Moisturizers are a huge part of any basic skincare routine. They range from the bland to the luxurious, the anti-acne to the anti-aging, the drug-store to the exclusive – but what do they actually do? Here we explain the three main groups of moisturizers – Emollients, Occlusives, and Humectants.
What Are Moisturisers?
The term ‘moisturizer’ was created by marketers to suggest a cream that increases skin moisture. It is not a scientific definition but is often used interchangeably with the Latin term ‘emollient’, meaning ‘a material designed to soften the skin’ , even though not all moisturizers are emollients.
Moisturizers are a key part of any skincare routine, especially for individuals who have an altered skin barrier function and reduced skin hydration. Contrary to popular belief, skin moisturization is essential for all skin types – including oily and acne-prone skin.
The Epidermis & Stratum Corneum Barrier Function.
The skin is the largest organ of the human body and acts as our first line of defense against various environmental pollutants. It is responsible for preventing excess water loss from the body and is made up of three main layers – the epidermis, the dermis, and the subcutaneous layer (or hypodermis) .
The epidermis is the outermost layer of the skin and is responsible for its barrier function. It is made up of four layers – the basal layer, the spinous layer, the granular layer, and the stratum corneum (or cornified layer).
Epidermal skin cells (keratinocytes) begin their life attached to a basal membrane in the basal layer. Here they multiply rapidly before detaching from the membrane and moving outward through the spinous layer and the granular layer until they undergo cornification (a programmed cell death that transforms keratinocytes into flatter corneocytes) and are eventually shed by the stratum corneum .
As the skin cells migrate through the layers of the epidermis, they release lipids into the spaces between them. Because of this, the stratum corneum is often said to have a ‘brick-and-mortar’ structure, where the corneocytes are the bricks and the lipids are the mortar-like substance that holds them together. Together, the corneocytes and lipids form a water-repellent barrier .
The corneocyte ‘bricks’ contain intracellular humectants (natural moisturizing factors/NMFs) that are essential for barrier function, skin hydration, and skin cell turnover. The lipid ‘mortar’ is arranged into lamella layers consisting of cholesterol, free fatty acids, ceramides, and sterol/wax esters. This lipid matrix is crucial for the prevention of transepidermal water loss (TEWL) .
For this reason, skin hydration is largely dependent on the presence of NMFs and the structure and composition of the lipid matrix.
NMFs represent 5-30% of the stratum corneum and consist of amino acids and their derivatives (e.g. lactic acid, urea, and sugars), while the lipid matrix is made up of ceramides (45-50% – by dry weight), cholesterol (25%), and free fatty acids (10-15%) .
How Do Moisturisers Work?
It was originally believed that moisturizers worked by preventing TEWL through occlusion (i.e. creating a layer on the skin that traps water inside). However, after the discovery that lipids (particularly ceramides) play a vital role in skin hydration , it is now known that there are multiple mechanisms at play.
These mechanisms include:
- Repairing the skin’s barrier
- Increasing water content
- Reducing TEWL
- Restoring the lipid barriers ability to attract, hold, and redistribute water .
There are three main groups of moisturizers – Emollients, Occlusives, and Humectants.
Emollients soften, smoothen, and condition skin by filling in the gaps between cells/skin flakes with droplets of oil. If applied heavily, they can also act as occlusive agents. Emollients are mainly oils and lipids, particularly lipids that are naturally present in the stratum corneum . This can help replenish depleted lipids and is particularly effective when ceramides, cholesterols, and fatty acids are combined together .
In fact, applying only one or two of these lipids to dehydrated skin may actually delay skin barrier recovery . Specifically, they need to be combined in a ratio of between 1:1:1 to 3:1:1 (ceramides: cholesterol: fatty acids) for maximum skin hydration and barrier repair . One study found that a topical cream that combined all three lipids was able to improve skin hydration within 30minutes of application .
Other popular emollients include aloe vera, shea butter, linoleic acid, oleic acid, stearic acid, and cetyl alcohol. In addition, some oils, such as mineral oil and jojoba oil, work as both emollients and occlusives.
