Room-Temperature Bonding for Plastic High-Pressure Microfluidic

However, bonding chips by thermal fusion places stringent requirements on the ...... Emma Mihevc , H. Trent Spencer , Christopher Doering , Wilbur A. ...
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Anal. Chem. 2007, 79, 5097-5102

Room-Temperature Bonding for Plastic High-Pressure Microfluidic Chips Dieudonne A. Mair,†,⊥ Marco Rolandi,‡ Marian Snauko,‡ Richard Noroski,§ Frantisek Svec,*,⊥ and Jean M. J. Fre´chet*,‡,⊥

Department of Chemical Engineering, Department of Chemistry, and Department of Mechanical Engineering, University of California, Berkeley, California 94720-1460, and The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720

A generic method for the rapid, reproducible, and robust bonding of microfluidic chips fabricated from plastics has been developed and optimized. One of the bonding surfaces is exposed to solvent vapor prior to bringing the mating parts into contact and applying a load. Nanoindentation measurements performed by atomic force microscopy show that a reversible material softening occurs upon exposure to solvent vapor. Subsequent exposure of the bonded chip to UV light then strengthens the bond between mating parts and increases the burst pressure by 50% due to partial cross-linking and chain scission reactions as measured by size exclusion chromatographymultiangle light scattering (SEC-MALS). Performing all steps of this procedure at room temperature eliminates channel distortion observed during thermal bonding and affords channels with highly uniform cross-sectional dimensions. Our technique enables chips resistant to pressures as high as 34.6 MPa. The advantages of miniaturizing chemical analysis include low consumption of expensive reagents, rapid analysis, and ability to run a large number of analyses simultaneously. Adapting current methods to a chip format also facilitates integration of multiple processes to yield a portable micro total analytical system (µTAS) capable of performing sophisticated analyses in the field.1-4 While the first microfluidic chips were made from silicon and glass, the multistep process of creating small channels and reservoirs in these substrates, i.e., cleaning, photolithography, and etching, is * To whom correspondence should be addressed. Phone: 510-643-3077 (J.M.J.F.). Fax: 510-643-3079 (J.M.J.F.). E-mail: [email protected] (J.M.J.F.); [email protected] (F.S.). † Department of Chemical Engineering. ‡ Department of Chemistry. § Department of Mechanical Engineering. ⊥ The Molecular Foundry. (1) Fintschenko, Y.; Choi, W. Y.; Cummings, E. B.; Fre´chet, J. M. J.; Fruetel, J. A.; Hilder, E. F.; Mair, D. A.; Shepodd, T. J.; Svec, F. Proc. SPIEsInt. Soc. Opt. Eng. 2003, 4982 (1), 196-207. (2) Lagally, E. T.; Scherer, J. R.; Blazej, R. G.; Toriello, N. M.; Diep, B. A.; Ramchandani, M.; Sensabaugh, G. F.; Riley, L. W.; Mathies, R. A. Anal. Chem. 2004, 76, 3162-3170. (3) Renzi, R. F.; Stamps, J.; Horn, B. A.; Ferko, S.; Van der Noot, V. A.; West, J. A. A.; Crocker, R.; Wiedenman, B.; Yee, D.; Fruetel, J. A. Anal. Chem. 2005, 77, 435-441. (4) Witek, M. A.; Llopis, S. D.; Wheatley, A.; McCarley, R. L.; Soper, S. A. Nucleic Acids Res. 2006, 34. 10.1021/ac070220w CCC: $37.00 Published on Web 05/27/2007

© 2007 American Chemical Society

slow and expensive. To address this concern, plastics are being used for device fabrication because they can be quickly and inexpensively processed by a variety of methods including micromilling,5,6 wire imprinting,7 in situ polymerization,8-10 soft lithography,11 laser ablation,12-14 hot embossing,15-22 and injection molding.16,18,23-25 Fabrication of microfluidic chips from polymers has been the subject of several reviews.11,18,26-32 A key step common to all chip fabrication methods is channel enclosure by bonding. The challenge of hermetically sealing the channel without deforming the small features or clogging the (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32)

