Facial serums are frequently marketed using claims about “deep penetration”, advanced delivery and powerful active ingredients. Clients may therefore assume that every serum travels through the skin and works deep within the dermis.

The reality is more complicated.

Whether an ingredient enters the skin, where it reaches and what it does when it gets there depend on several factors. These include its molecular size, chemical properties, concentration, stability, formulation and the condition of the skin barrier.

Some ingredients work mainly on or near the surface. Others can enter parts of the epidermis, follicles or deeper structures. A product does not, however, need to reach the deepest layers of the skin to produce a worthwhile cosmetic benefit.

For beauty therapists, understanding this distinction matters. It helps us recommend skincare more accurately, avoid repeating exaggerated marketing claims and explain product choices to clients with greater confidence.

What is a facial serum?

A facial serum is generally a lightweight topical product formulated to deliver one or more skincare ingredients. It may be water-based, oil-based, gel-like or contained within an emulsion.

The word 'serum' describes the product format. It does not automatically tell us:

• How concentrated the product is
• Whether its ingredients are stable
• How deeply those ingredients penetrate
• Whether the formula is more effective than a cream
• Whether it is suitable for a particular skin condition

A serum may contain beneficial ingredients, but the ingredient list alone cannot tell us the whole story. How those ingredients have been formulated is equally important.

For example, two products may both list vitamin C. One could contain pure L-ascorbic acid in a carefully controlled acidic formulation, while another might contain a vitamin C derivative in a different base. Their stability, tolerability and behaviour on the skin may be quite different.

How does the skin barrier affect serum penetration?

The skin is not designed to allow everything placed on it to pass freely into the body. One of its most important functions is to act as a protective barrier.

The outermost part of the epidermis is the stratum corneum. It is formed from flattened corneocytes surrounded by a lipid-rich matrix. It is often compared with bricks and mortar: the corneocytes are the bricks, while the lipids surrounding them form the mortar.

This structure helps to reduce water loss while limiting the entry of irritants, allergens, microorganisms and other external substances.

Consequently, a skincare ingredient must overcome a highly effective barrier before it can move into the lower layers of the epidermis. Human skin is particularly resistant to the passage of larger molecules, although size is only one of several factors affecting penetration.

This is why phrases such as 'absorbs instantly' and 'penetrates deeply' should not be accepted without question.

A product disappearing from the skin’s surface does not prove that every active ingredient has travelled deep into the tissue. Some of the product may have evaporated, remained within the upper stratum corneum, collected around follicles or simply become less noticeable as it spreads across the skin.

What determines whether a serum penetrates the skin?

There is no single rule that guarantees effective serum penetration. Several factors interact.

Molecular size

Molecular size is one of the most frequently discussed factors.

The often-quoted '500 Dalton rule' proposes that molecules generally need to have a molecular weight below approximately 500 Daltons to pass through the stratum corneum effectively. This is a useful scientific principle, but it should not be treated as an absolute promise that every molecule below this size will penetrate or that every larger molecule is completely inactive.

A small molecule may still penetrate poorly if its chemical properties or formulation are unsuitable. A larger ingredient may remain on the surface but still improve hydration, softness or barrier comfort.

Water and oil solubility

The stratum corneum contains both water-attracting and lipid-rich components. An ingredient’s balance between water solubility and lipid solubility influences how it interacts with the barrier.

Very water-soluble ingredients may struggle to move through the lipid regions of the stratum corneum. Highly oil-soluble ingredients may enter the barrier more readily but may not move easily into the more water-rich layers beneath it.

Good formulation is therefore about more than simply making an ingredient smaller.

Product pH

The pH of a formulation can influence the stability, ionisation, tolerability and penetration of certain ingredients.

Pure L-ascorbic acid is a well-known example. Research has found that it needs to be formulated at a sufficiently acidic pH to enter the skin effectively. In one frequently cited study, formulations below pH 3.5 were required for meaningful skin penetration, with absorption reaching a plateau at concentrations around 20%.

That does not mean every client should automatically use a strong, highly acidic vitamin C product. Suitability and tolerability still matter.

Ingredient stability

An ingredient cannot perform effectively if it has degraded before or shortly after it reaches the skin.

Light, heat, air, water, packaging and the surrounding formula can all influence stability. Vitamin C and retinoids are particularly associated with formulation and stability challenges.

This is one reason why a well-formulated product cannot be judged solely by the percentage printed on the packaging.

The product vehicle

The vehicle is the base that carries the active ingredients. It may be a gel, oil, cream, emulsion or another delivery system.

The vehicle can affect:

• How evenly the product spreads
• Whether the ingredient remains stable
• How long it remains in contact with the skin
• How much water is retained
• Whether the ingredient is released from the product
• How comfortable and tolerable the product is

A sophisticated ingredient in a poorly designed vehicle may perform less effectively than a more modest ingredient in a well-balanced formulation.

The condition of the skin

Healthy, intact skin behaves differently from damaged, inflamed or recently treated skin.

