Hydration is the foundation of any effective skincare routine. Without sufficient moisture, even the most potent active ingredients lose efficacy – enzyme systems work slower, barrier function suffers, and the skin's appearance becomes uneven and tired. It's not just about delivering water to the skin; the central question is how this water is bound, distributed, and retained in the tissue over time.

The market for hydrating serums has evolved considerably in recent years. Formulations considered technologically advanced in 2020 now seem one-dimensional. New active ingredient classes like polyglutamic acid, channel-forming peptides for aquaporin activation, and high-molecular-weight NMF complexes are shifting the standard. At the same time, basic chemistry remains unchanged: understanding how humectants, occlusives, and emollients work together allows for the classification of any formula on the market – regardless of packaging claims and marketing terminology.

This article provides you with the tools to evaluate hydrating serums according to scientific criteria. No product rankings, no advertising for individual preparations. Instead: the mechanisms, molecules, and formulation principles that matter.

The Hydration Triangle: Humectants, Occlusives, and Emollients

Moisturizing active ingredients in skincare products can be divided into three functional classes, which differ fundamentally in their mode of action. Understanding these differences is a prerequisite for correctly classifying a formula and adapting it to one's own skin type.

Humectants – The Water Binders

Humectants are hygroscopic substances: they draw water from the environment – and from deeper skin layers – into the upper epidermal layers, especially the stratum corneum. Established humectants include hyaluronic acid, glycerin, urea (up to 10%), panthenol, sorbitol, propylene glycol, and alpha-hydroxy acids in low concentrations. They measurably and immediately increase the water content of the stratum corneum.

An essential, often overlooked mechanism: In dry environments with low relative humidity, humectants can draw water from deeper tissue layers to the surface, which, if used exclusively without an additional sealant, can even increase transepidermal water loss (TEWL). For this reason, humectants in a serum context are ideally combined with a subsequent, richer formulation – such as the NATURFACTOR® Blue Crystal Drops, which are designed as a night care and can complement humectant serums as a final seal.

Occlusives – The Evaporation Barriers

Occlusives form a physical film on the skin's surface that mechanically inhibits the evaporation of water from the stratum corneum. Classic examples include petrolatum, lanolin, beeswax, squalane, dimethicone, and ceramides. In serums, occlusives are usually used in low concentrations to avoid a heavy finish – their primary role is protection in creams and overnight masks.

Ceramides hold a special position: they are not primarily occlusives in the classical sense but integral components of the lamellar lipid matrix of the stratum corneum. Their inclusion in formulations can help stabilize the barrier and thus indirectly reduce TEWL.

Emollients – The Texture Enhancers

Emollients fill the gaps between corneocytes, providing immediate smoothing of the skin surface. They improve skin texture and appearance by leveling microscopic roughness and cracks in the stratum corneum. Typical emollients in serums include plant oils (jojoba oil, argan oil, rosehip), esters like isopropyl myristate, and silicone-based compounds. Emollients contribute less to deep hydration than humectants but are relevant for the sensory experience and barrier stabilization.

Hyaluronic Acid: Why Molecular Size Determines Everything

Hyaluronic acid (HA) is the best-known hydrating molecule in modern skincare – and also the most frequently misunderstood in its positioning. The discussion usually revolves around concentration and origin, while the crucial parameter is rarely mentioned: molecular weight.

Hyaluronic acid is a linear glycosaminoglycan composed of alternating units of D-glucuronic acid and N-acetyl-D-glucosamine. In the skin, it naturally occurs in the dermis and epidermis, where it significantly contributes to the structure of the extracellular matrix and tissue hydration. In a cosmetic context, HA is produced biotechnologically through microbial fermentation.

High-Molecular-Weight HA (> 1,000 kDa) – Surface Action

High-molecular-weight HA fractions can barely penetrate the skin barrier due to their size. They remain on the skin's surface, where they form a hydrating film that temporarily binds moisture and improves skin feel. This effect is immediately noticeable – a "plumping" effect, softer skin feel – but limited to the surface.

Low-Molecular-Weight HA (< 50 kDa) – Deep Action

Molecules below approximately 50 kDa can penetrate deeper epidermal layers and contribute to the hydration of living cell layers there. Studies show that low-molecular-weight HA fractions can also trigger pro-inflammatory responses – an indication that they are more biologically active than their high-molecular-weight counterparts. In well-formulated serums, specific fractional ranges are therefore deliberately used.

Ultra-Low-Molecular-Weight HA and Oligomers (< 10 kDa)

The smallest HA fragments – sometimes referred to as oligomers or hydrolyzed HA – penetrate the epidermis most deeply. Their effect at a cellular level differs from intact HA: they can activate receptors like CD44 and RHAMM, thereby influencing cellular signaling pathways. Whether this is relevant in a cosmetic context depends on concentration and formulation.

