Langmuir 2009, 25, 3861-3866
3861
Novel Poly(dimethylsiloxane) Bonding Strategy via Room Temperature “Chemical Gluing” Nae Yoon Lee*,† and Bong Hyun Chung‡,§ Gachon BioNano Research Institute & DiVision of BioNano Technology and College of BioNano Technology, Kyungwon UniVersity, Gyeonggi-do 461-701, BioNanotechnology Research Center, Korea Research Institute of Bioscience and Biotechnology (KRIBB), Daejeon 305-600, and Nanobiotechnology, School of Engineering, Korea UniVersity of Science and Technology (UST), Daejeon 305-333, Korea ReceiVed August 28, 2008. ReVised Manuscript ReceiVed NoVember 29, 2008 Here we propose a new scheme for bonding poly(dimethylsiloxane) (PDMS), namely, a “chemical gluing”, at room temperature by anchoring chemical functionalities on the surfaces of PDMS. Aminosilane and epoxysilane are anchored separately on the surfaces of two PDMS substrates, the reaction of which are well-known to form a strong amine-epoxy bond, therefore acting as a chemical glue. The bonding is performed for 1 h at room temperature without employing heat. We characterize the surface properties and composition by contact angle measurement, X-ray photoelectron spectroscopy analysis, and fluorescence measurement to confirm the formation of surface functionalities and investigate the adhesion strength by means of pulling, tearing, and leakage tests. As confirmed by the above-mentioned analyses and tests, PDMS surfaces were successfully modified with amine and epoxy functionalities, and a bonding based on the amine-epoxy chemical gluing was successfully realized within 1 h at room temperature. The bonding was sufficiently robust to tolerate intense introduction of liquid whose per minute injection volume was almost 2000 times larger than the total internal volume of the microchannel used. In addition to the bonding of PDMS-PDMS homogeneous assembly, the bonding of the PDMS-poly(ethylene terephthalate) heterogeneous assembly was also examined. We also investigate the potential use of the multifunctionalized walls inside the microchannel, generated as a consequence of the chemical gluing, as a platform for the targeted immobilization.
Introduction Bonding is one of the important issues in microfluidic device fabrication. Because of a complicated fabrication process such as anodic or fusion bonding and high temperature and pressure and/or voltage involved for bonding glass-based or Si/SiO2based microdevices, polymer materials have received a great amount of attention in microdevice fabrication. Among many polymer materials, poly(dimethylsiloxane) (PDMS) has been most widely utilized for microdevice fabrication owing to its easy replicability, conformal contact with various substrates, high optical transparency, high thermal resistance, inexpensiveness, and biocompatibility.1-3 The bonding of the PDMS substrate can be easily achieved by generating a surface hydroxyl group by means of plasma treatment,4-6 corona discharges,7-9 and UV/ozone treatments,10-14 generally followed by thermal curing. * To whom correspondence should be addressed. Phone: +82-31-7508556. Fax: +82-31-750-8774. E-mail:
[email protected]. † Kyungwon University. ‡ KRIBB. § UST.
(1) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550. (2) Whitesides, G. M. Nature 2006, 442, 368. (3) El-Ali, J.; Sorger, P. K.; Jensen, K. F. Nature 2006, 442, 403. (4) Duffy, D. C.; McDonald, J. C.; Schueller, O. J. A.; Whitesides, G. M. Anal. Chem. 1998, 70, 4974. (5) Unger, M. A.; Chou, H.-P.; Thorsen, T.; Scherer, A.; Quake, S. R. Science 2000, 288, 113. (6) Makamba, H.; Kim, J. H.; Lim, K.; Park, N.; Hahn, J. H. Electrophoresis 2003, 24, 3607. (7) Hillborg, H.; Gedde, U. W. Polymer 1998, 39, 1991. (8) Bodas, D.; Khan-Malek, C. Sens. Actuators, B 2007, 123, 368. (9) Hillborg, H.; Ankner, J. F.; Gedde, U. W.; Smith, G. D.; Yasuda, H. K.; Wikstro¨m, K. Polymer 2000, 41, 6851. (10) Moorcroft, M. J.; Meuleman, W. R. A.; Latham, S. G.; Nicholls, T. J.; Egeland, R. D.; Southern, E. M. Nucleic Acids Res. 2005, 33, 75. (11) Yoo, J.-C.; Her, H.-J.; Kang, C. J.; Kim, Y. S. Sens. Actuators, B 2008, 130, 65. (12) Ouyang, M.; Yuan, C.; Muisener, R. J.; Boulares, A.; Koberstein, J. T. Chem. Mater. 2000, 12, 1591.
