Coated titanium dioxide (TiO₂) microparticles are commonly used as UV filter substances in commercial sunscreen products. The penetration of these microparticles into the horny layer and the orifice of the hair follicle was investigated. The distribution of the microparticles in the horny layer was analyzed using the method of tape stripping in combination with spectroscopic measurements. Deeper layers of the stratum corneum were devoid of TiO₂ even after repetitive application of sunscreen preparation when analyzing interfollicular areas. Only in the areas of the pilosebaceous orifices could microparticles be identified. The penetration of TiO₂ was investigated in histological skin sections. A biopsy was taken from a skin area from which the horny layer had been removed by tape stripping. In isolated areas, a penetration of coated TiO₂ into the open part of the follicle was observed. The amount of TiO₂ found in a given follicle was less than 1% of the applied total amount of sunscreens. A penetration of microparticles into viable skin tissue could not be detected.

A new method was developed to determine the horny layer profile of volunteers using tape stripping in combination with UV/visible spectroscopy. The optical absorbance and the weight of corneocyte aggregates were compared as parameters for the determination of the mass of the horny layer particles fixed to the individual tapes. It was shown that the potential disturbances influencing both parameters must be considered critically before calculating the correlation factor, found as R²_mean = 0.93 ± 0.05. It was proven that the absorbance in the visible range is better suited than the weight to quantify the amount of corneocyte aggregates removed by a single strip. The new method allows an exact anatomical localization of the individual tapes and all data obtained within the depth profile of the stratum corneum. This was exemplified by the determination of the penetration of chemical and physical UV filters into the horny layer.

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