The formulation of a cosmetic product is a complex process where each ingredient plays a specific role that can significantly influence the stability, aesthetics, and, crucially, the delivery of active ingredients to the skin. The vehicle, or the base of the formulation, interacts with the skin and the active compounds, affecting their release, penetration, and ultimately their efficacy. Ingredients such as water, alcohol, propylene glycol, surfactants, and emollients are key components that exert distinct effects on active delivery.
Water is the most common carrier and solvent in cosmetic formulations. It can hydrate the stratum corneum, which can influence percutaneous absorption by altering partitioning between the product and the skin. Water also acts as a solvent for many hydrophilic active ingredients, facilitating their incorporation into the product. However, the presence of water can also increase the risk of microbial contamination, necessitating the use of preservatives. The evaporation of water from an emulsion on the skin can lead to supersaturation of the active at the skin surface, potentially enhancing its penetration by creating a high concentration gradient.
Alcohol (e.g., ethanol) is another common solvent used in formulations. It can fluidise the stratum corneum, thereby altering its permeability and potentially enhancing the penetration of some active ingredients. Alcohols can also act as penetration enhancers themselves for certain compounds. However, they can also have a drying effect on the skin by disrupting the lipid barrier.
Propylene glycol is a versatile ingredient that functions as both a co-solvent and a humectant. As a co-solvent, it can help to dissolve active ingredients that are not readily soluble in water alone, improving their stability and availability within the formulation. Its humectant properties contribute to skin hydration, which can indirectly affect active penetration. Propylene glycol can also directly alter the permeability of the stratum corneum and influence the partitioning of active ingredients between the vehicle and the skin.
Surfactants, beyond their cleansing properties, play a critical role in active delivery. As emulsifiers, they help to create stable emulsions (like creams and lotions), which are common delivery systems for both hydrophilic and lipophilic actives. Surfactants can also enhance the solubility of lipophilic actives in aqueous formulations by forming micelles. Furthermore, some surfactants can interact with the stratum corneum, potentially altering its permeability and facilitating the penetration of active ingredients. The charge and structure of the surfactant can influence its interaction with both the active and the skin barrier.
Emollients are oily or lipid-based ingredients that soften and smooth the skin. They can act as carriers for lipophilic active ingredients, dissolving them and facilitating their application to the skin. By forming a film on the skin surface, some emollients can also enhance skin hydration by reducing TEWL, which can indirectly affect active penetration. Certain emollients may also directly interact with the stratum corneum lipids, potentially altering the skin barrier and influencing the uptake of active compounds. The partition coefficient of an active between the vehicle (which often includes emollients) and the stratum corneum is a key factor in its delivery. The delivery of active ingredients is a multi-step process involving partitioning from the vehicle into the stratum corneum, diffusion through the stratum corneum, partitioning into the viable epidermis, and diffusion through the epidermis and upper dermis. The formulation components significantly influence each of these steps. For instance, the solubility of the active in the vehicle determines its release from the product onto the skin surface. The presence of penetration enhancers, which can be incorporated into the formulation, can further facilitate the passage of actives through the stratum corneum. Occlusive ingredients, which create a barrier on the skin surface, can increase stratum corneum hydration, thereby influencing percutaneous absorption. The design of the delivery system (e.g., cream, ointment, serum) also impacts the contact time with the skin and the release kinetics of the active.