Human synthetic sebum formulation and stability under conditions of use and storage

The human skin surface and hair are generally coated with a thin film of liquid phase sebaceous lipids. This surface lipid film contributes to the cosmetic properties of the skin. Synthetic sebum has been used for studies on properties of skin and hair. However, there has been no standardized formulation of synthetic sebum and many of the synthetic sebum formulations that have been used do not closely resemble actual sebum. In this study, a formulation for a standardized and inexpensive synthetic sebum is proposed, and the chemical stability of this lipid mixture is demonstrated under conditions of use and storage. The proposed synthetic sebum consists of 17% fatty acid, 44.7% triglyceride, 25% wax monoester (jojoba oil) and 12.4% squalene. This lipid mixture takes up approximately 6% of its weight in water when equilibrated in an atmosphere saturated with water vapour. It is stable on exposure to the atmosphere at 32°C for at least 48 h, and it is also stable on storage at 4 or −20°C, either dry or in chloroform : methanol solution for at least 6 months. This synthetic sebum could be useful in studies on cosmetic properties of the skin surface or hair, on penetration of chemicals into the skin or in development of standardized tests of laundry detergent performance.     

Formulation and stability of a novel artificial sebum under conditions of storage and use

Materials in contact with liquids on the human skin surface may dissolve and permeate into skin. Release and permeation of chemicals in contact with skin is often estimated in vitro using artificial skin liquids, although sebum lipids are generally not included in these models. The purposes of this research were to develop a representative artificial sebum that contains the appropriate types of lipids at levels that match human values and quantitatively characterize the model to understand its utility for in vitro testing. Artificial sebum that consisted of 10 lipids at proportions that closely resembled human sebum was characterized using thin layer chromatography under a variety of storage and use conditions (dry and liquid, 4°C and 32°C, with and without vitamin E) for 28 days. Levels of sebum constituents maintained in solution and dry at 4°C were stable through the duration of the test period. Levels of all sebum lipids maintained dry at 32°C were stable in the presence of vitamin E; however, squalene oxidized rapidly in the absence of vitamin E. Liquids on the human skin surface consist of sebum and sweat with minor amounts of cellular debris and intercellular lipid from the stratum corneum. The relative importance of each component for release of chemicals from materials in contact with skin will depend upon the type of material (metal, organic, etc.). A model artificial sebum was formulated and characterized to aid researchers in understanding potential release of chemicals from materials in contact with skin and subsequent partitioning and absorption.     

Skin lipids

Concepts regarding the structure and function of the stratum corneum (SC) have changed from that of a tough film of loosely adherent cells to that of a two-compartment system of protein-enriched cells embedded in intercellular lipids. The two-compartment arrangement enlarged the role of epidermal lipids from that of the plasticizing component of the SC to that of the epidermal barrier governing water-holding properties as well as take-up of water, the differences in permeability of topically applied lipophilic and hydrophilic agents as well as cohesion and desquamation of the SC. Barrier properties of SC are dependent largely on the intactness of the lipid lamellae that surround the corneocytes. The pliability of the SC depends on a correct balance of lipids, hygroscopic water-soluble substances and water.
Mammalian differentiation involves characteristic changes in lipid composition consistent with the requirements for waterproofing. There is a progressive depletion of phospholipids and glycosphingolipids with enrichment in ceramides, cholesterol, free fatty acids, and small amounts of other polar (e.g. cholesterol sulphate) and non-polar species (e.g. hydrocarbons, cholesterol esters, triglycerides). The sphingolipids account for the most lipid by weight and are presumed to be of major importance for the water-retaining properties of the epidermal barrier. Decrease of sphingolipid content occurs in aged skin. Deficiency of essential fatty acids leads to enhanced transdermal water transport in addition to dryness and scaliness of the skin. A mixture of both saturated and unsaturated fatty acids produces the optimal barrier to water loss from the SC. The balance between solid crystal and liquid crystalline phases in epidermal lipids is determined by the degree of fatty acid unsaturation and the amount of water.     

