Linda D. Rhein

Self-perceived sensory responses to soap and synthetic detergent bars correlate with clinical signs of irritation.

BACKGROUND Epidemiologic studies indicate that after using soaps and other personal care products, many consumers experience irritation. In 50% of the cases the feelings of skin dryness, itching, and stinging occur in the absence of visible signs of irritation. OBJECTIVE We sought to determine the relation between self-perceived sensory responses of panelists to cleansing products and clinical signs of irritation. METHODS A combination of exaggerated arm-washing methods was designed to induce clinical signs of irritation with psychometric techniques developed to quantify sensations. RESULTS Two studies demonstrated that panelists could reproducibly differentiate between products on the basis of the sensations they felt and that there was a significant correlation (frequently r > 0.80) between these and the observable signs. In the case of skin dryness panelists differentiated products several washing cycles before observable differences were detected. CONCLUSION Sensory evaluations of irritation yield additional information on soap and detergent irritancy beyond clinical observations and expand understanding of the irritation process.

Human Cutaneous Response to a Mixed Surfactant System: Role of Solution Phenomena in Controlling Surfactant Irritation

Exposure of the skin to surfactant-based products can result in irritation. To control this effect researchers are probing mechanisms of surfactant action. In vitro studies show that mixing surfactants often results in less denaturation (swelling) of stratum corneum. We have explored the in vivo human irritation response (using a 21-day cumulative irritation test) to two of these surfactants-sodium lauryl sulfate (SLS) and (C12-C14) alkyl, 7-ethoxy sulfate (AEOS-7EO). Results demonstrate that addition of AEOS-7EO to a constant dose of SLS results in a significant reduction in erythema, hence producing a milder system. The reason for the synergism is unclear, but may related to experimentally determined alterations in the micellar solution properties of the SLS upon addition of AEOS-7EO.

Sequential order of skin responses to surfactants during a soap chamber test.

Differences in the response of distinct layers of the skin to surfactants were probed using a modification of the Frosch and Kligman soap chamber test. Soap and other surfactant‐containing cleansers corneum is readily damaged even by a mild insult when no erythema is induced. A more severe treatment, such as 24‐h exposure to a 5% solution, induced the maximal level of barrier damage but a submaximal level of erythema. Even 2 days of exposure to 5% soap does not elicit a maximal erythema response. These results suggest that the stratum corneum is more readily damaged than the dermis. Which is not unexpected because the stratum corneum is the readily damaged than the surfactant and skin. Furthermore, this study indicates that for discriminating among mild products, when a small degree of irritation is induced. The most effective measure is stratum corneum damage assessed by evaporimetry. However, for evaluating more irritating products, erythema is probably the more discriminating evaluation technique.

Surfactants and the stratum corneum lipids

Abstract A simplified mixture of model stratum corneum lipids was mixed with different surfactants to make a preliminary estimation of the influence of surfactants on the stratum corneum lipid structure. The results revealed differences between cationic and anionic surfactants and between anionic surfactants with different structures.

DIRECT ROLE OF LINOLEIC ACID IN BARRIER FUNCTION: EFFECT OF LINOLEIC ACID ON THE CRYSTALLINE STRUCTURE OF OLEIC ACID/OLEATE MODEL STRATUM CORNEUM LIPID

ABSTRACT An explanation for the direct role of linoleic acid in barrier function is offered based on the fact that the formation of a crystalline state of the hydrocarbon chains in the stratum corneum lipids strongly reduces or entirely removes the barrier. Addition of linoleic acid to a gel liquid crystalline structure of oleic acid/sodium oleate, a model for stratum corneum lipids, prevents crystallization of the hydrocarbon chains when the water content is reduced. Thus the presence of unsaturation in the fatty acid/soap is not sufficient to prevent cyrstallization, rather inclusion of the essential fatty acid, linoleic acid, will retain the liquid crystalline structure necessary for normal barrier function over a wide range of relative humidities.

Development of a stratum corneum lipid model to study the cutaneous moisture barrier properties