Kawazumi, H.; Tashiro, A.; Ogino, K.; Maeda, H. Lab Chip 2002, 2, 8-10. Lai, S. Y.; Cao, X.; Lee, L. J. Anal. Chem. 2004, 76, 1175-1183. Chen, Y. H.; Chen, S. H. Electrophoresis 2000, 21, 165-170. Chen, G.; Li, J. H.; Qu, S.; Chen, D.; Yang, P. Y. J. Chromatogr., A 2005, 1094, 138-147. Li, J. H.; Chen, D.; Chen, G. Anal. Lett. 2005, 38, 1127-1136. Xu, G. X.; Wang, J.; Chen, Y.; Zhang, L. Y.; Wang, D. R.; Chen, G. Lab Chip 2006, 6, 145-148. Sia, S. K.; Whitesides, G. M. Electrophoresis 2003, 24, 3563-3576. Klank, H.; Kutter, J. P.; Geschke, O. Lab Chip 2002, 2, 242-246. Roberts, M. A.; Rossier, J. S.; Bercier, P.; Girault, H. Anal. Chem. 1997, 69, 2035-2042. Sun, Y.; Kwok, Y. C.; Nguyen, N. T. J. Micromech. Microeng. 2006, 16, 1681-1688. Brown, L.; Koerner, T.; Horton, J. H.; Oleschuk, R. D. Lab Chip 2006, 6, 66-73. Chien, R. D. Sens. Actuatuators, A 2006, 128, 238-247. Grass, B.; Neyer, A.; Johnck, M.; Siepe, D.; Eisenbeiss, F.; Weber, G.; Hergenroder, R. Sens. Actuatuators, B 2001, 72, 249-258. Heckele, M.; Schomburg, W. K. J. Micromech. Microeng. 2004, 14, R1R14. Kricka, L. J.; Fortina, P.; Panaro, N. J.; Wilding, P.; Alonso-Amigo, G.; Becker, H. Lab Chip 2002, 2, 1-4. Lin, R.; Burns, M. A. J. Micromech. Microeng. 2005, 15, 2156-2162. Shah, J. J.; Geist, J.; Locascio, L. E.; Gaitan, M.; Rao, M. V.; Vreeland, W. N. Anal. Chem. 2006, 78, 3348-3353. Wang, J.; Pumera, M.; Chatrathi, M. P.; Escarpa, A.; Konrad, R.; Griebel, A.; Dorner, W.; Lowe, H. Electrophoresis 2002, 23, 596-601. Dang, F.; Shinohara, S.; Tabata, O.; Yamaoka, Y.; Kurokawa, M.; Shinohara, Y.; Ishikawa, M.; Baba, Y. Lab Chip 2005, 5, 472-478. Lee, D. S.; Yang, H.; Chung, K. H.; Pyo, H. B. Anal. Chem. 2005, 77, 54145420. Zhou, X. M.; Dai, Z. P.; Liu, X.; Luo, Y.; Wang, H.; Lin, B. C. J. Sep. Sci. 2005, 28, 225-233. Becker, H.; Gartner, C. Electrophoresis 2000, 21, 12-26. Becker, H.; Locascio, L. E. Talanta 2002, 56, 267-287. Fiorini, G. S.; Chiu, D. T. BioTechniques 2005, 38, 429-446. Reyes, D. R.; Iossifidis, D.; Auroux, P. A.; Manz, A. Anal. Chem. 2002, 74, 2623-2636. Rossier, J.; Reymond, F.; Michel, P. E. Electrophoresis 2002, 23, 858-867. Song, S.; Lee, K. Y. Macromol. Res. 2006, 14, 121-128. Sun, Y.; Kwok, Y. C. Anal. Chim. Acta 2006, 556, 80-96.

