© Copyright 2007 American Chemical Society
AUGUST 14, 2007 VOLUME 23, NUMBER 17
Letters Organized Monolayer of Thermosensitive Microgel Beads Prepared by Double-Template Polymerization Takamasa Sakai,† Yukikazu Takeoka,‡ Takahiro Seki,‡ and Ryo Yoshida*,† Department of Materials Engineering, Graduate School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, and Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya UniVersity, Furo-cho, Chikusa, Nagoya 464-8603, Japan ReceiVed February 14, 2007. In Final Form: June 22, 2007 A 2D close-packed array of thermosensitive microgel beads was prepared by the double-template polymerization method. First, a 2D colloidal crystal of silica beads with 10 µm diameter was obtained by the solvent evaporation method. This monolayer of colloidal crystals can serve as the first template for the preparation of macroporous polystyrene. The macroporous polystyrene trapping the crystalline order can be used as a negative template for fabricating gel beads arrays. A functional surface using thermosensitive poly(N-isopropylacrylamide) gel beads array was fabricated by the double-template polymerization method.
Introduction Recently, many functional surfaces have been developed by controlling the chemical composition or topography of a material surface. Loty et al.1 have formed a carbonated apatite layer on an apatite-wollastonite (AW) glass-ceramic surface. This surface composition supports the differentiation of osteogenic cells and the subsequent apposition of a bone matrix, which allows a strong bond from the AW to the bone. Superhydrophobic surfaces of polymer nanofibers and differently patterned aligned carbon nanotube films have also been developed.2 The surface topography of them was similar to that of lotus or rice leaves. Furthermore, there has been extensive research to create a stimuliresponsive surface by grafting a functional polymer (e.g., a polymer surface capable of forming surface relief by light * To whom correspondence should be addressed. E-mail: ryo@ cross.t.u-tokyo.ac.jp. Phone and Fax: +81-3-5841-7112. † The University of Tokyo. ‡ Nagoya University. (1) Loty, C.; Sautier, J. M.; Boulekbache, H.; Kokubo, T.; Kim, H. M.; Forest, N. J. Biomed. Mater. Res. 2000, 49, 423. (2) Feng, L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, D. AdV. Mater. 2002, 14, 1857.
irradiation,3 thermosensitive cell culture dishes that can regulate cell adhesion and detachment,4 etc.). Functional surfaces are often seen in nature. For example, the beautiful colors of the Jewell beetle or Neontetra originate from the ordered structure of the surface rather than from dyes or pigments. These colors are called “structural colors” and are generated by diffraction from the periodic structure of their submicrometer surfaces. Some species can change their color by changing the structure of their skin. Inspired by these natural functional surfaces, researchers have vigorously investigated the fabrication of surfaces with periodical structure. Nagayama et al.5 arrayed many kinds of spherical beads, with diameters ranging from nanometers to micrometers, into dense 2D-ordered monolayers on glass and mica surfaces. The governing factors for ordering the beads were attractive “immersion capillary forces” and convective transport. An ordered monostratal pattern of hydrogel microspheres was also developed by Kawaguchi et al.6 The gaps between the gel beads were (3) Seki, T.; Nagano, S.; Kawashima, Y.; Zettsu, Y.; Ubukata, T. Mol. Cryst. Liq. Cryst. 2005, 430, 107. (4) Yang, J.; Yamato, M.; Kohno, C.; Nishimoto, A.; Sekine, H.; Fukai, F.; Okano, T. Biomaterials 2005, 26, 6415. (5) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183.
10.1021/la700448t CCC: $37.00 © 2007 American Chemical Society Published on Web 07/11/2007
8652 Langmuir, Vol. 23, No. 17, 2007
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Figure 1. Fabrication method of a 2D close-packed silica colloidal crystal.
