Hydrophilic Composite Elastomeric Mold for High-Resolution Soft

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Langmuir 2006, 22, 9018-9022

Hydrophilic Composite Elastomeric Mold for High-Resolution Soft Lithography Nae Yoon Lee,† Ju Ri Lim,† Min Jung Lee,† Jong Bok Kim,‡ Sung Jin Jo,‡ Hong Koo Baik,‡ and Youn Sang Kim*,† Center for Intelligent Nano-Bio Materials, DiVision of Nano Sciences (BK 21) and Department of Chemistry, Ewha Womans UniVersity, 11-1 Daehyun-dong, Seodaemun-gu, Seoul, 120-750, Korea, and Department of Metallurgical System Engineering (BK 21), Yonsei UniVersity, 134 Shinchon-dong, Seodaemun-gu, Seoul, 120-749, Korea ReceiVed March 24, 2006. In Final Form: August 2, 2006 Here, we introduce a nanopatternable hydrophilic composite elastomer highly desirable for both nanostructure patterning via solvent-assisted micromolding (SAMIM) and microcontact printing of polar inks. This composite precursor is prepared by blending two UV-curable materials, Norland Optical Adhesives (NOA) 63 and poly(ethylene glycol) diacrylate (PEGDA), in an appropriate ratio; upon UV polymerization, a nanopatternable elastomer with preferential permeability both to aqueous and organic solvent is fabricated. Using this composite mold, nanoscale SAMIM of poly(4-vinylpyridine) (P4VP) and microcontact printing of a polar biomolecule, bovine serum albumin (BSA), was successfully demonstrated, paving the way for facile and efficient reproduction of various nanopatterns and a biomolecule-printed array platform.

Introduction An elastomeric mold with a nanopatternable surface which is capable of retaining water is highly desirable for various soft lithographic techniques1 such as solvent-assisted micromolding (SAMIM)2,3 and microcontact printing4 for facile replication of nanoscale patterns and transfer-printing polar biomolecules, respectively. So far, poly(dimethylsiloxane) (PDMS) has been the best choice of material for the mold or stamp used in soft lithographic techniques owing to conformal contact with substrate, low surface energy, and moderate solvent absorption, as well as outstanding transparency and simple fabrication. However, due to its low fidelity in reproducing submicrometer-scale structures resulting from its intrinsically low mechanical integrity5-7 and inherent hydrophobicity, nanostructure patterning and contact printing of polar molecules remain a considerable challenge. For these reasons, many efforts have been made to enhance the hardness of PDMS,8,9 modify the surface of the PDMS mold,10 or find a new nanopatternable material11-14 to compensate for these drawbacks. Yoo and co-workers11 attempted to use a urethane-related acrylate prepolymer as a new mold material capable of nanopatterning on its surface. Although a mold with a nanopatterned * Corresponding author. Tel: +82-2-3277-4131. Fax: +82-2-3277-3419. E-mail: [email protected]. † Center for Intelligent Nano-Bio Materials. ‡ Department of Metallurgical Systen Engineering (BK 21). (1) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550. (2) Kim, E.; Xia, Y.; Zhao, X.-M.; Whitesides, G. M. AdV. Mater. 1997, 9, 651. (3) Rogers, J. A.; Bao, Z.; Dhar, L. Appl. Phys. Lett. 1998, 73, 294. (4) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002. (5) Kumar, A.; Whitesides, G. M. Science 1994, 263, 60. (6) Delamarche, E.; Schmid, H.; Michel, B.; Biebuyck, H. AdV. Mater. 1997, 9, 741. (7) Hui, C. Y.; Jagota, A.; Lin, Y. Y.; Kramer, E. J. Langmuir 2002, 18, 1394. (8) Schmid, H.; Michel, B. Macromolecules 2000, 33, 3042. (9) Odom, T. W.; Love, J. C.; Wolfe, D. B.; Paul, K. E.; Whitesides, G. M. Langmuir 2002, 18, 5314. (10) Donzel, C.; Geissler, M.; Bernard, A.; Wolf, H.; Michel, B.; Hilborn, J.; Delamarche, E. AdV. Mater. 2001, 13, 1164. (11) Yoo, P. J.; Choi, S.-J.; Kim, J. H.; Suh, D.; Baek, S. J.; Kim, T. W.; Lee, H. H. Chem. Mater. 2004, 16, 5000.

