You may have come across skin experts or social media influencers challenging a collagen skincare product because 'the molecule to too large to penetrate the skin'. This statement often raises a broader and more important question: which cosmetic ingredients actually need to penetrate the skin in order to be effective, and does a product lose efficacy if its molecules cannot diffuse into the skin?
To answer this, it is useful to consider the physiology of the skin, particularly the top visible layer of the skin, the stratum corneum. The stratum corneum is often described using the “brick and mortar” model, in which corneocytes (the bricks) are embedded in an intercellular lipid matrix composed primarily of ceramides, cholesterol, and free fatty acids (the mortar). You can read up more on skin biology here: Strategies For A Healthy Skin Barrier. Molecular size, polarity, and solubility all influence whether an ingredient can diffuse across our skin barrier. In general, substances with a molecular weight greater than 500 Daltons show poor diffusion across the stratum corneum. But, importantly, not all skincare ingredients are designed to penetrate the skin, nor do they need to in order to provide benefit.
Active ingredients requiring epidermal penetration
Certain active compounds need to be formulated to reach the viable epidermis or upper dermis, where they can interact with living keratinocytes or dermal fibroblasts. Ingredient examples include ascorbic acid (vitamin C) and Vitamin C derivatives, retinoids, and certain signaling peptides.
These molecules have been shown to modulate biological pathways involved in skin aging and repair. For instance, ascorbic acid assists the enzymes required for collagen synthesis, while retinoids regulate gene expression through retinoic acid receptors (RARs), speeding up cell turnover and stimulating production of structural proteins such as collagen and elastin, which keep skin firm and hydrated. For these mechanisms to occur, they need to penetrate into the epidermis, which is why formulation strategies should focus on optimizing stability, solubility, and delivery. This is of even more importance where derivatives are used, eg. Vitamin C derivatives such as ascorbyl glucoside, sodium ascobyl phosphate etc, and retinoid derivatives such as retinol, retinaldehyde etc, because derivatives need additional conversion steps within the skin.
Barrier-supporting and occlusive ingredients
In contrast, ingredients designed to reinforce the skin barrier or reduce trans-epidermal water loss (TEWL) primarily function on the skin’s surface. Lipophilic compounds such as squalane, plant oils, and petrolatum form an occlusive film over the stratum corneum.
These ingredients improve skin hydration by improving water retention in the stratum corneum rather than by penetrating into deeper layers. Their efficacy is therefore largely dependent on their film-forming and barrier-modulating properties, not their ability to diffuse into the skin.
Exfoliating agents and corneocyte desquamation
Chemical exfoliants - including alpha hydroxy acids (AHAs), beta hydroxy acids (BHAs), polyhydroxy acids (PHAs), and enzymatic exfoliants - also primarily act at surface level. Their main mechanism involves weakening the cohesive forces between corneocytes, facilitating desquamation and promoting a smoother skin surface. Certain low molecular weight AHAs, particularly glycolic acid, can penetrate further into the epidermis due to their small molecular size. This deeper diffusion may enhance stimulation of epidermal renewal and dermal remodeling, which can improve fine lines and hyperpigmentation. However, increased penetration also correspondes to a higher potential for irritation, particularly in individuals with a compromised barrier function or sensitive skin.
Humectants and hydration agents
Humectant ingredients contribute to skin hydration by attracting and binding water in the stratum corneum. The extent of their penetration is often influenced by molecular size. Low molecular weight humectants such as glycerin readily diffuse into the superficial layers of the stratum corneum, where they interact with the natural moisturizing factor (NMF) and increase water retention within corneocytes.
In contrast, high molecular weight humectants such as hyaluronic acid remains predominantly on the skin surface due to its large polymeric structure. Despite limited penetration, it can still enhance hydration by forming a film that reduces trans epidermal water loss (TEWL) and binds environmental moisture, contributing to skin plumping and improved surface hydration.
Large molecules such as collagen
Proteins such as collagen have molecular weights that typically exceed 300 kDa, far surpassing the size threshold required for passive diffusion through the stratum corneum. Consequently, topically applied collagen cannot penetrate into the dermis to directly boost collagen levels or improve structure in the skin.
However, collagen can still contribute functional benefits in topical formulations. Due to its film-forming and hygroscopic properties, it can enhance surface hydration and improve skin feel by forming a moisture-retaining layer over the stratum corneum. In this context, collagen functions more appropriately as a hydrating and conditioning polymer rather than as a dermal collagen replacement.
The key takeaway
The effectiveness of a skincare ingredient is not determined only by its ability to penetrate the skin. Some ingredients must reach deeper layers to exert biological effects, while others are specifically intended to remain at the surface to support barrier function, hydration, or exfoliation. Understanding where an ingredient is designed to act within the skin provides a more accurate framework for evaluating skincare claims than focusing on molecular size alone.
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