Infrared spectroscopy of the skin: influencing the stratum corneum with cosmetic products

The stratum corneum is the outermost layer of the skin and, as such, represents the decisive barrier between the body and the environment. The combination of horny cells and lamellar lipid layers prevents water loss from the body and protects it against penetration by exogenous substances. For selective investigation of the thin outermost stratum corneum layer ATR-FTIR (attenuated total reflection Fourier transform infrared) spectroscopy has proved itself in practice. It provides information about the sebum content, type of fatty substances, water content and degree of order of the lamellar lipid film in the stratum corneum. Different types of skin (dry, normal and greasy) differ mainly in their sebum content but also in the composition of their fats and the degree of order of their lipids. A comparison with model lipid mixtures shows that the stratum corneum lipids are mainly present in a fixed gel form under physiological conditions. In cleansing processes a large part of the sebum is removed from the skin. The skin reacts to the defatting process by the rapid secretion of sebum. After cleansing with a mild face cleansing emulsion the re-achievement of the same fat status takes longer than with a surfactant shower gel. Skin creams cause alterations to the skin moisture and in the degree of order of the stratum corneum lipids. A lamellar cream with a structure similar to that of the stratum corneum lipids increases the degree of order of the alkyl chains of the skin lipid film (biomimetic principle), whereas a conventional w/o cream reduces this degree of order. Skin moisture increases after the use of the creams.     

Age‐related changes in skin barrier function – Quantitative evaluation of 150 female subjects

The protection against water loss and the prevention of substances and bacteria penetrating into the body rank as the most important functions of the skin. This so‐called ‘skin barrier function’ is the natural frontier between the inner organism and the environment, and is primarily formed by the epidermis. An impairment of the skin barrier function is often found in diseased and damaged skin. An influence of ageing on skin barrier function is widely accepted, but has not been conclusively evaluated yet. Therefore, the aim of this clinical study was to assess the potential influence of ageing on skin barrier function, including transepidermal water loss (TEWL), stratum corneum hydration, sebum content and pH value. One hundred and fifty healthy women aged 18–80, divided into five age groups with 30 subjects each, were evaluated in this study. TEWL, hydration level, sebum secretion and pH value of hydro‐lipid acid film were measured with worldwide acknowledged biophysical measuring methods at cheek, neck, décolleté, volar forearm and dorsum of hand. Whereas TEWL and stratum corneum hydration showed only very low correlation with subject's age, the sebum production decreased significantly with age, resulting in the lowest skin surface lipids levels measured in subjects older than 70 years. The highest skin surface pH was measured in subjects between 50 and 60 years, whereas the eldest age group had the lowest mean pH. The dorsum of the hand was the location with the highest TEWL and lowest stratum corneum hydration in all age groups. The results show that only some parameters related to skin barrier function are influenced by ageing. Whereas sebum production decreases significantly over lifetime and skin surface pH is significantly increased in menopausal woman, TEWL and stratum corneum hydration show only minor variations with ageing.     

Serum levels of IGF‐1 are related to human skin characteristics including the conspicuousness of facial pores

Conspicuous facial pores are one type of serious aesthetic defects for many women. However, the mechanism(s) that underlie the conspicuousness of facial pores remains unclear. We previously characterized the epidermal architecture around facial pores that correlates with the appearance of those pores in various ethnic groups including Japanese. The goal of this study was to evaluate the possible relationships between facial pore size, the severity of impairment of epidermal architecture around facial pores and sebum output levels to investigate the possible role of IGF‐1 in the pathogenesis of conspicuous facial pores. The subjects consisted of 38 healthy Japanese women (aged 22–41 years). IGF‐1 was measured using immunoradiometric assay. Surface replicas were collected to compare pore sizes of cheek skin and horizontal cross‐section images of cheek skin were obtained non‐invasively from the same subjects using in vivo confocal laser scanning microscopy and the severity of impairment of epidermal architecture around facial pores was determined. The skin surface lipids of each subject were collected from their cheeks and lipid classes were determined using gas chromatography/flame ionization detection. The serum level of IGF‐1 correlated significantly with total pore area (R = 0.36, P < 0.05), with the severity of impairment of epidermal architecture around facial pores (R = 0.43, P < 0.05) and with sebum output levels (R = 0.41, P < 0.01). The sebum output levels correlated with total pore area (R = 0.32, P < 0.05). Our study found that serum levels of IGF‐1 are correlated with facial skin characteristics including facial pore size and with the severity of impairment of epidermal architecture around facial pores.     

