Hydrolipidic film is an invisible, oil-water covering that protects the skin's surface. This film covers the entire body and is continuously renewed. Its pH is slightly acidic, ranging between 4.2 and 6.7. This film helps to prevent harmful bacterial colonisation of the skin and the body.
According to the 500 Dalton rule, where Daltons are a measure of the size of a molecule, studies have shown that any molecules large than 500 Daltons will not penetrate the skin's defensive barrier (1), the molecular weight of a compound must be less than 500 Dalton to allow skin absorption at the level of the stratum corneum (the outermost layer of the skin) Larger molecules cannot pass through the corneal layer. Our stratum corneum consists of three major lipids, namely ceramide, free fatty acid, and cholesterol that comprise approximately 50%, 25%, and 25% of the total lipid mass, respectively. The optimal composition of SC lipids is the vital epidermal barrier function of the skin. (2)
The active ingredients in any skincare product, therefore, must wholly bind to the stratum corneum by dissolving in its surface hydrolipidic film. After that, the active ingredients must pass either from cell to cell or between cells or (and usually) both.
This also shows why you have to be careful what you put where. On the eyelids, where the skin layers are very thin, then powerful ingredients will pass quickly and may harm the eyes (so we have special products for the eye area). If the cream is for the heels, then products can be heavier as it is the outer layers of the skin that you are aiming for. Other factors include how well your skin is hydrated before any application of any treatment and how often a product needs to be applied. Masks, for an example, work by basically, sitting on top of the skin and flooding it with the active ingredients.
As a summary, unless you have a specific skin type or condition that requires specific treatments or skin care product types (such as. for example, acne, dry skin or rosacea) select gentle skin care cosmetics that contain ingredients that support daily barrier reconstruction as well as being rich in emollients. Emollients, that is, moisturisers, form a protective layer on the surface, retaining water in the skin and protecting the skin. Avoid long baths in hot water - short and lukewarm baths or showers will be a better choice for the complexion. Use good moisturisers that help maintain this hydration and protect the hydrolipidic barrier from damage.
Examples of ingredients that help take care of the hydrolipid barrier are Ceramides, Cholesterol, Hyaluronic Acid, Squalane, Niacinamide, oils and butters such as Perilla Oil and Shea Butter.
(1) De Luca C, Valacchi G. Surface lipids as multifunctional mediators of skin responses to environmental stimuli. Mediators Inflamm. 2010;2010:321494. doi: 10.1155/2010/321494. Epub 2010 Oct 20. PMID: 20981292; PMCID: PMC2963132.
(2) Mohd Ariffin NH, Hasham R. Assessment of non-invasive techniques and herbal-based products on dermatological physiology and intercellular lipid properties. Heliyon. 2020 May 25;6(5):e03955. doi: 10.1016/j.heliyon.2020.e03955. PMID: 32478187; PMCID: PMC7251381.
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