Hydrogels Using Oil-in-Water Emulsions - American Chemical Society

Sep 19, 2007 - temperature (LCST) in the vicinity of 33 °C;1 the NIPA hydrogel swells in water below .... 1998, 120, 12694-12695. 11246. Langmuir 200...
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Langmuir 2007, 23, 11246-11251

Novel Synthesis of Macroporous Poly(N-isopropylacrylamide) Hydrogels Using Oil-in-Water Emulsions Hideaki Tokuyama* and Akifumi Kanehara Department of Chemical Engineering, Graduate School of Engineering, Nagoya UniVersity, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan ReceiVed May 21, 2007. In Final Form: July 28, 2007

Porous N-isopropylacrylamide (NIPA) hydrogels having a unique structure, that is, spherelike cavities distributed randomly and a homogeneous network in the gel phase, were successfully synthesized by means of an emulsion templating method; this method involves the synthesis of NIPA gels in an oil-in-water (O/W) emulsion by free radical copolymerization with a cross-linker, followed by washing (removal) of the dispersed oil as a pore template (porogen). The synthesis conditions, O/W volume ratio, amount of added surfactant, and monomer concentration affect the internal pore structure, equilibrium swelling, and swelling/shrinking kinetics. A porous hydrogel swollen at 10 °C has a pore diameter distribution in the range of 1-40 µm, which was observed with a scanning electron microscope. Scanning electron micrographs and swelling degree reveal that the pore size and porosity can be adjusted by varying the O/W volume ratios and surfactant amounts. The porous hydrogels show very rapid swelling/shrinking in accordance with the temperature swing. The fast response is attributed to the convection flow of water through the macropores. In addition to a faster response gel, the emulsion templating method can yield potentially intelligent gels in which the pores function as spaces for reaction, separation, and storage.

Introduction An N-isopropylacrylamide (NIPA) hydrogel is a typical thermosensitive hydrogel having a lower critical solution temperature (LCST) in the vicinity of 33 °C;1 the NIPA hydrogel swells in water below the LCST, and it shrinks as the temperature increases. NIPA hydrogels have attracted considerable attention for their potential applications in protein delivery systems,2 glucose sensors,3 separation operations,4 microfluidic actuators,5 and so on. In these applications, a fast response of the hydrogel to external stimuli is desirable. When homogeneous NIPA hydrogels undergo an increase in temperature across the LCST, changes in their morphology, that is, skin layer6 or bubble formation,7,8 are observed on the gel surface. These changes can slow down the response rate and induce structural disorders and functional decline of the gel. Porous NIPA hydrogels can achieve a fast response rate, since the water flows into/out of the porous hydrogel by convection through the pores rather than diffusion through the gel network. Several types of porous or heterogeneous NIPA hydrogels have been prepared as follows. Gotoh et al.9 reported a simple method to prepare heterogeneous NIPA hydrogels, that is, the radical copolymerization of NIPA with a cross-linking monomer above the LCST. Kishi et al.10 prepared heterogeneous NIPA hydrogels by γ-ray irradiation without using a cross-linking * To whom correspondence [email protected].

should

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E-mail:

(1) Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1984, 81, 6379-6380. (2) Wu, J. Y.; Liu, S. Q.; Heng, P. W. S.; Yang, Y. Y. J. Controlled Release 2005, 102, 361-372. (3) Suzuki, H.; Kumagai, A.; Ogawa, K.; Kokufuta, E. Biomacromolecules 2004, 5, 486-491. (4) Tokuyama, H.; Kanehara, A. React. Funct. Polym. 2007, 67, 136-143. (5) Harmon, M. E.; Tang, M.; Frank, C. W. Polymer 2003, 44, 4547-4556. (6) Kaneko, Y.; Yoshida, R.; Sakai, K.; Sakurai, Y.; Okano, T. J. Membr. Sci. 1995, 101, 13-22. (7) Hirose, H.; Shibayama, M. Macromolecules 1998, 31, 5336-5342. (8) Shibayama, M.; Nagai, K. Macromolecules 1999, 32, 7461-7468. (9) Gotoh, T.; Nakatani, Y.; Sakohara, S. J. Appl. Polym. Sci. 1998, 69, 895906. (10) Kishi, R.; Kihara, H.; Miura, T. Colloid Polym. Sci. 2004, 283, 133-138.

