The Corneotherapeutic Benefit of Glycerine
Feb 13, 2016
The term "Corneotherapy" was originally coined by Albert Kligman, who is referred to as the Grandfather of Corneotherapy. It was not until the latter half of the 20th century that the stratum corneum began to be viewed as much more than a dead, inert passive membrane. Dr Kligman discovered that simple emollients and moisturisers could treat common skin conditions. This post is in honour of Dr Kligman and the research he did on helping us understand how the skin works.
Glycerine or glycerol is currently demonised by many cosmetic brands on the market. Many of the companies who state that it is a "bad" ingredient and that it should be avoided do not understand how glycerine works. Glycerine does far more than just moisturise. Ironically Albert Kligman endorsed the use of glycerine as a corneotherapeutic agent and yet the current corneotherapeutic association states it should not be used. I have yet still to see one piece of research validating this considering the mountain of evidence supporting the use of glycerine.
The most important pharmacological properties of glycerol/glycerine are:
- Hydration of the SC, especially in emulsion systems [1, 3]
- Hygroscopicity [1, 2]
- Keratolytical effect by desmosome degradation [4]
- Smoothing effects [3, 5, 6]
- Protective function in emulsion-systems against irritations [7]
- Glycerol acts as a moisturizer by absorbing water. Glycerol, by its hygroscopic property, is able to bind water and to lower water evaporation [8]
Glycerol is well known to the cosmetic industry as a humectant that can take up 3 times its weight from a water-saturated atmosphere. It has been used extensively in dermatology practice to improve the condition of dry skin.Groundbreaking research done by Hara and Verkman has proved conclusively that glycerol is a major determinant of stratum corneum water retention and has other beneficial effects on stratum corneum biophysical properties [9]. Hara and Verkman studied aquaporin-deficient mice. The horny layer of these mice had multiple defects including increased TEWL, decreased hydration and poor elasticity. After topical applications of glycerol, these deficiencies were completely corrected, leading to a restoration of the skins normal level of watertightness. This study also found that glycerol enhanced the bio-synthesis of the physiologic lipids that are responsible for barrier function. Glycerol accelerates barrier repair. This paper showed that it is glycerol permeability, rather than water, that is important to keep skin watertight. Hara and Varkman also showed that Glycerol results in a doubling of the skin's elasticity.
Glycerine/glycerol also regulates the orderly shedding of corneocytes at the surface, thus keeping the horny layer at a steady state of thickness during desquamation. It accomplishes this by enhancing proteolytic activity and promoting the dissolution of corneodesmosomes near the surface, which are responsible for the cohesion of corneocytes below the surface. [10]
It is now firmly established that immature fragile corneocytes inevitably signify an impaired barrier [12]. Conditions such as psoriasis and atopic dermatitis have been found to have immature corneocytes in their histology. Hirao et al. in Japan have now demonstrated that cornified envelopes are critical elements in the construction of the barrier and in mediating its functions [11]. Glycerol also has effects on cornified envelopes. Glycerol has been shown to help differentiate the corneocytes properly.
If corneocytes are incubated ex vivo in a humidified environment for a few days, there is a rapid maturation of the corneocyte envelopes. The use of a moisturiser on the facial skin achieves the same degree of maturation of corneocytes. Using a moisturiser eliminates immature corneocyte envelopes, associated with improved barrier functions, such as decreased TEWL [13]. These findings are consistent with those of Rawlings and collaborators who showed that fragile corneocyte envelopes, which are increased in dry skin conditions, are converted into mature, rigid corneocytes by the application of moisturizers to the face [14].
Glycerol has been found to accelerate the barrier recovery of a damaged stratum corneum.
Two studies were performed to evaluate the influence of glycerol on the recovery of damaged stratum corneum barrier function. Measurements of transepidermal water loss and capacitance were conducted in a 3-day follow-up after tape stripping (study 1) and a 7-day follow-up after a barrier damage due to a repeated washing with sodium lauryl sulphate.
Glycerol creates a stimulus for barrier repair and improves the stratum corneum hydration; stratum corneum hydration is not strictly related to barrier homeostasis and can be optimized by different mechanisms and pathways. The observed effects were based on the modulation of barrier repair and were not biased by the humectant effect of glycerol. As the glycerol-induced recovery of barrier function and stratum corneum hydration were observed even 7 days after the end of treatment, glycerol can be regarded as a barrier stabilizing and moisturizing compound. Glycerol influences the skin surface mechanical properties by plasticizing the stratum corneum and inducing smoothing effect [16].