Epidermal tight junction: The master skin barrier regulator

IFSCC Magazine , 12 (2009) (2) 87–91
Tight junctions between adjacent epithelial cells control paracellular permeability of solutes and maintain cell polarity by fencing lipid bilayer components into demarcated apical and basolateral domains. In the present study we investigated the role of epidermal tight junctions in two skin physiological processes, 'formation of the epidermal calcium ion (Ca²⁺) gradient' and 'polarized lamellar body secretion'. The former is based on the fact that the epidermal Ca2+ gradient is closely related to epidermal differentiation and the latter on the knowledge that polarized lamellar body secretion is essential to supply intercellular lipids to the stratum corneum. We hypothesized that tight junctions might form the Ca²⁺ gradient in the epidermis by sealing cell-cell contact at the stratum granulosum and allow the lamellar bodies to be normally secreted toward the stratum corneum by giving granular cells polarity. In our experiments we discovered that tight junctions control not only both intercellular Ca²⁺ permeability and fluorescent ceramide analog (Cer-FL) secretion in cultured normal human keratinocyte cells but also both intercellular calcium distribution and polarized lamellar body secretion in the skin equivalent. Thus, tight junctions should be responsible for linking a series of processes from epidermal differentiation to stratum corneum barrier formation. Our findings suggest that tight junctions should have a crucial role not only in epidermal barrier function but also in both epidermal differentiation and stratum corneum barrier function.
Keywords: Calcium gradient, cell polarity, lamellar body, skin barrier, tight junction
IFSCC Basic Research Award winning paper presented at the 25th IFSCC Congress 2008, Barcelona, Spain     

Skin efficacy of liposomes composed of internal wool lipids rich in ceramides

Ceramides from intercellular lipids of skin stratum corneum are known to play an essential role in maintaining and structuring the lipid barrier of the skin. Internal wool lipids (IWL), which are also rich in ceramides, have a composition similar to that of the stratum corneum lipids. IWL extracted with chloroform/methanol azeotrope at the laboratory scale have been shown to be capable of forming liposomes with a stable bilayer structure. Furthermore, topical application of these IWL liposomes on intact and compromised skin has been demonstrated to improve barrier skin properties. In this study we evaluated the effect on human skin repair of different IWL extract compositions obtained by two extraction methodologies. The formation and characteristics of the liposomes prepared were greatly influenced by the IWL composition, primarily the sterol sulfate content. The IWL liposomes improved skin barrier integrity and increased skin hydration when applied onto intact skin. These improvements were slightly enhanced in the case of IWL liposomes that were richer in polar lipids.     

Stratum corneum fatty acids: their critical role in preserving barrier integrity during cleansing

Stratum corneum (SC) bilayer lipids, specifically fatty acids, ceramides and cholesterol, contribute to the permeability barrier function of the skin. Normal skin cleansing is associated with damage to the SC lipids because cleanser surfactants, in addition to providing the desired effect of solubilizing and facilitating the removal of sebum and skin soils, have a propensity to disrupt bilayer lipids by extracting endogenous skin lipids or intercalating into the bilayer. Disrupted SC lipids are associated with a variety of pathological skin conditions, as well as with dry skin induced by harsh cleansing. In an attempt to preserve the barrier and mitigate the damage caused by frequent normal cleansing, the incorporation of physiologically relevant lipids into skin cleansers has become common in leading cleansing products. It has been noted that fatty acids are more susceptible to surfactant‐induced removal than other lipids (eg, ceramides), an observation that may form the basis for a critically important strategy for replenishing SC lipids. This review will focus on the role of fatty acids in the structure and function of the SC, and the rationale for incorporation of stearic acid into moisturizing body cleansers to minimize their extraction by surfactants and replenish lost fatty acids to promote skin barrier preservation.     

