Emollient structure and chemical functionality effects on the biomechanical function of human stratum corneum

Abstract

OBJECTIVE: Cosmetic emollients are widely used in skincare formulations due to their ability to 'soften' the skin and modulate formulation spreadability. Though emollients are commonly used, little is known about their effects on the biomechanical barrier properties of human stratum corneum (SC), which play a critical role in consumer perception of formulation efficacy. Accordingly, our objective was to provide new insights with a study involving fourteen cosmetic emollient molecules with widely varying structures, molecular weights, SC diffusivities, topological polar surface areas (TPSAs), viscosities and chemical functionalities. METHODS: Mechanical stress in the SC was measured in vitro using a substrate curvature measurement technique. Stress development due to SC drying was measured before and after topical treatment with cosmetic emollients. Emollient diffusivity and alterations to lipid content in SC after treatment were measured via ATR-FTIR spectroscopy. The maximum penetration volume of emollient in SC was characterized to elucidate mechanisms underlying emollient effects on stress. RESULTS: The application of all cosmetic emollients caused a reduction in SC mechanical stress under dehydrating conditions, and a linear correlation was discovered between emollient penetration volume and the degree of stress reduction. These molecules also induced increases in stress equilibration rate, signalling changes to SC transport kinetics. Stress equilibration rate increases linearly correlated with decreasing intensity of the νCH2 band, indicating a previously unknown interaction between cosmetic emollients and SC lipids. Stress and penetration volume results were rationalized in terms of a multi-parameter model including emollient molecular weight, diffusivity, TPSA and viscosity. CONCLUSION: We provide a new rational basis for understanding the effects of cosmetic emollient choice on biomechanical properties affecting SC barrier function and consumer perception. We demonstrate for the first time that emollients very likely reduce SC mechanical stress through their ability to take up volume when penetrating the SC, and how molecular weight, SC diffusivity, TPSA and viscosity are predictive of this ability. As cosmetic formulations continue to evolve to meet the needs of customers, emollient molecules can be selected that not only contribute to formulation texture and/or spreadability but that also leverage this novel connection between emollient penetration and SC biomechanics.

From decoding the perception of tightness to a clinical proof of soothing effects derived from natural ingredients in a moisturizer

“…Previous studies have shown a strong connection between volume taken up by either water molecules or formulation components inside the SC and resulting drying stress. 22 Reduction in volume (i.e., losing water, lipids or proteins) is closely associated with an increase in stress while gaining volume (i.e., from water or components entering the SC) is related to a decrease in stress. Cleansers are known to strip lipids, particularly fatty acids, from the SC 13,23 and are expected to increase stress and perception of tightness.…”

“…We find that stiffness, film stress, and reported tightness all noticeably decrease following application of the moisturizer. This corresponds with an increase in penetration volume as not only do moisturizing components diffuse into the SC, but these hygroscopic molecules are also able to capture and retain water 22 . Furthermore, Osmoskin® contains 68% water and a mix of glycols, effectively dosing the skin with extra hydration.…”

“…This corresponds with an increase in penetration volume as not only do moisturizing components diffuse into the SC, but these hygroscopic molecules are also able to capture and retain water. 22 Furthermore, Osmoskin® contains 68% water and a mix of glycols, effectively dosing the skin with extra hydration. Given time, this oversaturation of water will slowly escape into the environment, which may explain the slight decrease in extensibility (Ue) over time as the SC loses this water and becomes stiffer (Figure 7d).…”

The Science of Selecting Excipients for Dermal Self-Emulsifying Drug Delivery Systems

“…Emollients are added to formulations for the purpose of improving spreadability, improving skin feel, as well as improving the hydration function of the SC [59][60][61]. The softening of skin brought about by emollients is attributed to the formation of an occlusive film on the skin that renders a reduced TEWL [62].…”

“…Emollients do not only enhance the character of the applied vehicle but also promote dermal penetration of incorporated drug(s) [63]. Moreover, emollients have demonstrated the capacity to reduce stress experienced by the SC, such as injuries or wounds, by penetrating the SC and replacing the lost water volume [61]. This emphasizes the profound relationship between the transport properties provided by emollients and their beneficial effect on repairing the barrier function of the outermost skin layer [61].…”