Electron resonance studies on the influence of anionic surfactants on human skin

The Skin Barrier and Moisturization: Function, Disruption, and Mechanisms of Repair

BACKGROUND

The anatomic layers of the skin are well-defined, and a functional model of the skin barrier has recently been described. Barrier disruption plays a key role in several skin conditions, and moisturization is recommended as an initial treatment in conditions such as atopic dermatitis. This review aimed to analyze the skin barrier in the context of the function model, with a focus on the mechanisms by which moisturizers support each of the functional layers of the skin barrier to promote homeostasis and repair.

SUMMARY

The skin barrier is comprised of four interdependent layers - physical, chemical, microbiologic, and immunologic - which maintain barrier structure and function. Moisturizers target disruption affecting each of these four layers through several mechanisms and were shown to improve transepidermal water loss in several studies. Occlusives, humectants, and emollients occlude the surface of the stratum corneum (SC), draw water from the dermis into the epidermis, and assimilate into the SC, respectively, in order to strengthen the physical skin barrier. Acidic moisturizers bolster the chemical skin barrier by supporting optimal enzymatic function, increasing ceramide production, and facilitating ideal conditions for commensal microorganisms. Regular moisturization may strengthen the immunologic skin barrier by reducing permeability and subsequent allergen penetration and sensitization.

KEY MESSAGES

The physical, chemical, microbiologic, and immunologic layers of the skin barrier are each uniquely impacted in states of skin barrier disruption. Moisturizers target each of the layers of the skin barrier to maintain homeostasis and facilitate repair.

Surfactant concentration and type affects the removal of Escherichia coli from pig skin during a simulated hand wash

The effect of surfactant type and concentration on a bland soap formulation's ability to remove bacteria from hands remains largely unstudied. Several combinations of surfactants and water were combined to test bacterial removal efficacy using a hand-washing device (two pieces of pig skin and a mechanical motor) to simulate a hand wash. A nalidixic acid-resistant, nonpathogenic strain of Escherichia coli (ATCC 11229) was used. Two anionic surfactants, sodium lauryl sulphate and sodium stearoyl lactylate, and two nonionic surfactants, poloxamer 407 and sorbitan monostearate, each in concentrations of 2, 5 and 10% were studied. A slight positive (r²  = 0·17) but significant (P = 0·03) correlation was observed between hydrophile-lipophile balance value and mean log reduction. No correlation was observed between pH of the treatment solution and the mean log reduction (r²  = 0·05, P = 0·25). A 10% sodium lauryl sulphate mixture showed the highest log reduction (x¯ = 1·1 log CFU reduction, SD = 0·54), and was the only treatment significantly different from washing with water (P = 0·0005). There was a correlation between increasing surfactant concentrations above the critical micelle concentration, and mean microbial reduction (r²  = 0·62, P = 0·001).

SIGNIFICANCE AND IMPACT OF THE STUDY

This study characterizes the role of surfactants in removing microbes during a hand wash. Numerous studies address how surfactants support antimicrobial effect in soap, or cause irritation of skin, but no published studies show which surfactants are best for removing microbes. We used pig skin as a model for human skin and a lathering device to simulate a hand wash. A 10% sodium lauryl sulphate mixture was the only treatment significantly different from a water wash. There was a strong correlation between increasing surfactant concentrations above the critical micelle concentration and mean microbial reduction.

α-Gel formation by amino acid-based gemini surfactants

Ternary mixtures being composed of surfactant, long-chain alcohol, and water sometimes form a highly viscous lamellar gel with a hexagonal packing arrangement of their crystalline hydrocarbon chains. This molecular assembly is called "α-crystalline phase" or "α-gel". In this study, we have characterized α-gels formed by the ternary mixtures of amino acid-based gemini surfactants, 1-hexadecanol (C16OH), and water. The surfactants used in this study were synthesized by reacting dodecanoylglutamic acid anhydride with alkyl diamines and abbreviated as 12-GsG-12 (s: the spacer chain length of 2, 5, and 8 methylene units). An amino acid-based monomeric surfactant, dodecanoylglutamic acid (12-Glu), was also used for comparison. At a fixed water concentration the melting point of the α-gel increased with increasing C16OH concentration, and then attained a saturation level at the critical mole ratio of 12-GsG-12/C16OH = 1/2 under the normalization by the number of hydrocarbon chains of the surfactants. This indicates that, to obtain the saturated α-gel, a lesser amount of C16OH is required for the gemini surfactants than for the monomeric one (the critical mole ratio of 12-Glu/C16OH = 1/3). Small- and wide-angle X-ray scattering measurements demonstrated an increase in the long-range d-spacing of the saturated α-gels in the order 12-Glu <12-G8G-12 < 12-G5G-12 < 12-G2G-12. In the three gemini surfactant systems, the decreased spacer chain length resulted in the increased maximum viscosity and elastic modulus of the saturated α-gels at a given water concentration. This is caused by the decreased amount of excess water being present outside the α-gel structure (or the increased amount of water incorporated between the surfactant-alcohol bilayers). To the best of our knowledge, this is the first report focusing on the formation of α-gel in gemini surfactant systems.

