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Hydrogels Containing Immobilized Bilayer Membranes Masaki Hayakawa,† Tomohiro Onda,‡ Toyoichi Tanaka,§ and Kaoru Tsujii*,† Tokyo Research Center, Kao Corporation, 2-1-3 Bunka, Sumida-ku, Tokyo 131, Japan, Recording and Imaging Science Laboratories, Kao Corporation, 2606, Akabane, Ichikai-machi, Haga-gun, Tochigi 321-34, Japan, and Department of Physics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 Received February 24, 1997. In Final Form: May 5, 1997X Novel polyacrylamide hydrogels containing immobilized bilayer membranes have been synthesized and characterized. Iridescent color appears to result from the Bragg diffraction of visible light by the periodic structure of bilayer membranes of a polymerizable surfactant (n-dodecyl glyceryl itaconate (DGI). The iridescent structure of DGI has been immobilized inside a polyacrylamide gel. The iridescent color of the gel thus obtained can be changed by controlling the swelling degree of the gel. Anisotropic hydrogels in the entire bulk body have been synthesized by polymerizing the gels under shear flow and immobilizing the macroscopically aligned bilayer membranes inside the gel networks.
A number of research works have been done on the bilayer membranes and polymer gels. They are both quite unique soft materials and show very different characteristics. Polymer gels undergo the volume phase transition with the change of environmental conditions1-4 and are expected to be applied to drug delivery systems5 and the chemo-mechanical devices.6,7 Bilayer membranes, on the other hand, are mainly studied as a model of biological membranes8 and used also as drug delivery systems9 and taste- and/or olfactory-sensors,10,11 etc. Any hybrid materials made of polymer gels and bilayer membranes have not yet been synthesized, and their properties and functions are unknown. The present letter deals with the synthesis and some unique properties of the bilayermembrane-immobilized polymer gels. One of the present authors (KT) and his collaborators studied previously the iridescent phenomena resulting from periodic structures of bilayer membranes.12-16 In the iridescent solutions, the bilayer membranes of surfactant are stacked regularly, having a spacing distance of sub-micrometer, and the iridescent color appears by the diffraction of visible light. One of the above systems consists of polymerizable surfactant, and its iridescent lamellar structure is immobilized inside a polyacrylamide †
Tokyo Research Center, Kao Corporation. Recording and Imaging Science Laboratories, Kao Corporation. § Massachusetts Institute of Technology. X Abstract published in Advance ACS Abstracts, June 15, 1997. ‡
(1) Tanaka, T.; Fillmore, D.; Sun, S. T.; Nishio, I.; Swislow, G.; Shan, A. Phys. Rev. Lett. 1980, 45, 1636. (2) Hirotsu, S.; Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1987, 87, 1392. (3) Annaka, M.; Tanaka, T. Nature 1992, 355, 430. (4) Shibayama, M.; Tanaka, T. Adv. Polym. Sci. 1993, 109, 1. (5) Okano, T. Adv. Polym. Sci. 1993, 110, 179. (6) Osada, Y.; Okuzaki, H.; Hori, H. Nature 1992, 355, 242. (7) Osada, Y.; Matsuda, A. Nature 1995, 376, 219. (8) Fendler, J. H. In Membrane Mimetic Chemistry; John Wiley & Sons: New York, 1982. (9) Sunamoto, J.; Sato, T.; Hirota, M.; Fukushima, K.; Hiratani, K.; Hara, K. Biochim. Biophys. Acta 1987, 898, 323. (10) (a) Nomura, T.; Kurihara, K. Biochemistry 1987, 26, 6135. (b) Nomura, T.; Kurihara, K. Biochemistry 1987, 26, 6141. (11) (a) Okahata, Y.; Ebato, H.; Taguchi, K. J. Chem. Soc., Chem. Commun. 1987, 1363. (b) Okahata, Y.; En-na, G. J. Chem. Soc., Chem. Commun. 1987, 1365. (12) Tsujii, K.; Satoh, N. In Organized SolutionssSurfactants in Science and Technology; Friberg, S. E., Lindman, B., Eds.; Marcel Dekker, Inc.: New York, 1992; p341. (13) Satoh, N.; Tsujii, K. J. Phys. Chem. 1987, 91, 6629. (14) Naitoh, K.; Ishii, Y.; Tsujii, K. J. Phys. Chem. 1991, 95, 7915. (15) Satoh, N.; Tsujii, K. Langmuir 1992, 8, 581. (16) Yamamoto, T.; Satoh, N.; Onda, T.; Tsujii, K. Langmuir 1996, 12, 3143.