monomer. The use of inert diluents as pore-forming agents during the copolymerization of NIPA and a cross-linking monomer was suggested for the preparation of porous hydrogels. Porous NIPA hydrogels were prepared in the presence of silica particles,11,12 which were subsequently removed by hydrogen fluoride treatment, poly(ethylene glycol),13,14 and nonionic surfactants.15 A promising synthesis method to prepare porous hydrogels is polymerization in continuous-discontinuous phase systems.16,17 An emulsion templating method to prepare porous NIPA hydrogels having a unique structure has been proposed in this study, which is schematically shown in Figure 1. Conventional free radical copolymerization of NIPA with cross-linking monomers in water is employed in the emulsion templating method. First, an oil-in-water (O/W) emulsion is formed by dispersing inert oil droplets as a pore template (porogen) into a pre-gel aqueous solution containing monomers. Second, the NIPA gel is synthesized in this aqueous solution by free radical copolymerization. Finally, a porous NIPA hydrogel is obtained by the washing (removal) of the dispersed oil. The emulsion templating method is conducted only using a low-toxic and inexpensive oil. The emulsion templating method yields porous NIPA hydrogels with the following potential unique features: (1) a homogeneous network structure in the gel phase similar to that in nonporous hydrogels and (2) easily controllable pore diameter (meso- or macropore) and porosity by changing the oil volume and the amount of added surfactant. Intelligent stimulisensitive copolymer gels such as biomolecule-sensitive gels18,19 (11) Kaneko, T.; Asoh, T. A.; Akashi, M. Macromol. Chem. Phys. 2005, 206, 566-574. (12) Takeoka, Y.; Watanabe, M. Langmuir 2002, 18, 5977-5980. (13) Dogu, Y.; Okay, O. J. Appl. Polym. Sci. 2005, 99, 37-44. (14) Zhang, X. Z.; Yang, Y. Y.; Chung, T. S.; Ma, K. X. Langmuir 2001, 17, 6094-6099. (15) Antonietti, M.; Caruso, R. A.; Goltner, C. G.; Weissenberger, M. C. Macromolecules 1999, 32, 1383-1389. (16) Okay, O. Prog. Polym. Sci. 2000, 25, 711-779. (17) Bennett, D. J.; Burford, R. P.; Davis, T. P.; Tilley, H. J. Polym. Int. 1995, 36, 219-226. (18) Kataoka, K.; Miyazaki, H.; Bunya, M.; Okano, T.; Sakurai, Y. J. Am. Chem. Soc. 1998, 120, 12694-12695.

10.1021/la701492u CCC: $37.00 © 2007 American Chemical Society Published on Web 09/19/2007

Synthesis of Macroporous NIPA Hydrogels

Figure 1. Schematic diagram of the preparation of porous hydrogels using the emulsion templating method.

and molecular imprinted gels20,21 require the homogeneous network structure in which ligands are randomly distributed. Porous hydrogels synthesized using the emulsion templating method can have spherelike cavities distributed randomly. Porous NIPA hydrogels in which the pore size and porosity can be successfully adjusted have potential applications such as reversible immobilization,22 molecular weight cutoff,22 and controlled release of macromolecular active agents; this is because the pores function as spaces for reaction, separation, and storage. In the present study, the feasibility of the preparation of porous NIPA hydrogels by using the proposed emulsion templating method was examined. The internal structure of the resulting porous hydrogels was observed with a scanning electron microscope (SEM). The equilibrium swelling diameter as a function of temperature and the kinetics for the swelling/shrinking of porous hydrogels by temperature swing were measured, and they were discussed in relation to the porous structure. Experimental Section Preparation of Porous NIPA Hydrogels. NIPA was kindly supplied by Kohjin Co. Ltd. The NIPA hydrogels were prepared by free radical polymerization. We used N,N′-methylenebisacrylamide (MBAA) as a cross-linker, N,N,N′,N′-tetramethylethylenediamine (TEMED) as an accelerator, and ammonium peroxodisulfate (APS) as an initiator. In the typical porous NIPA hydrogel synthesis method, three types of solutions are prepared. The pre-gel solution is 5.4 cm3 water containing NIPA, MBAA, and TEMED. The initiator solution is 0.6 cm3 water, which is 10% of the overall volume of the pre-gel aqueous solution, containing APS. The typical concentrations of NIPA, MBAA, TEMED, and APS are 1000, 50, 10, and 1 mol/m3water, respectively. The oil phase is 6 cm3 oleyl alcohol containing 0.01-0.167 g/cm3-oil polyoxyethylene(20) sorbitan monolaurate (Tween 20, HLB (hydrophile-lipophile balance) value 16.7) as the surfactant. These three types of solutions were maintained at 10 °C under a nitrogen atmosphere for 1 h. First, the oil phase was slowly added to the pre-gel solution in a test tube. Second, the initiator solution was injected into the pre-gel solution of lower phase using a syringe. The O/W emulsion was then immediately formed by mixing for 30 s with a tube mixer. The test tube was left at rest, and the polymerization was performed at 10 °C under a nitrogen atmosphere for 1 day. The resulting gels were washed with methanol using a Soxhlet extractor to remove the oil and unreacted monomers. The gels were then extensively washed with water to form the hydrogels. The volume ratio is shown as O/W ) 6/6 for the oil phase of 6 cm3 and aqueous phase of 6 cm3. The porous NIPA hydrogels were prepared under various conditions of O/W volume ratios, amount of added surfactant, and monomer concentration. Observation of the Porous Structure by Using a SEM. The internal structure of the porous hydrogels was observed by using the (19) Uguzdogan, E.; Denkbas, E. B.; Tuncel, A. Macromol. Biosci. 2002, 2, 214-222. (20) Watanabe, M.; Akahoshi, T.; Tabata, Y.; Nakayama, D. J. Am. Chem. Soc. 1998, 120, 5577-5578. (21) Tokuyama, H.; Fujioka, M.; Sakohara, S. J. Chem. Eng. Jpn. 2005, 38, 633-640. (22) Fanger, C.; Wack, H.; Ulbricht, M. Macromol. Biosci. 2006, 6, 393-402.

