CATIONIC HYDROGELS FOR CONTROLLED DELIVERY 425 SEM technique and sample preparation (SEM). A Hitachi S-4500 cold fi eld emission scan- ning electron microscope (FESEM) was used for this study. Image acquisition was carried out at a low accelerating voltage, a long working distance, and a short scanning time to minimize beam damage to the substrate. The various materials were stored and handled at an ambient temperature of 18.5°C to 19.5°C and ~50% RH prior to and during application to the hair fi bers. Representative hydrogel particles were coated on the precleaned glass coverslips in different environmental conditions and then air-dried. The samples were mounted in parallel on double-sided tape onto SEM stubs. All samples were subsequently coated with approximately 60 Å of platinum for better conductivity and improved contrast, and were viewed at suitable magnifi cations to display the characteristic physical nature of the nano-particulate depos- ited on the glass coverslip surface in the best possible way. Fluorescence measurements. Freeze-dried hydrogel particles were added to water-saturated pyrene solution at different pH levels and equilibrated for 12 hours. The pyrene fl uores- cence spectra were measured using a SPEX FluoroMax-2 spectrofl uorometer. The excitation wavelength was 335 nm and the emission was monitored between 350 nm and 600 nm. The ratio of the intensity of the third (I3 at 383 nm) to the fi rst (I1 at 373 nm) fl uorescence band of pyrene monomer (I3/I1) refl ects the polarity experienced by the fl uorescence probe (28–33). The peak around 480 nm is assigned to the excimer fl uorescence of pyrene, and the intensity ratio of the excimer fl uorescence (Ie at 480 nm) to the fi rst (I1 or Im at 373 nm) is the measure of pyrene excimer formation. RESULTS AND DISCUSSION Thermosensitive cationic hydrogels have been prepared using a one-pot synthesis tech- nique. The chemical structure of the hydrogel is illustrated in Figure 1. The chemical composition of the synthesized hydrogel was verifi ed using FTIR measurements. FTIR Figure 1 shows typical FTIR spectra of both starting monomers and hydrogel particles. The IR spectrum of the polymer particles shows the characteristic absorption bands of cationic monomer (AAPTAC) at 3085 cm−1, and 1646 cm−1, the characteristic peak of NIPAM at 3290 cm−1, and the characteristic peak of MBA at 1381 cm−1. This confi rms that the condition used for the synthesis of the thermosensitive cationic hydrogels is very effective in incorporating the comonomers into the structure. AFM AND SEM MEASUREMENTS Figure 2 illustrates the contact-mode AFM image of a glass surface covered with hydrogel particles. The image was obtained in ambient air. A clear and stable image could be ob- tained only when a relatively weak loading force was applied. When the loading force was

JOURNAL OF COSMETIC SCIENCE 426 Figure 2. (a) AFM and (b) SEM images of hydrogel particles. Figure 1. (a) Chemical structure of the hydrogel (b) FTIR spectra of (i) hydrogel, (ii) bis-acrylamide, (iii) acrylamide, and (iv) (3-acrylamidopropyl)trimethylammonium chloride.