Electrical Behavior of a Natural Polyelectrolyte Hydrogel: Chitosan

Mar 1, 2008 - Citation data is made available by participants in CrossRef's Cited-by Linking service. For a more comprehensive list of citations to th...
0 downloads 0 Views 1MB Size
Download PDF
1208

Biomacromolecules 2008, 9, 1208–1213

Electrical Behavior of a Natural Polyelectrolyte Hydrogel: Chitosan/Carboxymethylcellulose Hydrogel Jing Shang, Zhengzhong Shao, and Xin Chen* The Key Laboratory of Molecular Engineering of Polymers of MOE, Department of Macromolecular Science, Advanced Material Laboratory, Fudan University, Shanghai, 200433, People’s Republic of China Received November 1, 2007; Revised Manuscript Received January 24, 2008

An amphoteric hydrogel film was prepared by solution blending of two natural polyelectrolytes, chitosan and carboxymethylcellulose, and cross-linking with glutaraldehyde. The bending of the film in an electric field was studied in different electrolyte solutions. Because of its amphoteric nature, the hydrogel can bend toward either anode or cathode depending on the pH of the solution. Other factors such as ionic strength and electric field strength also influence the electromechanical behavior of the hydrogels. The equilibrium bending angle of the hydrogel was found to reach a maximum at about 90° in pH ) 6 Britton-Robinson buffer solution with an ionic strength of 0.2 M. The sensitivity of the films over a wide range of pH and the good reversibility of this natural amphoteric electric-sensitive hydrogel suggest its future use in microsensor and actuator applications, especially in the biomedical field.

Introduction Hydrogels are cross-linked, three-dimensional, and hydrophilic polymer networks that can swell but not dissolve in water.1 Hydrogels that can change their volumes and shape reversibly under external stimuli, such as temperature, pH, light, electric/magnetic field, and specific molecular recognition, are called environment-sensitive or smart hydrogels.2–7 Among the smart hydrogels, electric-sensitive hydrogels have attracted considerable attention in recent years because their electric response can be controlled relatively easily in a number of useful application geometries. These hydrogels can exhibit swelling, shrinking, or bending behavior in an electric field, which transforms the electrical energy into mechanical work. Electricsensitive hydrogels can thereby be used in the field of smart gel-based devices such as sensors, artificial muscles, membrane separation devices, and drug delivery systems.8–11 Several electric-sensitive hydrogels have been reported in the literature. For example, Mahaveer et al. prepared poly(acrylic acid)/poly(vinyl alcohol) copolymer membranes and found that they bent toward the cathode under a constant electric field.12 Yao and Krause synthesized sulfonated cross-linked polystyrene gel and studied its electromechanical behavior in different salt solutions.13 Moschou et al. developed a novel artificial muscle material composed of acrylamide/acrylic acid copolymer and polypyrrole/carbon black.14 It can be electrically actuated under low applied potential in a near neutral pH environment. Kim et al. have prepared a series of composite hydrogels based on both natural and synthetic polymers and investigated their electrical sensitivity. They found polycation hydrogels, such as chitosan (CS)/polyacrylonitrile15 and CS/poly(diallyldimethylammonium chloride),16 bent toward the anode, whereas polyanion hydrogels, such as hyaluronic acid/poly(vinyl alcohol)5 and alginate/ poly(methacrylic acid),17 bent toward the cathode under electric stimulus. Although many kinds of electric-sensitive hydrogels have been developed, most of them are based upon synthetic polymers or contain synthetic polymer components. Therefore, * To whom correspondence may be addressed. E-mail: chenx@ fudan.edu.cn. Telephone: +86-21-6564-2866. Fax: +86-21-6564-0293.

the application of those hydrogels in biological and pharmaceutical fields is quite limited due to their poor biocompatibility or latent toxic effect of the synthetic polymers. In addition, most of the electric-sensitive hydrogels reported were either polycationic18,19 or polyanionic,20–22 which only showed an electrical response in a specific pH range. This also limits their application in many fields. To overcome the limitations outlined above, we prepared a natural amphoteric hydrogel by solution blending of two natural polyelectrolyte, CS and carboxymethylcellulose (CMC), because both of them are polysaccharides that have good biocompatibility and have been used widely in biomedical fields.23–28 A CS/CMC blend is a polycation in acidic solutions but becomes a polyanion in basic solutions due to amino groups on CS and carboxymethyl groups on CMC macromolecular chains. Our results showed that such an amphoteric CS/CMC hydrogel exhibits a good electromechanical response over a wide pH range.