controllable by changing the length of hairy polymer grafted onto the beads. These bead monolayers (diameter >5 µm) were prepared by the convective assembly method, Langmuir-Blodgett method, electrophoretic bead deposition, and so forth. However, a 2D monolayer of gel beads of several micrometers to several tens of micrometers (near the cell size) has not been obtained because beads of this size are difficult to prepare and cannot disperse stably in solvent as a result of gravity. In this study, in order to fabricate a monolayer of thermosensitive microgel beads of several micrometers, we employed the double-template polymerization method. First, we attempted to establish the preparation method to obtain a 2D close-packed monolayer of silica beads with a diameter of 10 µm. Then, by transcribing the silica monolayer, a 2D close-packed monolayer of microgel beads was prepared. The double-template method was first developed by Colvin et al.7 to make uniformly sized inorganic beads and their 3D colloidal crystals. Although it was applied to the preparation of organic gel beads by Takeoka et al.,8 there is no reported preparation of a 2D monolayer of gel beads. A functional surface structure that consists of a gel bead monolayer is very attractive because biomimetic topography and stimuli sensitivity may be added. The fabrication method shown in this study is a versatile way to prepare a monolayer of gel and polymer beads. By using this method, we may fabricate a cellfunction-controllable surface, frictional characteristics of the surface, and an artificial compound eye. Experimental Section Preparation of the Porous Polystyrene Template. A glass plate and silicon ring (diameter ) 1 cm) were sonicated in ethanol for 5 min and then attached as shown in Figure 1 and dried. The glass plate was placed inside the oxygen plasma reactor (PR500, Yamato) and exposed to the oxygen plasma for 10 min. The power and the oxygen pressure were set to 300 W and 100 Pa, respectively. Silica beads (100 mg, diameter ) 10 µm) were dispersed in 5 mL of H2O. After sonication, 100 µL of the suspension was dropped inside the ring on the glass plate. By drying at room temperature for 10 h, a colloidal silica crystal was obtained. The glass plate with the colloidal silica was put on the bottom of the vial, and then styrene monomer (2.0 mL) including 2,2′-azobis-2-2,4-dimethylvaleronitrile (ADVN) (0.02 g) as the initiator was poured into the vial. Polymerization was carried out at 40 °C for 30 h. By immersing the obtained polymer film in the aqueous solution containing 5% hydrofluoric acid, colloidal silica in the polymer film was removed. The resulting macroporous polymer film was washed with water several times. Preparation of a 2D Thermosensitive Gel Bead Array. The glass plate was placed inside the oxygen plasma reactor and exposed to the oxygen plasma for 10 min (power ) 300 W, oxygen pressure ) 100 Pa). Then the glass plate was dipped into the solution including EtOH (95 g), H2O (5 g), acetic acid (200 µL), and a silane coupling agent (3-(trimethoxysilyl)propylmethacrylate, 2 g). Then, it was washed with EtOH and cured at 120 °C for 10 h. The pore in the polystyrene film supported by the silane-coupled glass was filled (6) Tsuji, S.; Kawaguchi, H. Langmuir 2005, 21, 8439. (7) Jiang, P.; Bertone, J. F.; Colvin, V. L. Science 2001, 291, 453. (8) Kumoda, M.; Takeoka, Y.; Watanabe, M. Langmuir 2003, 19, 525.
Figure 2. Preordered phase (a) and formation process of the reconstructed phase (b) at lower concentration (16 g/L). Preordered phase (c) and formation process of the reconstructed phase (d) at higher concentration (24 g/L). (R1, preordered phase; β, river; γ, reconstructed phase; R2, preordered phase with a small amount of the second layer). with a pre-gel solution containing N-isopropylacrylamide (NIPAAm) (456 mg), N,N′-methylenebis-acrylamide (31 mg), 2,2-dimethoxy1,2-diphenylethan-1-one (Irgacure 651) (51 mg), and EtOH (760 µL). Photopolymerization was carried out by irradiating UV light for 5 min. Then, polystyrene was etched by a large amount of toluene for 10 h. Finally, the gel bead array was immersed in water at 40 °C in order to cut the connecting portion. Preparation of a 2D PDMS Bead Array. Polydimethylsiloxane (PDMS) (SILPOD 184, Dow Corning, 900 µL) and the PDMS catalyst (100 µL) were mixed and degassed for 10 min. The pore in the polystyrene film supported by the silane-coupled glass was filled with the resulting mixture. After curing at 50 °C for 12 h, the styrene film was etched with toluene. Measurement of the Contact Angle. The glass substrate on the surface of which PDMS beads were arranged and the flat PDMS substrate were put on the stage of the contact angle meter (CA-W automatic contact angle meter, Face). After dropping water onto the surfaces, we measured, at room temperature, the contact angle of the water drop that is defined as the angle between the baseline and the tangent at the drop boundary. Observation of Surface Topography. The PNIPAAm gel beads array was immersed in pure water. The temperature was controlled by a water jacket connected to a thermostatic bath (Cool circulator CB-Jr AS ONE). The topography of the surface was observed via microscopy (DMIL Leica) at 20 and 40 °C.