J. Cosmet. Sci., 59, 217–224 (May/June 2008) In vivo spectrophotometric evaluation of skin barrier recovery after topical application of soybean phytosterols

The skin's uppermost thin layer, stratum corneum, plays a crucial role in protecting the body against unwanted influences from the environment. Disruption of the stratum corneum, by tape stripping or chemical injury, results in epidermal recovery of the skin barrier. Soy phytosterols are widely used in the cosmetic field as active ingredients in creams and lipsticks. Furthermore, they deserve an important place among nutracosmeceuticals; in fact, after their absorption from the diet they are transferred from the plasma to the skin, playing an important role in the constitution of skin surface lipids. The aim of the present work was to study the effect of the topical application of soybean phytosterols on skin barrier recovery in human volunteers using the extent of methyl nicotinate (MN)-induced erythema in damaged skin as a parameter to evaluate the rate of stratum corneum recovery. MN was chosen as an erythematogenous substance for its capability to cause an erythema whose intensity and duration are proportional to the quantity of the substance that has entered the living epidermis over time. MN-induced erythema was monitored using reflectance spectrophotometry as a non-invasive instrumental technique. The results show clearly that soy phytosterols exert positive results on skin repair; in fact, 3 days after tape stripping, the sites treated with a formulation containing phytosterols showed an appreciable recovery of barrier function compared to those treated with a vehicle control without soy phytosterols.     

Anti-microbial and -inflammatory activity and efficacy of phytosphingosine: an in vitro and in vivo study addressing acne vulgaris

Lipids are important constituents of the human epidermis. Either free and organized into broad lipid bilayers in the intercorneocytes spaces, or covalently bound to the corneocyte envelope, they play a crucial role in permeability barrier function and are major contributors to cutaneous anti-microbial defense. Free sphingoid bases are a recent addition to this family of active lipids, which emerged from studies of breakdown products from ceramides. Phytosphingosine (PS) is a lipid occurring naturally in the stratum corneum, both in its free form and as a part of the major fraction of ceramides. The biotechnological production of PS patented by Degussa yields to PS with the correct configuration present in the skin. So, application of a PS containing formulation leads to its integration into the natural lipid structures of the skin. In acne, different pathogenetic factors contribute to the inflammation process, defect in keratinization, increased sebaceous gland activity and increased colonization of Propionibacterium acnes. The results of in vitro and in vivo studies confirm the previous reports on strong anti-microbial effectiveness of skin-identical PS produced by Degussa in vitro and in vivo. In addition, PS shows excellent clinical results in the context of skin care in acne, based on both anti-inflammatory and anti-microbial activity. These results demonstrate the potential of PS to enhance or complement existing acne therapies acting as an active cosmetic ingredient.     

Modulation of sebum excretion from the follicular reservoir by a dichlorophenyl-imidazoldioxolan

The dichlorophenyl-imidazoldioxolan elubiol is used for skin and hair care in subjects with oily skin or dandruff. A study was conducted in 14 men to evaluate by non-invasive methods the effect of long-term use of elubiol on the sebum output at the skin surface. Measures were made using the Sebumeter SM810 and Sebutapes. Data show a progressive decrease in skin oiliness at the forehead site treated by elubiol compared to the contralateral control site. The function of the follicular reservoir appears to be affected without significant primary change in the sebaceous gland activity.     