Experimental Section Materials. CS (molecular weight ) 1100 kDa, deacetylation degree ) 75%) was purchased from Jinan Haidebei Marine Biological Product Co. Ltd. (Jinan, China). After further deacetylation,29 the deacetylation degree of CS was increased to 90% and the molecular weight was reduced to 460 kDa. Sodium CMC (NaCMC), sodium hydroxide, sodium sulfate, potassium chloride, boric acid, phosphoric acid, acetic acid, and glutaraldehyde were purchased from Shanghai Chemical Reagent Co. Ltd. (Shanghai, China) and were used without further purification. Preparation of CS/CMC Hydrogels. CS was dissolved in 2% (v/ v) acetic acid aqueous solution to make a 2% CS solution and NaCMC was dissolved in deionized water to make a 2% NaCMC aqueous solution. Then 2 volumes of NaCMC solution were added dropwise into 3 volumes of CS solution at 60 °C under stirring. In the meantime, glutaraldehyde (0.1% mole of the amino groups on CS) was added into the solution in order to cross-link the CS. Glutaraldehyde was chosen as the cross-linking agent because, under our preparation conditions, it cross-links mainly with the amino groups on the CS macromolecular chains, which makes the structure of the CS/CMC hydrogel relatively simple. After the complete mixing at 60 °C for 2 h,

10.1021/bm701204j CCC: $40.75  2008 American Chemical Society Published on Web 03/01/2008

Electrical Behavior of Chitosan/Carboxymethylcellulose Hydrogel

Figure 1. Schematic diagram for testing the bending behavior of hydrogels.

the mixture was poured into a poly(ethylene terephthalate) dish and dried at room temperature. Finally, the dried films were put into a 0.5% (w/w) NaOH aqueous solution to remove the remaining acetic acid and then washed repeatedly with deionized water. Characterization of CS/CMC Hydrogels. FTIR spectra were recorded with a Nicolet Nexus-470 spectrometer in transmission mode at 4 cm-1 resolution using 64 scans. Wide-angle X-ray diffraction measurements were performed with a Philips X’Pert PRO diffractometer using Ni-filtered Cu KR radiation (λ ) 1.5406 Å) with a 2θ range between 5° and 40° at 40 kV and 40 mA. The morphology of the CS/ CMC film was observed using a Philips XL30 scanning electron microscope (SEM) at 20 kV. The cross section was prepared by fracturing the film under liquid nitrogen and scanned after coating with a thin layer of gold. Swelling of CS/CMC Hydrogels. The CS/CMC hydrogels were dried in an oven at 60 °C to constant weight and then immersed in Britton-Robinson buffer solutions with different pH values (from 4 to 12) but constant ionic strength (0.1 M), or with different ionic strengths (from 0.1 to 0.3 M) but constant pH value (pH ) 6.0), respectively.30 After the excess solution on the hydrogel surface had been removed with filter paper, the weight of swollen samples was immediately measured. The swelling ratio was determined as follows:

swelling ratio ) (Ws - Wd) ⁄ Wd

(1)

where Ws and Wd are the weights of the samples in swollen and dry states, respectively. Measurement of Bending of CS/CMC Hydrogels in an Electric Field. A schematic diagram of the equipment used for studying the electrical response of hydrogels is shown in Figure 1. All hydrogel strips (10 mm long and 2 mm wide) were first swollen to equilibrium in an electrolyte solution (Britton-Robinson pH buffer solution or neutral salt solution) to be used later in the bending experiments. Two parallel carbon electrodes, 50 mm apart, were immersed in the electrolyte solutions and the hydrogel strip under investigation was mounted centrally between them. Upon application of a dc electric field, the degree of bending, θ, was measured by reading the angle of deviation from the vertical position. We define the value of bending angle as being positive when the hydrogel bends toward the anode and negative when it bends toward the cathode. The bending behavior was recorded with a digital camera (Kodak, USA).