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channel during the bonding process has engendered the development of several techniques. These methods range in complexity from simple use of adhesives,17,20,23,33-38 solvent,12,15,19,21,22,25,39-41 thermal fusion,3,7-10,14,20,42,43 and custom-made thermal adhesive,44 to more sophisticated methods including resin gas injection,6 microwave welding,45 UV degradation,46 surface modification,15,20 diode laser welding,47 and use of subcritical carbon dioxide.48 Since microfluidic chips are used to conduct sensitive analysis of biologically relevant molecules, bonding methods that have the potential to introduce chemical contaminants into the channel should be avoided.49 While bonding by thermal fusion circumvents this problem, the heating and cooling cycles significantly lengthen the fabrication time10,34 and introduce some lot-to-lot variability.21,42 The method of applying a solvent to the surface of interest is an important consideration in development of a solvent bonding technique because it strongly affects the quality and overall success of the bonding process, which is typically the yield-limiting step in the multistep device fabrication process. In fact, the strong tendency for the solvent to swell the surface can even lead to complete channel occlusion. The current literature described several techniques to quickly expose the bonding surface to liquid solvent by spin-coating,22 sprinkling,25 capillarity-induced flow,21,39 and solvent aspiration through the channels.21 However, when handling solvent in the liquid phase, extra precautions must be taken to protect the system against leaks and spills that may compromise the optical clarity of the chip.21,40 While we tried a variety of these methods with limited success, we eventually found that handling solvent in the vapor phase was much better controlled, more convenient, and very reliable. Although others have briefly mentioned the use of solvent vapor for bonding polymer chips at elevated temperatures, no optimization or evaluation of the procedure has been presented.50-52 In this report, we describe the development, evaluation, and characterization of (33) Brister, P. C.; Weston, K. D. Anal. Chem. 2005, 77, 7478-7482. (34) Dang, F. Q.; Tabata, O.; Kurokawa, M.; Ewis, A. A.; Zhang, L. H.; Yamaoka, Y.; Shinohara, S.; Shinohara, Y.; Ishikawa, M.; Baba, Y. Anal. Chem. 2005, 77, 2140-2146. (35) Gerlach, A.; Lambach, H.; Seidel, D. Microsyst. Technol. 1999, 6, 19-22. (36) Lim, Y. T.; Kim, S. J.; Yang, H.; Kim, K. J. Micromech. Microeng. 2006, 16, N9-N16. (37) Liu, R. H.; Yang, J. N.; Lenigk, R.; Bonanno, J.; Grodzinski, P. Anal. Chem. 2004, 76, 1824-1831. (38) Song, L. G.; Fang, D. F.; Kobos, R. K.; Pace, S. J.; Chu, B. Electrophoresis 1999, 20, 2847-2855. (39) Hiratsuka, A.; Muguruma, H.; Lee, K. H.; Karube, I. Biosens. Bioelectron. 2004, 19, 1667-1672. (40) Kelly, R. T.; Pan, T.; Woolley, A. T. Anal. Chem. 2005, 77, 3536-3541. (41) Liu, Y. J.; Ganser, D.; Schneider, A.; Liu, R.; Grodzinski, P.; Kroutchinina, N. Anal. Chem. 2001, 73, 4196-4201. (42) Chen, Z. F.; Gao, Y. H.; Lin, J. M.; Su, R. G.; Xie, Y. J. Chromatogr., A 2004, 1038, 239-245. (43) Martynova, L.; Locascio, L. E.; Gaitan, M.; Kramer, G. W.; Christensen, R. G.; MacCrehan, W. A. Anal. Chem. 1997, 69, 4783-4789. (44) Liu, J. K.; Sun, X. F.; Farnsworth, P. B.; Lee, M. L. Anal. Chem. 2006, 78, 4654-4662. (45) Yussuf, A. A.; Sbarski, I.; Hayes, J. P.; Solomon, M.; Tran, N. J. Micromech. Microeng. 2005, 15, 1692-1699. (46) Truckenmuller, R.; Henzi, P.; Herrmann, D.; Saile, V.; Schomburg, W. K. Microsyst. Technol. 2004, 10, 372-374. (47) Chen, J. W.; Zybko, J. M. Proc. SPIEsInt. Soc. Opt. Eng. 2005, 5718 (1), 92-98. (48) Yang, Y.; Lee, L. J.; Lu, W. J. Vac. Sci. Technol., B 2005, 23, 3202-3204. (49) Figeys, D.; Ning, Y. B.; Aebersold, R. Anal. Chem. 1997, 69, 3153-3160. (50) Liu, J. K.; Ro, K. W.; Nayak, R.; Knapp, D. R. Int. J. Mass Spectrom. 2007, 259, 65-72. (51) Ro, K. W.; Liu, H.; Knapp, D. R. J. Chromatogr., A 2006, 1111, 40-47.