Multiple stages of detergent-erythrocyte membrane interaction--a spin label study

The various stages of the interaction between the detergent Triton X-100 (TTX-100) and membranes of whole red blood cells (RBC) were investigated in a broad range of detergent concentrations. The interaction was monitored by RBC hemolysis-assessed by release of intracellular hemoglobin (Hb) and inorganic phosphate-and by analysis of EPR spectra of a fatty acid spin probe intercalated in whole RBC suspensions, as well as pellets and supernatants obtained upon centrifugation of detergent-treated cells. Hemolysis finished at ca. 0.9mM TTX-100. Spectral analysis and calculation of order parameters (S) indicated that a complex sequence of events takes place, and allowed the characterization of various structures formed in the different stages of detergent-membrane interaction. Upon reaching the end of cell lysis, essentially no pellet was detected, the remaining EPR signal being found almost entirely in the supernatants. Calculated order parameters revealed that whole RBC suspensions, pellets, and supernatants possessed a similar degree of molecular packing, which decreased to a small extent up to 2.5mM detergent. Between 3.2 and 10mM TTX-100, a steep decrease in S was observed for both whole RBC suspensions and supernatants. Above 10mM detergent, S decreased in a less pronounced manner and the EPR spectra approached that of pure TTX-100 micelles. The data were interpreted in terms of the following events: at the lower detergent concentrations, an increase in membrane permeability occurs; the end of hemolysis coincides with the lack of pellet upon centrifugation. Up to 2.5mM TTX-100 the supernatants consist of a (very likely) heterogeneous population of membrane fragments with molecular packing similar to that of whole cells. As the detergent concentration increases, mixed micelles are formed containing lipid and/or protein, approaching the packing found in pure TTX-100 micelles. This analysis is in agreement with the models proposed by Lasch (Biochim. Biophys Acta 1241 (1995) 269-292) and by Le Maire and coworkers (Biochim. Biophys. Acta 1508 (2000) 86-111).

Depth Dependence of Stratum Corneum Lipid Ordering: A Slow-Tumbling Simulation for Electron Paramagnetic Resonance

We investigated the structural ordering of stratum corneum (SC) lipid by means of electron paramagnetic resonance (EPR) slow-tumbling simulation in conjunction with spin probe studies. The SC of human mid-volar forearm was stripped consecutively from three to six times. The EPR probe method detected a characteristic peak of sebaceous matter in the first SC stripping. The order parameter values obtained by the slow-tumbling simulation (S(0)) showed significant differences between each layer compared with those indicated by the conventional order parameter (S) using hyperfine couplings. Although the conventional S values were in the range of 0.56 (outermost layer) to 0.61 (bottom layer), the S(0) values by the simulation changed from 0.22 to 0.96. The present results suggest that the structural ordering of the outermost SC layer is less tight, whereas the structure of inner layers becomes more rigid. Therefore, we concluded that the EPR probe method recognizes sebaceous matters, whereas EPR in conjunction with the simulation allows quantitative evaluation of SC lipid ordering in relation to skin depth.

Personal cleanser technology and clinical performance

Cleansing is essential for health and hygiene. Personal cleansing agents, such as soap, one of the earliest to be developed, were initially expected to deliver only cleansing benefits, but consumer expectations came with time to encompass health and cosmetic benefits. This demand led to the development of an array of milder cleansing agents with a broader spectrum of application. Cleansing products are sold around the world in different forms such as bars (the most widely used), liquids, gels, and creams. The present paper discusses the chemistry, formulation, product attributes, and benefits of various types of personal wash products.