Results and Discussion Characteristics of CS/CMC Hydrogels. Both CS and CMC are polysaccharides that have a range of functional groups such as amino, hydroxyl, and carboxyl groups. It is well documented that CS and CMC can form an intermacromolecular complex through the strong electrostatic and hydrogen bonding interactions between those groups.31,32 To enhance the strength of CS/ CMC hydrogels, we used glutaraldehyde to cross-link the amino groups on the CS component of the hydrogel to form a semiinterpenetrating polymer network (semi-IPN).33 A simplified scheme for the formation of a CS/CMC semi-IPN is suggested as follows (Figure 2).31,34,35

Biomacromolecules, Vol. 9, No. 4, 2008

1209

The FTIR spectrum and WAXD pattern of dry CS/CMC semi-IPN film shown in Figure 3 demonstrate the interactions between CS and CMC in the film. For example, The -NH3+ characteristic absorption band at 1589 cm-1 and -COOcharacteristic absorption band at 1599 cm-131,36 combine to give a new adsorption band at 1593 cm-1 in CS/CMC film (Figure 3a). Moreover, the WAXD pattern of CS/CMC film shows lower crystallinity than pure CS (Figure 3b), which also suggests that the intermolecular interaction between CMC and CS macromolecular chains destroys the original crystalline structures of CS. Because of the strong intermolecular interactions between CS and CMC in CS/CMC film, the morphology of the film was quite uniform and no obvious macrophase separation was found (Figure 4). The Bending Behavior of CS/CMC Hydrogels. After an electric field is applied to the CS/CMC hydrogel strip in electrolyte solution, the hydrogel quickly bends toward one electrode, showing good electrical sensitivity. The hydrogel bends in different directions depending on the pH of the buffer solution. When pH < 7, the hydrogel bends toward the anode, but when pH g 7, the hydrogel bends toward the cathode. Figure 5 shows a typical response of the hydrogels to an electric field in acidic and basic buffer solutions in the form of bending angle as a function of time. The bending rates of the hydrogel are quite fast, taking less than 60 s to reach the equilibrium deformation in both acidic and basic buffer solutions. However, if we put the hydrogel in a neutral salt solution such as 0.1 mol/L aqueous Na2SO4 solution, the bending behavior of the CS/CMC hydrogel is quite different from those in the pH buffer solutions (Figure 6). In this case, the bending behavior is quite complicated: first bending toward the cathode and then reversing and bending toward the anode. Also, it took more time for the hydrogel to reach its equilibrium bending angle in aqueous Na2SO4 solution compared to those in pH buffer solutions. From the literature, a reasonable explanation for the bending behavior of hydrogel in an electric field is the osmotic pressure difference inside and outside the hydrogel that occurs as a consequence of mobile ion transport to the anode or cathode side.8,37 According to Flory’s theory, the osmotic pressure, π, due to ionic distribution, is given by the following equation.

π ) RT

(∑ C - ∑ C ) i in

i

j out

(2)

j

where R is the gas constant, T is the absolute temperature, Ciin is the concentration of mobile ion i in the gel, and Cjout is the concentration of mobile ion j in the surrounding solution. When an electric field is applied, the counterions of the polyions in the hydrogel and the free ions in the solution move toward their counter-electrodes, which results in an ionic gradient along the direction of the electric field. The ionic concentrations inside the gel and outside the gel are thereby different. According to eq 2, this difference in ionic concentrations induces an osmotic pressure difference ∆π between the anode and cathode sides of the hydrogel (eq 3), which generates the driving force for the bending of the hydrogel.

∆π ) π1 - π2

(3)

j where π1 ) RT (∑i Ciin - ∑j Cout anode side), represents the osmotic pressure between the hydrogel and the solution on the j anode side and π2 ) RT (∑i Ciin - ∑j Cout cathode side), represents the osmotic pressure between the hydrogel and the solution on the cathode side. For the polyanionic hydrogel, π1 increases to give ∆π > 0, such that the hydrogel swells on the anode side

1210

Biomacromolecules, Vol. 9, No. 4, 2008

Shang et al.