Penetration may increase when the barrier has been disrupted by excessive exfoliation, irritation, dermatitis, needling, peeling or other procedures. Increased penetration is not automatically beneficial. It may also increase stinging, inflammation and the risk of an adverse reaction.

Beauty therapists should therefore consider recent treatments, homecare use, sensitivity, contraindications and visible barrier condition before recommending active serums.

Does a serum need to reach the dermis to work?

No.

This is one of the most important points for therapists and clients to understand.

Cosmetic products are intended primarily to clean, protect, perfume, maintain or alter the appearance of the external parts of the body. Products placed on the Great Britain market must meet the relevant cosmetic safety and regulatory requirements.

A cosmetic ingredient may deliver a valuable result by:

• Supporting hydration within the stratum corneum
• Reducing surface water loss
• Improving the feel and flexibility of the skin
• Encouraging more even shedding of corneocytes
• Working within the epidermis
• Entering the follicular opening
• Influencing cell communication or visible skin behaviour

It does not have to flood the dermis to be useful.

In fact, confidently claiming that an ordinary cosmetic serum travels into the deepest layers and changes the structure or function of the body could move the language closer to a medicinal claim. In the UK, the MHRA considers both product composition and the claims made when determining whether a product should be regarded as medicinal.

Therapists should therefore avoid dramatic promises and use accurate cosmetic language.

How does Hyaluronic Acid work in a serum?

Hyaluronic acid is widely used because of its ability to attract and retain water. However, not all hyaluronic acid behaves in exactly the same way.

Traditional high-molecular-weight hyaluronic acid is a large molecule. Much of it is expected to remain on or close to the skin’s surface, where it can support hydration and improve the skin’s feel.

Lower-molecular-weight forms may move further into the stratum corneum or epidermis. Research comparing different molecular weights found that lower-molecular-weight hyaluronic acid showed greater permeation than high-molecular-weight forms.

More recent reviews also suggest that molecular weight influences both penetration and biological activity. However, study methods, molecular-weight ranges and formulations differ, so broad claims that all low-molecular-weight hyaluronic acid reaches the dermis should be treated cautiously.

The practical message is not that high-molecular-weight hyaluronic acid is inferior. Surface hydration is still useful. A formula containing different molecular weights may be intended to support hydration at different levels, but its performance depends on the complete formulation rather than a fashionable label alone.

How does Vitamin C penetrate the skin?

Vitamin C is used in skincare for its antioxidant activity and its role in pathways associated with collagen production and pigmentation.

Pure vitamin C is known as L-ascorbic acid. It is water-soluble and chemically unstable, which makes effective topical formulation difficult.

Penetration depends partly on acidity and concentration. As already noted, research has demonstrated better absorption when L-ascorbic acid is formulated below pH 3.5, with no proportional absorption benefit beyond a certain concentration.

Vitamin C derivatives may be selected because they are more stable or more comfortable to use. They must generally be converted into a biologically usable form within the skin, and their behaviour differs according to the derivative and formulation.

A review of vitamin C formulations found that stability, percutaneous absorption and skin effects vary considerably between pure ascorbic acid and its derivatives.

Therefore, '20% vitamin C is not automatically better than 10% vitamin C”' Therapists should consider the specific form, pH, packaging, stability and client tolerance.

How do retinoids work in skincare?

Retinoids are a family of vitamin A derivatives. They include retinol, retinaldehyde, retinyl esters and prescription-strength retinoic acid.

Different retinoids require different conversion steps before they reach the biologically active form that interacts with retinoid receptors.

In simplified terms:

• Retinyl esters require several conversion stages.
• Retinol is converted into retinaldehyde and then retinoic acid.
• Retinaldehyde requires one principal conversion step.
• Retinoic acid is already active but is generally regulated as a medicine rather than an ordinary cosmetic ingredient.

The conversion pathway is only part of the story. Stability, packaging, concentration and tolerability affect how successfully a cosmetic retinoid can be used.

Retinoids have been extensively researched for photoaged skin, but they can also cause dryness, flaking, irritation and increased sensitivity, particularly when introduced too aggressively.

A professionally recommended retinoid should therefore be selected according to the client’s skin, experience, routine and ability to use it consistently—not because the strongest option sounds more impressive.

Do peptides penetrate the skin?

Peptides are short chains of amino acids. They are often described as signalling ingredients because particular peptides may interact with processes associated with skin condition and appearance.

However, the word 'peptide' covers a wide range of molecules. They differ in size, structure, solubility, stability and intended function.

Some peptides are too large or too water-soluble to pass easily through an intact stratum corneum. Manufacturers may alter the peptide, attach it to a lipid component or use an encapsulated delivery system to improve stability or skin interaction.

This means that a product containing peptides cannot be assumed to stimulate collagen simply because the word appears on the ingredient list.

Therapists should be wary of turning a proposed biochemical mechanism into a guaranteed client result. The quality of the finished formulation and the evidence supporting that specific product matter.

What is the difference between AHAs and BHAs?

Alpha hydroxy acids and beta hydroxy acids are commonly included in exfoliating serums.

