely by the evaporation of unreacted solvent. Altogether, with the use of both a photoinitiator and a thermal initiator, features are replicated in TPE in less than 10 min, and the complete cure of the TPE resin is obtained in just 2 h, for a total processing time of ∼3 h. Access holes are molded into the TPE during the casting procedure by placing solid stainless steel rods or PDMS cylindrical posts onto the silicon wafer in the desired locations. As stated in the Experimental Section, pieces of transparency film are used to produce a flat and level top barrier for the resin to mold against, and a piece of PDMS is used as a mold surround to confine the resin to the desired area. After placing the posts on the wafer and placing the transparency film in position on top of the surround, TPE resin is poured into the surround, around the posts, and under the transparency film. To provide additional structural support, the posts are left in place when peeling the piece off of the master and during the subsequent steps for bonding and curing of the chip. To produce an enclosed chip, a patterned piece of TPE is brought into contact with a flat piece. At this point in the procedure, both of the pieces are semicured by UV lightsthe features are stable, but the surfaces are slightly tacky and can undergo further cross-linking reactions. Once in contact with each other and exposed to additional UV light, covalent bonding occurs as the cross-linking reaction resumes at the interface of the two semicured pieces. After the bonded piece is cured by UV light, it is further thermally cured for 2 h in an oven. Surface Characterization of Thermoset Polyester. (A) Contact Angle Measurements. A hydrophilic surface facilitates the filling of the microchannels and is required for many applications in microfluidics that use aqueous solutions. Because native TPE is hydrophobic, we explored the use of oxygen plasma to increase its surface charge. The static contact angle of deionized water was ∼61° on untreated TPE and ∼42° on oxygen plasma treated TPE (5 min at 30 W and ∼0.3 mbar), for an overall decrease of ∼20° (see Figure 3, insets). Oxidizing the surface of PDMS in oxygen plasma is a commonly used procedure both for bonding two pieces of PDMS together and for increasing it surface charge.11 In comparison with
Figure 3. XPS C1s spectra of the surface of thermoset polyester before (A) and after (B) treatment in oxygen plasma for 5 min. Peak assignment: CsH, CsC at 285.0 eV; CsO at 286.6 eV; OsCdO at 289.2 eV; aromatic C at 292.0 eV. Insets show contact angle images of thermoset polyester before (A) and after (B) treatment in oxygen plasma for 5 min.
TPE, the surface of native PDMS is more hydrophobic with a contact angle of ∼108° but is made more hydrophilic with a contact angle of ∼30° after plasma oxidation.74 TPE therefore undergoes a significant but less drastic change in its wetting properties following plasma oxidation than PDMS. This improved wetting facilitates the filling of the TPE microchannels in our experiments. (B) XPS Analysis. To more thoroughly investigate the compositional changes that occur during plasma treatment, we analyzed TPE samples by XPS from both before and after plasma treatment. A survey scan determined that the elemental composition of TPE prior to treatment was 75.4 ( 0.4% carbon and 23.0 ( 1.4% oxygen. After treatment, the elemental composition was 67.2 ( 0.3% carbon and 31.8 ( 0.4% oxygen, indicating that treatment of TPE in oxygen plasma introduces additional oxygen content, while decreasing the C/O ratio at the surface. Silicon was detected at ∼1% for both conditions, most likely due to contamination in the form of silicone through the daily use of PDMS elastomer in our laboratory. Figure 3A shows the high-resolution scans of the C1s binding energy region before plasma treatment, in which the peaks at 285.0, 286.6, 289.2, and 291.8 eV correspond to hydrocarbon, ether, ester/acid, and aromatic functional groups, respectively. The chemical shifts of 1.6 and 4.2 eV agree well with literature (74) Morra, M.; Occhiello, E.; Marola, R.; Garbassi, F.; Humphrey, P.; Johnson, D. J. Colloid Interface Sci. 1990, 137, 11-24.
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Figure 4. Measurements of electroosmotic mobility (electric field strength, 500 V/cm) over 5 days of native thermoset polyester and polyester treated in oxygen plasma for 5 min.