Figure 2. Schematic diagram of the formation of CS/CMC semi-IPN.

Figure 3. FTIR spectra (a) and wide-angle X-ray diffraction patterns (b) of dry CS, CMC, and CS/CMC films.

Figure 5. Typical response of the CS/CMC hydrogels to an electric field in different pH buffer solutions (ionic strength ) 0.1 M) at 15 V. (a) pH ) 6; (b) pH ) 10. Figure 4. SEM image of the cross-section of dry CS/CMC film. The insert figure indicates the thickness of the dry CS/CMC film (between two white arrows).

and shrinks on the cathode side, so causing it to bend toward the cathode side. Conversely, for the polycationic hydrogel, ∆π < 0, thereby it bends to the anode side. The amphoteric hydrogel can act as a polyanionic or polycationic hydrogel depending on the pH value. In our case, the amino groups (-NH2) on CS in CS/CMC hydrogel are protonated to become -NH3+ in acidic solution, while the carboxyl groups on CMC remain as -COOH, so it behaves as a polycationic hydrogel that bends toward the anode. Conversely, the carboxyl groups in the CS/CMC hydrogel are

ionized to -COO- in basic solution, but the amino groups remain in their original uncharged form to give a polyanionic hydrogel, which bends toward the cathode. However, the situation is different in a neutral electrolyte solution. As we used sodium CMC to prepare CS/CMC hydrogel, some of the sodium ions may transfer from the hydrogel into the aqueous Na2SO4 solution to leave net -COO- ions in the hydrogel. Thus, the CS/CMC hydrogel starts as a polyanionic hydrogel in neutral aqueous Na2SO4 solution, so it first bends to the cathode as it does in the pH buffer solution. We know that electrolysis occurs in aqueous solutions when an electric field is applied:38 Anode: 2H2O f O2 + 4H+ + 4e-. Cathode: 4H2O + 4e- f 4OH- + 2H2.

Electrical Behavior of Chitosan/Carboxymethylcellulose Hydrogel

Biomacromolecules, Vol. 9, No. 4, 2008

1211

Figure 8 shows the effect of ionic strength on the swelling ratio and equilibrium bending angle of the CS/CMC hydrogels.

Figure 6. Response of the CS/CMC hydrogels in 0.1 mol/L aqueous Na2SO4 solutions at 15 V.

The pH changes in aqueous Na2SO4 solution in an electric field affect the swelling of the hydrogel. The OH- ions generated around the cathode cause more carboxyl groups in CS/CMC hydrogel to ionize to -COO-. This increasing fraction of -COO- ions on the cathode side enhances the electrostatic repulsion in the hydrogel and swells the cathode side. Meanwhile, the H+ ions generated around the anode protonate the -COO- ions to -COOH in the hydrogel, thereby inducing shrinkage of the hydrogel on the anode side. Because these two processes occur at different rates, the hydrogel gradually changes its bending direction and finally bends to the anode. Although such electrolysis also happens in pH buffer solutions, it has little effect due to the buffer effect and almost has no influence on the bending direction of CS/CMC hydrogel. Effect of pH on the Bending Behavior of CS/CMC Hydrogels. As an amphoteric hydrogel, the type and amount of charge on CS/CMC hydrogel depends on the local pH value, so the bending behavior of the hydrogel is also greatly influenced by the pH of the buffer solution. As described above, the hydrogel is positively charged when pH < 7. It swells in the buffer solution by the repulsion between protonated amino groups on CS; a lower pH gives a higher swelling ratio due to the increased number of NH3+ ions on the hydrogel (Figure 7a). However, we did not observe the same tendency for the equilibrium bending angle of the hydrogel (Figure 7b). The angle showed a maximum value at pH 5-6 and then decreased with the further decreases in the pH. The reason for this effect is suggested to be that, although the net charge increases with the decrease in pH, the volume of the hydrogel increases more significantly at the same time (the swelling of the hydrogel increases dramatically when pH < 5, see Figure 7a). Therefore, the charge density in the hydrogel actually decreases when pH < 5, which results in decrease in the bending angle.19 The bending behavior in basic buffer solutions is simpler than that in acidic buffer. Both the swelling ratio and the equilibrium bending angle increase slightly with increase in the pH value in the buffer solution. We also found that those two effects were smaller than those in the acidic environment due to the strong hydrogen bonding between CS macromolecular chains in basic solution. Effect of Ionic Strength on the Bending Behavior of CS/CMC Hydrogels. In electrolyte solutions with different ionic strength, the number of movable ions in an electric field is different, which results in dissimilar osmotic pressure differences between the hydrogels and the solutions. Because osmotic pressure difference is the driving force on the deformation of hydrogels in an electric field, the ionic strength is another factor that influences the bending behavior of the hydrogels.

