Skin Structure and Barrier Function

To fully develop a robust cosmetic product, the formulation must be evaluated not only as a bulk material, but also by how it interacts with the surface on which it is applied. Many cosmetic formulations form a continuous or semi-continuous film on the skin, hair, or nails through wetting, spreading, deposition, adsorption, adhesion, and subsequent drying or setting. To understand these formulation–surface interactions, we first need to consider the basic structure of the skin.

Figure 1 illustrates the major layers of human skin. The epidermis is the outermost layer of the skin and provides a protective barrier against the entry of foreign substances, water loss, and physical and mechanical stresses. The stratum corneum (SC) is the outermost layer of the epidermis and consists primarily of corneocytes, which are terminally differentiated keratinocytes. Corneocytes, sometimes referred to as squames, are flattened, anucleated cells organized within an extracellular lipid matrix that together form the principal permeability barrier of the skin.

The stratum corneum provides a barrier against the penetration of potentially harmful environmental substances while limiting excessive loss of water from the underlying tissue. Sometimes referred to as the “horny layer,” the SC also provides mechanical strength and resilience, helping protect the viable epidermis from friction and other routine physical stresses.

Keratinocytes originate in the deeper epidermis and progressively differentiate as they migrate toward the surface, ultimately becoming corneocytes within the stratum corneum. Corneocytes are subsequently shed from the surface through desquamation and continuously replaced by cells migrating from the underlying epidermis. Residence within the stratum corneum is approximately two weeks, although epidermal turnover varies with anatomical location, age, physiological condition, and other factors.

Aggressive or repeated exfoliation can disturb this barrier-renewal process by removing corneocytes faster than normal barrier recovery occurs. Resulting disruption of the stratum corneum can increase transepidermal water loss and skin permeability and may increase susceptibility to irritation or contact dermatitis.

Figure 1. Formulation–skin interaction and barrier disruption. Schematic representation of cosmetic application and removal, progressing from the intact skin barrier (Step 1) through film formation (Step 2) and mechanical disruption (Step 3) of the stratum corneum to increased penetration of formulation ingredients, irritants, and allergens and potential pathways for microbial entry (Step 4).

Formulation–Skin Interaction

Step 1 – Clean Skin: Baseline Barrier

As illustrated in Figure 1, Step 1, intact skin provides the baseline surface onto which the cosmetic formulation will be applied. The stratum corneum forms the outer barrier of the epidermis and presents a chemically and physically heterogeneous surface influenced by corneocytes, intercellular lipids, desquamation, sebum, perspiration, and environmental exposure. Surface conditions can therefore influence subsequent wetting, spreading, deposition, and adhesion of a cosmetic formulation.

Step 2 – Makeup Application: Film Formation

During application (Figure 1, Step 2), the formulation wets and spreads across the skin surface, depositing ingredients that may form a continuous or semi-continuous film through adsorption, adhesion, drying, setting, or other formulation-dependent processes. Film performance depends upon both the formulation and the condition of the substrate. Cleaning, conditioning, or other surface preparation may improve wetting, deposition, adhesion, and film uniformity. Conversely, inadequate surface preparation or incompatibility between the formulation and substrate may contribute to poor adhesion, cracking, transfer, premature film degradation, or loss of functionality.

Step 3 – Removal and Mechanical Stress: Barrier Disruption

During wear and subsequent removal (Figure 1, Step 3), the deposited film and underlying stratum corneum are subjected to mechanical and environmental stresses. Rubbing, cleansing, water exposure, perspiration, and repeated application and removal can progressively alter the cosmetic film and may also remove or disrupt corneocytes and intercellular lipids. Repeated or aggressive mechanical treatment can compromise stratum corneum integrity, increasing skin permeability and potentially increasing exposure of the viable epidermis to formulation ingredients.

Step 4 – Increased Penetration and Exposure

As illustrated in Figure 1, Step 4, disruption of the stratum corneum may increase penetration or diffusion of certain formulation ingredients, irritants, or allergens into the underlying viable epidermis. This becomes particularly important when repeated application, friction, prolonged contact, or formulation composition increases exposure to substances having irritation or sensitization potential. Barrier disruption may also create physical pathways that increase susceptibility to microbial entry or colonization; however, microbial entry should be distinguished from the partitioning and molecular diffusion mechanisms governing penetration of dissolved chemical substances.

The durability and integrity of the deposited film depend on both the formulation and the substrate. On skin, film performance may be affected by desquamation, sebum, perspiration, water exposure, and mechanical rubbing. On hair, deposited films may be progressively altered or removed by washing, brushing, thermal styling, sunlight, humidity, and other environmental stresses.

Formulation Films on Skin and Hair

On skin, cosmetic products such as foundations, lip products, eyeliners, adhesives, and other topical formulations may form continuous or semi-continuous films through wetting, spreading, deposition, adsorption, adhesion, and subsequent drying or setting. Film integrity and performance may be influenced by the condition of the stratum corneum, sebum, perspiration, desquamation, water exposure, mechanical rubbing, and environmental conditions. These interactions can affect adhesion, appearance, transfer resistance, wear, and eventual removal of the formulation.

On hair, conditioners, colorants, styling products, sprays, serums, and related formulations deposit ingredients onto the surface of the hair fiber and may form films over the cuticle. Depending upon the formulation and condition of the hair, deposited materials may also interact with damaged or exposed regions of the cuticle and underlying structures. Film performance can subsequently be affected by washing, brushing, friction, thermal styling, humidity, sunlight, and other environmental stresses.

For both skin and hair, surface condition and formulation compatibility are therefore integral components of product development. Effective formulation requires consideration not only of bulk physical and chemical properties, but also of wetting, deposition, adhesion, film formation, durability, removal, and the changes that occur at the formulation–substrate interface during actual use.

Figure 2. Cosmetic formulation films on skin and hair. Illustration of continuous or semi-continuous cosmetic films deposited on skin and hair, depicting formulation–substrate interactions and the effects of surface condition, environmental exposure, and mechanical stress on film adhesion, integrity, durability, and performance.