Langmuir, Vol. 23, No. 22, 2007 11247 SEM. The sample gel specimens were prepared by freeze-drying the swollen hydrogel to avoid shrinkage in the drying process. The swollen hydrogels at 10 °C (or other temperatures) were frozen using liquid nitrogen. The frozen gels were dried under reduced pressure for several hours and then split into 1-mm-thick pieces. Observation of Oil Droplets by Using a Stereomicroscope. The size consistency of the pores, which was observed by using the SEM, and oil droplets as a pore template were examined. An O/W emulsion was formed in a similar manner as that in the preparation of porous hydrogels; however, APS was absent in the pre-gel solution. To obtain a better view of the dispersed oil droplets, an extremely small amount of the O/W emulsion was put on the water in a petri dish at 10 °C. The oil droplets spreading on the water surface were then observed with a stereomicroscope. Measurement of the Swelling Diameter. The swelling diameter of the cylinder-shaped hydrogels was measured as a function of temperature. The gels were synthesized in a glass tube with an inside diameter of 6.0 mm. The ratio of the diameter to the length of the gel was 1:1. The hydrogels were initially immersed in water at 10 °C. The diameter, at equilibrium, was measured with a microscope; the central point was used as the measuring point. The temperature was then increased in increments, and the diameter was measured at each temperature. In addition, the swelling degrees were determined from the ratio of the weights of the swollen gels to the corresponding dried gels. The kinetics for the swelling/shrinking of the cylinder-shaped hydrogels by temperature swing was investigated. The swelling/ shrinking of the hydrogels was initiated by placing the hydrogels from water at 50/10 °C into water at 10/50 °C; the instants at which the immersions took place were considered as time zero. The diameters of the hydrogels were measured using a digital camera. Measurement of the Shear Modulus. The shear modulus of cylinder-shaped hydrogels was measured by compression.23,24 A weight was loaded onto the hydrogels, which was vertically placed in water at 10 °C, in a length direction. The shear modulus, µ [Pa], of the hydrogels was evaluated from the linear relation dP ) 3µ(dl/l) between changes in stress dP and length dl/l.

Results and Discussion SEM Micrographs of Porous NIPA Hydrogels. The NIPA gels were prepared under the conditions of O/W ) 0/12, 2/10, 4/8, 6/6, and 8/4 with 0.05 g-Tween/cm3-oil and O/W ) 6/6 with 0.01-0.167 g-Tween/cm3-oil. The concentrations of NIPA and MBAA in the pre-gel aqueous solution were 1000 and 50 mol/ m3-water, respectively, unless otherwise noted. The swollen hydrogel of O/W ) 0/12, that is, the nonporous hydrogel, is transparent in water below the LCST, while the hydrogels prepared using the oil phase are opaque. This implies that porous hydrogels are obtained by using the emulsion templating method. The synthesis of the gel of O/W ) 10/2 was unsuccessful due to poor emulsification. Figure 2 shows SEM micrographs of porous NIPA hydrogels. The influence of the O/W volume ratio on the porous structure of the hydrogels is as follows: in the hydrogel of O/W ) 0/12, no pores are observed, while the porous structure is clearly observed for the hydrogels synthesized using the oil phase. Porous NIPA hydrogels can be successfully synthesized by using the emulsion templating method. The pore seems to be the site of the oil droplet. In the gel of O/W ) 6/6 with 0.05 g-Tween/cm3oil, the pore diameters are distributed in the range of 1-40 µm. The amount of polymer freeze-dried obviously decreases with an increase in the oil volume. The amount of added surfactant obviously has a significant impact on the pore structure in the micrographs for the hydrogels (23) Tanaka, T.; Hocker, L. O.; Benedek, G. B. J. Chem. Phys. 1973, 59, 5151-5159. (24) Sakohara, S.; Takioka, T.; Nikai, T.; Takatani, K. Kagaku Kogaku Ronbunshu 2003, 29, 62-69.

