The aim of the present study is the development of a method to determine quantitatively in vivo the influence of homogeneity of the distribution of sunscreen containing UV filters on the sun protection factor (SPF). The SPF of a sunscreen applied either topically or inside an optical cell (pure or in a solvent) fixed above the skin is determined in vivo. In both cases, in vivo measurements using the erythema formation are carried out. Identical optical parameters of the skin are realized in both experiments. In addition, both in vitro (using tape stripping) and in vivo microscopic measurements are performed to analyze the homogeneity of distribution of the topically applied substances. An SPF of 8 is measured in the experiment applying the UV filters topically, whereas this value increases by a factor of 10 if the same amount of filter substances is distributed homogeneously in solution inside the optical cell. Tape strips removed from skin treated with the sunscreen reflect the inhomogeneous distribution of the topically applied substances on the skin. The direct correlation of homogeneity of distribution with the SPF opens up the possibility to increase the SPF by optimizing the formulation.

Penetration profiles of topically applied drugs and cosmetic products provide important information on their efficacy. The application of tape stripping in combination with UV/VIS spectroscopy is checked to determine the local position of topically applied substances inside the stratum corneum, the penetration profile. The amount of corneocytes removed with each tape strip is quantified via the particle-dependent absorption, the pseudoabsorption, in the visible spectral range. The concentration of a typical UV filter substance, 4-methylbenzylidene camphor, is determined by optical spectroscopy using the tape strips removed originally. In this case, a time-dependent increase in the absorbance must be taken into account. Laser scanning microscopic investigations confirm that the nonhomogeneous distribution of the filter substance, on the strips, can explain this spectroscopic behavior. When reaching a homogeneous distribution, the UV spectroscopic signal reflects the correct concentration. These spectroscopic values are compared with high performance liquid chromatography (HPLC) data. The values obtained with both methods for the concentrations of 4-methylbenzylidene camphor are in good agreement. The data obtained are used to illustrate the determination of a penetration profile of a UV filter substance. The results demonstrate that the described protocol is well suited to characterize, in a simple manner, topically applied substances that have a characteristic UV/VIS absorption band.