Abstract
Hydration is a key aspect of the skin that influences its physical and mechanical properties. Here, we investigate the interplay between molecular and macroscopic properties of the outer skin layer – the stratum corneum (SC) and how this varies with hydration. It is shown that hydration leads to changes in the molecular arrangement of the peptides in the keratin filaments as well as dynamics of C-H bond reorientation of amino acids in the protruding terminals of keratin protein within the SC. The changes in molecular structure and dynamics occur at a threshold hydration corresponding to ca. 85% relative humidity (RH). The abrupt changes in SC molecular properties coincide with changes in SC macroscopic swelling properties as well as mechanical properties in the SC. The flexible terminals at the solid keratin filaments can be compared to flexible polymer brushes in colloidal systems, creating long-range repulsion and extensive swelling in water. We further show that the addition of urea to the SC at reduced RH leads to similar molecular and macroscopic responses as the increase in RH for SC without urea. The findings provide new molecular insights to deepen the understanding of how intermediate filament organization responds to changes in the surrounding environment.
Highlights
The skin is a large interfacial film separating the human body and the outside environment
We aim at the characterization of hydration-induced changes on molecular organization and dynamics within the keratin filaments inside the corneocytes of stratum corneum (SC) at varying hydration conditions
We investigate the effects of adding urea to slightly dehydrated SC and corneocyte samples
Summary
The skin is a large interfacial film separating the human body and the outside environment. It is highly likely that these molecular changes in the keratin filaments strongly impact its swelling and mechanical properties, and this interplay is investigated in the present study. The molecular properties of the keratin can be altered by the addition of other small polar molecules, e.g. urea and glycerol[31,37] These molecules are naturally present in skin as part of the so-called “Natural Moisturizing Factor” (NMF), and they are commonly used in skin care products as “humectants”. We aim at deepened understanding of the nature of hydration-induced changes in keratin filaments inside the corneocytes, and of how these molecular changes can lead to alterations in macroscopic material properties. Based on the combination of results obtained from these techniques, we can correlate observed changes in SC macroscopic properties to molecular effects in terms of dynamics, structure and conformation, thereby deepening our understanding of SC hydration
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