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Figure 3. Photographs of oil droplets spreading on the water surface when a small amount of O/W emulsion was put on water. The O/W volume ratio and the surfactant concentration are shown in each photograph.

Figure 2. SEM micrographs of nonporous and porous NIPA hydrogels. The preparation conditions of gels, O/W volume ratio, surfactant concentration, and temperatures of the swollen/shrunken hydrogels in water before freeze-drying are shown in the photographs.

of O/W ) 6/6 with 0.01-0.167 g-Tween/cm3-oil. A large pore diameter of ∼100 µm is observed for 0.01 g-Tween/cm3-oil. The pore diameter decreases with an increase in the surfactant amount. As compared to the O/W volume ratio, the surfactant amount plays an extremely important role in determining the pore size. The deformation of pores depending on temperature was examined using the hydrogel of O/W ) 6/6 at 10, 28, and 50 °C. The porous hydrogel was macroscopically swollen/shrunken at 10/50 °C; the swollen hydrogel at 28 °C has the same size as the synthesized one (described in detail hereinafter). The pore

diameter at 28 °C is in the range of 1-40 µm, which may correspond to the diameter of the oil droplet. The porous structure was also observed at 50 °C, but large pores having diameters of several tens of micrometers were not observed. The microscopic expansion/contraction of the pore can correspond to the macroscopic swelling/shrinking of the porous hydrogel. Size of the Oil Droplet in an O/W Emulsion. The size consistency of the pore and oil droplet as a pore template was examined. A small amount of the emulsion of O/W ) 6/6 with 0.01-0.167 g-Tween/cm3-oil was placed on water. The photographs of the oil droplets spreading-on the water surface are shown in Figure 3. For 0.05 g-Tween/cm3-oil, the droplet diameters are distributed in the range of 1-20 µm. The droplet diameter distribution corresponds to that of the pores shown in Figure 2, considering the expansion of the pore due to freezedrying of the gel phase. It is assumed that the oil droplets act as the pore template. The droplet diameter largely decreases with an increase in the additive amount. The amount of added surfactant induces substantial changes in the size of the oil droplet. With regard to the synthesis conditions other than the surfactant amount, the droplet size slightly decreases with an increase in the volume of the oil phase (Figure 3) and/or the agitation intensity. The photograph for O/W ) 6/6 with 0.05 g-Tween/cm3-oil in Figure 3 shows that the droplet distribution can be divided into two groups: ∼1 and 20 µm. The agitation intensity of the tube mixer used in this study may be relatively low. Sub-micro- or nanodroplets having a narrow distribution may be formed by increasing the agitation intensity by the use of a mechanical agitator, emulsification equipment, and an ultrasonic homogenizer. Equilibrium Swelling. Figure 4 shows the temperature dependence of the swelling diameter of the cylinder-shaped porous NIPA hydrogels in water. In Figure 4a, the gels were prepared under various O/W volume ratios with 0.05 g-Tween/cm3-oil. The diameter of the nonporous hydrogel of O/W ) 0/12 decreases

Synthesis of Macroporous NIPA Hydrogels

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Figure 5. Swelling degree of porous NIPA hydrogels as a function of temperature. The gels were prepared under various O/W volume ratios with 0.05 g-Tween/cm3-oil.

