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This article was originally published in The Tan Sheet

Executive Summary

SPF TESTING: SURGICAL ADHESIVE TAPES AS SUBSTRATES HAVE "LIMITATIONS," Katherine Kelley, et al. concluded in a study, "In Vitro Sun Protection Factor Evaluation of Sunscreen Products," published in the May/June issue of the Journal of the Society of Cosmetic Chemists. The researchers found that "the use of surgical adhesive tapes as substrates for determining SPF in vitro has limitations." Problems with the substrates included chemical absorption into the tapes and that "oil- and alcohol-based products give especially erratic results, probably due to vehicle interactions with the tapes." The scientists are affiliated with Allergan Herbert, University of Cincinnati College of Pharmacy, and Oclassen Pharmaceuticals. Kelley, et al., used two surgical adhesive tapes -- 3M Company's Transpore and Blenderm -- in a procedure based on a study by B. L. Diffey and J. Robson, published in 1989. The modified procedure used a 150 watt xenon arc solar simulator and an ultraviolet V radiometer. To determine the in vitro SPFs of a number of sunscreen formulations, the researchers calculated the ratio of light transmission through untreated tape to that of sunscreen-treated tape. They compared the values to SPFs determined through in vivo methods. "Correlation between in vivo and in vitro SPF data varied tremendously," the scientists stated. Transpore is UV-transparent with perforations that form a "bumpy" surface to mimic the topography of skin. Blenderm is similar to Transpore but is not perforated and is smoother. Since Transpore is perforated, the estimates of SPFs are "potentially dependent on the sunscreen application technique," the researchers said. However, Blenderm did not increase the correlation between in vitro and in vivo SPFs. Kelley, et al., studied 17 commercial and investigational sunscreens and the FDA standard homosalate sunscreen. The surgical adhesive tapes were mounted in plastic photographic film slide frames. Sunscreens were applied to the non-adhesive sides of the tapes at a uniform rate. The researchers directed the solar simulator at the surgical tape, which was placed in front of the radiometer's UVB detector head. The percent of light transmitted through the tape was calculated. The Transpore tape technique results "did not consistently yield SPF values that could be correlated to the label claim SPFs, and the errors associated with the measurements were unacceptably large," the scientists said. They added: "In addition to the unacceptable high errors, . . . the correlation coefficients were low (r = 0.587 on Transpore tape and r = 0.397 on Blenderm). Diffey's correlation coefficient was much higher, r = 0.943." The researchers determined that "since the sunscreen and the substrate were exactly the same but the technician, light source, and UV detectors were different, the discrepancy in the results may either be attributed to the application of sunscreen to the tape by the technician or to the detector." The scientists noted that "a modified version of the experimental setup may yield better results," recommending that a more sensitive detector be used. Another substrate that should be tested, according to Kelley, et al., is custom ground quartz glass plates, which would eliminate the problem of absorption of the sunscreen into the substrate while providing a "bumpy" surface. The plates also would offer the necessary UV-transparent qualities and could be washed and reused.

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