CATIONIC HYDROGELS FOR CONTROLLED DELIVERY 427 increased, the image showed an increasing number of horizontal lines (streaks), indicative of the mechanical sweeping out of hydrogel particles by the AFM tip. The result shows that a uniform hydrogel particle submonolayer can be formed on the glass surface and that the hydrogel–glass bonds provide strong enough binding for the particle to tolerate the contact-mode tip scanning. The measured width from the AFM topographic cross section is approximately 1.5 μ, substantially larger than the value determined from the SEM image (200 nm) shown in Figure 2. This change in particle diameter is attributed to the swollen state of hydrogels due to hydration. Figure 3 shows the AFM and SEM micrographs of hydrogel particles taken directly from the preparation. The colloidal particles appear to have collapsed and there is material bridging the particles together. It is believed that the straight-chain copolymers that were unable to form the particles due to confi gurational restraints were coated on the hydrogel particles, thus acting as the bridging materials. These entangling materials were removed during the centrifugation and redispersion processes. In Figure 4 the same hydrogel particles are shown after successive centrifugations and redispersions in distilled water. The particles are found to be monodisperse, with a diam- eter of about 1.7 μm. During the centrifugations and redispersion processes, the straight- chain copolymers, which were acting as the bridging elements and were responsible for making the solution viscous, were preferentially removed due to the differences in density. Furthermore, the unutilized monomers and indigenously formed salts were also separated during these processes. Therefore, the diameter of the hydrogel particles increased mar- ginally due to encapsulation of a larger amount of water. Figure 3. (a) AFM and (b) SEM images of hydrogels as synthesized.

JOURNAL OF COSMETIC SCIENCE 428 STABILITY PARAMETER The variation of the stability parameter with pH is illustrated in Figure 5. As seen here, the hydrogel dispersion was very stable in the pH range studied. The stability of the hy- drogels is attributed to the stronger electrostatic repulsion between positively charged units located on the copolymer particles. Figure 4. AFM images of hydrogels after fi ve times successive centrifugations. Figure 5. The variation of the stability parameter of hydrogels “n” with pH at 25°C.