With increases in ionic strength, the swelling ratio of the CS/ CMC hydrogel also increases due to the antipolyelectrolyte behavior of polyampholyte hydrogels (Figure 8a).39 The addition of salt weakens the electrostatic attractions between oppositely charged ionic groups (-NH3+ and -COO-) so that the hydrogel becomes less compact and the swelling ratio increases. However, the effect of ionic strength on the equilibrium bending angle of the CS/CMC hydrogel is not as simple as that on swelling ratio. As ionic strength increases, the equilibrium bending angle first increases, reaching the maximum value when the ionic strength (I) was 0.2 M, and then decreases with further increases in ionic strength (Figure 8b). We know that the increase in electrolyte concentration in the solution induces an increase in the amount of free ions moving to their counterelectrodes. This increases the osmotic pressure difference between the anode and cathode side of the hydrogels that could increase the equilibrium bending angle. On the other hand, an increasing number of ions can have a shielding effect on the fixed charges in the hydrogels. Therefore, when the ionic strength is low (I < 0.2 M), the shielding effect is not significant, so the equilibrium bending angle of the CS/CMC hydrogel increases with increases in ionic strength. When I > 0.2 M, in addition to the obvious shielding effect, the volume of the hydrogel is enlarged due to significant swelling (Figure 8a), which decreases the charge density in the hydrogel. As a consequence, the equilibrium bending angle of the CS/CMC hydrogel decreases with further increases in ionic strength when I > 0.2 M. Effect of Electric Voltage on the Bending Behavior of CS/CMC Hydrogels. The electric field provides the power for the electric-sensitive hydrogels to bend in an electrolyte solution. The ions in the solution move faster as the electric voltage increases, thereby generating an ionic gradient more quickly, which should then result in increases in both bending rate and angle. Figure 9 shows the influence of electric voltage on the bending rate of the CS/CMC hydrogels. Various functions have been used to fit the kinetic curves in order to determine kinetic parameters. Of these, exponential decay functions are mostly commonly used.40 The kinetic curves in Figure 9 can be fitted quite well with a first-order exponential decay function (eq 4) and the time constant for the bending decreased from 35.4 to 25.6 s with increasing voltage (a lower time constant means a faster bending rate).

bending angle ) A0(1 - e-t⁄τ)

(4)

where A0 is the equilibrium bending angle, t is the time, and τ is the time constant. In addition, the equilibrium bending angle increased from 11.6° to 81.5° as the voltage increased, thus confirming that a higher applied electric voltage gives a faster bending rate and a greater equilibrium bending angle. Reversible Bending Behavior of CS/CMC Hydrogels. The reversible bending behavior of the hydrogels was examined by alternately applying an electric voltage of 15 V to the CS/CMC hydrogels for 60 s and then removing the voltage for a further 60 s. Figure 10 shows that hydrogels bend and return to their initial position quickly as the electric field is turned on and off several times. The shape of each cycle is very similar, which indicates a good reversibility. This suggests that CS/CMC hydrogels have the potential in applications such as microsensors, actuators, and artificial muscles, etc.

1212

Biomacromolecules, Vol. 9, No. 4, 2008

Shang et al.

Figure 7. Effect of pH on the swelling ratio (a) and equilibrium bending angle at 15 V (b) of CS/CMC hydrogels at different pH buffer solutions (ionic strength ) 0.1 M).

