CATIONIC HYDROGELS FOR CONTROLLED DELIVERY 423 EXPERIMENTAL MATERIALS N-Isopropylacrylamide (NIPAM, 99%, Aldrich) was purifi ed by recrystallization from a 60:40 toluene:hexane mixture. We employed (3-acrylamidopropyl)trimethylammonium chloride (AAPTAC, 75 wt%, Aldrich), N,N-methylenebisacrylamide (MBA, 99+%, Aldrich), sodium dodecyl sulfate (SDS, 98%, Aldrich), and ammonium persulfate (APS, 99%, Sigma) as received. The water used in the synthesis was of Millipore Milli-Q grade. METHODS Hydrogel preparation. Polymerization was conducted in a 250-ml three-necked fl ask equipped with a refl ux condenser, a thermometer, and a nitrogen-bubbling tube. NIPAM (1.1316 g), SDS (1 g of 0.1M SDS), AAPTAC (0.1012 g) and MBA (0.075 g) were all dissolved in 100 ml of water. The solution was heated while being stirred to the polymeriza- tion temperature of 70°C and maintained at 70°C using a thermostated oil bath. Nitrogen was bubbled into the solution for 30 minutes to remove oxygen completely before the APS aqueous solution was added. An appropriate amount of APS was dissolved in 10 ml of water and injected to initiate the polymerization. The polymerizations were carried out for six hours under nitrogen atmosphere. After cooling, all hydrogels were puri- fied by at least five cycles of ultracentrifugation (Beckman model L7-55, 45 min at 12000g), decantation, and redispersion in water. The hydrogels were lyophilized and stored at 4°C. Fourier transform infrared analysis. FTIR spectra of hydrogels and all the monomers were determined using a Nicolet 560 FTIR spectrophotometer. At least 256 scans were ob- tained to achieve an adequate signal-to-noise ratio. The spectral resolution was 2 cm−1. Spectroscopic grade KBr was dried under vacuum for 48 hours before use to make sure that all the water had been removed completely. One hundred milligrams of pre-dried KBr and 10 mg of each dried sample were mixed thoroughly using mortar and pestle. The KBr pellet was made using a pellet-making press. The attenuated total refl ectance spectra of the KBr pellets were collected from 400 cm−1 to 4000 cm−1. Absorbance measurements. Absorbance measurements were made at different temperatures using a Shimadzu UV-240 spectrophotometer (cell length 5 cm). The hydrogel suspen- sion was diluted to get an absorbance of approximately ≅0.25 at 500 nm at 20°C. Then the samples were equilibrated to the required temperature values by immersing the sus- pension in a water bath for 30 minutes. The stability of the hydrogel was evaluated by using a spectrophotometric protocol. For this purpose, the absorbance of a diluted dispersion containing 0.1% (w/w) copolymer parti- cles at different pHs was measured in the wavelength range of 300–700 nm. These mea- surements were conducted in the pH range of 4–10 by adjusting the pH with aqueous NaOH or HCl solution. The stability parameter, n, was calculated according to equation 1 by using the plots including the linear variation of absorbance with the wavelength, where A is the absorbance at a certain wavelength, λ:

JOURNAL OF COSMETIC SCIENCE 424 −d = λ log A n d log (1) The thermoresponsive behavior of the hydrogels was also monitored by a UV-visible spec- trophotometer. The absorbance measurements were carried out at 500 nm under both heating and cooling conditions. Zeta potential measurements. The zeta potential of hydrogel dispersion (solid content: 0.1%, w/w) was measured at different pH values at 25°C in a Zeta Sizer (Malvern Instruments, 3000 HSA, London, UK). The hydrogels were dispersed in 10−3M KNO3 solution to keep the electrolyte concentration constant. The pH was adjusted by the addition of 0.01 M Na(OH) or 0.01 M HCl. The zeta potential was calculated from the average electropho- retic mobility for ten particles per point. The average value of at least three measurements was taken at a given pH value. Dynamic light-scattering measurements. The hydrodynamic particle diameters of hydro- gels were determined both under heating and cooling conditions using dynamic light scattering (DLS) at a detector angle of 90°. A Lexel 95 ion laser operating at a wave- length of 488 nm and a power of 100 mW was used as the light source. Correlation data were analyzed using a BI-9000AT digital autocorrelator, version 6.1 (Brookhaven Instruments Corp.), and the CONTIN statistical method was used to calculate the particle size distributions. The hydrogels used in this study were highly monodis- perse. Samples were prepared in thoroughly cleaned vials by suspending a small quantity of lyophilized hydrogels in fi ltered 10−3 M KCl. Sample pH values were adjusted using 0.1 M HCl and NaOH. The samples were temperature controlled to ± 1°C. Decalin, a refractive index matching liquid, was used to reduce light bending at the glass interfaces. At least fi ve replicate measurements were conducted for each sample the experimental uncertainties represent the standard deviation of the replicate measurements. The sam- ples were thermostated for at least 30 minutes to equilibrate the system. From light-scattering measurements a ζ-average translational diffusion coeffi cient, D0, was calculated (26). According to the Stokes-Einstein equation, the translation diffusion coeffi cient at zero concentration depends on the effective hydrodynamic sphere radius, Rh, which can be calculated from the equation for D0 given below: = π η0 k T D Rh B 0 6 (2) where kB is the Boltzmann constant, T is the absolute temperature, and η0 is the solvent viscosity (27). The system used was a BI-9000AT (Brookhaven Instrument Corp.), which allows DLS measurements at various scattering angles. Atomic force microscopy (AFM). An atomic force microscope (AFM) (Digital Instrument Nanoscope (3A)) was used in contact mode to observe the topology of the hydrogel par- ticles. Samples were prepared by coating the particles on previously cleaned glass surfaces at room temperature. The tips used for the experiments were silicon nitride tips.