surface was successfully fabricated with high fidelity and the microcontact printing of oil-soluble molecules was successfully performed, it was unsuitable for patterning polar biomolecules. Rolland and co-workers14 introduced a new rigid mold with low surface-adhesion property capable for nanopatterning and easy replication with great simplicity; however, due to its high resistance to organic solvents, it was not suitable for microcontact printing. Therefore, finding an appropriate material capable of creating high-feature nanostructures on its surface and, at the same time, transfer-printing polar inks is a substantial criterion for the construction of a successful mold applicable for versatile high-resolution soft lithographic techniques. Here, we introduce a hydrophilic composite elastomer with a nanopatterned surface capable of patterning polymeric nanostructures via SAMIM as well as transfer-printing polar biomolecules via microcontact printing, both of which are two promising soft lithographic techniques requiring an elastomeric mold. This composite mold15 is composed of two UV-curable materials, Norland Optical Adhesives (NOA) 6316 and poly(ethylene glycol) diacrylate (PEGDA).17 NOA 63 provides high mechanical strength, and the PEGDA hydrogel provides high permeability to water; the cured mixture in an appropriate ratio gives rise to an elastomer with the preferential permeability to water and ethanol. By employing this new composite mold, the physical patterning of nanostructures was performed with poly(4-vinylpyridine) (P4VP) via the SAMIM method, and the chemical patterning of a polar biomolecule was performed with bovine serum albumin (BSA) via microcontact printing. (12) Csucs, G.; Kunzler, T.; Feldman, K.; Robin, F.; Spencer, N. D. Langmuir 2003, 19, 6104. (13) Trimbach, D.; Feldman, K.; Spencer, N. D.; Broer, D. J.; Bastiaansen, C. W. M. Langmuir 2003, 19, 10957. (14) Rolland, J. P.; Hagberg, E. C.; Denison, G. M.; Carter, K. R.; De Simone, J. M. Angew. Chem., Int. Ed. 2004, 43, 5796. (15) Lee, N. Y.; Lim, J. R.; Lee, M. J.; Park, S.; Kim, Y. S. Langmuir 2006, 22, 7689. (16) Park, J.; Kim, Y. S.; Hammond, P. T. Nano Lett. 2005, 5, 1347. (17) Revzin, A.; Russell, R. J.; Yadavalli, V. K.; Koh, W.-G.; Deister, C.; Hile, D. D.; Mellott, M. B.; Pishko, M. V. Langmuir 2001, 17, 5440.

10.1021/la060790b CCC: $33.50 © 2006 American Chemical Society Published on Web 09/09/2006

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Figure 2. Schematic illustrations for (a) physical patterning of P4VP via SAMIM and (b) chemical patterning of BSA via microcontact printing using the NOA 63-PEGDA composite mold.

Figure 1. Schematic illustrations for the fabrication of the NOA 63-PEGDA composite mold.

Experimental Section Materials. Poly(ethylene glycol) diacrylate (PEGDA, Mw ) 575) and 2-hydroxy-2-methylpropiophenone (HOMPP) were purchased from Aldrich. Poly(4-vinylpyridine) (P4VP, Mw ) 60 000) was purchased from Sigma-Aldrich. Fluorescein isothiocyanate-conjugated bovine albumin (FITC-BSA) was purchased from Sigma. Norland Optical Adhesives (NOA) 63 was purchased from Norland Company. Fabrication of a NOA 63-PEGDA Composite Mold. The schematic for the fabrication of a NOA 63-PEGDA composite mold is illustrated in Figure 1. First, the nanoscale line patterns on the Si wafer whose smallest width was approximately 160 nm were replicated on a urethane-related acrylate prepolymer11 on a polyester film (SK Chemical) and cured for 30 min using a 365-nm, 135mW/cm2 UV light source. After the pattern release from the Si wafer, the pattern was post-cured for 12 h. Next, the above-prepared polymeric thin mold was put inside a PDMS frame, and the mixture of NOA 63 prepolymer and PEGDA hydrogel precursor solution was poured on this patterned polymeric mold surrounded by the PDMS frame and cured under UV light (λ ) 365 nm) for 30 min to obtain negative relief patterns of the polymeric mold. The composite mold was prepared by blending the NOA 63 prepolymer and a PEGDA hydrogel precursor solution at varying ratioss5:5, 6:4, 7:3, 8:2, and 9:1 (w/w). The PEGDA hydrogel precursor solution was prepared by mixing PEGDA and the photoinitiator, HOMPP, at a ratio of 95:5 (v/v). This ratio of PEGDA and HOMPP was chosen to endow the composite mold with high transparency after UV polymerization.