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a generic, high-yield method for hermetically sealing plastic microfluidic chips at room temperature. A negligible amount of solvent was applied to one of the bonding surfaces by exposure to the vapor phase prior to bringing the mating parts into contact and applying a load. Subsequent exposure of the bonded chip to UV light considerably improved the bond strength presumably due to cross-linking reactions at the bonding interface. EXPERIMENTAL SECTION Mold Insert and Plastic Chip Fabrication. The mold insert fabrication is similar to that reported previously.54 Briefly, a stainless steel wafer (Valley Design Corporation, Santa Cruz, CA) was dehydrated, plasma cleaned, and coated with Cr/Au (30 nm each) by thermal evaporation. The chromium functions as an adhesion layer to improve adhesion of Au to the stainless steel substrate while gold facilitates photoresist adhesion to the stainless steel substrate.53 Negative-tone photoresist SU-8 2075 was spincoated on the substrate using static dispense method. After allowing the puddle of resist to settle, the resist was spread and finally spun to achieve the desired film thickness of 120 ( 5 µm. After the pre-exposure bake (65 °C, 12 h, 0.5 °C/min), the wafer was allowed to cool by natural convection. Following exposure (800 mJ/cm2, 265 nm) through an optical filter (Omega Optical, Battleboro, VT) wafers were placed on a plate for postexposure bake (65 °C, 12 h, 0.5 °C/min). After the wafers cooled to room temperature by natural convection they were developed (10 min, SU-8 developer) and rinsed in 2-propanol. The patterned substrate was then electroplated, cut, and loaded onto an injection-molding tool for rapid fabrication.54 Chip parts consisting of a flat cover plate and a structured plate with channels and integrated I/O ports were fabricated from COC pellets (Topas 8007 × 10, Ticona, Florence, KY) using a Roboshot 30R-I injection-molding machine (FANUC America Corporation, Chicago, IL). Chip Bonding. In order to minimize surface contamination, gloves were worn and the parts were blown off with high-pressure nitrogen prior to bonding. The unstructured half of the chip (part A) was exposed to solvent vapor in a chamber containing a reservoir of cyclohexane (Sigma-Aldrich, Saint Louis, MO). After a prescribed amount of time, part A was removed from the chamber and brought into contact with the mating half of the chip (part B) to enclose the channels. The enclosed chip was then placed in a custom-made bonding jig and covered with a sheet of soft material (neoprene, McMaster-Carr, Atlanta, GA) to equalize the pressure distribution. This assemblysbonding plate, chip, neopreneswas then placed in a press (Carver, Wabash, IN) for 3 min at 178 kPa. Immediately after bonding, some chips were irradiated (13.5 J/cm2 at 260 nm) with deep UV (DUV) light using a semiconductor lithography light source (Optical Associates, Inc., San Jose, CA) fitted with a 500 W Hg-Xe lamp (USHIO America, Cypress, CA). Several chips were bonded for each of the six various bonding conditions. (52) Morales, A. M; Simmons, B. A.; Wallow, T. I; Krafcik, K. L.; Domier, L. A. In Micro Total Analytical Systems; Baba, Y., van den Berg, A., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2006; Vol. 1, pp 155157. (53) Kim, S. J.; Yang, H.; Kim, K.; Lim, Y. T.; Pyo, H. B. Electrophoresis 2006, 27, 3284-3296. (54) Mair, D. A.; Geiger, E.; Pisano, A. P.; Fre´chet, J. M. J.; Svec, F. Lab Chip 2006, 6, 1346-1354.

