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j. Cosmet. Sci., 55,463-471 (September/October 2004) Comparative study between the viscoelastic behaviors of different lipid nanoparticle formulations E. B. SOUTO, S. A. WISSING, C. M. BARBOSA, and .. R. H. MULLER, Department of Pharmacez/tics, Biopharmacez/tics and Biotechnology, Free University of Bet/in, Ke/chstrasse 31, D-12169 Bet/in, Germany (E.B.S., S.A.W., R.H.M.), and FacMty of Pharmacy, Department of Pharmacez/tica/ Technology, Oporto University, Rz/a An/ba/ Cz/nha 164, P-4050-047 Porto, Portz/ga/ (E.B.S., C.M.B.). Accepted for pz/blication JMy 7, 2004. Synopsis Application of drug substances to the skin for systemic absorption or action in a particular layer of the skin is a rather old approach. However, over the last years it has received much more attention, as a consequence of the development of new membrane-moderated and matrix reservoir devices. As new reservoir systems, solid lipid nanoparticles (SLN TM) and nanostructured lipid carriers (NLC TM) have been successfully tested for derreal application of different physicochemical substances. The knowledge obtained from theological investigations of these systems may be highly useful for the characterization of the newly developed topical formulation. In the present study, an oscillation frequency sweep test was used for the evaluation of storage modulus (G'), loss modulus (G"), and complex viscosity ('q*) of twelve different SLN and NLC formulations, over a frequency range from 0 to 10 Hz. The lipidic aqueous dispersions were prepared using three different solid lipids (Softisan©138, Compritol©888, and stearyl alcohol) as matrix material. Miglyol©812, tocoph- erol, sunflower oil, and long-chain triacylglycerols were the chosen liquid lipids for NLC preparation. The objective of the present work was to investigate the effect of these different liquid lipids on the rheological properties of aqueous dispersions of NLC as model systems. It was found that the liquid oil component of the formulation has a strong influence on the viscoelastic parameters, which are dependent on the particle size, zeta potential, and crystallinity of the lipid particles, as well as on the solid lipid used. INTRODUCTION The skin is the largest organ of the body, considered as a natural protective barrier against either the penetration of dangerous exogenous compounds or the loss of excessive amounts of water and other essential compounds from the body. At the same time, it can also be a promising portal of entry of active substances to the systemic circulation. A drug penetrates the stratum corneum and eventually it diffuses across the viable under- lying tissues, according to its physicochemical properties (1). Address all correspondence to R. H. Miiller. 463