Figure 4. Swelling diameters of cylinder-shaped porous NIPA hydrogels in water as a function of temperature. (a) The gels were prepared under various O/W volume ratios with 0.05 g-Tween/cm3oil. (b) The gels of O/W ) 6/6 were prepared under various surfactant amounts. (c) The gels of O/W ) 6/6 were prepared at various monomer concentrations.

with an increase in temperature, and a drastic change occurs at around 30 °C due to the hydrophilic/hydrophobic transition. The porous hydrogels prepared by the emulsion templating method show a thermosensitive swelling pattern similar to that of the nonporous hydrogels. This fact indicates that the porous hydrogel can be similar in network structure to the nonporous hydrogel. The diameters of the porous hydrogels increase to some extent with an increase in the volume of the oil phase during preparation. The presence of pores can contribute to the swelling of the hydrogels. The influence of the amount of added surfactant on the swelling diameter of the porous hydrogels of O/W ) 6/6 is shown in Figure 4b. The surfactant amount induces substantial changes in the pore size, as shown in Figure 2; however, it has a slight influence on the macroscopic size of the hydrogel. In the investigated surfactant concentration range, the swelling diameter for 0.05 g-Tween/cm3-oil was somewhat larger than the others over the investigated range of temperature. The reason for this is still unclear.

The influence of monomer composition on the swelling diameter of the porous hydrogels is shown in Figure 4c. In addition to the case of NIPA/MBAA ) 1000/50 mol/m3-water mentioned above, the porous hydrogels of NIPA/MBAA ) 1000/30, 1000/ 100, 800/40, and 1200/60 mol/m3-water, which were prepared under the conditions of O/W ) 6/6 with 0.05 g-Tween/cm3-oil, were used. In the investigated monomer concentration range, the monomer composition had a slight influence on the macroscopic size and pore size below the LCST, although their SEM micrographs are omitted in this report. On the other hand, the diameter of the shrunken hydrogel above the LCST increases with an increase in the MBAA concentration. The rigid gel phase in the porous hydrogel could be generated by increasing the amount of cross-linker, resulting in the prevention of hydrogel shrinking. In fact, the shear modulus of the porous hydrogels of NIPA/MBAA ) 1000/100, 1200/60, 1000/50, 800/40, and 1000/ 30 mol/m3-water were 2.70, 2.43, 1.75, 1.61, and 0.817 kPa, respectively, while that of the nonporous hydrogel was 27.3 kPa. Figure 5 shows the temperature dependence of the swelling degree of porous NIPA hydrogels, which were prepared under various O/W volume ratios with 0.05 g-Tween/cm3-oil. The thermosensitive pattern of the swelling degree is the same as that of the swelling diameter shown in Figure 4a. The swelling degree increases with an increase in the oil volume used during the preparation of the gel. This fact indicates that the sites of the oil droplets become the pore template. The temperatures at which the swelling diameter is equal to 6.0 mm (see Figure 4a) are approximately 10, 25, 30, and 30 °C for O/W ) 0/12, 4/8, 6/6, and 8/4, respectively. At these temperatures, the swelling degrees for O/W ) 4/8, 6/6, and 8/4 are approximately 1.5, 2, and 3 times greater than that for O/W ) 0/12. This demonstrates that the porosity in hydrogels corresponds to the O/W volume ratio. Swelling/Shrinking Kinetics. Figure 6 shows the shrinking rate of the porous NIPA hydrogels by the temperature swing from 10 to 50 °C. The gels were prepared under various O/W volume ratios with 0.05 g-Tween/cm3-oil. The longitudinal axis shows the normalized hydrogel diameter: (dt - d50)/(d10 - d50), where dt is the hydrogel diameter at time t, and d10 and d50 are the equilibrium diameters at 10 and 50 °C, respectively. For the nonporous hydrogel of O/W ) 0/12, the swelling equilibrium was attained in about 1 day, where the shrinking rate was promoted after a crack appeared. Since the swelling equilibrium was attained within 1 min for the porous hydrogels of O/W ) 4/8, 6/6, and 8/4, the shrinking rate is clearly facilitated by the formation of porous structures. The shrinking rate of the hydrogel of O/W ) 2/10 is relatively low. It could be said that a porosity greater than 33% is indispensable to convect water.

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Figure 6. Shrinking rate of porous NIPA hydrogels by the temperature swing from 10 to 50 °C. The gels were prepared under various O/W volume ratios with 0.05 g-Tween/cm3-oil.