Burst Pressure Measurement. The pressure at which the chip fails was measured as described elsewhere.54 Briefly, a pressure transducer was connected in-line with a pump and a chip containing a single channel and two integrated I/O ports. After pumping water through the device at a flow rate of 1 µL/min for 1 to 2 min, a plug fitting that blocked flow was threaded into the remaining port of the device and the internal pressure buildup was recorded. The burst pressure is defined as the maximum internal pressure recorded prior to dramatic device failure due to delamination at the bonding interface. The burst pressure reported for each bonding condition is the average of 10 measurements. The yield for each bonding condition is calculated by dividing this number by the number of chips bonded using the particular bonding condition. Film Preparation for SEC-MALS and AFM. In an effort to reconstruct the chip interface for further characterization, polymer films (50 µm thick) were prepared by spin-coating (1500 rpm, 30 s) a 20 wt % COC solution in cyclohexane onto silicon wafers at 1500 rpm for 30 s. Films were dried on a hotplate (75 °C, 30 min) before being exposed to cyclohexane vapor for a prescribed time ranging and irradiated for 15 min. The residual solvent was removed by baking (75 °C, 30 min). For characterization by size exclusion chromatography (SEC) the dry film was carefully removed from the wafer and dissolved in cyclohexane. Molar Mass Characterization Using SEC-MALS. Characterization of molar mass was performed using a SEC system including multiangle light scattering (MALS) and refractive index (RI) detectors for determination of absolute molar mass and concentration, respectively. The SEC system consisted of a pump (Waters, Milford, MA) and injector (Rheodyne, Rohnert Park, CA) fitted with a 100 µL sample loop. Two columns were connected in series (PL-gel 103 and 105 Å, 5 mm, 7.8 × 300 mm, Polymer Laboratories, Church Stretton, Shropshire, U.K.) and thermostated at 70 °C to lower the pressure. The MALS detector (Dawn EOS, Wyatt Technology Corporation, Santa Barbara, CA) was used at ambient conditions, and the RI detector (Optilab DSP, Wyatt Technology) was thermostated at 35 °C. Cyclohexane was used as the mobile phase and delivered to the system at a flow rate of 1.0 mL/min. All sample solutions were prefiltered through a 0.2 µm pore size PTFE filter (Whatman, Clifton, NJ) prior to injection. The data were analyzed using commercial software (ASTRA v4, Wyatt Technology) using measured dn/dc value of 0.118 mL/g for the Zimm plots and calculation of molar mass. Atomic Force Microscope Nanoindentation. An atomic force microscope (AFM) method similar to that reported by Brown et al.15 was used to monitor the film hardness as it was exposed to solvent vapor. Film AFM was performed using a Digital Instruments Multimode instrument with a Nanoscope IIIA controller. A single-crystal quartz fluid cell was used to enclose the controlled atmosphere applied to the sample surface. Intentionally blunted diamond-like carbon-coated tapping mode probes (Veecoprobes) with an original radius of curvature of ca. 100 nm and force constant of 20-80 N/m were used. The force constant of the cantilevers k was precisely determined using the Sader method.55 The resonance frequency and the Q factor were measured in air using the AFM, and the lateral dimensions were verified using a scanning electron microscope (SEM). The SEM was also used to determine the geometry of the apexes to derive

the indentation area Ah versus indentation depth h function. For the probe used in the reported measurements, k ) 65 N/m and tip radius of curvature of ca. 300 nm were estimated. The tip was modeled as a sphere assuming the conical part did not indent the surface. The maximum penetration depth of about 200 nm was always smaller than the radius of curvature of the probe. The indentation area was calculated as

Ah ) π[R2 - (R - h)2] where R is the measured radius of curvature of the tip. Experiments were performed by flowing either dry nitrogen or nitrogen saturated with cyclohexane through the fluid cell to expose the samples to the different environment. Particular care was taken in order to avoid any macroscopic solvent condensation on the surfaces. The hardness was calculated by measuring the maximum load at the end of the indentation curve divided by the indentation area at the deepest penetration value. RESULTS AND DISCUSSION Bonding Quality. Thermal fusion is one of the most popular bonding techniques for sealing the channels of plastic microfluidic chips because it is easily implemented and precludes the potential for chemical contamination from adhesives applied in bonding techniques that use an intermediate layer. However, bonding chips by thermal fusion places stringent requirements on the bonding apparatus and microfabricated part that challenge its implementation for volume manufacture of high-quality devices. Because the plastic chip is heated close to or slightly above the glass transition temperature (Tg) of the material, the entire chip becomes soft and very sensitive to small heterogeneities in load distribution during the bonding process. To evenly distribute the load, the chip, bonding jig, and plates of the bonding press must all be precisely parallel across the length of the chip; this requirement is difficult to achieve. Deviations from these specifications result in severe feature distortion as illustrated in a comparison of the panel A with panel C in Figure 1. The anticipated reduction in device performance caused by such channel irregularities59 motivated development of a new technique to obviate the use of heat to bond polymer-based microfluidic chips. Comparison of panels A, C, and E in Figure 1 clearly illustrates the ability of this method to eliminate undesirable channel distortion defects. Another stringent requirement imposed by thermal bonding is minimal residual stress in the parts to be bonded.42 Residual internal stress prior to bonding can lead to bulk material reflow and warpage of the parts upon reheating above Tg. This phenomenon is common in thin parts formed by hightemperature microfabrication processes such as microinjection molding where nonisothermal flow of the polymer melt in the mold cavity introduces stresses that are partly frozen-in upon cooling.57 In order to circumvent bulk material reflow observed during thermal bonding, we optimized and evaluated a room-temperature (55) Sader, J. E.; Chon, J. W.; Mulvaney, P. Rev. Sci. Instrum. 1999, 70, 3967-69. (56) Ross, D.; Ivory, C. F.; Locascio, L. E.; Van Cott, K. E. Electrophoresis 2004, 25, 3694-3704. (57) Zoetelief, W. F.; Douven, L. F.; Housz, A. J. Polym. Eng. Sci. 1996, 36, 1886-96.

