Skin Barrier: What Is It and Why Does It Matter?
In simple terms, it is the skin's protective system that helps keep water in and unwanted substances out. One of its most important components is the stratum corneum, the outermost layer of the epidermis. Its structure plays a central role in controlling water loss and the movement of substances through the skin. (Cosmetic Formulation: Principles and Practice, 2019, Heather A. E. Benson, Michael S. Roberts, Vânia Rodrigues Leite-Silva and Kenneth A. Walters, eds.)
What is the skin barrier made of?
The outer layer of the skin is made up largely of flattened cells called corneocytes, surrounded by an organised mixture of lipids.
These lipids include ceramides, cholesterol and fatty acids and are important for creating the barrier between the skin and the external environment.
A useful way to picture this structure is the familiar “bricks and mortar” analogy. The corneocytes act as the bricks, while the surrounding lipids help form the mortar.
This organisation is important because the skin barrier is not simply a layer sitting on top of the skin. It is a structured system that controls how easily water and other substances can move through it.
Why is the skin barrier important?
The skin constantly interacts with the environment around us. At the same time, it needs to prevent excessive water loss.
The stratum corneum is therefore important for maintaining the skin's hydration and protecting it from external influences. Its condition also affects the permeability of the skin.
When the barrier is intact, water loss through the skin is relatively well controlled. When the barrier is disrupted, water can escape more readily and the permeability of the skin can increase. (Dermatologic, Cosmeceutic, and Cosmetic Development: Therapeutic and Novel Approaches, 2008, Kenneth A. Walters and Michael S. Roberts, eds.)
This is why barrier function and skin hydration are so closely connected.
What happens when the skin barrier is weakened?
A weakened or disrupted barrier can result in increased water loss from the skin.
The skin may consequently feel dry, tight or uncomfortable, and its surface properties can change. Barrier disruption can also affect how readily substances move through the skin.
The skin is capable of repairing itself after barrier disruption. This involves a number of biological processes, including changes in lipid production and organisation within the stratum corneum. (Cosmetic Formulation: Principles and Practice, 2019, Heather A. E. Benson, Michael S. Roberts, Vânia Rodrigues Leite-Silva and Kenneth A. Walters, eds.)
For cosmetic formulation, this is an important consideration: the condition of the skin influences how a topical product interacts with it.
What is TEWL?
One of the most commonly used ways of evaluating skin barrier function is transepidermal water loss, or TEWL.
TEWL measures the amount of water vapour passing through the skin. In general, lower TEWL is associated with an intact barrier, while increased TEWL can indicate barrier disruption.
This makes TEWL particularly useful in cosmetic research and product testing when the effect of a formulation on the skin barrier needs to be evaluated. (Cosmetic Formulation: Principles and Practice, 2019, Heather A. E. Benson, Michael S. Roberts, Vânia Rodrigues Leite-Silva and Kenneth A. Walters, eds.)
However, TEWL measurements are sensitive to external conditions. Temperature, humidity, body location and measurement technique can all influence the result. Proper testing therefore requires controlled conditions rather than simply taking a measurement and interpreting it in isolation.
How do moisturizers support the skin?
This is where cosmetic formulation becomes particularly interesting.
A moisturizer does not simply “put water into the skin.” Different types of ingredients can help maintain or improve the hydration of the stratum corneum in different ways.
Humectants attract and retain water. Glycerin is a familiar example.
Emollients help improve the feel and appearance of the skin and can contribute to the smoothness of the skin surface.
Occlusive ingredients help reduce water loss by forming a film over the skin. Oils and petrolatum are examples of materials that can have an occlusive effect.
These approaches can be combined in the same formulation to create a desired moisturizing effect. (Cosmetic Formulation: Principles and Practice, 2019, Heather A. E. Benson, Michael S. Roberts, Vânia Rodrigues Leite-Silva and Kenneth A. Walters, eds.)
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Why does the formulation matter?
This is perhaps the most important point from a cosmetic development perspective.
The ingredient is only one part of the product. The formulation around it matters too.
The same moisturizing ingredient can behave differently depending on the formulation in which it is incorporated. The vehicle can influence how an ingredient interacts with the skin, how the product feels and how long the desired effect lasts.
For example, the moisturizing performance of ingredients such as urea can vary depending on the formulation used to deliver them. (Cosmetic Formulation: Principles and Practice, 2019, Heather A. E. Benson, Michael S. Roberts, Vânia Rodrigues Leite-Silva and Kenneth A. Walters, eds.)
This illustrates a broader principle of cosmetic science:
A good ingredient does not automatically make a good product.
The ingredient, formulation, stability and intended use all need to work together.
Can skincare products affect the skin barrier?
Yes. Cosmetic products interact directly with the outer layers of the skin, so their formulation matters.
Cleansers are a good example. Their purpose is to remove substances from the skin, but cleansing ingredients also interact with the stratum corneum. The formulation and choice of cleansing agents therefore matter, particularly when a product is intended for frequent use.
Moisturizers work in the opposite direction, helping to maintain hydration and reduce water loss.
The goal of formulation is therefore not simply to add as many “good” ingredients as possible. It is to create the right balance for the intended product and its use.
How can cosmetic products be tested?
If a product is designed to improve skin hydration or support barrier function, its performance can be evaluated using appropriate testing methods.
TEWL is one important measurement for barrier function. Other non-invasive techniques can be used to assess skin hydration and other properties of the skin.
This distinction is important for cosmetic development. A product may feel moisturizing, but a measurable cosmetic claim should be supported by an appropriate testing approach.
Objective measurements can help formulators understand whether a finished formulation is actually producing the intended effect.
Why skin barrier science matters in formulation
The skin barrier is a good example of why cosmetic product development involves much more than selecting attractive ingredients.
A successful formulation needs to take into account the skin, the ingredients, the vehicle, stability, sensory properties and the intended use of the finished product.
At A&T Formulation, this is why we approach skincare development as formulation development rather than simply ingredient selection. The objective is to create a complete product in which the ingredients and formulation system work together to deliver the intended cosmetic experience and performance.
The key takeaway
The skin barrier is a sophisticated protective system that helps control water loss and the movement of substances through the skin.
Its condition affects hydration, skin feel and permeability. For cosmetic products, this means that what you put into a formula matters — but how you formulate it matters just as much.
A well-designed skincare product starts with understanding both the ingredients and the skin they are designed to interact with.
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