Hydration is a fundamental principle in skincare. It supports the skin’s barrier function, underpins its suppleness and elasticity, and plays a critical role in both how a product performs and how it feels. Yet in the lab, hydration is never treated as a one-size-fits-all solution.

The cosmetic scientists in our R&D labs approach hydration as a formulation discipline that requires technical precision, physiological understanding, and real empathy for how skin behaves across different people, environments and life stages. In this article, Dr. Catherine Leray, our Account Head of R&D, shares what that looks like behind the scenes.

Dry vs Dehydrated Skin: Why the Distinction Matters in Formulation

One of the most important and most misunderstood distinctions in skincare is the difference between dry and dehydrated skin. In the lab, this distinction influences how we formulate as well as how we guide brands on claims and education.

• Dry skin is a skin type characterised by reduced lipid production, especially ceramides and sebum. While genetics play a key role, environmental factors and hormonal shifts can also influence lipid levels over time. This compromises the skin’s ability to form an effective barrier, making it prone to flaking, tightness, and irritation. Dry skin needs lipid restoration i.e. emollients, butters, oils and barrier-building actives.
• Dehydrated skin is a temporary condition: it can affect any skin type, even oily or blemish-prone. It occurs when the skin lacks water, often due to environmental stress, over-exfoliation or harsh cleansing. Dehydrated skin benefits from humectants like glycerin, hyaluronic acid and NMF mimetics, alongside protective lipids or film-formers to reduce TEWL.

In our research, we often see consumers using mattifying or drying products for oily skin, only to end up with surface dehydration. Educating around this difference is essential in helping brands build trust and ensures the products are solving the right problem.

Understanding the Skin’s Hydration Mechanisms

The stratum corneum, the skin’s outermost layer, is often misunderstood as just a passive barrier, but it's remarkably dynamic. It contains a complex matrix of Natural Moisturising Factors (NMFs) and intercellular lipids that help regulate water retention and barrier flexibility.

In our labs, we sometimes describe this layer as the skin’s first language because it responds and signals discomfort so quickly when hydration levels drop: tightness, flaking, dullness. It’s something consumers notice instantly, and it often drives first impressions of a product.

In development, we look at:

• The integrity and condition of the skin’s lipid barrier
•  How effectively moisture is attracted and retained within the stratum corneum
• Selecting ingredient systems that support optimal hydration without disrupting natural skin renewal

The science may be complex, but the goal is simple: create products that keep water where it needs to be.

Hydration Claims and Regulatory Considerations

Hydrating claims are popular, but they’re not soft science and substantiated them with data is considered best practice, especially for claims around moisture retention over time. Depending on the specificity of the claim, corneometry, TEWL measurements or user perception trials can be used to support product performance and build consumer trust.

We’re often asked whether a product can be called “hydrating” if it simply feels moisturising. The answer is, it depends on the claim, the context and the product format. It’s good practice to have credible substantiation behind them, especially if you’re suggesting specific benefits like long-lasting hydration or barrier reinforcement. In cases where the sensorial profile or the chosen active ingredients suggests hydration, but no instrumental data is available, we advise caution with a minimum of a user trial test.

The Formulator’s Toolkit: Building a Hydrating Product

Hydration formulas are built on a precise blend of humectants, emollients and barrier supporting agents. While this sounds straightforward, the challenge lies in achieving the right ratio for the target skin type, market and usage occasion.

At the core of any hydrating product are three essential ingredient categories:

• Humectants (e.g. glycerin, hyaluronic acid) attract water from the environment when humidity allows or from deeper skin layers in drier conditions, which is why pairing them with protective lipids is so important in low-humidity climates.
• Emollients (e.g. esters, silicones, plant oils ) to smooth and soften
• Barrier-supporting agents (e.g. butters, petrolatum or alternatives, film-formers) to help reduce moisture loss and reinforce skin resilience

By understanding the physical and chemical behaviour of these ingredients (their water affinity, spreadability and sensorial profile) as chemists we can create formulations that are not only effective, but elegant.

A gel cream for humid climates, for example, would need non-sticky humectants that don’t attract too much moisture from the environment to avoid that sweaty, tacky feel while pairing them with lightweight emollients that provide slip without shine.

Modern innovations include:

·       Cold-process emulsification to help preserve temperature-sensitive ingredients including delicate hydrating actives and improve production performance

·       Biomimetic emulsifiers that mimic the skin’s natural lipid structure, supporting barrier integrity and skin compatibility

·       Encapsulation systems that enhance the delivery, stability and targeted release of hydration-boosting actives

How Hydration Influences Skin Aesthetics

Hydrated skin looks better, full stop. It reflects light more evenly, feels softer to the touch and helps smooth out the appearance of fine lines.

From a consumer’s point of view, this can often be more compelling than long-term anti-ageing claims. I’ve seen firsthand how a well-formulated hydrating serum transforms someone’s perception of their skin overnight, not because it changes the structure, but because it visibly plumps and brightens dullness to improve texture and radiance. In this way, hydration is both a biophysical effect and a visual one, with many consumers using hydration as a proxy for product performance. This perception is reinforced through:

• Soft, silky textures
• Quick absorption with low tack
• A fresh, dewy finish

Our cosmetic scientists consider these sensorial cues carefully, which is why we build sensorial testing into our development process. A product may have the numbers behind it, but if it feels heavy, greasy or takes too long to absorb, it won’t land with consumers. Sensory elegance is part of the science.

Environmental Pressures on Skin Hydration

Modern skincare must contend with increasingly complex environmental conditions, from arid, air-conditioned interiors to urban pollution and long-haul travel. Humidity, pollution and UV exposure all compromise hydration.

Each stressor impacts hydration:

• Cold weather and low humidity increase TEWL
• UV exposure degrades NMFs and lipids
• Pollution causes oxidative stress and microinflammation that gradually compromises the barrier function and hydration retention.

Advanced hydration formulas often include:

• Antioxidants
• Film-formers create a protective barrier to help reduce moisture loss.
• Climatic adaptogens to buffer against temperature extremes

We often advise clients building global anges to consider seasonal or regional SKUs. What works in the UK winter won’t necessarily satisfy users in Asia, for example. This is where dual-launch concepts come into play: one ultra-rich barrier balm for dry climates and a lighter, antioxidant-boosted emulsion for pollution-prone cities.

These kinds of adjustments are subtle but impactful and they show that good hydration formulation is never generic.

ABOUT THE AUTHOR

Dr. Catherine Leray 
Account Head of R&D, THG LABS 

With over 20 years in the beauty industry, Dr Catherine Leray , Account Head of Research & Development at THG LABS, is passionate about the science behind truly effective cosmetic products. Specialised in haircare, she enjoys turning research into practical solutions that deliver tangible results, developing products with proven benefits without overlooking the importance of sensorial experience. Her work is driven by a deep understanding of consumer needs and the scientific principles that make effective haircare possible.