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Figure 1. Optical microscope images of double-offset tee region of chips before bonding (A), after thermal bonding (C), and after solvent vapor bonding (E). SEM images of channel cross before bonding (B), after thermal bonding (D), and after solvent vapor bonding (F). All SEM images were taken at a magnification of 800×.

solvent vapor bonding method that relegates the high polymer chain mobility required for bonding to the bonding interface. A comparison of the cross-sectional SEM micrographs in panels Band D in Figure 1 illustrates the severe channel shrinkage caused by thermal bonding, whereas the high feature fidelity is afforded by solvent vapor bonding as illustrated in panel F. In order to quantify the fidelity of the channel cross-sectional geometry along its length, a chip was cut into 5 mm increments and imaged by SEM. Figure 2 shows the average channel height and width of 82 ( 2 µm and 59 ( 3 µm, respectively. The low relative standard deviations of 2% and 5%, respectively, demonstrates high size uniformity of channels. Burst Pressure Measurement and Yield. The ability for a microfluidic system to sustain high pressure is important when pumping liquids through the channels containing porous materials. Burst pressure measurements enable us to evaluate the effect that solvent exposure time and DUV irradiation has on bond 5100

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strength. Figure 3A shows that the average burst pressure is significantly improved after 90 s of exposure to solvent vapor and levels off thereafter. A further positive shift of about 50% in burst pressure is observed for chips irradiated with DUV light, presumably resulting from UV-induced chain scission and cross-linking reactions at the bonded interface. Figure 3B shows evolution of the internal pressure for a device bonded using 90 s solvent exposure and 15 min of DUV irradiation. This device exhibited no leakage prior to a dramatic failure at 34.6 MPa, the highest value of in-channel back pressure of any microfluidic device reported to date. In the process of optimizing this bonding process, we found that extended solvent exposure times resulted in parts that were too soft to bond and extended irradiation times did not markedly improve bond strength. Thermal and solvent vapor bonding methods were compared by plotting bond strength and yield for each bonding condition. The burst pressure, yield, and bonding conditions for thermally

Figure 2. Channel height (0) and width (9) along the channel measured using SEM micrographs of chip cross sections taken at 5 mm intervals.

Figure 4. Molar mass of DUV irradiated COC thin films (b) and original COC prior to DUV irradiation (O).

Figure 5. Surface hardness of COC films exposed to solvent vapor determined by AFM nanoindentation.

Figure 3. Burst pressure measurements for chips bonded by exposure to solvent vapor with (9) and without DUV irradiation (0) (A), the maximum burst pressure for chips exposed to solvent vapor for 90 s and DUC irradiation for 15 min (B), and yield and burst pressure for several thermal (9) and solvent vapor (0) bonding conditions (C).

