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Squeeze-Film Hydrogel Deposition and Dry Micropatterning Zhenwen Ding,†,‡ Amani Salim,‡,§ and Babak Ziaie*,‡,| Department of Physics, School of Electrical and Computer Engineering, Weldon School of Biomedical Engineering, and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907 In this technical note, we demonstrate a squeeze-film based spacer-free method for creating controllable submicrometer hydrogel films on planar substrates that can be used to photolithographically fabricate hydrogel microstructures. This new technique improves the photolithographic resolution and yield by providing a uniform and low-defect hydrogel film. The optimum polymerization initiation time for achieving such a layer was determined to be around 1 min. For patterning, the dried hydrogel film was coated with a parylene-C masking layer. Subsequent etching in oxygen plasma was used to transfer selected patterns of hydrogel to the substrate in a batch scale. Hydrogels are three-dimensional, hydrophilic, polymeric networks capable of imbibing large amounts of water (usually more than 20% of the total weight) or biological fluids while remaining insoluble.1-3 Three dimensional networks are usually formed by chemical or physical cross-linking of hydrophilic polymer chains.4 Some hydrogels due to the presence of different thermodynamically active functional groups on polymer chains have additional properties, such as swelling or deswelling in response to the changes in environmental conditions.5-7 These hydrogels can be designed to respond to a wide range of environmental stimuli, such as pH,8 temperature,9 specific ionic strength,10,11 electric field,12 magnetic field,13 light,14,15 and so on. The applications of hydrogel as a smart component in micrototal-analysis-systems * Corresponding author. Phone: (765)404-0726. E-mail:
[email protected]. † Department of Physics. ‡ Birck Nanotechnology Center. § Weldon School of Biomedical Engineering. | School of Electrical and Computer Engineering. (1) Peppas, N. A.; Bures, P.; Leobandung, W.; Ichikawa, H. Eur. J. Pharm. Biopharm. 2000, 50, 27–46. (2) Peppas, N. A. Hydrogels in Medicine and Pharmacy; CRC Press: Boca Raton, FL, 1986. (3) Ottenbrite, R. M.; Huang, S. J.; Park, K. Hydrogels and Biodegradable Polymers for Bioapplications; American Chemical Society: Washington, DC, 1996. (4) Hennink, W. E.; van Nostrum, C. F. Adv. Drug Delivery Rev. 2002, 54, 13–16. (5) Osada, Y.; Russ-Murphy, S. B. Sci. Am. 1993, 268, 82–87. (6) Osada, Y.; Gong, J. Prog. Polym. Sci. 1993, 18, 187–226. (7) Qiu, Y.; Park, K. Adv. Drug Delivery Rev. 2001, 53, 321–339. (8) Richter, A.; Paschew, G.; Klatt, S.; Lienig, J.; Arndt, K.-F.; Alder, H.-J. P. Sensors 2008, 8, 561–581. (9) Klouda, L.; Mikos, A. G. Eur. J. Pharm. Biopharm. 2008, 68, 34–45. (10) Holtz, J. H.; Asher, S. A. Nature 1997, 389, 829–832. (11) Mayes, A. G.; Blyth, J.; Millington, R. B.; Lowe, C. R. Anal. Chem. 2002, 74, 3649–3657. (12) Murdan, S. J. Controlled Release 2003, 92, 1–17. 10.1021/ac100579v 2010 American Chemical Society Published on Web 03/23/2010
(µTAS) is generally dependent on the volume change resulting from the uptake or loss of water from a hydrogel network which is fundamentally a diffusive process.16,17 In macroscopic systems, the application of hydrogel has been hindered by the slow response time due to the long diffusion paths. In order to achieve a much shorter response time, it is therefore imperative to scale down the hydrogel component to microscale dimensions.18,19 Patterning hydrogel using standard photolithographic processes not only helps to scale down the hydrogel dimensions to the micro/nano regime but can also allow for selective creation of specific hydrogel compositions on targeted areas. UV lithography through a shadow mask has been used for creating smart hydrogel structures inside microfluidic channels for active flow control16,18,20 and biosensing applications.21 Although in some aspect a convenient method, this technique lacks resolution beyond tens of micrometers and the requirement for photopolymerizability of hydrogel increases the complexity of the chemical system and limits its application. An alternative technique is the deposition of a thin hydrogel layer and its subsequent patterning using dry oxygen plasma. Central to all high-resolution lithographic techniques is the ability to deposit a thin and uniform hydrogel layer. This has typically been achieved through spin coating or spacer casting. Although spinning has been used to create thin films of hydrogel down to 25 nm,22 in order to achieve submicrometer thicknesses, it is imperative to use UV photopolymerizable hydrogels since other formulations result in a fast dry-out before full polymerization and cross-linking. In addition, spinning aqueous hydrogel solutions requires careful consideration of surface chemistry and polymer rheology (in particular viscosity) in order to achieve a uniform hydrogel layer. The thickness of spacer cast hydrogel on the other hand is limited by the spacer thickness and is typically several tens of micrometers. Furthermore, existence of a thin extra (13) Liu, T.-Y.; Hu, S.-H.; Liu, T.-Y.; Liu, D.-M.; Chen, S.-Y. Langmuir 2006, 22, 5974–5978. (14) Suzuki, A.; Tanaka, T. Nature 1990, 346, 345–347. (15) Szilagyi, A.; Sumaru, K.; Sugiura, S.; Takagi, T.; Shinbo, T.; Zrinyi, M.; Kanamori, T. Chem. Mater. 2007, 19, 2730–2732. (16) Eddington, D. T.; Beebe, D. J. Adv. Drug Delivery Rev. 2004, 56, 199–210. (17) Calvert, P. Adv. Mater. 2009, 21, 743–756. (18) Beebe, D. J.; Moore, J. S.; Bauer, J. M.; Yu, Q.; Liu, R. H.; Devadoss, C.; Jo, B. H. Nature 2000, 404, 588–590. (19) Baldi, A.; Gu, Y.; Loftness, P.; Siegel, R. A.; Ziaie, B. IEEE J. Microelectromech. Syst. 2003, 12, 613–621. (20) 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. (21) Lee, W.; Choi, D.; Lee, Y.; Kim, D. N.; Park, J.; Koh, W. G. Sens. Actuators, B 2008, 129, 841–849. (22) Zhang, N.; Knoll, W. Anal. Chem. 2009, 81, 2611–2617.