Figure 3. (a) Young’s modulus and (b) MHC and EHC of the NOA 63-PEGDA composite mold at different blending ratios of NOA 63 and PEGDA. Measurements. Young’s Modulus. First, tensile tests were performed as per ASTM-D-638 typeswith a gauge of 50 mm and cross-head speed of 50 mm/min at a temperature of 23 ( 2 °C and a relative humidity (RH) of 50 ( 5% using a Universal Testing Machine (LR 100K Lloyd Instruments Ltd., UK). The Young’s modulus was obtained by measuring the slope of the axial stressstrain curve in the elastic region obtained from the tensile tests. Seven tests were performed for each sample.

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Figure 4. SEM images of physical patterning realized by transferring nanopatterns from (a) the NOA 63-PEGDA composite mold onto (b) a coated thin film of P4VP on a Si wafer. (c) The NOA 63-PEGDA composite mold having arrays of box patterns which contain repeating units of 1-µm line patterns. The inset shows the OM of the line patterns on the mold. (d) The result of SAMIM performed on a coated thin film of P4VP over a large area (2.5 cm × 5 cm) on a Si wafer. The inset shows the OM of the line patterns formed via SAMIM on the Si wafer. Moisture/Ethanol Holding Capacity. The moisture holding capacity (MHC) and ethanol holding capacity (EHC) were measured by immersing in water and ethanol, respectively, for 30 min using the composite molds (15 mm × 15 mm × 1 mm) made from different blending ratios of NOA 63 and PEGDA as stated above. The MHC and EHC were calculated using the following equation: M ) [(Ww - Wd)/Wd] × 100, where M is the absorbed moisture or ethanol (wt %) in the mold and Ww and Wd are the weights of the wet and dry molds, respectively. Contact Angle Measurement. The contact angle was measured by the sessile drop technique using distilled water utilizing a Phoenix 300 surface angle analyzer (Surface Electro Optics, Korea) and analyzed with ImagePro 300 software. The measurements were quadruplicated. Physical Patterning via SAMIM. Physical patterning via the SAMIM process was carried out as shown in Figure 2. A 10 wt % solution of P4VP, dissolved in ethanol, was dropped on a Si wafer treated with an oxygen plasma (50 W, 0.2 Torr, 10 min) and spincoated at 3000 rpm for 40 s. The P4VP film was heated on a hot plate at 80 °C for approximately 30-40 min to evaporate the residual ethanol in the coated P4VP film. The above-prepared composite mold was soaked in ethanol for 10 min followed by drying by blowing with air. The composite mold was brought into conformal contact with the P4VP-coated Si wafer and heated to 65 °C, which is below the boiling point of ethanol (78.3 °C), for 10 min. Field emission scanning electron microscopy (FE-SEM) images were obtained using JSM-6700F (JEOL). Chemical Patterning via Microcontact Printing. Chemical patterning was realized via microcontact printing (Figure 2b), employing dot (d ) 20 µm)- and line (w ) 2 µm)-patterned composite molds. They were soaked in water for 30 min and dried with air. A solution of fluorescein isothiocyanate (FITC)-tagged BSA (FITCBSA) (110 µg/mL), prepared in phosphate-buffered saline (PBS; 10

mM sodium phosphate buffer, 2.7 mM KCl, and 137 mM NaCl; pH 7.4), was dropped on the surface of the patterned composite mold and left undisturbed at 4 °C for 40 min and spin-coated at 500 rpm for 10 s and then at 3000 rpm for 30 s to ensure the homogeneous coating of FITC-BSA on the surface of the mold. After drying the surface of the mold, it was brought into conformal contact with a glass slide and then released. The mold was presoaked in water so that a large amount of polar ink could be absorbed in the mold, enabling multiple contact-printing without re-inking the biomolecule. Fluorescent microscopy images were obtained using an inverted fluorescence microscope (Axiovert 200 MAT, Carl Zeiss).

Results and Discussion Characterization of the Composite Mold. Young’s Modulus. Figure 3a shows the results of the Young’s modulus for the composite molds. A low value of the Young’s modulus represents high elasticity of the mold. To obtain the composite mold with the lowest possible Young’s modulus, various blending ratios for NOA 63 and PEGDA were examined. As shown in Figure 3a, the lowest Young’s modulus of 9.6 MPa was obtained when the blending ratio of NOA 63 to PEGDA was 6:4 (w/w). NOA 63 has a high modulus of elasticity of approximately 1655 MPa18 which imparts hardness and rigidity to the mold. On the other hand, PEGDA, the hydrogel precursor, endows the mold with some degree of elasticity. As a result, a higher proportion of PEGDA gave rise to a lower Young’s modulus; however, when the blending ratio was 5:5 (w/w), the Young’s modulus slightly increased (14 MPa) and the cured mixture became brittle. This (18) Kim, Y. S.; Lee, N. Y.; Lim, J. R.; Lee, M. J.; Park, S. Chem. Mater. 2005, 17, 5867.

