Abstract
Cyclic siloxanes (D4, D5, D6) are widely used in skin products. They improve skin sensory properties and alleviate dry skin, but there is still one report (published 2019), which regards their effects on the destruction of the skin barrier, by using fluorescence microscopy and attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR). A new skin-imaging technique, digital holographic microscopy (DHM), was used for the first time to investigate the impact of D4, D5, and D6 on the skin barrier. We observed irreversible damage of the stratum corneum due to the interaction with cyclic siloxanes. These substances changed: (a) the first level of the skin barrier through destabilization of the intercellular lipid lamellae and destruction of the corneocyte structure (measured with axial nanometer resolution), (b) the second level by collapse of not only corneocytes but also of a significant part of the clusters, leading to the loss of the stratum corneum integrity and formation of the lacunae, (c) the third level as an effect of the change in the surface geometrical topography of the stratum corneum and disruption of the integrity of this skin layer, measured with lateral micrometer resolution. DHM allowed also to identify an important pathway for substances to penetrate into the skin through canyons surrounding the clusters. Our investigations provide advanced information for understanding the mechanisms by which various substances pass the skin barrier, including uncontrolled diffusion into the skin.
Highlights
It is known that the human skin is composed of characteristic layers and the stratum corneum (SC) is crucial for the skin barrier and responsible for protection against various factors entering the skin
The consequence of impact of cyclic siloxanes on the skin barrier is presented in Figure 1 which shows the following images: intensity, wrapped phase, and unwrapped phase
The canyons surrounding the clusters have been identified as an important pathway for penetration of substances into the skin. This increases our knowledge about substances that are able to overcome the skin barrier by a pathway different from the well-known transcellular, intercellular, and transappendageal ones
Summary
It is known that the human skin is composed of characteristic layers and the stratum corneum (SC) is crucial for the skin barrier and responsible for protection against various factors entering the skin (e.g., xenobiotics, allergens, microorganisms). Siloxanes improve skin sensory properties and alleviate dry skin, but there is still one report (published 2019), regarding their effects on skin barrier properties, including interaction with lipids and proteins of the SC, proved by using fluorescence microscopy and attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) [11] To study this in more detail, we selected the cyclic siloxanes D4, D5, and D6, based upon the current state of knowledge about these compounds; especially with respect to: (a) widespread use in medical products for skin, cosmetics and personal care products intended for adults (e.g., Zeraderm Rofil Medical, La Roche-Posay Hydreane Riche cream, Vichy Aqualia Thermal cream, Garnier Regenerating body lotion) and children (e.g., Aderma Epitheliale Pierre Fabre Cosmetique), and for infants (e.g., Penaten cream, Emolium) and (b) significant environmental pollution (in particular water, sediment, soil, air, and dust), increasing the risk of human exposure to these compounds, especially on exposure through the skin, posing even toxicological problems [11,12,13,14,15]
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