Alpha hydroxy acids

Common AHAs include glycolic acid, lactic acid and mandelic acid.

They help to reduce the adhesion between cells within the upper epidermis, supporting exfoliation and improving surface texture. The acids differ in molecular size and chemical behaviour.

Glycolic acid is relatively small and can act quickly, but that may also increase irritation for some skins. Lactic acid is larger and has useful humectant properties. Mandelic acid is larger again and is often selected when a slower or more controlled approach is preferred.

The result depends on more than the acid’s name. Concentration, pH, free-acid value, contact time and overall formula all affect performance.

Beta hydroxy acids

Salicylic acid is the best-known BHA used in skincare. It is oil-soluble and is often selected for oily, congested or blemish-prone skin because it can work within lipid-rich areas and follicular openings.

Neither an AHA nor a BHA needs to penetrate into the deep dermis to perform its main cosmetic function. Their value largely comes from their effects within the epidermis and follicular environment.

Do nanotechnology and encapsulation improve serum delivery?

Advanced delivery systems may include liposomes, lipid carriers, encapsulated ingredients and nano-sized structures.

Depending on the system, these technologies may help to:

• Protect an unstable ingredient
• Improve dispersal within the formula
• Control how quickly an ingredient is released
• Improve contact with the skin
• Reduce irritation
• Support delivery to a more appropriate site

They do not guarantee that every ingredient will penetrate more deeply or produce a superior clinical result.

'Nanotechnology' can sound persuasive in marketing, but therapists should still ask basic questions. What ingredient is being delivered? What is the intended target? Is the system designed to improve stability, tolerability or penetration? Is there evidence for the finished formula rather than only its individual ingredients?

Good science is more valuable than impressive terminology.

What should beauty therapists tell their clients?

Clients do not need a lecture on molecular chemistry. They do, however, deserve an honest explanation.

Instead of saying: “This serum penetrates into the deepest layers of your skin.”

A more accurate explanation would be: “I’ve selected this serum because its formulation and ingredients are suited to your current skin concerns. It is designed to support hydration and improve the appearance of the skin when it is used consistently.”

Or: “This contains an active form of vitamin C in protective packaging. We’ll introduce it gradually because the strength of a product is only useful when your skin can tolerate it.”

This language is still confident. It simply avoids pretending that cosmetic marketing is settled biological fact.

How should therapists choose a professional serum?

Before recommending a serum, consider:

• The client’s primary skin concern
• Current skin condition and barrier health
• Previous reactions and known sensitivities
• The active ingredient and its specific form
• Concentration and formulation pH, where relevant
• Supporting ingredients
• Packaging and product stability
• Manufacturer instructions
• Contraindications and precautions
• Compatibility with the client’s existing routine
• Whether the client can use it consistently
• The quality of evidence behind the product claims

The most expensive serum is not automatically the best. Neither is the serum with the longest ingredient list or highest advertised percentage.

The best choice is the one that is suitably formulated, appropriately recommended and realistically usable by the individual client.

The bottom line on serum penetration

Facial serums can be valuable, but their performance cannot be judged by texture, price or marketing language alone.

The skin barrier deliberately restricts penetration. Molecular size matters, but so do solubility, stability, pH, vehicle, delivery system and skin condition. Some ingredients work mainly at the surface, while others may enter the epidermis or follicular structures.

That does not make surface action unimportant. Hydration, barrier support, exfoliation and protection can all create meaningful visible improvements without an ingredient travelling into the deepest layers of the skin.

For professional beauty therapists, the aim should not be to make the biggest claim. It should be to understand the product well enough to make an appropriate recommendation, set realistic expectations and explain the science without misleading the client.

That is the difference between repeating skincare marketing and providing professional skincare advice.

References and Further Reading

UK regulatory guidance

1. Office for Product Safety and Standards. Regulation 1223/2009 and the Cosmetic Products Enforcement Regulations 2013: Great Britain.
2. UK Government. Making cosmetic products available to consumers in Great Britain.
3. Medicines and Healthcare products Regulatory Agency. Decide if your product is a medicine or a medical device.
4. UK legislation. Regulation (EC) No 1223/2009 on cosmetic products.

Scientific research

5. Bos JD, Meinardi MMHM. The 500 Dalton rule for the skin penetration of chemical compounds and drugs. Experimental Dermatology. 2000.
6. Essendoubi M, et al. Human skin penetration of hyaluronic acid of different molecular weights as probed by Raman spectroscopy. Skin Research and Technology. 2016.
7. Zanchetta C, et al. Hyaluronic Acid in Topical Applications: The Various Forms and Biological Effects. 2025.
8. Pinnell SR, et al. Topical L-ascorbic acid: percutaneous absorption studies. Dermatologic Surgery. 2001.
9. Stamford NPJ. Stability, transdermal penetration and cutaneous effects of ascorbic acid and its derivatives. Journal of Cosmetic Dermatology. 2012.
10. Riahi RR, et al. Cosmetic retinoid use in photoaged skin: a review of the compounds, efficacy and safety. 2024.