assignments for CsO and OsCdO species, respectively.75,76 On average, 60.1% of the carbon content of untreated TPE was from hydrocarbon bonds (CsC and CsH), 24.2% was from carbon involved in ether bonds, and 14.5% was contributed by carbon in ester or acid functional groups. A small percentage (1.2%) was attributed to a shake-up satellite at 291.8 eV, which is consistent with the presence of aromatic groups from either aromatic diacids in the polyester polymer chains or from styrene used as the crosslinking monomer. Following plasma treatment (Figure 3B), the hydrocarbon content changed from 60.1 to 50.1%, the ether content from 24.2 to 26.7%, and the ester/acid content from 14.5 to 22.2%; the aromatic content did not show substantial change. Overall, XPS elemental scans indicated that the ratio of oxygen to carbon atoms increases following plasma treatment, and the C1s scans indicated that more of the carbon atoms on the surface become bonded to oxygen in ether, ester, or acid functional groups than were prior to treatment. The increase in oxygen-containing functional groups on the surface agrees well with the increase in hydrophilicity demonstrated by the contact angle measurements. Electroosmotic Flow and Capillary Electrophoresis in Thermoset Polyester Devices. (A) Electroosmotic Flow. TPE microchannels generate EOF toward the cathode, similar to EOF in PDMS and glass, which indicates the presence of negative surface charges. Figure 4 shows measurements of the electroosmotic mobility of TPE microchannels over a period of 5 days. The EOF mobility of native TPE microchannels was 1.3 × 10-4 cm2 V-1 s-1. Immediately following treatment in oxygen plasma (5 min at 30 W and 0.3 mbar), the channels were easier to fill, and the EOF mobility increased 2-fold to ∼2.6 × 10-4 cm2 V-1 s-1. Over the period that we monitored EOF following plasma oxidation (5 days), the EOF was very stable. The microchannels used for these EOF stability measurements were stored dry for 24 h between measurements. The magnitude of EOF in TPE chips (∼2.6 × 10-4 cm2 V-1 s-1) is smaller than that of PDMS (∼6 × 10-4 cm2 V-1 s-1, oxidized for 1 min) or fused-silica capillaries (∼7 × 10-4 cm2 V-1 s-1) as determined by the same method using identical buffer solutions. Glass capillaries or channels have stable (75) Chen, J. R.; Wang, X. Y.; Tomiji, W. J. Appl. Polym. Sci. 1999, 72, 13271333. (76) Gupta, B.; Hilborn, J.; Hollenstein, C.; Plummer, C. J. G.; Houriet, R.; Xanthopoulos, N. J. Appl. Polym. Sci. 2000, 78, 1083-1091. (77) Lim, D. S. W.; Kuo, J. S.; Chiu, D. T. J. Chromatogr., A 2004, 1027, 237244.
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Figure 5. Capillary electrophoresis separation of amino acids tagged with FITC in a thermoset polyester microchannel. Applied electric field strength 200 V/cm; distance between detection and injection 3 cm; Gly ) glycine, Glu ) glutamic acid, and Asp ) aspartic acid.
EOF with proper maintenance and can be reconditioned with NaOH. In contrast, the EOF of oxidized PDMS microchannels is stable only over short periods of time,11 but the hydrophilic state of oxidized PDMS can be prolonged by keeping the surface in contact with water. TPE devices have stable EOF for at least 5 days (our period of observation) following plasma treatment, often a suitable time frame for most standard applications in microfluidics. The increase in EOF in TPE microchannels following plasma treatment correlates well with both contact angle measurements and XPS analysis. Contact angle measurements showed an increase in the hydrophilicity of the surface, while the XPS data showed an increase in the oxygen-containing functional groups on the surface, such as esters, acids, and ethers, which are known to carry negative charges under appropriate pH conditions. (B) Capillary Electrophoresis. To verify that TPE microfluidic devices were suitable for capillary electrophoresis applications, we separated a mixture of amino acids (glycine, glutamic acid, aspartic acid) tagged with FITC in an oxidized TPE chip (15 min at 30 W and 0.3 mbar). Figure 5 shows the resultant electropherogram. At an electric field strength of 200 V/cm, the mixture was separated and the three species are easily distinguished. The separation was complete in less than 160 s. The magnitude of µEOF for TPE devices that are oxidized for 5 min, as reported in the previous section, is a factor of ∼2-3 times slower than that of glass or PDMS because of its lower surface charge density. At a given electric field strength, this lower µEOF for TPE channels means that a correspondingly shorter channel length is required to achieve the same degree of separation. This ability to use shorter channels minimizes the need to pack long serpentineshaped channels, which tends to cause band broadening,3 onto the limited space available in a chip-based format. The tradeoff is that at a much slower EOF rate the electrophoretic motion of highly negatively charged analytes may be faster than EOF and in the opposite direction of EOF, thus preventing their detection. Optical Transparency of Thermoset Polyester. Optical transparency is a characteristic often required in microfluidic applications, because the small sample volume present in microchannels typically necessitates sensitive optical methods of detection, such as laser-induced fluorescence. Figure 6 shows optical transmission measurements of a 2-mm-thick sample in the 200-
Table 2. Comparison of TPE, PDMS, and Glass as Materials Used for Microfluidic Devicesa
Figure 6. Optical transparency of 2-mm- and 10-µm-thick TPE and the blank glass coverslip (dashed line) on which the 10-µm-thick film of TPE was spin-coated.
rapid prototyping capability (ease, speed, cost) range of feature sizes small features complex features multilayer features elastomeric optical clarity gas permeability thermal conductivity77 stability of surface properties compatible with solvents a
TPE
PDMS
glass
+
++
-
++ + + ++ + + + +
++ ++ ++ ++ ++ ++ ++ -
++ ++ ++ ++ ++ ++ ++
Rating system: ++, excellent; +, good; -, poor.
Table 1. Solvent Compatibility of Thermoset Polyester
a
solvent
swelling ratio
water acetonitrile tetrahydrofuran methanol ethanol 2-propanol acetone ethyl acetate acetic acid, 1 M hydrochloric acid, 1 M ammonium hydroxide, 1 M sodium hydroxide, 1 M dichloromethane chloroform cyclohexane n-heptane benzene toluene
1.0 1.4 1.5 1.1 1.0 1.0 1.4 1.3 1.0 1.0 1.0 1.0 n/aa n/a 1.0 1.0 1.1 1.0
n/a, not applicable.
