The skin, the largest organ of the body, acts as the primary interface between our internal environment and the external world. Its outermost layer, the epidermis, and particularly its most superficial layer, the stratum corneum (SC), play a critical role as a highly effective permeability barrier. This barrier function is essential for maintaining overall health by preventing the loss of vital substances such as water and electrolytes from the body, while simultaneously protecting against the ingress of harmful environmental factors, including toxins, pathogens, and even topically applied substances like cosmetic ingredients and drugs. Understanding the structure and function of the stratum corneum is therefore paramount in the field of cosmetic dermatology, as it directly influences the efficacy of skincare products designed to enhance the skin’s health and aesthetic properties.
The stratum corneum is a unique tissue composed primarily of corneocytes, which are non-nucleated, flattened cells derived from keratinocytes in the underlying epidermal layers. These corneocytes are embedded in a matrix of lipid-enriched intercellular domains, forming a structure often described as a “brick and mortar” model. The corneocytes, representing the “bricks”, are enriched with cross-linked proteins, providing resilience and structural integrity. The intercellular lipids, synthesised in the nucleated epidermal layers and extruded into the spaces between the corneocytes, form continuous membrane bilayers, acting as the “mortar” that holds the structure together and is crucial for the barrier function. This highly organised, lipid-rich environment creates a formidable obstacle for most substances attempting to penetrate the skin.
The primary function of this epidermal barrier is to regulate the passage of molecules both into and out of the skin. This regulation is vital for maintaining hydration levels within the skin and the body as a whole. The prevention of excessive water loss, known as transepidermal water loss (TEWL), is a key aspect of this barrier function. A compromised barrier can lead to dehydration of the stratum corneum, resulting in dry, scaly skin and potentially exacerbating conditions like sensitive skin and eczema. Conversely, the barrier also prevents the absorption of many external substances, which is generally beneficial for protection against harmful chemicals and irritants. However, this barrier function also presents a significant challenge for the delivery of active ingredients intended to exert their effects within the skin, whether on the surface, within the epidermis, or even in the deeper dermal layers.
For cosmetic ingredients to be effective, they must, in most cases, penetrate beyond the stratum corneum to reach their target sites. Various factors influence the degree to which a substance can permeate this barrier. The physicochemical properties of the ingredient itself are crucial determinants of its ability to penetrate. Molecules with a lower molecular weight (generally less than 500 Daltons) tend to penetrate more readily than larger molecules. Solubility is another key factor; the stratum corneum has both hydrophilic (water-loving) and lipophilic (oil-loving) components, and ingredients with a balance between these properties often exhibit better penetration. The charge of a molecule can also affect its ability to cross the lipid-rich environment of the stratum corneum.
The condition of the skin barrier also plays a significant role in penetration. A compromised barrier, due to factors such as environmental damage, age, or certain skin conditions, can lead to increased permeability. For example, exposure to harsh surfactants found in some cleansers can disrupt the intercellular lipids, impairing the barrier function and potentially increasing the penetration of other substances, which may lead to irritation. Conversely, cosmetic products are often formulated to support and restore the skin barrier, aiming to improve skin health. Moisturisers, for instance, work by adding water to the stratum corneum and/or preventing water loss. Some moisturising ingredients can also interact with the lipid matrix of the stratum corneum, potentially altering skin permeability.
Furthermore, the vehicle in which the cosmetic ingredient is formulated significantly impacts its penetration. The vehicle, which includes components like water, oils, and emulsifiers, can influence the solubility of the active ingredient and its ability to partition from the product into the stratum corneum. Penetration enhancers are often incorporated into formulations to facilitate the passage of active ingredients through the skin barrier. These enhancers can work through various mechanisms, such as disrupting the structure of the stratum corneum lipids or increasing the solubility of the active within the skin. Examples of penetration enhancers include certain alcohols, glycols, and fatty acids.
Novel delivery systems are also being developed to overcome the limitations of the skin barrier. These systems, such as liposomes, nanoparticles, and microcapsules, aim to encapsulate active ingredients and facilitate their delivery to specific layers of the skin, potentially enhancing efficacy and minimising systemic absorption. These technologies are designed to interact with the skin in ways that promote penetration while maintaining skin integrity. In conclusion, the stratum corneum represents a sophisticated and crucial barrier that protects the body. However, it also poses a significant challenge for the delivery of active ingredients in cosmetic products. A thorough understanding of the stratum corneum’s structure, function, and the factors influencing ingredient penetration is essential for the development of effective skincare products that can improve skin health and address various cosmetic concerns. Ongoing research continues to explore novel formulations and delivery systems to enhance the penetration of beneficial ingredients while ensuring safety and maintaining the integrity of this vital barrier.