Examples of Emollients:
- Astringent Emollients:
- Isopropyl Myristate
- Octyl Octanoate
- Dry Emollients:
- Decyl Oleate
- Isopropyl Palmitate
- Isostearyl Alcohol
- Fatty Emollients:
- Propylene Glycol
- Octyl Stearate
- Glyceryl Stearate
- Jojoba Oil
- Castor Oil
- Protective Emollients:
- Isopropyl Isostearate
- Diisopropyl Dilinoleate
Occlusives create a barrier on top of the skin in order to reduce/physically block TEWL and are most effective when applied to wet skin . The most widely-used and effective occlusive agent is petroleum jelly (a.k.a. Vaseline) which can reduce TEWL by more than 98% when used at a minimum concentration of 5% . The next best occlusive agents are lanolin, mineral oil, and silicones (e.g. dimethicone). However, these only reduce TEWL by 20-30% .
While occlusive agents are effective at reducing TEWL, they often have a very greasy consistency which makes them less cosmetically appealing and potentially comedogenic.
List of Occlusive Agents:
- Petroleum Jelly
- Mineral Oil
- Caprylic/Capric Triglyceride
- Fatty Acids:
- Stearic Acid
- Lanolin Acid
- Fatty Alcohols:
- Cetyl Alcohol
- Stearyl Alcohol
- Polyhydric Alcohols:
- Propylene Glycol
- Vegetable Waxes:
- Wax Esters:
- Stearyl Stearate
Humectants are ‘hygroscopic’ which means that they attract water into the epidermis from the dermis and/or from humid environmental conditions . In addition, many humectants also have emollient properties .
Due to the fact that humectants draw water from the dermis, excessive use may exacerbate dry skin conditions . Glycerol is considered to be one of the most effective humectants , although hyaluronic acid is probably the most well-known. As well as acting as a humectant by drawing water from the dermis into the epidermis, hyaluronic acid is able to form a film on the skin to reduce TEWL .
As mentioned earlier, NMFs are made up of amino acids and their derivatives (e.g. lactic acid, urea, and sugars). These are low-molecular-weight hygroscopic substances that are important for skin hydration. For this reason, these humectants are often included in moisturizers .
Urea and lactic acid are hydrating humectants at low concentrations, however, at higher concentrations they act as chemical exfoliants and increase skin cell turnover .
Examples of Humectants:
- Hyaluronic Acid
- Propylene Glycol
- Butylene Glycol
- Hydroxy Acids:
- Glycolic Acid
- Lactic Acid
- Sodium Pyrrolidine Carboxylic Acid (Sodium PCA)
Emollients, Occlusives, and Humectants in Combination.
The majority of moisturizers contain some combination of emollients, occlusives, and humectants. The combination of occlusive and humectants can enhance the water-holding capacity of the skin and some emollients may improve or stabilize a moisturizers active ingredients .
There are two common types of delivery-system used for moisturizers – lotions and creams.
Lotions and creams are both referred to as ‘emulsions’ due to their two-phase system containing two immiscible (non-mixing) liquids – oil and water .
Lotions are oil-in-water emulsions, meaning that oil droplets are dispersed in water, whereas creams are water-in-oil emulsions, meaning that water droplets are dispersed in oil . Lotions tend to be thinner and more ideal for daytime use, whereas creams tend to be made with heavier lipids and are more ideal for use at night.
The stratum corneum barrier function and skin hydration are largely dependent on the presence of natural moisturizing factors and the structure and composition of the lipid matrix.
The ideal moisturizer should repair the skin’s barrier, increase water content, reduce transepidermal water loss and restore the lipid barriers ability to attract, hold, and redistribute water.
There are three main types of moisturizing agents – emollients, occlusives, and humectants.
Emollients smooth skin by filling in the gaps between corneocytes and are usually made up of oils and lipids that are naturally present in the stratum corneum.
Occlusives create a barrier on top of the skin which traps the water inside the skin and prevents it from evaporating.
Humectants draw moisture from the dermis into the epidermis and/or from the environment if there is adequate humidity.
These moisturizing agents are often used in combination in either lotions (oil-in-water emulsions) or creams (water-in-oil emulsions) for increased skin-hydrating effects.
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