Bonding based on these methods is fast, robust, and nearly permanent. Also, these methods not only are highly convenient for bonding PDMS-PDMS homogeneous assemblies, but also have wide applicability in bonding PDMS with various hard and soft substrates such as glass, plastic, metal, and silicon wafers. Despite many advantages, however, the above-mentioned bonding method requires immediate contact because the oxidized surfaces undergo hydrophobic recovery with time.7–9,14,15 Also, a subsequent thermal curing process is generally required for permanent bonding. Thermal curing, on some occasions, could become a great disadvantage when immobilization of heatsensitive cells or biomolecules or packing of thermally weak structural features such as electrodes and waveguides follows prior to the bonding. To overcome these shortcomings, many researchers have endeavored to find alternative methods to realize bonding between the PDMS substrates. Norton group16 generated an aluminum film-deposited PDMS surface and activated the surface via Ar plasma prior to bonding because the aluminum thin film acted as a barrier to the hydrophobic recovery of the PDMS for a relatively long period of time. The Zare group17 and Stemme group18 both tried, in slightly different ways, to use the PDMS prepolymer itself as an adhesive to bond two PDMS substrates permanently. Although the method itself is very simple, it still requires heating for the bonding, and if not thermally cured, it takes an overnight period of time to be cured completely. (13) Phely-Bobin, T. S.; Muisener, R. J.; Koberstein, J. T.; Papadimitrakopoulos, F. AdV. Mater. 2000, 12, 1257. (14) Hillborg, H.; Tomczak, N.; Ola´h, A.; Scho¨nherr, H.; Vancso, G. J. Langmuir 2004, 20, 785. (15) Bodas, D.; Khan-Malek, C. Microelectron. Eng. 2006, 83, 1277. (16) Patrito, N.; McLachlan, J. M.; Faria, S. N.; Chan, J.; Norton, P. R. Lab Chip 2007, 7, 1. (17) Wu, H.; Huang, B.; Zare, R. N. Lab Chip 2005, 5, 1393. (18) Samel, B.; Chowdhury, M. K.; Stemme, G. J. Micromech. Microeng. 2007, 17, 1710.
10.1021/la802823e CCC: $40.75 2009 American Chemical Society Published on Web 02/17/2009
3862 Langmuir, Vol. 25, No. 6, 2009
Lee and Chung
Here, we propose a novel PDMS bonding strategy via room temperature “chemical gluing”. This new bonding strategy is realized by anchoring amine-terminated silane on one PDMS substrate and epoxy-terminated silane on the other PDMS substrate via silane coupling reaction followed by amine-epoxy bond formation. It is well-known that the reaction between the amine and the epoxy groups leads to the formation of a strong amine-epoxy bond.19,20 In our previous study,21-23 we employed amine-epoxy bond formation for generating an antiadhesion thin layer on a nanopatterned mold for imprint lithography, which was proven to be highly effective. Although we performed the reaction at an increased temperature, it is also known that amine-epoxy bond formation can proceed at room temperature.19,20 In this study, we show the reliability of room temperature chemical gluing based on amine-epoxy bonding by performing pulling, tearing, and leakage tests and characterize surface functionalities by contact angle measurement, X-ray photoelectron spectroscopy (XPS) analysis, and fluorescence measurement. In addition, we demonstrate the potential use of the multifunctionalized walls inside the microchannel, generated as a consequence of chemical gluing, as a platform for targeted immobilization.