Location and nature of the epidermal permeability barrier

The lowest region of human and pig stratum corneum has been isolated as an integral layer, referred to as the stratum compactum. This preparation is resistant to disruption by enzymes, 6M urea, Triton X-100 and organic solvents. Our evidence suggests that penetration of materials to and through the corneum depend on the state of cohesion between cells, and of the organization of intercellular lipid species. As the cells move up towards the outside of the stratum corneum, the cohesive forces are reduced due to desmosome degradation and lipid modifications, with ultimate dyshesion and sloughing of individual cells.
A number of linoleate-rich lipids have been identified within human and pig epidermis which could contribute to the formation or maintenance of an epidermal permeability barrier. Our current belief is that the molecule key to barrier function is a hydroxylated linoleate derivative which appears to be formed in the stratum compactum region.
Situation et nature de la barrière de perméabilitéépidermique     

Dandruff: infection or not?

The presence of apparently normal corneocytes as well as parakeratotic material in dandruff scales suggests that atypical keratinization in the scalp epidermis has not been of long duration. Abundance of hair fat on the scalp after puberty, colonization by fat-splitting micro-organisms (notably yeasts of the genus Pityrosporum) and transitory upsurges of epidermal mitosis in response to fatty acids produced by lipolysis of sebum triglycerides within the hair follicles, especially those in the anagen phase, seem likely to be implicated in the generation of dandruff scaling. Antidandruff efficacy apparently calls not only for antifungal potency demonstrable in vitro but also for the ability, when incorporated in a suitable formulation, to reach the microflora within the hair follicles and to inhibit the metabolic activity of these microorganisms even in the presence of sebum. The in vivo demonstration of an alleged ‘anti-hyperproliferative'effect of an antidandruff compound has been described but probably has only coincidental relationship to its normal function in the control of dandruff, which seems more likely to depend on its antimicrobial properties. The available evidence thus tends to favour the view that the scalp microflora have a predominant role in the generation of dandruff. Pellicules: origine microbienne ou non microbienne?     

Physiological background to skin ageing

Ageing and ultimate death, when survival offers no reproductive advantages to the species, are essential to the evolutionary process. Yet the prolongation of youth has always been a human dream. The prevention of ageing in visible areas of skin is desirable not for physiological but for social reasons; the skin proclaims our sexuality and asserts our social image. Has the aspiration any hope of success? Ageing of the skin involves changes in the dermis, epidermis, pigment cells, hair follicles, sebaceous and sweat glands, and sense organs. Some of these changes appear intrinsic to the organs concerned; some are dependent on hormonal and other systemic mechanisms; some involve environmental factors such as radiation. If changes of the first type appear to be inevitable, those of the second may be modifiable, and those of the third may, to some extent, be preventable. L'histoire physiologique du vieillissement de la peau     

Topical proteolytic enzymes affect epidermal and dermal properties

Although proteolytic enzymes have a history of use in skin care products it is not known whether they simply induce superficial exfoliation or with continued use can alter epidermal and dermal skin properties. We examined herein whether enhanced exfoliation resulting from treatment with an aspartyl dependent acid protease produces appearance improvements and over time, changes in the epidermis and dermis. Test participants applied 15% enzyme containing serum and a simple moisturizer twice daily for 3 months; a matched control group applied the same serum (without enzyme) and moisturizer. Changes in skin smoothness and texture, the depth and number of lines and wrinkles, and epidermal and dermal firmness and thickness were examined in a double-blind fashion. Treatment with 15% enzyme product(s) resulted in significant improvement in epidermal properties after 1 month and both epidermal and dermal properties after 3 months. The control group showed modest improvements in surface properties only. These results demonstrate that significant appearance benefits can be derived from use of exfoliative proteolytic enzymes. Such improvements are the result of changes to the epidermis and dermis and are strikingly similar to results observed with higher concentrations of AHAs (alpha hydroxy acids).     