800-nm wavelength region. TPE passes up to 90% of the incident light in the visible region (400-800 nm), which is comparable to glass (borosilicate coverslips) and PDMS, but the transmittance quickly decreases at wavelengths below 400 nm, whereas glass coverslips transmit down to ∼300 nm and PDMS transmits down to ∼240 nm. A standard thickness (>2 mm) TPE device is therefore only suitable for applications in the visible light region. This limitation may be overcome, however, by using a thin layer (tens of micrometers) of TPE supported on a suitable substrate (e.g., glass coverslips). The transmission profile of a 10-µm-thick layer of TPE spin-coated on glass is similar to that of the blank glass by itself (Figure 6). This thin layer of TPE on glass transmits light down to 300 nm, where glass begins to absorb, with a 50% transmittance at 325 nm instead of at 390 nm for the thick TPE sample. A modified fabrication procedure using spin-coated TPE could be a solution for applications requiring the use of long-wave ultraviolet light. Solvent Compatibility of Thermoset Polyester. We have tested the compatibility of TPE with a wide range of solvents. To determine the swelling ratio (Table 1), cylindrical pieces of cured TPE (∼7 mm in diameter, 2 mm thick) were imaged prior to submersion to record the original dimensions, then subjected to 18 different solvents for 24 h, and finally imaged again to record
the dimensions of the piece following the 24-h period, similar to the procedure used by Lee et al. to determine the swelling ratios for PDMS.56 We found TPE performed very well (no observable changes after immersion) with water, acids, and bases, several alcohols, and some of the hydrocarbon solvents. Acetonitrile, tetrahydrofuran, acetone, and ethyl acetate caused cracking and swelling of TPE, while the chlorinated solvents (chloroform, dichloromethane) dissolved the TPE sample pieces as anticipated. Overall, TPE shows good solvent resistance, but not as good as glass. Compatibility with hydrocarbon solvents (cyclohexane, n-heptane, toluene) is an interesting result in that PDMS is known to have severe swelling issues with these same solvents. TPE chips may be useful for applications in which a nonpolar solvent is required and PDMS is not an option. Overall Performance of Thermoset Polyester Microfluidic Devices. To become a commonly used material for microfabrication, TPE must rival other commonly used materials on several criteria or show excellent capabilities in areas that other materials do not perform as well in. Table 2 summarizes the properties of TPE, alongside the properties of glass and PDMS. Glass is commonly chosen as a material for microfluidics because of its excellent performance in almost all aspects; however, the procedure required for fabrication in glass typically involves specialized equipment and extremely thorough cleaning prior to bonding, and the polished glass substrates are expensive. In contrast, rapid prototyping in PDMS is well known for its low cost and ease of fabrication, and once a silicon master is generated, the time for device production is only ∼2 h. Fabrication in TPE, using the procedure outlined in Figure 1, is easy and quick, similar to fabrication in PDMS. Similarly to PDMS, TPE pieces can be bonded together easily and with high yield. All three materials can produce features in a wide range of sizes, but multilayer features are more easily replicated in PDMS and TPE than in glass. When cured, TPE is a rigid material similar to glass, while PDMS is elastomeric. Based on EOF measurements, TPE has a more stable surface than PDMS (over 5 days) but is likely less stable than glass over longer periods (weeks to months). The optical properties and UV transmittance of both PDMS and glass (or fused silica) are excellent; TPE shows good transmission in the visible light region but blocks UV light. Glass is one of the best materials to use in terms of solvent resistance, while PDMS Analytical Chemistry, Vol. 76, No. 16, August 15, 2004
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is largely limited to aqueous solutions or other low-solubility solvents;56 the solvent resistance of TPE lies between glass and PDMS. CONCLUSIONS Besides rapid prototyping in PDMS, a number of efficient and high-throughput replication methods have been developed and reviewed in the literature for the fabrication of microdevices, including embossing and injection molding. Here we focused our discussion on PDMS systems both because of the extraordinary versatility PDMS offers in the fabrication of complex threedimensional and multilayer microfluidic networks and because of its ease of implementation and popularity within the microfluidics community. For pressure-driven flows and in cell-based applications, PDMS will likely be the material of choice because of the ease of forming interconnects in PDMS and the biocompatibility and oxygen permeability of PDMS. Glass chips will retain an advantage over plastic or polymer devices in applications that
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involve harsh chemicals or physical conditions or when reusability and robustness is desired. We believe thermoset polyester systems fill an important unmet need in which both rapid fabrication and some of the characteristics exhibited by glass chips are required, such as stable electroosmotic flow, solvent resistance, surface stability, and material rigidity. Such needs are especially acute in analytical applications, such as electrophoreticbased separations that are becoming increasingly important in the context of proteomics and metabolomics. ACKNOWLEDGMENT We gratefully acknowledge NIH for support of this work.
Received for review January 16, 2004. Accepted April 28, 2004. AC0498922
