POLYMER EFFECT ON SPF AND WATER RESISTANCE 215 formulation. The control formula did not contain a polymer, whereas the test formulation and the standard contained VA/butyl maleate/isobornyl acrylate copolymer and acrylates/ octylacrylamide copolymer, respectively. Since the VA/butyl maleate/isobornyl acrylate copolymer is supplied as a 50% solution, the actual percentage of polymer solids in the formula was 1%. Absorption measurement. Films of sunscreen formulations were spread on PMMA plates at an application rate of 1.20 mg/cm2. The sunscreen product was deposited on the PMMA plate by weight, and spread evenly using the index fi nger covered with a fi nger cot. The fi lm was allowed to dry at room temperature for 15 minutes for the fi lm to set up before any measurements were taken Absorption spectra were acquired for each plate from 290 to 400 nm at 1-nm increments, and the areas under the curve were calculated. Four mea- surements were taken per plate by rotating the plate on each side. Three plates were evaluated per formulation. Image capturing. Digital images of the plates were taken using a digital microscope. The images were taken at 100× magnifi cation at multiple spots on each plate. Images were taken before and after absorption at the same exact area. This was achieved by marking certain areas on the plates with permanent ink to enable repositioning on the same spot. Image analysis. All images were fi rst converted to a gray scale, and further analysis was done by generating pixel histograms. The area under the curve was calculated and was used as a means to quantify differences in fi lms properties between plates. Contact angle measurements. Contact angles were measured by the sessile drop method, us- ing a manual disposable syringe with a 1-mm-diameter needle. Several substrates, namely, porcine skin, silicone fi lms, Vitro skin®, porcine stratum corneum, human cadaver skin, and PMMA plates were screened as substrates. All the data collected showed similar trends, and so for simplicity, only PMMA values were reported. PMMA plates with and without products were used as substrates onto which the water droplets were deposited. Reported surface tension values represent the average of 20 trials. Measurements were conducted at room temperature. In vitro water-resistance measurements. An updated version of the method used by Markovic et al. (9), which was developed by this laboratory, was used to measure water resistance. The initial UV absorbance of sunscreen fi lms deposited on PMMA plates was measured with a UV-Vis spectrophotometer, and the area under the curve (290–400 nm) was calcu- lated. The samples were then submerged in a 25°C water bath, with circulation for 80 minutes. The water bath was equipped with a mixing blade that rotated at 50 rpm, and the PMMA plates were positioned so that the surface with the fi lm faced the current. After immersion, the plates were removed from the water bath, were placed on their sides, and allowed to dry at room temperature, after which absorbance was measured and the area under the curved calculated. The percent water resistance is calculated from the following equation: Percent water resistance = (Abs. after immersion / Initial Abs.) × 100 In vivo static SPF and high-water-resistance testing. Testing was conducted as outlined in a US monograph (10) on a fi ve-subject panel with Fitzpatrick skin types between I and III. A 150-watt Xenon arc solar simulator (Solar Light® Co., Philadelphia, Model 14S) was used as the artifi cial light source. The device is equipped with a dichroic mirror, which

JOURNAL OF COSMETIC SCIENCE 216 refl ects radiation below 40 nm, and a WG-320 fi lter that absorbs radiation below 290 nm. UV radiation was monitored continuously during exposure using a DCS-1 Meter/Dose Controller (Solar Light® Co.). An 8% homosalate formulation was used as a standard. Products were applied using volumetric syringes at doses of 2 mg/cm2. Protected and unprotected sites were irradiated to determine the SPF values. Immediate responses were monitored and fi nal readings were recorded 24 hours after irradiation. High-water-resistance SPF values were determined by the product’s ability to resist an 80-minute immersion in a whirlpool with a water temperature of 23°–32°C. All in vivo testing was conducted at AMA Laboratories (New City, NY). Statistical analysis. Normal distribution and equality of variance were tested using the Wilk-Shapiro test, and the Bartlet’s test, respectively. When normality failed, a Kruskall- Wallis non-parametric evaluation was performed. Tukey’s test was used for multiple comparisons. Statistical analysis was performed using Sigma Plot® 11 software (Systat® Software, Inc., San Jose, CA). RESULTS AND DISCUSSION Absorbance spectra of the control and test product were generated before and after water immersion, as presented in Figure 1. An examination of the graph indicates that the control had lower absorbance than the test product, as demonstrated by the area under the curve and the peak height. The areas under the curve before immersion for the control and test samples were 98.49 and 117.09, respectively. After immersion, the areas under the curve were 94.63 and 118.22 for the control and test samples, respectively. This indicates a very small loss in absorbance in the control sample and no loss in absorbance in test samples after water immersion. With regard to peak height before immersion, the control peaked at 1.11, whereas the test product peaked at 1.37. It appears from the data that the polymer boosted the in vitro absorbance of the formula when measured on PMMA plates. Figure 1. Polymer effect on in vitro absorbance before and after immersion.