Experimental Section Materials. SU-8 2050 and SU-8 developer were purchased from MicroChem (Newton, MA). PDMS prepolymer (Sylgard 184) and a curing agent were purchased from Dow Corning (Midland, MI). (3-Aminopropyl)triethoxysilane (APTES; 97%), (3-glycidoxypropyl)trimethoxysilane (GPTMS; 98%), and N-[3-(dimethylamino)propyl]-N′-ethylcarbodiimide hydrochloride (EDC) were purchased from Aldrich. N-Hydroxysulfosuccinimide (NHS; 98.5%) was purchased from Fluka. Amine-functionalized fluorosphere and carboxylate-functionalized fluorosphere were purchased from Molecular Probes. Phosphate-buffered saline (PBS) buffer solution (pH 7.4) was obtained from Invitrogen. The poly(ethylene terephthalate) (PET) film was purchased from SKC (Korea). Bonding Strategy. The PDMS microchannel was fabricated using conventional photolithography and replica molding.24 In brief, the silicon master structure was fabricated using a negative photoresist, SU-8 2050, and a 10:1 (w/w) mixture of PDMS prepolymer, and a curing agent was poured onto the master. After thermal curing at 80 °C for 30 min, the PDMS replica was peeled off. Figure 1 shows the schematic flow of chemical gluing. First, the surfaces of the PDMS replica and a flat PDMS were treated with oxygen (O2) plasma for 1 min (Figure 1a). After generation of hydroxyl groups on the surfaces of two PDMS substrates, 1% (v/v) aqueous solutions of APTES and GPTMS were poured onto the O2-plasma-treated PDMS surfaces (Figure 1b). After reaction for 20 min, APTES- and GPTMSanchored PDMS surfaces were washed thoroughly with distilled water and dried completely using an air gun. Next, the APTESanchored PDMS and GPTMS-anchored PDMS substrates came into conformal contact (Figure 1c) and were bonded for 1 h at room temperature (Figure 1d). Surface Characterizations. Contact Angle Measurement. The water contact angles were measured on the surfaces of bare PDMS, O2-plasma-treated PDMS, APTES-anchored PDMS, and GPTMSanchored PDMS by the sessile drop (5 µL) technique using a Kru¨ss DSA10-MK2 contact angle measuring system (Kru¨ss GmbH, Germany) and analyzed with drop shape analysis software. Three measurements were made and averaged. (19) Mateo, C.; Torres, R.; Ferna´ndez-Lorente, G.; Ortiz, C.; Fuentes, M.; Hidalgo, A.; Lo´pez-Gallego, F.; Abian, O.; Palomo, J. M.; Betancor, L.; Pessela, B. C. C.; Guisan, J. M.; Ferna´ndez-Lafuente, R. Biomacromolecules 2003, 4, 772. (20) Mateo, C.; Abian, O.; Ferna´ndez-Lorente, G.; Pedroche, J.; Ferna´ndezLafuente, R.; Guisan, J. M. Biotechnol. Prog. 2002, 18, 629. (21) Lee, M. J.; Lee, N. Y.; Lim, J. R.; Kim, J. B.; Baik, H. K.; Kim, Y. S. AdV. Mater. 2006, 18, 3115. (22) Lee, N. Y.; Kim, Y. S. Macromol. Rapid Commun. 2007, 28, 1995. (23) Lee, N. Y.; Kim, Y. S. Nanotechnology 2007, 18, 415303.
Figure 1. (a) Surface hydrophilization of PDMS surfaces by O2 plasma treatment for 1 min. (b) APTES and GPTMS anchoring on the O2plasma-treated PDMS surfaces. (c) Conformal contact of the APTESanchored and GPTMS-anchored PDMS substrates at room temperature for 1 h. (d) Bonding of the APTES-anchored and GPTMS-anchored PDMS substrates.