Figure 8. Effect of ionic strength on the swelling ratio (a) and equilibrium bending angle at 15 V (b) of the CS/CMC hydrogels in pH 6.0.

Figure 9. Effect of electric voltage on the bending behavior of the CS/CMC hydrogel in the same pH buffer solution (pH ) 6, ionic strength ) 0.1 M). The experimental data were fitted by the firstorder exponential decay function as shown in eq 4. (a) 5 V; (b) 10 V; (c) 15 V; (d) 20 V.

Conclusions Natural amphoteric CS/CMC hydrogels were prepared by a simple solution blending method, and it gave a mechanical response in an electric field. The hydrogels were responded quickly (bending to the electrode) in different electrolyte solutions and over a wide pH range. The bending behavior of the hydrogel was different in different electrolyte solutions, i.e., acidic buffer solutions, basic buffer solutions, or neutral salt solutions. The equilibrium bending angle of CS/CMC hydrogels was influenced by pH, ionic strength, and electric voltage. The equilibrium bending angle of the hydrogels reached a maximum in the buffer solution at pH 5-6 with 0.2 M ionic strength. Their sensitivity over a wide pH range and good reversibility

Figure 10. Reversible bending behavior of the CS/CMC hydrogels in pH buffer solution (pH ) 6, ionic strength ) 0.1 M) at 15 V.

in an electric field may expand the scope for practical application of electric-sensitive hydrogels and shows their potential to be used as microsensors, actuators, and artificial muscles, particularly in biomedical fields because all the materials are from nature. Acknowledgment. This work is supported by the National Natural Science Foundation of China (Nos. 20674011 and 20525414), the Program for New Century Excellent Talents in University of MOE of China (NCET-06-0354), and the Program for Changjiang Scholars and Innovative Research Team in University of MOE of China. We thank Dr. Jingrong Yao and Dr. Lei Huang for helpful advice and discussions and Dr. David Porter for reading and correcting the manuscript.

References and Notes (1) Kim, S. J.; Shin, S. R.; Lee, S. M.; Kim, I. Y.; Kim, S. I. Smart Mater. Struct. 2004, 13, 1036–1039.

Electrical Behavior of Chitosan/Carboxymethylcellulose Hydrogel (2) Ruel-Gariepy, E.; Shive, M.; Bichara, A.; Berrada, M.; Le Garrec, D.; Chenite, A.; Leroux, J. C. Eur. J. Pharm. Biopharm. 2004, 57, 53–63. (3) Liang, H. F.; Hong, M. H.; Ho, R. M.; Chung, C. K.; Lin, Y. H.; Chen, C. H.; Sung, H. W. Biomacromolecules 2004, 5, 1917–1925. (4) Tomatsu, I.; Hashidzume, A.; Harada, A. Macromolecules 2005, 38, 5223–5227. (5) Kim, S. J.; Yoon, S. G.; Lee, Y. M.; Kim, H. C.; Kim, S. I. Biosens. Bioelectron. 2004, 19, 531–536. (6) Menager, C.; Sandre, O.; Mangili, J.; Cabuil, V. Polymer 2004, 45, 2475–2481. (7) Zhu, Y.; Zheng, L. Y. J. Drug DeliVery Sci. Technol. 2006, 16, 55– 58. (8) Shiga, T. Deformation and viscoelastic behavior of polymer gels in electric fields. In Neutron spin echo spectroscopy Viscoelasticity rheology; Advances in Polymer Science, Vol. 134; Springer-Verlag: New York, 1997; pp 131–163. (9) Lira, L. M.; de Torresi, S. I. C. Electrochem. Commun. 2005, 7, 717– 723. (10) Wallmersperger, T.; Kroplin, B.; Gulch, R. W. Mech. Mater. 2004, 36, 411–420. (11) Shahinpoor, M. Smart Mater. Struct. 1994, 3, 367–372. (12) Mahaveer, D. K.; Jae-Rock, L.; JaeHung, H. Smart Mater. Struct. 2006, 15, 417–423. (13) Yao, L.; Krause, S. Macromolecules 2003, 36, 2055–2065. (14) Moschou, E. A.; Madou, M. J.; Bachas, L. G.; Daunert, S. Sens. Actuators, B 2006, 115, 379–383. (15) Kim, S. J.; Shin, S. R.; Lee, J. H.; Lee, S. H.; Kim, S. I. J. Appl. Polym. Sci. 2003, 90, 91–96. (16) Kim, S. J.; Yoon, S. G.; Kim, S. I. J. Polym. Sci., Part B: Polym. Phys. 2004, 42, 914–921. (17) Kim, S. J.; Yoon, S. G.; Lee, Y. H.; Kim, S. I. Polym. Int. 2004, 53, 1456–1460. (18) Lee, C. K.; Kim, S. J.; Kim, S. I.; Yi, B. J.; Han, S. Y. Smart Mater. Struct. 2006, 15, 607–611. (19) Sun, S.; Mak, A. F. T. J. Polym. Sci., Part B: Polym. Phys. 2001, 39, 236–246. (20) Li, L.; Hsieh, Y. L. Nanotechnology 2005, 16, 2852–2860.

