Cross-Linked vs Non-Cross-Linked Hyaluronic Acid — Why a Cosmetic Isn't a Filler

Hyaluronic acid (HA) is currently the most popular humectant in cosmetics. At the same time, it is the key component of the dermal fillers used in aesthetic medicine. The problem is that these are two completely different substances — and cosmetic marketing deliberately blurs the line, promising a "filler effect" from a serum costing 49 zł.

What is hyaluronic acid cross-linking?

Native hyaluronic acid is a linear polysaccharide made up of repeating units of D-glucuronic acid and N-acetyl-D-glucosamine. In the body it is enzymatically degraded by hyaluronidases — its half-life in the skin is a mere 24–48 hours.

Cross-linking is a chemical process in which HA chains are joined by chemical bridges, forming a three-dimensional network. The most commonly used cross-linking agent is BDDE (1,4-butanediol diglycidyl ether), which forms ether bonds between the hydroxyl groups of adjacent HA chains.

The effect of cross-linking is dramatic: cross-linked HA becomes a gel with viscoelastic properties, resistant to enzymatic degradation. Its residence time in tissue extends from hours to 6–18 months.

Cross-linked HA = a medical device, NOT a cosmetic

Dermal fillers (Juvederm, Restylane, Teosyal) are cross-linked HA preparations classified as Class III medical devices in the European Union. They are subject to strict regulatory requirements, including:

• CE certification required to place them on the European market.
• Clinical trials confirming their safety and efficacy.
• Administration by doctors only — they require subcutaneous or intradermal injection.
• Residual BDDE control — the concentration of unreacted BDDE must be below 2 ppm (parts per million) due to its cytotoxic potential.

Cross-linked HA acts mechanically: it fills tissue volumetrically, lifting the skin from within. No cosmetic applied to the surface of the skin can reproduce this mechanism — no matter what the advertising suggests.

Non-cross-linked HA in cosmetics

The hyaluronic acid in serums and creams is native, non-cross-linked HA of varying molecular weight. Its action depends on the size of the molecules:

Molecular weight fractions

• High-molecular-weight HA (>1000 kDa) — does not penetrate the stratum corneum. It forms an occlusive film on the skin surface that reduces transepidermal water loss (TEWL). The hydrating effect is immediate, but superficial and temporary.
• Medium-molecular-weight HA (100–1000 kDa) — partially penetrates the upper layers of the stratum corneum. It hydrates better than the high-weight fraction, with some smoothing effect.
• Low-molecular-weight HA (10–100 kDa) — penetrates deeper into the epidermis. Studies (Pavicic et al., 2011) showed that HA with a molecular weight of 50 kDa stimulates the expression of genes associated with collagen and elastin production.
• Nano-HA (<10 kDa) — penetrates deepest, but at very low concentrations. Note: HA fragments below 20 kDa can act pro-inflammatory (Stern et al., 2006), activating TLR2 and TLR4 receptors.

The best strategy is to use a blend of fractions (multi-weight HA) — combining surface hydration with deeper action. You can read more about active ingredients in our guide to active ingredients.

Misleading marketing — the "filler effect" in a cream

More and more cosmetics use phrases like "filler effect", "botox-like" or "filling wrinkles from within". These promises are fundamentally misleading for the following reasons:

• The mechanism of action is different — a filler works mechanically (by volume), whereas a serum works at the level of epidermal hydration.
• The scale of the effect is incomparable — 1 ml of filler changes the geometry of the face, while a serum can only gently smooth fine lines.
• Durability — a filler lasts for months, while a serum's effect fades after a few hours without reapplication.

EU Regulation 1223/2009 on cosmetic products prohibits attributing the properties of medical devices to cosmetics. However, enforcement is weak, and manufacturers walk the legal line by using suggestive but imprecise wording.

How to read the label — HA in the INCI

On the ingredient list, hyaluronic acid appears under various names that indicate different fractions:

• Sodium Hyaluronate — the sodium salt of HA, most often medium/high molecular weight.
• Hydrolyzed Sodium Hyaluronate — a hydrolyzed form, low molecular weight.
• Sodium Hyaluronate Crosspolymer — lightly cross-linked cosmetic HA (not to be confused with a filler!). It forms an elastic mesh on the skin.
• Sodium Acetylated Hyaluronate — acetylated HA, which binds better to the lipids of the epidermis.

Check the full analysis of each of these ingredients in our cosmetic ingredient database.

Summary — realistic expectations

Hyaluronic acid in cosmetics is an excellent humectant — but not a substitute for aesthetic medicine procedures. Non-cross-linked HA hydrates the epidermis, smooths fine lines and improves the skin barrier. Cross-linked HA is a completely different category — a medical device administered by injection by doctors. Any cosmetic that promises a "filler effect" should put you on your guard.

Want to find out which HA fraction your serum contains? Enter the product name in the PurScore search engine — we'll analyze the formula and explain what you can realistically expect.