XPS Analysis. X-ray photoelectron spectroscopy (XPS) analysis was conducted using a PHY 5700 (PHI, Chanhassen, MN) equipped with an aluminum X-ray radiation source (1486.6 eV) and pass energy of 23.5 eV. The pressure in the chamber was below 1.3 × 10-9 Torr before the data were taken, and the voltage and current of the anode were 15 kV and 26.7 mA, respectively. The takeoff angle was set at 45°. The binding energy of Au 4f7/2 (84.0 eV) was used as the reference. The resolution for the measurement of the binding energy was about (0.6 eV. XPS analyses were interpreted using an ESCA1 (PHI). Fluorescence Measurement. Surface modification was further confirmed by fluorescence measurement using amine- and carboxylate-functionalized fluorospheres. Amine-functionalized fluorospheres were diluted 50-fold with distilled water, and carboxylatefunctionalized fluorospheres were diluted 50-fold in a 1:1 (v/v) mixture solution of 150 mM EDC and 60 mM NHS, both prepared in PBS buffer solution (pH 7.4), to enhance the binding of the carboxylate-functionalized fluorospheres with amine functionality by activating the carboxyl (-COOH) group.25 After 30 min, the excess EDC and NHS were removed by rinsing with PBS buffer and then with distilled water. Fluorescence was measured using a Nikon Eclipse TE 2000-U fluorescence microscope and was analyzed using NIS-Elements software. Bonding Strength Analysis. The bonding strength was analyzed by means of pulling, tearing, and leakage tests. The pulling test was conducted using a texture analyzer (QTS 25, Brookfield, Middleboro, MA). A thick twine was inserted into the PDMS prepolymer and cured thermally in the process of PDMS curing. After bonding of (24) Lee, N. Y.; Yamada, M.; Seki, M. Anal. Sci. 2004, 20, 483. (25) Lee, N. Y.; Lim, J. R.; Kim, Y. S. Biosens. Bioelectron. 2006, 21, 2188.
PDMS Bonding Via Room Temperature Chemical Gluing
Langmuir, Vol. 25, No. 6, 2009 3863
Figure 2. Contact angle measurements. Water contact angles measured on the surfaces of (a) bare PDMS, (b) O2-plasma-treated PDMS, (c) APTES-anchored PDMS, and (d) GPTMS-anchored PDMS.
two PDMS substrates, they were pulled at a speed of 100 mm min-1. The leakage test was performed by introducing an ink solution at various flow rates. Red ink was used for visual effect. A syringe pump (KDS 200, KD Scientific, New Hope, PA) was used to introduce the ink solution into the microchannel. Flow rates were varied as follows: 25, 125, 250, 500, 1250, 2500, and 5000 µL min-1. The retention time inside the microchannel was controlled to be approximately 5 min for each flow rate condition. Targeted Immobilization inside a Microchannel. First, an APTES-anchored PDMS microchannel and GPTMS-anchored flat PDMS were bonded by chemical gluing. Next, an amine-functionalized fluorosphere solution was introduced into the microchannel using a disposable syringe. After reaction for 20 min, the microchannel was washed thoroughly with water and dried completely with air. Subsequently, a carboxylate-functionalized fluorosphere solution was introduced, reacted for 20 min, and washed thoroughly with water and dried. After introduction of both solutions consecutively, the fluorescence was detected inside the microchannel at a fixed position using two fluorescence filters.
Results and Discussion Surface Characterizations. Figure 2 shows the results of water contact angle measurements. The water contact angles measured on the surfaces of bare PDMS (Figure 2a), O2-plasmatreated PDMS (Figure 2b), APTES-anchored PDMS (Figure 2c), and GPTMS-anchored PDMS (Figure 2d) were 116.6°,
