CATIONIC HYDROGELS FOR CONTROLLED DELIVERY 435 the effects of urea to explore the mechanisms of temperature dependence of hydrogel swelling at different pH values (Figure 15) and found that urea reduces the temperature effect signifi cantly on the phase separation of hydrogel suspensions at pH 4.0 and pH 10.0. On the other hand, at pH 7.0, a small reduction in absorbance was observed after the LCST even in the presence of 20% urea. Compared to higher pH levels, the effect of urea is more predominant at pH 4.0 due to the presence of a maximum number of positive charges and hence maximum hydrogen bonding. Furthermore, it has been seen that the effects of urea in the phase separation of a hydrogel suspension is irreversible in nature. Even after fi ve days of dialysis against distilled water, the absorbance curve did not re- turn to its original position (Figure 15a). These results confi rm that intra/intermolecular hydrogen bonding plays an important role during the hydration and dehydration pro- cess of hydrogels. Molecular level sensitivity towards different stimuli makes these hydrogels potential can- didates for different delivery and toxic scavenging applications. Furthermore, it will be possible to tailor these hydrogels further to meet the need of specifi c applications by the proper selection of monomers. CONCLUSIONS In this study, multi-sensitive NIPAM-based uniform hydrogel particles were synthesized by using a relatively new amide-type cationic monomer. The obtained hydrogel dispersion Figure 14. (a) AFM and (b) SEM images of hydrogel particles in the presence of an excess amount of NaCl.

JOURNAL OF COSMETIC SCIENCE 436 exhibited colloidal stability in a wide pH range, comparable to that of similar cationic hydro- gels. The hydrogel particles showed a very sharp volume transition over a temperature range of 25°C–40°C and a reversible swelling and deswelling behavior when the temperature was cycled between 20°C and 40°C. Fluorescence spectroscopy revealed the dispersion’s pH-dependent hydrophobic/hydrophilic behavior. It was also found that the ionic strength of the hydrogel suspension should be kept lower than 0.1 M to observe a strong effect on hydrogel swelling. Incorporation of temperature, pH, and ionic-strength-sensitive monomers in a single network provides fl exibility for controlling swelling behavior by controlling related var- iables. Hence, by the proper selection of monomers, it will be possible to tailor a temper- ature-sensitive hydrogel to meet specifi c delivery or scavenging applications. ACKNOWLEDGMENT We are thankful to Prof. P. Somasundaran, Department of Earth and Environmental Engineering, Columbia University, New York, for allowing us to use AFM, light-scattering, and fl uorescence spectrometry equipment. REFERENCES (1) R. H. Pelton and P. Chibante, Colloids Surf., 20, 247–256 (1986). (2) R. Pelton, Adv. Colloid Interface Sci., 85, 1–33 (2000). (3) C. D. Jones and L. A. Lyon, Macromolecules, 33, 8301–8306 (2000). (4) D. Gan, and L. A. Lyon, J. Am. Chem Soc., 123, 7511–7517 (2001). (5) T. Tanuguchi, D. Duracher, T. Delair, A. Elaissari, and C. Pichot, Colloids Surf. B, 29, 53 (2003). (6) S. Rossi, C. Lorenzo-Ferreira, J. Battistoni, A. Elaissari, C. Pichot, and T. Delair, Colloid Polym. Sci., 282, 215 (2004). Figure 15. Change in absorbance as a function of temperature at (a) pH 4.0, (b) pH 7.0, and (c) pH 10.0.