Influence of the temperature in the adsorption of sodium dodecyl sulfate on phosphatidylcholine liposomes

The influence of the temperature on the adsorption of monomeric and micellar solutions of the anionic surfactant sodium dodecyl sulfate (SDS) on phosphatidylcholine (PC) liposomes was investigated using the fluorescent probe 2-(p-toluidinyl)-naphthalene-6-sodium sulfonate (TNS). The number of adsorbed molecules was quantified by measuring changes in the electrostatic potential (Psi(o)) of the liposomes/probe during an incubation with SDS at varying temperatures. At low surfactant concentrations (from 0.05 to 0.25 mM), the increase in temperature reduced the number of surfactant molecules incorporated per vesicle regardless of the incubation time, whereas at high surfactant concentrations (from 0.50 to 1.0 mM) the incubation time has an opposite effect on this process. Thus, after 10s, the surfactant adsorption decreased with temperature, yet it increased progressively with time. The adsorption was linear with temperature below critical micellar concentration (CMC) of SDS and this linear tendency did not change above CMC. This suggests an adsorption of SDS monomers regardless of the surfactant concentration.

Analysis of electron spin resonance spectra of alkyl spin labels in excised guinea pig dorsal skin, its stratum corneum, delipidized skin and stratum corneum model lipid liposomes

The electron spin resonance (ESR) spectra of alkyl spin labels were observed in the excised guinea pig dorsal skin, its stratum corneum, delipidized skin and stratum corneum model lipid liposomes. The spectrum of 5-doxylstearic acid (5-NS) in the stratum corneum and order parameter obtained from the spectrum, indicated that the spin label was present in highly ordered lipid lamella. On the other hand, the spectrum of methyl ester of 5-NS (5-NMS) and its apparent rotational correlation time calculated from the spectrum, showed only a weakly immobilized component in the stratum corneum as well as in the whole excised skin. The ester spin label seemed to be scarcely present in the rigid lipid lamella, but mainly in the relatively fluid environment. On the other hand, cationic alkyl spin labels showed quite different spectra depending on their alkyl chain lengths. Long-chain 4-(N,N-dimethyl-N,-pentadecyl)ammonium-2,2,6,6-tetramethylpiperidine-1-oxyl (CAT-15) seemed to be present in the protein region of the stratum corneum as we recently reported, whereas hydrophilic quaternary ammonium spin label 4-trimethylammonium-2,2,6,6-tetramethylpiperidine-1-oxyl (CAT-1) seemed to be present in the bulk water of the skin, even in delipidized skin. These findings indicated that the different interaction and different localization of the alkyl spin labels depended on their electronic charge as well as their alkyl chain lengths.

Spectroscopic techniques in the study of membrane solubilization, reconstitution and permeabilization by detergents

This review focuses on the use of spectroscopic techniques for the study of membrane solubilization, reconstitution, and permeabilization by detergents. Turbidity and light scattering, visible and infrared spectroscopic methods, fluorescence, nuclear magnetic resonance, electron spin resonance and X-ray diffraction are examined from the point of view of their applicability to the above detergent-mediated phenomena. A short introduction is provided about each of the techniques, and references are given for further study.

Effect of surfactants on human stratum corneum: electron paramagnetic resonance study

Electron paramagnetic resonance (EPR) spectra of nitroxide spin probes are useful for studying biological membranes and chemical-membrane interactions. Recently, we established a stripping method to remove stratum corneum (SC) for this purpose. To assess this stripping method with EPR and correlate with standard methods, we quantified the irritant effects of three types of surfactants by measurements of visual score and transepidermal water loss (TEWL), SC hydration and chromametry and studied EPR spectra measurements of surfactant-treated cadaver SC (C-SC) and stripped off SC (S-SC) on patch tested sites. 5-Doxyl stearic acid was the spin label. The order parameter S obtained from the spectra of S-SC correlated with those of C-SC and TEWL values. The results suggest that this method is capable of evaluating the fluidity of SC and correlates with the above bioengineering parameters.

Biophysical characterization of skin damage and recovery after exposure to different surfactants

The majority of adverse skin reactions to personal-care products are presumed to be caused by irritant substances, like surfactants. In this study, different aspects of the irritant reaction after a single exposure to 8 surfactants were characterized during 2 weeks. Solutions of 2% sodium lauryl sulfate, 5% sodium C12-15 pareth sulfate, 5% sodium cocoyl isethionate, 10% disodium laureth sulfosuccinate, 10% sodium cocoamphoacetate, 10% cocamide DEA, 10% cocamidopropyl betaine and 10% lauryl glucoside, respectively, were applied to the forearm of 12 volunteers. Clinical assessment, an evaporimeter, a laser Doppler flowmeter and a corneometer were used for evaluation. The surfactants induced different degrees of irritation. Erythema, transepidermal water loss and skin blood flow exhibited a similar time course, which seemed to be inversely related to the delayed scaling and reduced skin capacitance. The mechanism of the damaging effect of the surfactants seems to be similar, although some minor differences were noted.