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signifies that an increased amount of PEGDA above a certain point might lower the elasticity of the material owing to the inherent property of the hydrogel precursor, PEGDA.19 Besides the Young’s modulus, various other mechanical properties such as the tensile strength and elongation at break were obtained from the stress-strain curves for the composite mold and PDMS (see also Supporting Information). Moisture/Ethanol Holding Capacities. Figure 3b shows the MHC and EHC of the composite mold at different blending ratios of NOA 63 and PEGDA. As can be seen in Figure 3b, both the MHC and EHC were increased as the proportion of PEGDA was increased, owing to the extremely hydrophilic nature of PEGDA. The MHC of the composite mold for 9:1, 8:2, 7:3, 6:4, and 5:5 (w/w) molds were 0.70, 0.95, 1.1, 1.8, and 2.6%, respectively. The MHC of a mold indicates the mold’s ability to transfer polar molecules. Given that almost all biomolecules are water-soluble, the MHC of a mold is of considerable importance when performing microcontact printing of biomolecules. Considering that NOA 63 itself has a relatively low MHC of 0.46% and that the MHC of the NOA 63-PEGDA composite mold increased with an increasing proportion of PEGDA, it can be inferred that PEGDA played a significant role in the enhancement of the MHC. In general, the MHC values of the composite mold and NOA 63 itself were always greater than that of PDMS (0.23%) regardless of the blending ratio of NOA 63 and PEGDA. This clearly demonstrates the good water permeability of the composite mold and its potential for patterning polar biomolecules, eliminating the needs for the complicated pretreatment of the mold. The EHC of the composite mold increased with the increase in the amount of PEGDA, and those of the 9:1, 8:2, 7:3, 6:4, and 5:5 (w/w) molds were 0.31, 0.55, 0.73, 1.1, and 1.3%, respectively. The EHC of the PDMS mold, cut into the same dimension as those of the composite molds, was 2.2%. Considering that solvent absorption of 1-2% is sufficient for performing SAMIM, 5:5 and 6:4 (w/w) molds seemed appropriate for performing SAMIM in this experiment. Solvent permeability of the mold determines the mold’s ability to perform the SAMIM process since SAMIM is achieved via the melting and subsequent wicking of a coated polymer film into the grooves of a nanopatterned elastomeric mold during the evaporation process of the solvent soaked in the mold. Therefore, the EHC of a mold is a driving force for the precise filling of the melted polymer into the nanopatterns on the surface of an elastomeric mold. Although the 5:5 (w/w) mold displayed higher values in both MHC (2.6%) and EHC (1.3%) than the 6:4 (w/w) mold, 1.8% and 1.1%, respectively, the 6:4 (w/w) mold seemed more suitable as a mold for performing SAMIM and microcontact printing when considering the elasticity of the mold. The MHC of the 6:4 (w/w) mold was almost 8-fold higher than that of PDMS under the same experimental conditions. The water contact angle for the 6:4 (w/w) mold was measured to be approximately 65°, which is lower than that of PDMS (θ ) 110°), representing the surface hydrophilicity20 of the 6:4 (w/w) mold. High values of MHC and EHC for the 6:4 (w/w) mold did not deform the nanopatterned surface. On the basis of the results obtained from the Young’s modulus and MHC/EHC, the ratio of 6:4 (w/w) was selected as the optimum blending ratio for the fabrication of the composite mold to carry out nanoscale SAMIM and microcontact printing of a polar biomolecule throughout the experiment. (19) Lee, N. Y.; Jung, Y. K.; Park, H. G. Biochem. Eng. J. 2006, 29, 103. (20) Burton, Z.; Bhushan, B. Nano Lett. 2005, 5, 1607.

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Figure 5. Chemical patterning results of FITC-BSA (110 µg/mL) using the NOA 63-PEGDA composite mold. Fluorescent microscopy images of (a and b) dot patterns (d ) 20 µm) and (c) line patterns (w ) 2 µm) on glass slides.