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Figure 1. Schematic illustration showing the process of squeezefilm for generation of hydrogel thin film on a planar surface.
hydrogel layer on top of the spacer limits the achievable minimum thickness and hinders further processing capability. In dry pattern transfer methods, masking layer is also a critical component. This is in light of the fact that dried hydrogel must be totally isolated from any contact with water or other aqueous solvents until the very end of the process. In this technical note, we address the above-mentioned problems through a combination of squeeze-film based spacerfree method for creating controllable submicrometer hydrogel films on planar substrates and the use of parylene-C as the masking layer. Parylene protects hydrogel from any contact with water during the lithography process and is easily patternable using oxygen plasma. MATERIALS AND METHODS Our spacer-free casting and parylene-masking method can be applied to almost all of the hydrogel systems. Hydrogel compositions reported in this work include but are not limited to the following pH sensitive hydrogel system. Poly(methacrylic acid-co-acrylamide) (poly(mAA-co-AAm), pH sensitive hydrogel): The pregel solution was prepared by adding 334.5 mg of acrylamide (AAm, Sigma-Aldrich), 100.8 µL of methacrylic acid (mAA, Sigma-Aldrich, distilled after received), 4 µL of ethylene glycol dimethacrylate (EGDMA, cross-linker from Polysciences Inc.), and 100 µL of N,N,N′,N′-tetramethylethylenediamine (TEMED, accelerator from Sigma-Aldrich) to 1.2 mL of deionized (DI) water. This mixture was called solution A. Solution B consisted of 80 mg/mL ammonium persulfate (APS, initiator from Polysciences Inc.) in DI water. Solutions A and B were mixed in a volume ratio of 5.9:1 to make the final pregel solution. With the high concentration of the accelerator and initiator, the pregel solution started to polymerize in 60 s. Squeeze flow of the pregel solution between two planar surfaces with a certain applied pressure was used to accurately control the hydrogel film thickness. In order to make a defectfree hydrogel film, the applied hydrogel system was designed to begin polymerization at around 1 min. Figure 1 shows the schematic of the squeeze-film process. To improve the adhesion between the hydrogel and the patterning surface (glass or silicon), the substrate was treated with an adhesion promoter by soaking in a 10 vol % solution (in acetone) of an organosilane coupling agent, γ-methacryloxypropyl trimethoxysilane (γ-MPS, Sigma3378
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Figure 2. Experimental results showing the thickness of dehydrated hydrogel (poly(mAA-co-AAm)) film versus the loading mass.
Aldrich) for 1 h followed by rinsing in acetone, methanol, and isopropyl alcohol. The substrate was then baked in a 120 °C oven for 10 min. In the above process, organosilanes such as γ-MPS form chemical bridges between hydrogel and the surface containing hydroxyl functional groups by chemical bonds. Subsequently, a pregel droplet (∼200 µL) was dispensed on the substrate, followed by carefully laying a transparency film on the substrate (from one side to the other in order to avoid the air bubble) to spread the pregel solution. The use of transparency film allows for easy separation after hydrogel is polymerized. A glass slide was then placed on top of the transparency film to allow a uniform pressure distribution over the substrate. Finally, a known weight was placed on top to provide a controllable pressure. The whole process had to be completed before the pregel solution started to polymerize. A time of 10 min was allowed for the hydrogel film to be fully polymerized. The transparency film was then peeled off the substrate. The substrate with hydrogel film was left in a nitrogen box for 24 h to allow a full dehydration. The above process was repeated with different applied pressures. Figure 2 shows the thickness of dried hydrogel vs applied weight (substrate area was 6.45 cm2). The dehydrated hydrogel thickness was measured by Alpha-Step (KLA-Tencor Corp. Alpha-Step IQ) surface profilometer. Six replicate measurements were done to obtain the error bars in the figure. The average values of the dry hydrogel thickness were smoothly connected by a Bezier curve. As expected, the thickness of the dry hydrogel film is inversely proportional to the applied pressure (loaded mass). We were able to achieve a hydrogel thickness down to around 428 nm with a normal force of 450 g. The measured uniformity across the whole substrate was
