Infrared spectroscopic studies of sodium dodecyl sulphate permeation and interaction with stratum corneum lipids in skin

The barrier function of skin is primarily provided by the lamellar lipid matrix of the stratum corneum (SC), which has been shown in previous infrared (IR) and related studies to consist predominantly of ordered lipids packed in orthorhombic and hexagonal domains. In the current work, we investigate the effects of the anionic surfactant, sodium dodecyl sulphate (SDS), on SC lipid packing and phase behaviour, using FT-IR spectroscopy. The use of acyl chain perdeuterated SDS allows unequivocal spectroscopic detection of both endogenous lipid and exogenous material in intact tissue. IR spectra were acquired as a function of temperature from isolated human SC exposed to SDS for various incubation periods at 34°C. SDS is found to enter the SC and is observed to be in a more ordered state in the SC than in solution, indicating that the SDS interacts with the ordered SC lipids. The results reveal that SDS reduces the amount of orthorhombic phase in the SC and increases the amount of hexagonally packed lipid at physiologically relevant temperatures. In addition, a small decrease in the lipid T(m) (acyl chain melting temperature) is observed. Furthermore, these SDS-induced changes were found to be strongly dependent on the time of exposure.     

Hydration of the stratum corneum

Topically applied water, occlusion and topically applied glycerol were used to investigate and characterize some of the changes which occur in the hydrated stratum corneum. The effects of these treatments were monitored using non-invasive techniques under controlled conditions. The Servomed Evaporimeter was used to determine natural water flux from the skin surface before and after treatment. The performance of the Evaporimeter in this type of study had previously been improved by attaching a paper baffle to the detector. This eliminated the variance in output caused by atmospheric movement. Experiments were carried out at temperatures below the threshold of thermal sweating and emotional sweating was minimized. Skin surface topography was characterized by means of a new type of profilometer. The instrument's design allowed a diamond stylus to traverse the living skin surface without significantly altering its structure. Changes in skin surface roughness were further elucidated using scanning electron microscopy and macrophotography. In vivo penetration of glycerol was assessed by chemical analysis of stratum corneum layers of treated skin. Samples were obtained by sequential stripping of the stratum corneum using adhesive tape. Topically applied water produced only a transient benefit because of rapid evaporation. More prolonged hydration was achieved by suppressing transepidermal water loss with polyethylene film. This occlusive hyperhydration was characterized by a significant reduction in profile roughness and by a smoother macroscopic appearance. Glycerol achieved the same effects by reducing the magnitude of the natural water flux from the skin surface and by reducing the rate of evaporation of water from applied aqueous glycerol solution or cosmetic product. Both effects were seen as the result of lowered water activity in the proximity of glycerol. Smoothing effects of glycerol on the skin surface, and improved appearance, persisted for at least 24 h. This persistence was explained by evidence for diffusion of glycerol into the stratum corneum where it formed a reservoir. Hydration of the skin is known to affect its barrier function and thereby exert a profound effect on penetration of both lipophilic and hydrophilic molecules. Clinically, this effect may be achieved using liberal applications of occlusive petroleum jelly and ointments. The results presented in this paper suggest that the use of humectants could achieve useful hydration using cosmetically acceptable materials.     

Role of ceramides 2 and 5 in the structure of the stratum corneum lipid barrier

We utilized Fourier transform infrared (FTIR) spectroscopy to investigate headgroup and chain interactions in model SC lipid barriers containing equimolar amounts of deuterated hexadecanoic acid, cholesterol, and ceramide 2 (non-hydroxy sphingosine) or ceramide 5 (alpha-hydroxy sphingosine). In the ceramide 2 model the thermotropic response of the CD _ 2 and CH _ 2 stretching modes indicates that hexadecanoic acid begins to disorder at 42 degrees C while ceramide 2 remains ordered until 52 degrees C. Additionally, splitting of the CD _ 2 bending and CH _ 2 rocking modes provides evidence for separate orthorhombic hexadecanoic acid and ceramide domains. The ceramide amide I mode (1650 cm ; -1) is split into two components indicating strong intermolecular hydrogen bonding between headgroups. In the ceramide 5 model, the CH _ 2 and CD _ 2 stretching frequencies again reveal highly conformationally ordered ceramide 5 and hexadecanoic acid chains. Splitting of both the CD _ 2 bending and CH _ 2 rocking modes is observed. However, the CH _ 2 rocking frequencies indicate distorted packing of the ceramide. The collapse of these highly ordered phases, and the onset of conformational disorder, occurs at 50 degrees C for both ceramide 5 and hexadecanoic acid. The amide I and II frequencies of ceramide 5 indicate strong H-bonding, although neither mode is split. Our results demonstrate that model SC lipid systems have quite different physical properties depending on whether they contain ceramide 2 or 5. From this we infer that ceramide 2 and 5 make distinct contributions to the structural biophysics of the SC lipid barrier. Our observation of ordered lipid domains is also consistent with the recently proposed domain mosaic model of the skin barrier.     