A comparison of females and males for anti-perspirant efficacy and sweat output

For human skin, high water content and low sebum secretion are considered to be main features of fair skin. To explore the proper personal care regimen for facial skin, we investigated the change of skin physiologic parameters after cosmetic application by measuring the skin water content, transepidermal water loss, and skin sebum secretion on facial skin before and after the cosmetic application using the Corneometer, Tewameter, and Sebumeter, respectively. The results indicated that the cosmetics application kept a higher water content and a lower transepidermal water loss, and at the same time, a lower sebum secretion 4 h and 8 h after the cosmetic application, compared with those before it. The situation was maintained in the succeeding three week continuous use of the cosmetics. It could be concluded that the cosmetic application on human facial skin might provide some moisturizing effect and at the same time an anti-sebum effect, which favors the maintenance of good skin physiological function after applying skin care products. Our results might provide a scientific personal care regimen for human facial skin to prompt the balance for the hydrolipid film on skin.     

Contribution à l'étude expérimentale de la séborrhée. Implications éventuelles en cosmétologie

The sebaceous gland is an androgen-dependent structure and a major site for androgen metabolism. Androgens are involved in stimulation of sebaceous secretion in humans. The object of this study is to contribute to the understanding of the role of these hormones and their metabolites in seborrhea and to comment on the activity of certain inhibitors. Methods used are sebaceous secretion and ¹⁴C-lipogenesis in animals, ¹⁴C-testosterone or DHA metabolism in animal and human skin. Our results showed that: – by subcutaneous administration in rats, there was a relative specificity of weak androgens for sebaceous gland stimulation unlike potent androgens.     

Expert opinion and review article: The timing of comedone extraction in the treatment of premenstrual acne – a proposed therapeutic approach

The face is the most exposed part of our body, and deterioration of facial appearance, through disease or ageing, causes psychological distress. Acne lesions and subsequent pigmentation changes contribute to the unevenness of skin colour that is detrimental to facial attractiveness and leads to rejection and stigmatization in our society. The purpose of this review is to look at the root causes of premenstrual acne and, for the first time ever, propose that the specific timing of treatment can, at least in part, resolve the acne lesions. We postulate that it is the timing of the manual extraction that matters in resolving premenstrual acne and that the effectiveness of this therapy could be improved. Based on the reviewed evidence, we propose that it is important to remove the comedones at the time of ovulation, prior to the reduction of the size of the sebaceous orifice and epidermal barrier function, to counteract the onset of increased sebum production, prevent blockage of the pores and subsequent bacterial colonization and inflammation. If performed successfully, the extraction may contribute to a reduction in inflamed acne lesions and thus benefit women by increasing their facial attractiveness, well‐being, social function and thereby decrease psychological stress.     

On the effects of a plant extract of Orthosiphon stamineus on sebum‐related skin imperfections

Overproduction of sebum is very common and results in an undesirable oily, shiny complexion with enlarged pores. Sebum secretion is basically under the control of 5‐α reductase, and more particularly under that of type 1 isozyme. But it is also highly sensitive to environmental factors such as temperature, humidity and food. Moreover, in Asia, the edicts of a flawless facial skin turn oily skin into a major concern for Asian women. We identified Orthosiphon stamineus leaf extract as an interesting ingredient for reducing the oily appearance of skin thanks to its ability to reduce 5‐α reductase type 1 expression in normal human epidermal keratinocytes in vitro. This was confirmed ex vivo, where Orthosiphon stamineus leaf extract was shown to reduce 5‐α reductase activity as well as the production of squalene, one of the main components of sebum that was used as a tracer of sebum. To evaluate the efficacy of Orthosiphon stamineus leaf extract at reducing sebum‐related skin imperfections in vivo, we performed two different clinical studies, one in France on a panel of Caucasian volunteers and the other one in Thailand on a panel of Asian volunteers. Using instrumental techniques as well as clinical evaluation and self‐evaluation, we could highlight that an O/W cosmetic formula containing 2% of Orthosiphon stamineus leaf extract could visibly reduce the oily appearance of skin as well as the size of pores, thus leading to a significant improvement of complexion evenness and radiance. Overall, the results obtained were better than those observed with the same formula containing 1% of zinc gluconate, an ingredient frequently used in oily skin care products.