S0743-7463(97)00195-9 CCC: $14.00
gel.14 Some novel properties of the above hybrid gel are reported in the present Letter. A novel soft material developed in this work consists of the bilayer membranes of polymeric surfactant being stacked periodically in the chain networks of the polymer gel. Dodecyl glyceryl itaconate (DGI; n-C12H25OCOCH2C(dCH2)COOCH2CH(OH)sCH2OH) monomer molecules form an iridescent lamellar liquid crystal in water in the concentration range 1-2 wt % in the presence of a small amount (0.2-2.0 wt % in DGI) of ionic surfactant.14 The above iridescent structure of DGI is maintained even in the aqueous solution of the monomers of acrylamide and N,N-methylenebis(acrylamide) (a cross-linker). The iridescent solution of DGI containing the above monomers (700 mM acrylamide and 10 mM methylenebis(acrylamide)) was photopolymerized by UV light, keeping its iridescent color.14 The iridescent color of the gel interestingly shifts to the blue side depending on the shrinking degree of the gel, as demonstrated in Figure 1. The volume of the polyacrylamide gel decreases with increasing concentration of ethanol, and the spacing distance between bilayer membranes incorporated in the gel networks becomes smaller also, being accompanied by the gel shrinking. There have been reported so far more than 10 surfactant systems which show the iridescent phenomena.12-24 The color, however, can be changed only by the surfactant concentration in most of the systems, with only one exception.16 The iridescent color in the present system can be altered by controlling the swelling degree of the gel. This kind of iridescent material is, of course, firstly synthesized and characterized in this work. This result indicates that the iridescent color can be controlled by the environmental conditions which affect the volume of the polymer gels. Anisotropic gels are crucially important for practical applications of polymer gels. For example, one-dimen(17) Lasson, K.; Krog, N. Chem. Phys. Lipids 1973, 10, 177. (18) Nagai, M.; Ohnishi, M. J. Soc. Cosmet. Chem. Jpn. 1984, 18 (1), 19. (19) Suzuki, Y.; Tsutsumi, H. Yukagaku 1984, 33 (11), 48. (20) Thunig, C.; Hoffmann, H.; Platz, G. Prog. Colloid Polym. Sci. 1989, 79, 297. (21) Imae, T.; Sasaki, M.; Ikeda, S. J. Colloid Interface Sci. 1989, 131, 601. (22) Platz, G.; Thunig, C.; Hoffmann, H. Prog. Colloid Polym. Sci. 1990, 83, 167. (23) Strey, R.; Schomacker, R.; Nallet, F.; Roux, D.; Olsson, U. J. Chem. Soc., Faraday Trans. 1 1990, 86, 2253. (24) Berlepsch, H.; Strey, R. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 1403.
© 1997 American Chemical Society
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Letters
Figure 1. Photographs of the polyacrylamide gels containing immobilized bilayer membranes immersed in the water/ethanol mixtures of various ratios. The ethanol concentrations in wt % are denoted in the figure. The gels were put in the solvent for 2 weeks to attend the equilibrium. The iridescent color appears resulting from the periodic structure of the bilayer membranes and shifts to the blue side owing to the shrinkage of the gel with increasing concentration of ethanol.
