State of the art in Stratum Corneum research: The biophysical properties of ceramides.

Graphical abstract Highlights • A cumulative assessment of results of the past 10 years in the field of ceramide biophysics /physical chemistry is provided. • Implications of these results for the field of skin barrier research are discussed. • Little to no redundant properties of the different ceramide species can be demonstrated, despite minor chemical differences. • A new theoretical model, the tri-layer model, is introduced, based on this cumulative assortment of data. • The entropy vs. enthalpy concept explaining the physicochemical function of the skin barrier is introduced. Abstract This review is summarizing an important part of the state of the art in stratum corneum research. A complete overview on discoveries about the general biophysical and physicochemical properties of the known ceramide species' is provided. The ceramides are one of the three major components of the lipid matrix and mainly govern its properties and structure. They are shown to exhibit very little redundancy, despite the minor differences in their chemical structure. The results are discussed, compared to each other as well as the current base of knowledge. New interesting aspects and concepts are concluded or suggested. A novel interpretation of the 3-dimensional structure of the lipid matrix and its influence on the barrier function will be discussed. The most important conclusion is the presentation of a new and up to date theoretical model of the nanostructure of the short periodicity phase. The model suggests three perpendicular layers: The rigid head group region, the rigid chain region and, a liquid-like overlapping middle layer. The general principle of the skin barrier function is highlighted in regard to this structure and the ceramides biophysical and physicochemical properties. As a result of these considerations, the entropy vs. enthalpy principle is introduced, shedding light on the function as well as the effectiveness of the skin barrier. Additionally, general ideas to effectively overcome this barrier principle for dermal and transdermal delivery of actives or how to use it for specific targeting of the stratum corneum are proposed. [ABSTRACT FROM AUTHOR]