PHYSICAL SUNSCREENS 65 Figure 6. Ultrathin section of a skin biopsy after topical application of sunscreen preparation 1. Acicular TiO 2 crystals seem to arrange preferentially at the periphery of the oil phase. In this case, one spherical microstructure has penetrated the widened intercellular spaces of the outer horny layer of the skin (TEM) (scale bar = l •m). spheres released prismatic TiO 2 crystals that could homogeneously coat the underlying structures. This could be verified around hair fibers (Figure 9) as well as along the horny layer of the skin surface (Figure 10). Enlargements of such areas demonstrated the nonagglomeration of the individual TiO2 particles, even after topical application onto human skin (Figure 11). When the prismatic titanium dioxide was added to a classic emulsion, the crystalline arrangement appeared to be quite different in sunscreen preparations 1 and 2. Actually, no microstructure was noticeable in sunblock preparation 3- In TEM, the tiny mono- crystals or small clusters appeared to be randomly and uniformly distributed in the oily phase of the cream (Figure 12). After topical application we could subsequently observe a similar organization of the TiO2 crystallites upon the horny cells of the skin (Figure 13), i.e., the inorganic component more or less evenly dispersed in the emulsion layer. Nevertheless, some areas exhibited thinner crystalline depositions along the skin surface (Figure 14). DISCUSSION Today titanium is largely used for orthopedic and dental implants. Hence, numerous studies considered the histological responses to titanium (10-13). However, titanium is spontaneously covered by a passivating oxide layer mainly composed of rutile and anatase, which are both titanium dioxides (14). Thus the titanium implant/biological

66 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 7. After application of sunscreen preparation I onto the skin, the corneocytes may be coated by a regular and thin layer of acicular metallic oxide crystals (TEM) (scale bar = 100 nm). 'c 7: Figure 8. Cross-section of a natural brown hair. Presence of a microsphere (H•liosides ©) filled with prismatic TiO 2 crystals at the surface of the cuticle (C) after topical application of sunscreen preparation 2. Uranyl acetate and lead citrate staining (TEM) (scale bar = 1 •xm).