Stratum corneum occlusion induces water transformation towards lower bonding state: a molecular level in vivo study by confocal Raman microspectroscopy

Abstract

Objective It is conventionally understood that occlusive effects are the retention of excessive water in the stratum corneum (SC), the increase of SC thickness (swelling) and a decrease of the transepidermal water loss. However, the influence of occlusion on water binding properties in the SC is unknown. Methods The action of plant‐derived jojoba and almond oils, as well as mineral‐derived paraffin oil and petrolatum topically applied on human skin, is investigated in vivo using confocal Raman microspectroscopy. To understand the oils’ influence on the SC on the molecular level, the depth‐dependent hydrogen bonding states of water in the SC and their relationship to the conformation of keratin, concentration of natural moisturizing factor (NMF) molecules and lipid organization were investigated. Results A significant SC swelling was observed only in petrolatum‐treated skin. The water concentration was increased in oil‐treated skin in the intermediate SC region (40–70% SC depth). Meanwhile, the amount of free, weakly and tightly bound water increased, and strongly bound water decreased in the uppermost SC region (0–30% SC depth). The NMF concentration of oil‐treated skin was significantly lower at 50–70% SC depth. The lateral organization of lipids in oil‐treated skin was lower at 0–30% SC depth. The secondary structure of keratin was changed towards an increase of β‐sheet content in mineral‐derived oil‐treated skin and changed towards an increase of α‐helix content in plant‐derived oil‐treated skin. Conclusion The occlusive properties can be summarized as the increase of free water and the transformation of water from a more strongly to a more weakly hydrogen bonding state in the uppermost SC, although some oils cause insignificant changes of the SC thickness. The accompanied changes in the keratin conformation at the intermediate swelling region of the SC also emphasize the role of keratin in the SC’s water‐transporting system, that is the water in the SC transports intercellularly and intracellularly in the intermediate swelling region and only intercellularly in the uppermost non‐swelling region. Bearing this in mind, almond, jojoba and paraffin oils, which are not occlusive from the conventional viewpoint, have an occlusion effect similar to petrolatum on the SC.

Retaining Skin Barrier Function Properties of the Stratum Corneum with Components of the Natural Moisturizing Factor—A Randomized, Placebo-Controlled Double-Blind In Vivo Study

“…All changes were significant (p < 0.01). This increase could be due to an occlusion effect directly after treatment, reducing the evaporation of water and resulting in swelling of the SC [41,48] (Figure 2).…”

“…All changes were significant (p < 0.01). This increase could be due to an occlusion effect directly after treatment, reducing the evaporation of water and resulting in swelling of the SC [41,48] (Figure 2). Average stratum corneum (SC) thickness for untreated skin (black) and skin treated with the placebo (red) and verum (green) after 1 h, 4 h, 7 days and 14 days after beginning of the treatment.…”

“…The treatment resulted in significant changes in NMF concentration in different SC depths. to the dilution of NMF in the occluded SC due to oil treatment [41]. This is based on previous studies [40,52] and taking into consideration that these highly aqueous soluble molecules are present in a lipophilic environment in the SC.…”

“…Here, it has been shown, that these parameters are inhomogeneous, exposing a maximal lipid ordering in the depth 20-40% of the SC thickness, which correlates with the NMF content and the hydrogen bonding state of water. The SC swelling is also not homogenous and only the intermediate SC depths were found to be responsible for swelling of the SC [32,40], as it is most probably due to the loosing of hydrogen bonds of strongly bound water, which increase the concentration of weakly bound water due to the occlusion [41]. Age-dependent differences have also been reported [42].…”

“…Additionally, to healthy untreated human skin, the parameters were determined in skin topically treated with oils, which did not penetrate deep into the SC [43], but acted as an occluding medium, preventing from dehydration [41]. The influence of oils on barrier function-related parameters of the SC were determined in in vivo experiments [44].…”