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Langmuir 2008, 24, 9154-9161
Internal Modification of Poly(dimethylsiloxane) Microchannels with a Borosilicate Glass Coating J.-B. Orhan,* V. K. Parashar, J. Flueckiger, and M. A. M. Gijs Laboratory of Microsystems, Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland ReceiVed April 28, 2008. ReVised Manuscript ReceiVed June 4, 2008 We report on an original technique for the in situ coating of poly(dimethylsiloxane) (PDMS) microchannels with borosilicate glass, starting from an active nonaqueous and alkali-free precursor solution. By chemical reaction of this active solution inside the microchannel and subsequent thermal annealing, a protective and chemically inert glass borosilicate coating is bonded to the PDMS. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and nuclear magnetic resonance spectroscopy of the active solution show that it is composed of a silicon oxide network with boron connectivity. Thermal gravimetric analysis demonstrates the absence of organic content when curing is done above 150 °C. The borosilicate nature of the glass coating covalently bonded to the PDMS is demonstrated using ATR-FTIR spectroscopy and X-ray photoelectron spectroscopy. Atomic force microscopy and scanning electron microscopy show a smooth and crack-free coating. The latter is used as an efficient protective barrier against diffusion in PDMS of fluorescent rhodamine B dye that is dissolved either in water or in toluene. Moreover, the coating prevents swelling and consequent structural damage of the PDMS when the latter is exposed to harsh chemicals such as toluene.
Introduction Microfluidics is of increasing interest for chemical and biological applications. Microfluidic devices in glass are robust and chemically inert, but require time-consuming clean-room microfabrication techniques.1 For economic reasons, polymeric materials have also been investigated,2 but most of them cannot be used in harsh chemical conditions. In particular, poly(dimethylsiloxane) (PDMS) replica molding3 allows rapid prototyping of microfluidic chips.4 PDMS is easy to prepare and is mechanically stable and transparent in the ultraviolet (UV)-visible region.4 However, chips in PDMS are restricted in use to polarsolvent-based solutions, as nonpolar solvents interact with the polymer network. This leads to diffusion of solvent into the PDMS, inducing swelling and structural damage,5 which explains the interest of microchannel wall protection to reduce these effects. A modification of the bulk PDMS has been studied to improve its compatibility with the use of organic solvents in such modified PDMS microfluidic chips. This transformation can be performed by blending of polymers, by copolymerization, or by forming interpenetrating networks.6,7 However, this approach is not appropriate here, either because we lose the benefits of replication of pure PDMS or because a modification of the complete polymer network is simply not necessary. A surface method by grafting a polymer layer on PDMS after an activation treatment, to retard swelling, has also been investigated. Lee et al.8 modified PDMS with poly(urethane acrylate), but this technique presents the (1) Madou, M. J. Fundamentals of Microfabrication: The Science of Miniaturization, 2nd ed.; CRC Press LLC: London, 2002. (2) Olivier Geschke, H. K.; Tellemann, P. Microsystem Engineering of Labon-a-Chip DeVices; Wiley-VCH: Weinheim, Germany, 2004. (3) Orhan, J. B.; Parashar, V. K.; Sayah, A.; Gijs, M. A. M. J. Microelectromech. Syst. 2006, 15(5), 1159–1164. (4) McDonald, J. C.; Duffy, D. C.; Anderson, J. R.; Chiu, D. T.; Wu, H.; Schueller, O. J. A.; Whitesides, G. M. Electrophoresis 2000, 21(1), 27–40. (5) Lee, J. N.; Park, C.; Whitesides, G. M. Anal. Chem. 2003, 75(23), 6544– 6554. (6) Abbasi, F.; Mirzadeh, H.; Katbab, A. A. Polym. Int. 2001, 50(12), 1279– 1287. (7) Roman, G. T.; Hlaus, T.; Bass, K. J.; Seelhammer, T. G.; Culbertson, C. T. Anal. Chem. 2005, 77(5), 1414–1422. (8) Lee, J.; Joon Kim, M.; Lee, H. H. Langmuir 2006, 22(5), 2090–2095.
disadvantage of requiring an oxygen plasma treatment. Moreover, the polymer used is also not optimum for biological applications and cannot easily be functionalized. Improving the surface properties of PDMS microfluidic devices was also investigated using sol-gel technology for surface treatment. A 300 ( 200 nm thick glasslike coating on PDMS microchannels using transition metal sol-gel chemistry has been demonstrated.7,9 However, all the reported methods present a relatively low annealing temperature7,9,10 (100 ( 5 °C), leading to coatings that are still in the gel form11 and consequently susceptible to swelling. Furthermore, the reported methods do not lead to covalent bonding of the coating to the polymer matrix7,9 or require the application of an external activation treatment.10 These two factors reduce the ability of such coatings to prevent swelling, diffusion, and PDMS network destruction in harsh chemical conditions. Conventional sol-gel techniques do not allow the formation of a glass layer at a sufficiently low temperature (