Physical Patterning via SAMIM. The scanning electron microscopy (SEM) images of the results of the physical patterning of P4VP are shown in Figure 4. Owing to the NOA 63-PEGDA composite mold’s moderate elasticity and high permeability to ethanol, P4VP patterning via SAMIM was easily realized below its glass transition (Tg), 137 °C. In general, PDMS is employed for the SAMIM process because of good conformal contact with the substrate and high permeability to organic solvent; however, due to its inherent softness, high-resolution nanoscale patterning is not easily realized (see also Supporting Information). On the other hand, nanopatterns as small as 160 nm were fabricated with high feature-fidelity (Figure 4a) on the surface of the 6:4 (w/w) composite mold because its hardness (9.6 MPa), represented by the Young’s Modulus, was higher than that of PDMS (1.5 MPa), and were successfully transferred onto the P4VP-coated Si wafer (Figure 4b). In addition, the inherently low surface energy of NOA 63 at elevated temperature16 facilitated the mold release, dispensing with additional anti-adhesion coating of the mold, which makes it highly appropriate for SAMIM-based physical patterning. The EHC of the 6:4 (w/w) mold (1.1%) was

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sufficient for performing the SAMIM process, as confirmed by the concrete and precise nanofeatures shown in the SEM images (Figure 4b). Large area patterning was also successfully performed via SAMIM using the composite mold (Figure 4c) as shown in Figure 4d. Arrays of box patterns which contain repeating units of 1-µm line patterns in each box were patterned onto the P4VPcoated Si wafer over the area of 2.5 cm × 5 cm. The inset optical micrograph (OM) in Figure 4d confirms the precise patterning via the SAMIM process. Chemical Patterning via Microcontact Printing. The fluorescent microscopy images for the results of microcontact printing of FITC-BSA are shown in Figure 5. Dots (20-µm) (Figure 5a,b) and 2-µm lines (Figure 5c) were successfully patterned with high sensitivity and selectivity. Using the 6:4 (w/w) mold, dots were patterned over a large area of the glass slide with high regularity (Figure 5a). Some of the dot patterns seemed missing and distorted, and this may have been due to the low printing resolution used during the mask fabrication; therefore, as long as the mask is fabricated with high-resolution, this kind of problem might be resolved easily. For the contact-printing, ∼1 kPa was applied homogeneously over the whole area of the composite mold. The elasticity of a mold is the key to successful microcontact printing. Although the Young’s modulus of the 6:4 (w/w) mold was approximately 6-fold higher than that of PDMS, the elasticity of the composite mold was sufficient to achieve conformal contact with the flat substrate. Moreover, the slightly higher modulus of the composite mold prevented lateral collapse21,22 of the mold when performing microcontact printing which usually occurs with PDMS mold. The homogeneous coating of biomolecules on the surface of the patterned composite mold is another important parameter for successful microcontact printing. In this experiment, FITC-BSA was spin-coated first at 500 rpm for 10 (21) Hsia, K. J.; Huang, Y.; Menard, E.; Park, J.-U.; Zhou, W.; Rogers, J.; Fulton, J. M. Appl. Phys. Lett. 2005, 86, 154106. (22) Huang, Y. Y.; Zhou, W.; Hsia, K. J.; Menard, E.; Park, J.-U.; Rogers, J. A.; Alleyne, A. G. Langmuir 2005, 21, 8058.

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s and then at 3000 rpm for 30 s to obtain homogeneous distribution of the FITC-BSA solution. Also, up to 6 repeated printings were realized without re-inking the FITC-BSA, by presoaking the mold in water for 30 min. An MHC value of 1.8% seemed sufficient to perform microcontact printing of polar biomolecules, signifying the potential of the composite material as a mold for printing a variety of polar biomolecules with great simplicity and high efficiency.

Conclusions In this work, we have introduced a hydrophilic composite elastomer with a nanopatterned surface. This composite mold enables numerous soft lithographic techniques to be performed for a variety of patterning applications, such as facile micro- and nanopattern replication and contact-printing of polar biomolecules in high density with great ease. Owing to the versatile functionalities and wide applicability, the use of this composite mold could be extended to multipurpose stamping. Especially, the hydrophilic nature of the NOA 63-PEGDA composite mold exhibits good potential for the printing of various polar biomolecules with great simplicity and high efficiency for biosensor applications. Acknowledgment. This work was equally supported by the SRC program of the Korea Science and Engineering Foundation (KOSEF) through the Center for Intelligent Nano-Bio Materials at Ewha Womans University (Grant No. R11-2005-008-020030) and the Korea Research Foundation (Grant No. KRF-2004005-C00093). The authors are supported by the Brain Korea 21 (BK 21) fellowship from the Ministry of Education of Korea. Supporting Information Available: The stress-strain curves for the composite molds and PDMS mold as well as fabrication results of nanopatterned molds using PDMS and the composite material are provided. This material is available free of charge via the Internet at http://pubs.acs.org. LA060790B