A Scientific Look at Moisturizers Pt. 2

Emollients and Occlusives

Emollients

Emollients, which are mainly lipids and oils, hydrate and improve the appearance of the skin by contributing to skin softness, enhanced flexibility, and smoothness. The “skin slip” or lubricity of some moisturizers, contributes to consumer satisfaction and product preference.5 Consumers desire smooth skin following moisturizer application. Emollients serve to fill the cracks between clusters of desquamating corneocytes and are not usually occlusive unless applied heavily.

Long chain saturated fatty acids and fatty alcohols are commonly used in topical pharmaceuticals and cosmetic formulations. They exert their benefits through effects on the skin barrier, partially through improved repair, and on permeability. Examples include stearic, linoleic, linolenic, oleic, and lauric, which can be found in palm oil, coconut oil, and wool fat. A sterol-enriched fraction from canola oil reduced clinical signs of sodium lauryl sulphate (SLS)-induced irritation. Other lipids (e.g., fish oil, petrolatum, shea butter, and sunflower seed oil) had no effect on the degree of irritation. Loden and Andersson suggested that canola oil assisted the skin in supplying the damaged barrier with adequate lipids. Essential fatty acids (i.e., linoleic and alpha-linoleic acids) influence skin physiology and pathology via their effects on skin barrier functions, eicosanoid production, membrane fluidity, and cell signaling.

Occlusives

Occlusives reduce TEWL by creating a hydrophobic barrier over the skin and contributing to the matrix between corneocytes, and have the most pronounced effect when applied to slightly dampened skin. There is a wide range of agents with occlusive properties. Their main limitations include odor, potential allergenicity, and the greasy feel associated with most occlusives.

Astringent Emollients
Cyclomethicone, dimethicone, isopropyl myristate, octyl octanoate

Dry Emollients
Decyl oleate, isopropyl palmitate, isostearyl alcohol

Fatting Emollients
Castor oil, glyceryl stearate, jojoba oil, octyl stearate, propylene glycol

Protective Emollients
Diisopropyl dilinoleate, isopropyl isostearate

Protein Rejuvenators
Collagen, elastin, keratin

Common substances with emollient properties
Fatty Acids
Lanolin acid, stearic acid
Fatty Alcohols
Cetyl alcohol, lanolin alcohol, stearyl alcohol
Hydrocarbon Oils/ Waxes
Caprylic/capric triglyderide, mineral oil, paraffin, petrolatum, silicone derivatives (cyclomethicone, dimethicone), squalene

Common substances with occlusive properties

Petroleum jelly, in a minimum concentration of 5%, reduces TEWL by more than 98% and is the most effective occlusive, followed by lanolin, mineral oil, and silicones (e.g., dimethicone), which only reduce TEWL by 20%-30%. Occlusives are thought to diffuse into the intercellular lipid domains, thus contributing to their efficacy. Petrolatum is widely used as a classic moisturizer. Lanolin, a complex structure of esters, diesters, and hydroxyesters of high molecular weight, lanolin alcohols, and lanolin acids, is also widely used and quite effective.

Humectants

Humectants are able to attract water from two sources: they enhance water absorption from the dermis into the epidermis, and in humid conditions they also help the SC to absorb water from the external environment. Many humectants also have emollient properties. The most effective humectant is the trihydroxylated molecule, glycerol. Immature corneocytes are fragile but mature into more resilient and protective cells as they migrate through the SC. Glycerol hastens the maturity of corneocytes through the activation of residual transglutaminase activity in the SC.18 Also, by facilitating the digestion of desmosomes and subsequently enhancing desquamation, glycerol reduces the scaling associated with xerosis.

Gelatin
Glycerin
Honey
Hyaluronic acid
Panthenol
Propylene glycol
Sodium and ammonium lactate
Sodium pyrrolidine carboxylic acid
Sorbital
Urea

Common substances with humectant properties
Found in the NMF, pyrrolidine carboxylic acid hydrates the skin, and has been shown to improve xerosis.20 Urea is another important humectant. In double-blind studies moisturizers with urea have been shown to reduce TEWL in atopic and ichthyotic patients,21,22 and reduce SLS-induced skin irritation.

Alpha hydroxy acids (e.g., lactate) are effective agents for the treatment of dry skin; following treatment with lotions containing D-, L-lactic acid, the SC prevents xerosis more effectively. Lactic acid, particularly the L-isomer, stimulates ceramide biosynthesis leading to higher SC ceramide levels that result in a superior lipid barrier and more effective resistance against xerosis.

One major drawback of humectants is that some of them can increase TEWL3 by enhancing water absorption from the dermis into the epidermis where it can then be lost into the environment. For this reason, they are almost always combined with an occlusive agent. Occlusive and humectant ingredients work together to enhance epidermal hydration and barrier function.