Introduction: Cosmetic formulation ingredients including emollients and humectants are widely used in skin-care formulations due to their efficacy in promoting skin health and a pliant skin feel through moisturization and maintenance of the skin barrier function. Individual ingredient efficacy has been closely linked to the degree an ingredient penetrates human stratum corneum (SC) and alters the SC biomechanical properties critical to consumer sensorial perception of skin tightness. Specific ingredient molecular features, for example, molecular weight, diffusivity, topological polar surface area (TPSA), and even viscosity can be used together to enable effective predictions of an ingredient’s ability to penetrate the SC, reduce the development of skin stress and promote skin health and barrier function. However, understanding is lacking about how individual ingredients affect optimum product performance when applied as part of a full skin-care formulation. In this case, ingredients may act together with other components including emulsifiers, polymers and/or other molecules to amplify individual effects.
Since consumer perception of formulation efficacy is strongly affected by skin hydration and dry skin stress, a key quantifiable metric by which to optimize formulation performance is the amount of biomechanical stress developed in the SC during dehydration. Formulations that include ingredients with strong synergistic interactions should have the highest potential to significantly reduce drying stress in the SC and yield positive perceptions of skin comfort and softness. Determining a rational basis for understanding, predicting, and leveraging these ingredient interactions is a critical step to guide the design and customization of new, advanced cosmetic formulations that maintain or improve skin health. Accordingly, our objective was to provide new insights with a holistic study involving fourteen cosmetic formulations containing ten cosmetic emollients with widely varying properties and molecular structures, most of whose individual effects on SC properties were previously characterized for comparison.

Methods: Salient properties of the SC including mechanical stress were measured in vitro using a substrate curvature measurement technique. Stress development due to SC drying was measured before and after topical treatment with fourteen different cosmetic formulations, with special attention given to changes in the maximum stress and stress rate values that affect consumer perception. The maximum penetration volumes of formulations in the SC were characterized to compare with known mechanisms underlying individual ingredient effects on SC stress.

Results: Remarkably, the correlation between penetration volume and stress reduction known for individual emollients was found to extend to formulations. Greater penetration volumes were observed after application of the formulation such that all formulations strongly enhanced the degree of SC mechanical stress reduction over previous observations of individual ingredients. These formulations also caused the transient rate of stress development to significantly decrease compared to individual ingredients, thus indicating the benefit of multiple ingredient interactions in full formulations for managing water transport kinetics. The stress and penetration volume results were understood through a multi-parameter model considering the molecular weight, diffusivity, TPSA and viscosity of the ingredient, along with ingredient logP value (or partition coefficient) to enable the prediction of how lipophilic or hydrophilic ingredient choice impacts moisturizing formulation effects on SC biomechanical properties.

Discussion and Conclusion: The study offers new insights on how ingredient choice affects cosmetic formulation performance and reduces drying stresses while also demonstrating a robust ability to predict these effects based on ingredient characteristics. By utilizing the same chassis formulation and only exchanging the selected emollient and, in some cases additionally introducing humectant molecules, the formulations were standardized as best possible to avoid confounding factors and enable analysis of a given ingredient’s role in enhancing formulation efficacy. We establish how multiple ingredients behaving synergistically in formulations amplify penetration of substances into the SC and strongly reduce the development of the biomechanical stress that affects SC barrier function and consumer perception. This understanding is vital to ensure that when designing new cosmetic formulations, emollients and other formulation components are selected accordingly to deliver optimum performance via ingredient interactions that maximize effects which positively impact SC biomechanics, help to maintain skin health, and meet customer needs.