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.