Biomacromolecules, Vol. 9, No. 4, 2008

1213

(21) Kim, S. J.; Lee, K. J.; Kim, S. I.; Lee, Y. M.; Chung, T. D.; Lee, S. H. J. Appl. Polym. Sci. 2003, 89, 2301–2305. (22) Li, R. X.; Zhang, X. Z.; Zhao, J. S.; Wu, J. M.; Guo, Y.; Guan, J. J. Appl. Polym. Sci. 2006, 101, 3493–3496. (23) Ravi Kumar, M. N. V. React. Funct. Polym. 2000, 46, 1–27. (24) Ichikawa, S.; Iwamoto, S.; Watanabe, J. Biosci. Biotechnol. Biochem. 2005, 69, 1637–1642. (25) Arica, M. Y.; Yilmaz, M.; Bayramoglu, G. J. Appl. Polym. Sci. 2007, 103, 3084–3093. (26) Bayramoglu, G.; Arica, M. Y.; Bektas, S. J. Appl. Polym. Sci. 2007, 106, 169–177. (27) Arica, M. Y.; Yilmaz, M.; Yalcin, E.; Bayramoglu, G. J. Membr. Sci. 2004, 240, 167–178. (28) Nadagouda, M. N.; Varma, R. S. Biomacromolecules 2007, 8, 2762– 2767. (29) Chen, C. H.; Wang, F. Y.; Ou, Z. P. J. Appl. Polym. Sci. 2004, 93, 2416–2422. (30) Frugoni, J. A. C. Gazz. Chim. Ital. 1957, 87, L403. (31) Rosca, C.; Popa, M. I.; Lisa, G.; Chitanu, G. C. Carbohydr. Polym. 2005, 62, 35–41. (32) Yan, L.; Qian, F.; Zhu, Q. Polym. Int. 2001, 50, 1370–1374. (33) Chen, X.; Liu, J. H.; Feng, Z. C.; Shao, Z. Z. J. Appl. Polym. Sci. 2005, 96, 1267–1274. (34) Berger, J.; Reist, M.; Manyer, J. M.; Felt, O.; Peppas, N. A.; Gurny, R. Eur. J. Pharm. Biopharm. 2004, 57, 19–34. (35) Rokhade, A. P.; Agnihotri, S. A.; Patil, S. A.; Mallikarjuna, N. N.; Kulkarni, P. V.; Aminabhavi, T. M. Carbohydr. Polym. 2006, 65, 243– 252. (36) Smitha, B.; Sridhar, S.; Khan, A. A. Eur. Polym. J. 2005, 41, 1859– 1866. (37) Shiga, T.; Kurauchi, T. J. Appl. Polym. Sci. 1990, 39, 2305–2320. (38) Choi, O. S.; Yuk, S. H.; Lee, H. B.; Jhon, M. S. J. Appl. Polym. Sci. 1994, 51, 375–379. (39) Das, M.; Kumacheva, E. Colloid Polym. Sci. 2006, 284, 1073–1084. (40) Chen, X.; Knight, D. P.; Shao, Z. Z.; Vollrath, F. Biochemistry 2002, 41, 14944–14950.

BM701204J