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j. Cosmet. sci., 52, 313-324 (September/October 2001) Investigations on the occlusive properties of solid lipid nanoparticles (SLN) S. A. WISSING, A. LIPPACHER, and R. H. MULLER, Department of Pharmaceutics, Biopharmaceutics and Biotechnology, The Free University of Berlin, Kelchstrasse 31, D-12169 Berlin, Germany. Accepted for publication May 31, 2001. Synopsis Skin hydration can be influenced to a great extent by occlusive compounds. Conventional highly occlusive compounds tend to have an unacceptable appearance. Therefore, the development of innovative occlusive topicals is an essential issue regarding the formulation of cosmetic and pharmaceutical preparations. Solid lipid nanoparticles (SLN TM) represent a novel carrier system for controlled release of topical cosmetic and pharmaceutical compounds. In addition to their controlled release characteristics, it has been found that SLN have an occlusive effect. The extent of the occlusive effect depends on various factors such as particle size, applied sample volume, lipid concentration, and crystallinity of the lipid matrix. These factors have been investigated in detail by an in vitro test, and the obtained data give insight into their importance. INTRODUCTION Solid lipid nanoparticles (SLN TM) have been introduced as a novel carrier system for the controlled release of pharmaceutical and cosmetic active compounds (1). Various routes of application, such as oral, i.v., J.p., and dermal have been investigated (2,3). A recent development is the use of SLN in topical cosmetic and pharmaceutical preparations, e.g., creams and gels (4,5). Advantages of SLN in topical formulations are: ß protection of labile incorporated compounds against chemical degradation ß controlled release of incorporated compounds ß a white pigment effect (covering undesired colors of compounds or their degradation products) ß an occlusive effect, inducing enhanced skin hydration, thus leading to wrinkle smoothing and an enhanced penetration into--or specific localization of compounds in--specific skin layers. There are many reports about incorporation of cosmetic and pharmaceutical compounds into SLN and about SLN-related benefits (2,3,6,7). However, very little has been re- ported about the occlusion effect and the SLN parameters affecting the extent of occlu- sion. Occlusion is an important factor for skin hydration and the subsequent effects of the modified penetration of compounds (8-10), and therefore this paper investigates in detail the occlusive properties of SLN and the factors affecting occlusion. 313