From contact angle titration to chemical force microscopy: a new route to assess the pH-dependent character of the stratum corneum

Despite of its complex multicomponent organization and its compact architecture, the Stratum corneum (SC) is not completely impermeable to substances directly applied on the skin surface. A huge number of works have been dedicated to the understanding of the mechanisms involved in substance permeation by exploring deeper layers than the SC itself. Surprisingly, there is a poor interest in studies relating to interactions which may occur in the near-surface region (i.e. approximately 1 nm depth) of the SC. In this work, equilibrium proton-transfer reactions have been used as probes to define in a fundamental point of view the nature of the SC interactions with its environment. Such titration curves are investigated on 'in vitro' SC (isolated SC from abdominal skin tissue) and on 'in vivo' volar forearm (a sebum poor area). The results are discussed in term of work of adhesion and surface pKa values. Because SC can 'reconstruct' under heating, influence of the temperature on titration curves is investigated and the role of the different components is discussed. Different sigmoidal transitions were observed. Two common pKa values (pKa(1) = 4 and pKa(2) = 11.5) were clearly identified in both cases and associated to an acid-base character. By playing with the temperature of 'in vitro' SC, the 'accessibility' of polar functions was increased, thus refining the results by revealing an amphoteric character with an acid-to-base transition at pH 3.5 and two acid transitions at pH = 6.5 and pH = 11.5. Adhesion forces between an Atomic Force Microscopy (AFM) tip and a single isolated corneocyte through buffered liquid media were also investigated to better understand the role of the individual corneocytes.     

Nanostructure of the stratum corneum extracellular lipid matrix as observed by cryo-electron microscopy of vitreous skin sections

In recent years, high-resolution cryo-electron microscopy of vitreous skin sections has been used to visualize the formation and structure of the human stratum corneum extracellular lipid matrix. The aim of the present work was to summarize these findings. It is proposed that skin barrier formation does not take place as a 'lamellar body' fusion process, but as a lamellar 'unfolding' of a small lattice parameter lipid 'phase' with cubic-like symmetry with subsequent 'crystallization' and concomitant lamellar re-organization of the extracellular lipid matrix.     

Infrared photoprotection properties of cosmetic products: correlation between measurement of the anti-erythemic effect in vivo in man and the infrared reflection power in vitro

Measurements were carried out on 19 commercial cosmetic samples. Initially, their anti-infrared properties were quantified, in vivo , on healthy volunteers, by measuring variations in the Colorimetric Erythema Index ΔCEI, before and after application of a normalized quantity of product. Subsequently, the physico-chemical response of the product was tested in vitro by measuring the infrared reflection coefficient ΔIR at 800 nm.
The correlation established between these two parameters enables products to be ranked in three categories. A good correlation was demonstrated between reflective power (ΔIR) and anti-erythemic properties (—ΔCEI) of the products in the first two groups.
A qualitative examination of the diffraction of X-rays confirms the presence of titanium dioxide, TiO₂ rutile, in most products with high IR protection properties whereas TiO₂ anatase was detected in a sample with no IR reflection properties.     

Influence of cleansing on stratum corneum tryptic enzyme in human skin

Desquamation in human skin is a well-balanced process of de novo production of corneocytes and their shedding from the skin surface. The proteolysis of corneodesmosomes is an important step in the final desquamation process. In the degradation of these adhesion molecules, the stratum corneum tryptic enzyme (SCTE) plays a key role. In initial studies with extracts of porcine epidermis, SCTE was shown to be inactivated by low concentrations of sodium lauryl ether sulphate (SLES). These in vitro findings were supported by in situ results obtained by measuring the release of fluorescent dyes coupled to trypsin-specific substrates incubated on human skin cross-sections. Moreover, in further studies, it could be demonstrated that the SCTE activity in the human horny layer decreases after in vivo application of cleansing products containing SLES. After repeated washing of human volunteers with tap water, a standard market cleansing product (SLES/betaine system) or a new improved cleansing product (SLES/betaine/disodium cocoyl glutamate system), the specific SCTE activity was determined in extracts from the uppermost layers of the stratum corneum. It could be shown that after application of the new formula the remaining SCTE activity was significantly higher than after use of the standard market formula. This ex vivo approach has proven to be very helpful for measuring surfactant effects on human skin enzymes. Using this assay, we developed an improved shower gel formula, which leads to a significantly higher skin enzyme activity after application, compared to a standard market formula.     

Dispersions of lamellar phases of non-ionic lipids in cosmetic products

Although aqueous dispersions of lipids in the form of particles having a lamellar structure (liposomes) are already known as excellent vehicles for pharmaceutical substances, their usefulness in cosmetic formulations has not been demonstrated. The present work shows the advantages obtained by application of such systems to the skin, and in particular the use of non-ionic lipids in aqueous dispersions. Thus, in comparison with classical formulations such as emulsions, these systems exhibit lower toxicity and permit closer control of the availability of active substances at the stratum corneum. As examples, compositions suitable for skin moisturising and for tanning products are presented. Dispersions de phases lamellaires de lipides non-ioniques en cosmétique     

Distinct locational differences observable in biophysical functions of the facial skin: with special emphasis on the poor functional properties of the stratum corneum of the perioral region