sional swelling-shrinking behavior is essential in the applications for artificial muscles. Bilayer membrane systems are essentially anisotropic in nature. But the anisotropic domains are randomly oriented in bulk, and the entire solution in a vessel shows the isotropic properties. We have tried to make an anisotropic orientation of bilayer membranes in the bulk phase and to immobilize the anisotropic structure in the polymer gels. The iridescent solution containing the monomers of DGI (3.0 wt %), acrylamide, and N,N-methylenebis(acrylamide) was placed in the reaction cells under shear flow by sucking the monomer solution and was photopolymerized by UV light. The sucking rate of the sample solution was 0.5 mm/s, and the shear rate was roughly estimated to be ∼1 s-1. The macroscopically oriented anisotropic lamellar structure of the polymeric DGI was immobilized inside the network of polyacrylamide. Figures 2 and 3 show the microscopic photographs under crossed polarizers of the anisotropic hybrid polyacrylamide gels. The gel was rotated on the sample stage and observed by two kinds of crossing angle between the flow direction and the direction of the detecting polarizer. One was 0° (the darkest position), and the other was 45° in the counterclockwise direction (the brightest one). In the rectangular pillar sample (Figure 2), the two cross sections parallel to the flow direction showed similar textures, and a strong contrast in brightness between the crossing angles of 0° and 45° was observed. Furthermore, the texture seemed to be in one-dimensional alignment. The
Table 1. Some Mechanical Properties of the DGI (3 wt %)-Immobilized and Simple Polyacrylamide Gelsa elastic force at the extension at modulus/ breakdown the breakdown -2 gram wt cm point/gram wt point/% polyacrylamide gel DGI-gel:parallelb DGI-gel:perpendicularc
66.2 483 378
8.0 19.8 20.8
163 586 300
a A test piece of a gel (1 mm × 10 mm × 50 mm) was extended with a tensile tester at the rate of 10 mm/min. b A gel sample was elongated in parallel with the flow direction on the occasion of the sample preparation. c A gel sample was elongated perpendicularly to the flow direction.
cross section perpendicular to the flow direction, on the other hand, showed no aligned texture and almost the same contrast in brightness at the crossing angles of 0° and 45°. The strong contrast in the top view between the crossing angles of 0° and 45° was also observed in the cylindrical shaped gel (Figure 3). In the cylindrical gel, the cross sectional view showing the big cross like pattern is particularly interesting. This pattern covers the entire cross section of the gel sample, indicating that the anisotropic orientation takes place in the whole gel of 1 mm diameter. These results observed in both rectangular and cylindrical shaped gels were very much different from those of the homogeneous gels which were prepared without flow treatment for orientation. The polarizing microscopic textures of homogenous gels were quite similar to those of ordinary lamellar liquid crystals
Letters
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Figure 3. Microscopic photographs (magnification: ×100) under crossed polarizers of an anisotropic polyacrylamide gel of cylindrical shape. The top and the cross sectional views of the cylindrical gel are exhibited in the top (a) and bottom (b) figures. The photos in the left side column were taken at the crossing angle of 0° between the flow direction and the direction of the detecting polarizer, and those in the right side column at 45° in the counterclockwise direction.
Figure 2. Microscopic photographs (magnification: ×100) under crossed polarizers of an anisotropic polyacrylamide gel of rectangular pillared shape. The top and the side view of the cross section parallel to the flow direction are shown in the top (a) and middle (b) figures. The bottom photo (c) shows the cross section perpendicular to the flow direction. The three photos in the left side column were taken at the crossing angle of 0° between the flow direction and the direction of the detecting polarizer, and those in the right side column at 45° in the counterclockwise direction.
of surfactant. Our hydrogels containing immobilized bilayer membranes show the clear anisotropy in the microscopic observations but are still far from the perfect anisotropy. The present work, however, provides us a guiding principle to synthesize the anisotropic polymer
gels which are useful and important in the practical applications of hydrogels. Preliminary experiments on the swelling behavior and the mechanical properties of the hybrid gels show the anisotropy to some extent. Some results of such experiments are shown in Table 1. The hybrid gels are much stronger than simple polyacrylamide gel. In addition, the membrane-immobilized gel seems stronger when elongated in parallel with the flow direction. Studies on the swelling, mechanical, and volume phase transition behaviors of the membrane-immobilized gels will be reported in the near future. Acknowledgment. The authors thank Mr. Sodebayashi of Kao Corporation for taking photos. LA970195X