Results and Discussion At first, a 2D close-packed colloidal crystal silica was prepared by solvent evaporation from the water suspension. The procedure is shown in Figure 1. A silicon O-ring was bonded to a glass surface. The proper amount of the silica suspension was dropped inside the O-ring. By using the O-ring, water evaporation occurs from the center of the droplet. During evaporation, a concave
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Figure 3. (a) Photograph and SEM images of the obtained silica colloidal crystal. (b) Fabrication process of a 2D monolayer of gel beads by the double-template polymerization method. (Photograph: microscope observation).
and rather flat meniscus is formed at the liquid-air interface. The volume and concentration of the suspension dropped inside the ring were determined to obtain the dense monolayer of beads after the evaporation of water. The suspension was well stirred just before being dropped because the silica beads were not stabilized in the water suspension. The process of crystal formation consists of the following three stages. The first stage is the formation of a preordered phase. As mentioned above, 10 µm silica beads cannot disperse stably in water. Thus, a few minutes after the suspension is dropped, the beads precipitate, and a loosely hexagonally packed monolayer including some defects or second layers, depending on the concentration of the silica beads, appeared on the substrate (Figure 2a,c). When short beads exist (concentration ) 16 g/L), the preordered phase has some line defects (Figure 2a, R1), but when excess beads exist (24 g/L), the phase has a second layer with some defects (Figure 2c, R2). This difference affects the formation process of the final ordered structure. In the second stage, when the liquid phase became thinner by evaporation, the nucleus of the ordered phase appeared in the center of the ring. This phenomenon was well explained by Kralchevsky et al.9 for water-dispersive beads. The upper surface of the thinning aqueous layer presses the beads toward the glass surface. When the beads are partially immersed in a liquid layer on a substrate, the deformations of the liquid-gas interface cause strong long-range attractive capillary forces (immersion force) among the beads. This capillary force forms the nucleus of the ordered phase (γ). The same mechanism may be applicable for precipitated beads because beads concerned with this mechanism are precipitated. (9) Kralchevsky, P. A.; Nagayama, K. AdV. Colloid Interface Sci. 2000, 85, 145.
The third stage is the reconstruction of the ordered phase. In the region where the beads are partially immersed in the liquid layer (reconstructed phase), evaporation from the concave menisci between the beads must increase the local curvature of the menisci. To decrease this local curvature, water flows intensively from the thicker liquid layer to the reconstructed region. This convective flow carries the beads in the preordered region toward the reconstructed region. In this stage, the mechanism of crystal formation dramatically bifurcates with concentration. When the concentration is low and there are fewer beads, three different phases that consist of a preordered phase (R1), a freely mobile area of beads (river) (β), and a reconstructed phase (γ) appear from the edge to the center (Figure 2b). In this state, as beads in the nucleus assemble more tightly with each other, the river becomes wide. Then beads in the preordered phase move to the reconstructed phase through the river by convective flow. In this case, the beads are so large that they cannot be transported as easily as smaller beads. Thus when the river becomes wide, beads stop halfway up the river to form a new island. As a result, when fewer beads existed in the ring, the ordered structure was never obtained because of the broadening river. However, when the concentration is high and there is a small surplus of beads, two different phases consisting of a preordered phase (R2) and a reconstructed phase (γ) appear (Figure 2d). In this state, because the preordered phase is a double-layered structure to some extent, the river does not appear. After nucleation, beads in the second layer of the preordered phase move to the reconstructed phase by immersion force. When beads arrive at the boundary between the preordered and reconstructed phases, the beads are captured in the outermost layer of the reconstructed phase. However, in this case the border between the preordered phase and the reconstructed phase was crowded with beads, and the second
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Figure 4. Change in the surface topography of a 2D gel bead monolayer in response to the temperature change between 20 °C (swelling state) and 40 °C (deswelling state), as observed via microscopy.