Polyglutamic Acid, Panthenol, Urea – The Underestimated Hydrators

In addition to hyaluronic acid, several active ingredients exist that are equivalent or superior in their hydrating effect in certain contexts – yet receive significantly less marketing attention. A scientifically oriented product evaluation explicitly considers these substances.

Polyglutamic Acid (PGA)

Polyglutamic acid is a bacterially fermented biopolymer originating from Japanese fermentation research. It is a polypeptide composed of glutamic acid units and exhibits exceptionally high water-binding capacity – comparable to or, in some studies, slightly superior to HA at the same concentration. PGA penetrates the skin more efficiently than high-molecular-weight HA and can additionally support endogenous hyaluronic acid synthesis in keratinocytes by inhibiting hyaluronidase activity.

In modern serum formulations, PGA is increasingly used as a complement to HA to extend hydration duration. Its viscoelastic properties also allow for pleasant textures without the sticky feeling associated with some HA-heavy formulations.

Glycerin

Glycerin is the most underestimated humectant in skincare. As one of the oldest cosmetic moisturizers, it is often perceived as a "cheap ingredient" – yet the data shows a different picture. Glycerin is a natural component of the NMF, acts as an aquaporin-3 substrate (more on this in section 5), improves barrier function, and in clinical studies shows a hydrating effect comparable to hyaluronic acid. In concentrations of 5–15% in aqueous formulations, glycerin achieves measurable and lasting hydration results.

Panthenol (Provitamin B5)

Panthenol – the alcoholic precursor of pantothenic acid – is oxidized to pantothenic acid in the skin, a component of coenzyme A. Its hydrating effect is based on its function as a humectant as well as its ability to support keratinocyte proliferation and promote barrier regeneration. Panthenol also exhibits anti-inflammatory properties and is well tolerated by sensitive skin. In serums, it is typically used in concentrations of 0.5–5%.

Urea

Urea (carbamide) is an NMF component and acts as a humectant in low concentrations (2–10%); in higher concentrations (> 10%), it also acts as a keratolytic. In a serum context, concentrations of 2–5% are common and aim for hydration without a keratinolytic effect. Urea demonstrably increases the penetration of other active ingredients by temporarily loosening the structure of the stratum corneum – a formulation-technically relevant synergistic effect that can be specifically utilized in combination products.

The Natural Moisturizing Factor: Hydration from Within

The Natural Moisturizing Factor (NMF) is not a single active ingredient but a complex of water-soluble compounds found within the corneocytes – the keratinized cells of the stratum corneum. They are formed as a breakdown product of the structural protein filaggrin, which is degraded into amino acids and their derivatives during the terminal differentiation of keratinocytes.

The composition of the NMF is known and well-studied: Free amino acids make up about 40% (including serine, alanine, glutamic acid, threonine, and proline), pyrrolidone carboxylic acid (PCA) about 12%, lactate about 12%, urea about 7%, as well as smaller proportions of citrate, sugars, peptides, inorganic ions, and other compounds. This mixture is highly efficient in binding water and maintaining the optimal water content of the stratum corneum.

Crucially: The NMF is not a static structure. It can be depleted by repeated washing, the use of surfactants, UV exposure, age, and genetic factors (especially filaggrin mutations, which are common in atopic dermatitis). Therefore, supplementing the NMF through topical formulations is not only sensible but essential for certain skin types.

NMF-Analogous Ingredients in Serums

Aquaporins and Formulation Technology for Deep Hydration

Aquaporins are transmembrane proteins found in almost all living organisms and function as selective water channels in cell membranes. In human skin, Aquaporin-3 (AQP3) and Aquaporin-9 (AQP9) play a central role in moisture distribution. AQP3 is concentrated in the basal layer of the epidermis and transports not only water but also glycerin – which is why glycerin, as a natural AQP3 substrate, has particular relevance in hydration biochemistry.

With increasing age, AQP3 expression in the skin decreases, which is sometimes used to explain the reduced hydrating capacity of aged skin. Topical approaches aimed at supporting aquaporin function are an active field of research – and increasingly a formulation approach in premium serums.

Formulation Approaches for Enhanced Penetration

Deep penetration of moisturizing active ingredients into the epidermis is a formulation challenge. The stratum corneum is evolutionarily designed as a barrier – meaning that even hydrophilic active ingredients do not easily penetrate. Modern formulation technologies address this problem in various ways:

Liposomal and Nanoparticle Encapsulation: Active ingredients such as HA fractions or PCA are encapsulated in liposomes or nanoparticles, whose lipid membrane exhibits increased affinity to the skin barrier. This can improve penetration depth and active ingredient release at the site of action.

Hydrosomes and Multi-Lamellar Vesicles: Multi-layered vesicular systems imitate the lamellar lipid structure of the stratum corneum and allow gradual penetration into deeper layers. The release kinetics can thus be controlled.

pH-Optimized Formulations: The physiological skin pH is between 4.5 and 5.5. Formulations adjusted to this range work with the natural barrier source instead of against it. Deviations from physiological pH can impair NMF function and destabilize the barrier.