The shrinking kinetics was analyzed in terms of the first-order rate analysis.11

ln[(dt - d50)/(d10 - d50)] ) -kt

(1)

where k is the rate constant. The value of k for the porous hydrogel of O/W ) 6/6 was approximately 20 times greater than that of the nonporous hydrogel. The influence of the amount of added surfactant and the monomer composition on the shrinking rate were investigated for the porous hydrogels of O/W ) 6/6, which were prepared under the conditions shown in Figure 4b and 4c. For these hydrogels, the swelling equilibrium was attained within 1 min, although their results are not shown here. The shrinking rates of the hydrogels of O/W ) 6/6 remained nearly unaffected by the surfactant amount and the composition within the investigated range. As shown in the SEM micrographs, the pores in the hydrogels synthesized using the emulsion templating method are not through-bores but spherelike cavities distributed randomly. Kaneko et al.11 reported that the shrinking rate of porous hydrogels prepared using silica nanoparticles with a network morphology was greater than that of porous hydrogels perforated using spherical silica nanoparticles by means of a stepwise water transfer. It could be assumed that the shrinking rate of porous hydrogels synthesized by the emulsion templating method is fully facilitated by the macropores, which are mutually independent cavities distributed randomly. Figure 7 shows the swelling rate, that is, the time course of the normalized hydrogel diameter, of porous NIPA hydrogels by the temperature swing from 50 to 10 °C. In Figure 7a, the gels were prepared under various O/W volume ratios with 0.05 gTween/cm3-oil. For the nonporous hydrogel of O/W ) 0/12, the swelling equilibrium was attained in about 1 day. The swelling rate is also facilitated with an increase in the oil volume. In the hydrogel of O/W ) 4/8, however, the degree of swelling rate enhancement is less than that of the shrinking rate. If the water flow into the porous hydrogel by convection through the pores enhances the swelling rate, the presence of the pores in the shrunken hydrogel must contribute toward faster swelling. In Figure 4a, the swelling diameter of O/W ) 4/8 at 50 °C is smaller that that of O/W ) 6/6. This fact implies that the pore in the shrunken hydrogel of O/W ) 6/6 is large enough to convect water while that of O/W ) 4/8 is not.

Figure 7. Swelling rate of porous NIPA hydrogels by the temperature swing from 50 to 10 °C. (a) The gels were prepared under various O/W volume ratios with 0.05 g-Tween/cm3-oil. (b) The gels of O/W ) 6/6 were prepared under various surfactant amounts. (c) The gels of O/W ) 6/6 were prepared at various monomer concentrations.

The influence of the amount of added surfactant on the swelling rate of the porous hydrogels of O/W ) 6/6 is shown in Figure 7b. The hydrogels used are the same as those shown in Figure 4b. The swelling rate for 0.05 g-Tween/cm3-oil is conspicuously the fastest. In Figure 4b, the swelling diameter for 0.05 g-Tween/ cm3-oil at 50 °C is the largest. As mentioned above, the existence of the larger pores in the shrunken gel, which can be estimated from the macroscopic swelling diameter, plays a crucial role in the fast swelling kinetics of the porous hydrogels. The influence of monomer composition on the swelling rate of the porous hydrogels is shown in Figure 7c. The hydrogels used are the same as those shown in Figure 4c. The swelling rate increases with an increase in MBAA concentration. This order also corresponds to the swelling diameter at 50 °C shown in Figure 4c.

Synthesis of Macroporous NIPA Hydrogels

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Conclusions

Figure 8. Swelling diameter of a porous NIPA hydrogel (O/W ) 6/6) with time in the swelling/shrinking process by the temperature swing between 10 and 50 °C.

Figure 8 shows the repetitive swelling and shrinking in the porous hydrogel of O/W ) 6/6 with 0.05 g-Tween/cm3-oil by the temperature swing between 10 and 50 °C. The porous hydrogel swells and shrinks reversibly at a constant rate with a change in temperature.

Porous NIPA hydrogels having spherelike cavities distributed randomly and the homogeneous network in the gel phase were successfully synthesized by the emulsion templating method proposed in this study. For instance, in the swollen hydrogel prepared under the conditions O/W ) 6/6 with 0.05 g-Tween/ cm3-oil, the pore diameters are distributed in the range of 1-40 µm and the porosity corresponds to the O/W volume ratio. It was confirmed that the oil droplets act as a pore template. The pore size and porosity are controllable by varying the O/W volume ratio and the amount of added surfactant. The swelling/shrinking rate of porous hydrogels is considerably facilitated, since water into/out of the porous hydrogel can flow by convection through the macropores. In addition to a faster response gel, the emulsion templating method can yield potentially intelligent gels in which the pores function as spaces for reaction, separation, and storage. Acknowledgment. The authors wish to acknowledge the support of a Grant-in-Aid for Scientific Research from JSPS (Japan Society for the Promotion of Science) (Grant No. 10363029). LA701492U