bonded chips have been omitted here for brevity but can be found in our previous work.54 As illustrated in Figure 3C, the solvent vapor bonding method is superior to thermal bonding both in burst pressure and yield. The yield for the solvent vapor bonding method is exceptionally favorable, typically exceeding 85%. It is also important to note that chips bonded using solvent vapor and DUV are considerably more robust. Chips that were incompletely bonded due to human error were not tested and lowered the yield. A process parameter that is critical to successful chip bonding is the time that elapsed between part removal from the solvent vapor exposure chamber and load

application in the bonding press. When this transport time exceeded the range of 30-45 s, the exposed surface was not tacky because the solvent at the surface had both evaporated into the atmosphere and diffused into the bulk of the part. This phenomenon leads to reduced chain mobility at the bonding interface and is primarily responsible for incomplete bonding and suboptimal yields. The high yield we obtained with this bonding method give us confidence that the human effect introduced by this process restriction can be readily eliminated by automation in an industrial setting. Molecular Characterization Using SEC-MALS. Chain scission and cross-linking reactions in polymers exposed to UV light has been well-documented.55-60 To investigate the mechanism of bond strengthening by DUV irradiation of the interface, a model system was created by spin-coating a thin film of COC onto a silicon wafer. These films were processed in a manner similar to the chips and subsequently used for SEC-MALS characterization of the molar mass of the polymer. Comparison with the material not exposed to DUV light in Figure 4 indicates that DUV irradiation in the absence of solvent vapor is sufficient to induce limited cross-linking. As hypothesized, the presence of solvent expedites cross-linking and chain scission reactions simultaneously as evidenced by the molar mass fluctuation between 107 000 and 95 000. While the change in Mw is not large enough to unequivocally conclude that DUV irradiation induces crosslinking and chain scission reactions, the data also suggests that these mechanisms are a plausible explanation for the improvement in bond strength upon irradiation with DUV light. (58) (59) (60) (61)

Allen, N. S. Eng. Plast. 1995, 8, 247-286. Onyiriuka, E. C. J. Appl. Polym. Sci. 1993, 47, 2187-2194. Torikai, A. Angew. Makromol. Chem. 1994, 216, 225-241. Andrady, A.; Amin, M. B.; Hamid, S. H.; Hu, X. Z.; Torikai, A. Ambio 1995, 24, 191-196. (62) Ferry, L.; Vigier, G.; Bessede, J. L. Polym. Adv. Technol. 1996, 7, 493-500. (63) Rabello, M. S.; White, J. R. Polym. Degrad. Stab. 1997, 56, 55-73.

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Nanoindentation with AFM. The use of solvent vapor is a convenient room-temperature method for bonding microfluidic chips. The main advantage is the minimal amount of solvent used that facilitates controlled exposure of the bonding surface and prevents channel clogging. The volume of solvent used is very small and unlikely to trigger any permanent changes in channel shape. This hypothesis was tested by AFM to measure the surface hardness of COC films exposed to cyclohexane vapor. Initially, three measurements of the hardness were carried out with no solvent in nitrogen flow. The COC film was then exposed to solvent in situ by flowing nitrogen saturated with cyclohexane through the AFM fluid cell. After 15 min, pure nitrogen was flushed through the chamber and recovery of the substrate hardness was observed. Figure 5 clearly shows a reduction in surface hardness upon film exposure to solvent vapor followed by a return to the original values upon removal of solvent vapor from the fluid cell. These measurements also document that the change in surface properties of the film is reversible. This is an important factor affecting the bond strength important for use of the chips. CONCLUSION The bonding method we have developed is reliable, convenient, and rapid to seal plastic microfluidic chips at room temperature while preserving the molded features. The ability to relegate the induced softness exclusively to the bonding surface in a controlled manner also relaxes the minimal residual internal stress requirement for molded parts, thereby facilitating the use of injection

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molding for fabrication. Postbonding irradiation with DUV light then significantly strengthens the bond. Although demonstrated with chips fabricated from COC, this technique is generic and can be easily applied to the bonding of chips made of other plastics such as methyl methacrylate, polyethylene, and polypropylene. Chips resistant to high pressure demonstrated in this work now open the door to pursue processes on a microfluidic chip that require pressurized flow such as high-performance liquid chromatography. Results of this research will be published elsewhere. ACKNOWLEDGMENT Support of this research by a Grant of the National Institute of Biomedical Imaging and Bioengineering, National Institutes of Health (EB-006133) is gratefully acknowledged. The authors also thank the National Center for Electron Microscopy, Lawrence Berkeley National Laboratory for use of the microtome. Nanoindentation studies by AFM were made possible by an INTEL postdoctoral fellowship extended to M.R. Work at the Molecular Foundry was supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

Received for review February 2, 2007. Accepted April 13, 2007. AC070220W