We are exposed to a wide range of environmental conditions. One of the key functions of the stratum corneum is to prevent uncontrolled moisture loss. Glycerol penetrates the skin and creates a ‘reservoir’ in the depth of the stratum corneum within the lipid bilayers without disruption of liquid crystallinity ⁄ lamellar structure. The lipid bilayer is made up of different proportions of saturated and unsaturated fatty acids. It forms what is known as liquid crystalline structure. The balance between the liquid crystalline and solid crystal phases is determined by the degree of fatty acid unsaturation, the amount of water and other factors. So during hot weather it is much more fluid but in cold weather it freezes and becomes less flexible. When it is cold there is rapid water loss from the skin due to the lipid bilayer losing its fluidity. It can no longer move to fill in gaps in the skin. The result is massive loss of mositure from the skin. Glycerol has the ability to prevent what is known as humidity-induced crystal phase transitions [2]. It improves the barrier function of the skin. Glycerol does not act as a humectant at low humidity. It keeps the lipid bilayer fluid.
The video below illustrates the fluidity of the lipid bilayer and how it moves to fill in the gaps.
[video width="1920" height="1080" mp4="http://www.educatedtherapists.com/wp-content/uploads/2016/02/lipid-bilayer.mp4"][/video]
There is a dose dependency relationship with the hydrating abilities of glycerine. The maximum benefit is obtained at concentrations of 20–40% glycerol in a formula [22]. It is ONLY application of undiluted glycerol results in dehydration of the skin, based on osmotic water extraction from the SC [23]
The SC water content in a healthy skin is around 20–30% by weight [14]. The SC needs to be hydrated to maintain its integrity. SC hydration variations can influence the SC barrier function [15]. Glycerol prevents damaging effect on the SC. Glycerol pre treatment decreases irritancy caused by alkali solution (e.g., sodium hydroxide), dimethyl sulfoxide, and sodium lauryl sulphate (SLS) [15]. Glycerol leads to a more rapid reconstitution of the protective skin barrier following mechanical (tape stripping) or chemical (repeated SLS application, acetone) damage. It can absorb water and thus creating water flux in the SC which may lead to a stimulus for barrier repair [17]. The beneficial effect of glycerol on skin barrier function, disrupted by acute chemical, acts through the increase of water holding capacity [18].
Froebe et al. [19] showed, from an in vitro experiment, that by glycerol adding to the SC lipids, the transition of the lipophilic components from the liquid crystalline phase to the solid crystalline phase can be prevented at low relative humidity. Glycerol can interact with polar head groups of the lipid bilayers rather than by penetrating the alkyl chains. Consequently, maintaining the fluidity of the lipid membrane improves skin conditions in dry climates. Thus, glycerol decreases SC permeability to water but enhances SC barrier function. Glycerol-containing moisturizers continue to improve barrier function for at least a week after cessation of treatment [20].
Glycerol does not act as a humectant at low humidity.
Glycerine can also be used to prevent skin barrier disruption by surfactants. The addition of 10% glycerol to an aqueous solution of SLS prevents the skin barrier perturbation in vitro. It does this by reducing the skin aqueous pore radius and the aqueous pore number density [21]. Glycerol present in the SC is able to bind water in the SC and thus reduce the mobility of water. The glycerol-hydrating property occurs not only on healthy skin but also on skin affected by xerosis.
Glycerol seems to have keratolytic properties. It does this by increasing the activation of SC protease activity. This occurs simply by elevated water activity. This protease is responsible of the regulation of corneocytes desquamation, resulting in more efficient reduction in SC thickness. The desmosome degradation is essential to maintain a healthy skin which requires an equilibrium between degradation and synthesis of desmosome. Furthermore, glycerols desmolytic effect, demonstrated by Rawlings et al. [22], causes a decrease in intraepidermal pressure on the bilamellar intercellular lipids and, therefore, indirectly causes an increase in the liquid crystalline state lipids. The dry skin scaliness is thus reduced, and the SC barrier is maintained in xerotic skin
In conclusion, there is a vast amount of research supporting the use of Glycerine in the treatment of dry skin and as an adjunct for healthy skin. Glycerol does not just work as a humectant and has many valuable skin benefits. The demonisation of this ingredient does not make any sense given the current research.
About the Author
Jacine Greenwood is an internationally recognised educator who is known within the industry for her up to date knowledge and her ability to deliver training in an easy to understand method.
Jacine holds 4 Diplomas and a Bachelor of Nursing and her knowledge is well respected by her peers. With over 19 years experience in the industry and a background of cosmetic formulation, Jacine has an immense knowledge of current trends in research and new developments in the industry.
Jacine has been continually educating herself in all aspects of skin function and cosmetic chemistry for the past 21 years. Jacine’s knowledge is current and has a vast knowledge of the active ingredients that are being released onto the market.
References:
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Froebe CL, Simion AF, Ohlmeyer H, Rhein LD, Mattai J, Cagan RH, et al. Prevention of stratum corneum lipid phase transitions in vitro by glycerol - an alternative mechanism for skin moisturization. J Soc Cosm Chem 1990; 41: 51 -65.
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