layer was formed on the reconstructed phase. As a result, the ordering of the preordered phase is very important in this process. When the proper number of beads was dropped inside the ring, a fine close-packed monolayer of silica beads was obtained (Figure 3a). The colloidal crystal was almost a perfect monolayer and showed structural color, although it had some line defects in the radial direction and some point defects. The resulting silica-air crystal was used as the first template for the preparation of a macroporous polystyrene film that became the second template to obtain a 2D monolayer of microgel beads (Figure 3b). The polymerization of styrene was carried out in the space of the colloidal crystal. Then silica colloids in the polystyrene film were removed by HF etching, and a macroporous polystyrene template was obtained. After it was washed and dried, the pre-gel solution including the NIPAAm monomer, cross linker, and photoinitiator was allowed to infiltrate the macroporous template. After covering it with the silane-coupled glass plate, gel beads were synthesized by irradiating with UV light. In this state, the obtained gel beads covalently interconnect with each other, but, we can separate them by shrinking under dry conditions. Furthermore, by etching the surrounding polystyrene template with toluene, a 2D monolayer of gel beads on the glass plate was obtained. Gel beads were separated from each other but were chemically immobilized on the surface. Figure 4 shows a monolayer of thermosensitive PNIPAAm gel beads. It was observed that the topography of the surface changed with temperature. At higher temperature above the lower critical solution temperature (LCST, 32 °C), dotted and hexagonal patterns appeared on the surface. The mean diameter of the gel beads at 40 °C was 6.8 µm. However, at temperatures below the LCST, the gel beads became larger and were packed tightly. From a macroscopic point of view, it seemed like a thin gel film on the substrate. The mean diameter of the beads at 20 °C became
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larger than 10 µm but could not be estimated precisely because the shape of the gel beads was deformed. This change from the dotted pattern to tight packing occurred within several seconds. The change was reversible, and detachment of the gel beads from the surface was not observed after repeated temperature changes. In general, the wettability of the surface can be changed by controlling the surface topography. Practically, the contact angle of the monolayer of polydimethylsyloxane (PDMS) beads fabricated by same method (138°) was larger than that of the flat PDMS surface (114°). In the case of the PDMS bead array, the obtained bead array also transcribed the colloidal crystal of silica. For the temperature-responsive surface prepared by terminal grafting of the linear PNIPAAm, the surface wettability as a function of temperature was measured by Okano et al.10 The contact angles (cos θ) below and above the LCST were 0.63 and 0.05, respectively. When NIPAAm gel beads are used, because not only a hydrophilic change but also a morphological change in the gel beads can contribute to wettability, the change in wettability may become more remarkable. The details are under investigation. So far, this surface may be able to change not only the periodic structure but also the wettability, coefficient of friction, optical properties, and so forth by changing the temperature. It is also advantageous that any kind of gel bead monolayer can be obtained by using this method. Thus, there are several applications utilizing these periodic structures such as DNA separation,11 scaffold of cell proliferation,12 and superhydrophobic surfaces.
Conclusions In this study, a 2D monolayer of thermosensitive microgel beads was prepared by the double-template polymerization method for the first time. The fabrication method demonstrated here was so versatile that any kind of gel bead can be obtained. This fabrication method of 2D microgel bead arrays may be a key technology in creating new functional surfaces. Acknowledgment. This work was supported in part by a Grant-in-Aid for Scientific Research to R.Y. from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (no. 15205027). LA700448T (10) Takei, Y. G.; Aoki, T.; Sanui, K.; Ogata, N.; Sakurai, Y.; Okano, T. Macromolecules 1994, 27, 6163. (11) Kaji, N.; Tezuka, Y.; Takamura, Y.; Ueda, M.; Nishimoto, T.; Nakanishi, H.; Horiike, Y.; Baba, Y. Anal. Chem. 2004, 76, 15. (12) Saito, A.; Taketani, S.; Arai, K.; Tanaka, M.; Shimomura, M.; Sawa, Y. J. Mol. Cell. Cardiol. 2005, 39, 1022.
