DSC MEASUREM ENT Differential scanning calorimetry (DSC 214 Polyma, Netzsch, Germany) was used in thermal analysis of formulated PLO gel. Temperatures ranged between –6 ~ 90°C in the fi rst heating, 90 ~ -60°C in the fi rst cooling, and 60 ~ 90°C in the second heating cycles at a heating rate of 10°C/min. The temperature ranges were selected to assess the change in the physical properties of PLO gel by temperature, and measurements on nitrogen were carried out. RHEOLOGY MEASUREMEN T To assess rheologic al properties including viscosity and elasticity in PLO gel formula- tions, viscoelasticity was measured using a rheometer (Rheolaser Master, Formulaction, Toulouse, France). In general, scatterers (particles, droplets, fi bers, etc.) are constantly in motion because of Brownian motion in samples with viscoelastic properties. This constant motion of scatterers results in deformation of speckle image by time. The speed of scatterer movement varies by viscoelastic properties and also affects the deformation speed of speckle image. Therefore, the rheological properties of the sample including viscoelasticity can be assessed by measuring the deformation speed of speckle image. The Rheolaser Master uses diffusing-wave spectroscopy (DWS), which is an optical technique derived from dynamic light scattering (DLS). The DWS method is based on microrheology (28). The measurement of rheological properties was performed by converting the change in the mean square displacement (MSD) over a period of time into numerical values. Measurement was performed at room temperature (25°C) for 3 h. IN VITRO SKIN PERMEA TION TEST The transdermal Fran z diffusion cell system (FDC-6T, Logan, Somerset, NJ) was used to determine the skin permeation effi ciency of the formulated PLO gel. An artifi cial mem- brane (Strat-M membrane) was set in the diffusion cell array system, and in vitro percuta- neous absorption test was carried out by applying 400 μL of test solution in the donor and 50% ethanol solution in the receptor at 32 ± 1°C. The receptor fl uid in the receptor compartment was collected at 2, 4, and 8 h after applying the test solution, and skin penetrant in the donor solution and all membranes was collected 8 h after applying the test solution. The collected solutions were analyzed with the HPLC system (Alliance HPLC, Waters, Santa Clara, CA) under the conditions presented in Table IV. Table IV HPLC Analysis Condition Instrument Conditions Column C18 4.6 mm × 250 mm, 5.0 μm Column temp. 40°C Mobile phase Methanol: 0.01% Trifl uoreoacetic acid in Water (5:95) Detector PDA Detector (261 nm) Flow rate 1.0 mL/min Injection volume 10 μL JOURNAL OF COSMETIC SCIENCE 332

STATISTICAL ANALYSIS Ex periments were repeat ed three times, and data were presented as means ± SD. Signifi - cance in difference was tested by Student’s t-test. Differences were considered signifi cant at * p 0.05, ** p 0.01, and *** p 0.001. RESULTS AND DISCUSSION S TABILITY EVALUATION Sta bility evaluation was conducted at low temperature (4°C), room temperature (25°C), and constant temperature (45°C) each. PLO gel remained the same and stable at room (25°C) and constant (45°C) temperatures, but separated and turned to liquid at low tem- perature (4°C), showing that the gels were not suitable ingredients for cosmetic formula- tions. In the polyol selection experi ment, stability evaluation by temperature revealed that the all-formulated organogels were stable at room (25°C) and constant (45°C) temperatures. Excluding PEG-400, phase separation began to occur from day 7 at low temperature (4°C), and complete separation occurred 3 weeks after starting observation of time- elapsed change (Figure 4). Based on these fi ndings, PEG-400 was considered as the most appropriate PLO gel, and the experiment was additionally carried out by choosing PEG- 400 as a polyol. In the cycling test, phase separation also occurred in all PLO gel, except for PEG-400 (data not shown). When PEG (polyethylene glycol) is adsorbed to the colloidal surface, the colloidal activi- ties are substantially reduced, and the growth rate of the colloids is limited in certain aspects. In the experiment, the addition of PEG changes the kinetics of the growth pro- cess by inducing the rapid growth of nucleation and the aggregation of nanoparticles. Therefore, the addition of PEG can promote the crystallinity of samples and change the product morphology (29). The present study confi rmed that the addition of PEG-400 maintains the stability of PLO gel at low temperature. Figure 4. After 3 weeks’ stability at low temperature (4°C) (PLO gel #1). PREPARATION AND EVALUATION OF PLURONIC LECITHIN ORGANOGELS 333