The face is composed of complicated anatomical components, presenting unique portions, such as the eyes, nose and mouth in a relatively narrow area. Moreover, the facial skin is densely populated by the pilosebaceous units and sweat glands, and its stratum corneum (SC) is much thinner than that of the trunk and limbs, although it is always exposed to the environment. Among various portions of the facial skin, some are more easily irritated than others by environmental stimuli, or are more often affected by certain dermatoses. However, the functional aspects of the different portions of the facial skin have not been studied in detail under a strictly controlled environment in sufficiently large numbers of subjects covering different age groups. Thus, we conducted studies in winter with various biophysical techniques, such as transepidermal water loss (TEWL), as a parameter for SC barrier function, high-frequency conductance as that for skin surface hydration state, skin surface lipids, pH, blood flow and skin surface temperature on the forehead, mid-portion of the cheek (cheek in short), nasal tip (nose in short), nasolabial fold and chin of 20 healthy Japanese females aged 22-37 years (average 25 years) in a climate chamber adjusted to 21 degrees C and 50% relative humidity. Thereafter, we studied the influence of ageing on these biophysical parameters by collecting data of TEWL, high-frequency conductance and size of superficial corneocytes on the cheek, nasolabial fold and chin of 303 healthy Japanese female volunteers of different ages. The obtained results showed that the barrier function of the SC was best on the cheek, presenting the lowest TEWL, which was significantly higher on the nasolabial fold and chin than on the cheek. TEWL showed a decrease with age. In contrast, skin hydration state was higher on the nose, but it tended to be lower on the nasolabial fold, showing a mild age-related increase. The corneocytes on the nasolabial fold and chin were smaller than those on the cheek. They revealed a clear increase in size with age. Skin surface lipids were richest on the nose, whereas the superficial pH on the nose was the lowest among the regions tested. The skin temperature was lowest on the cheek than on other areas of the face; although, together with the nose, its blood flow was higher than that of the others. These data indicate great regional differences observable in SC functions on the face. In general, the SC barrier function increases with age, probably because of a decreased epidermal turnover rate as recognized by the increase in corneocyte size. Among the various sites, the skin of the nasolabial fold and chin, whose SC consisted of the smallest corneocytes, showed poorest SC properties in barrier function, suggesting the presence of mild invisible inflammation. It is understandable that this area easily develops not only the complaint of sensitive skin to cosmetics but also dermatitis because of various external agents.     

An in vivo assessment of the biomechanical properties of human skin modifications under the influence of cosmetic products

A new device using a torsional technique has been developed to record the biomechanical properties of the skin.
After determination of the optimal experimental conditions, effects of cosmetic products on the stratum corneum biomechanical properties are described.     

Location‐related differences in structure and function of the stratum corneum with special emphasis on those of the facial skin

Between the two different kinds of the skin covering the body, the glabrous skin is found only on the palmo‐plantar surface because of its rather simple function to protect the underlying living tissue with its remarkably thick stratum corneum (SC) from strong external force and friction. Thus, its barrier function is extremely poor. In contrast, the hair‐bearing skin covers almost all over the body surface regardless of the presence of long hair or vellus hair. In regard to its SC, many dermatologists and skin scientists think that it is too thin to show any site‐specific differences, because the SC is just present as an efficient barrier membrane to protect our body from desiccation as well as against the invasion by external injurious agents. However, there are remarkable regional differences not only in the living skin tissue but also even in such thin SC reflecting the function of each anatomical location. These differences in the SC have been mostly disclosed with the advent of non‐invasive biophysical instruments, particularly the one that enables us to measure transepidermal water loss (TEWL), the parameter of the SC barrier function, and the one that evaluates the hydration state of the skin surface, the parameter of the water‐holding capacity of the SC that brings about softness and smoothness to the skin surface. These in vivo instrumental measurements of the SC have disclosed the presence of remarkable differences in the functional properties of the SC particularly between the face and other portions of the body. The SC of the facial skin is thinner, being composed of smaller layers of corneocytes than that of the trunk and limbs. It shows unique functional characteristics to provide hydrated skin surface but relatively poor barrier function, which is similar to that observed in retinoid‐treated skin or to that of fresh scar or keloidal scars. Moreover, there even exist unexpected, site‐dependent differences in the SC of the facial skin such as the forehead, eyelid, cheek, nose and perioral regions, although each location occupies only a small area. Between these locations, the cheek shows the lowest TEWL in contrast to the perioral region that reveals the highest one. Moreover, these features are not static but change with age particularly between children and adults and maybe also between genders. Among various facial locations, the eyelid skin is distinct from others because its SC is associated with poor skin surface lipids and a thin SC cell layer composed of large corneocytes that brings about high surface hydration state but poor barrier function, whereas the vermillion borders of the lips that are covered by an exposed part of the oral mucosa exhibit remarkably poor barrier function and low hydration state. Future studies aiming at the establishment of the functional mapping in each facial region and in other body regions will shed light on more delicate site‐dependent differences, which will provide us important information in planning the strategy to start so called tailor‐made skin care for each location of the body.