Abstract A skin lipid model to study barrier properties of stratum corneum has been developed. Research that led to the evolution of this model is presented along with highlights of recent findings. At normal water content of skin, the model lipid exists as a liquid crystal with only a small amount of solid crystals present. As the water content is reduced, for example by exposure to a low humidity environment, more of the solid crystal phase is found. Further X-ray diffraction studies identified the location of specific lipids in the model layered structure. Triglycerides and squalene are found in the hydrophobic methyl layer, while fatty acids, cholesterol and ceramides are located between the fatty acid chains. Water uptake was significantly enhanced when extracted stratum corneum lipids or model lipids were combined with the delipidated corneocytes, compared with water uptake of the lipids or delipidated corneocytes alone. Water uptake of the combined system was similar to that of isolated, intact stratum corneum. We determined the effect of glycerol, a well known skin moisturizer, on the model. While glycerol did not alter the water loss of the model at low relative humidity (6% RH), it maintained the liquid crystalline state of the lipid at the extreme condition; in the absence of glycerol the model showed substantial crystallization and exhibited multiple phases at 6% RH. Glycerol did not exhibit humectant behavior under these conditions. This study suggests that an alternate mechanism for moisturization may be to maintain the liquid crystalline structure under dry environmental conditions. Future studies using the model will examine the role of specific lipids and proteins in enhancing barrier properties; parameters measured will be phase behavior, water uptake/loss and alterations in diffraction patterns under different environmental conditions. Additives will be sought that improve the moisturization capacity of skin by altering biophysical properties of the lipid.

Ethoxylated alcohol (Neodol-12) and other surfactants in the assay of protein kinase C

Most commonly used surfactants were found to be inhibitors of partially purified rat brain protein kinase C at or above their critical micellar concentrations (CMC). These include sodium lauryl sulfate, deoxycholate, octyl glucoside, dodecyl trimethylammonium bromide, linear alkylbenzene sulfonate and Triton X-100. Several detergents, including the nonionic surfactants digitonin and Neodol-12 (ethoxylated alcohol), did not inhibit protein kinase C activity, even at concentrations greater than their CMC, while the anionic surfactant, AEOS-12 (ethoxylated alcohol sulfate), inhibited enzyme activity only slightly (less than 8%). Since these latter surfactants have little or no inhibitory effect on protein kinase C, they may be of value in solubilizing cells and tissues for the determination of enzyme activity in crude extracts. Among the detergents tested, sodium lauryl sulfate and linear alkylbenzene sulfonate significantly stimulated protein kinase C activity in the absence of phosphatidylserine and calcium. This was found to be dependent on the presence of histone in the protein kinase C assay. These detergents failed to stimulate protein kinase C activity when endogenous proteins in the partially purified rat brain extracts were used as the substrate. Our results indicate that activity of protein kinase C can be modified by the conditions of the assay and by the detergents used to extract the enzyme.

CHARACTERIZATION OF THE SUB-MICELLAR SPECIES IN MIXED SURFACTANT SOLUTIONS I: MIXTURES OF SODIUM LAURYL SULFATE AND AN ALKYL POLYETHOXY SULFATE

Abstract The formation of micelles and the composition and concentration of sub-micellar components of surfactant solutions was studied for sodium lauryl sulfate (SLS), alkyl polyethoxy (average of 7 EO) sulfate (AEOS 7EO) and mixtures of these two anionic surfactants both above and below the critical micelle concentrations (c.m.c). Radiolabeled surfactant was added to the solutions to follow the concentration and identity of the components. To separate micelles from sub-micellar components, an Amicon microfiltration apparatus containing either a 2, 5, or 30 thousand molecular weight cut off membrane was used. Results using a hydrophobic fluorescent probe demonstrated minimal disruption of the micelles during separation. Separations for single surfactants showed breaks in the concentration curves at the c.m.c. The observed sub-micellar composition and concentration were evidence of non-ideal behavior. Concentration dependence and surfactant selectivity in micellization (the “micro c.m.c.”phenomena) is des...

WATER UPTAKE INTO STRATUM CORNEUM; PARTITION BETWEEN LIPIDS AND PROTEINS

ABSTRACT Water uptake in natural and reaggregated stratum corneum was determined by weight difference after storage in an atmosphere of controlled relative humidity. Interlayer spacing in separated lipids as a function of their water content was determined by low-angle X-ray diffractometry. These values were used as a calibration curve to determine the water content of the lipid bilayers in reaggregated stratum corneum. The results revealed different behavior of the lipid models compared to natural lipids of the stratum corneum. The additional water taken up after reaggregation of equilibrated lipids and proteins, was equally partitioned between the protein and the natural lipid fraction, while the models gave a proportionally higher water uptake into the lipids at high relative humidity. It is obvious that the models, so far, do not mimic all the properties of the natural stratum corneum lipids.

Myelin subfractions isolated from mouse brain: Analysis of the lipid composition at three developmental stages

Lipids were examined in whole myelin and 8 myelin subfractions isolated from mouse brain at 18–24, 44–48 and 80–90 days of age. Relative to protein, total lipid was lowest in whole myelin isolated from the oldest animals as well as from subfractions isolated at greater sucrose densities, thus partially accounting for the observed myelin subfraction distribution pattern which shifted during development from an average peak density banding between 0.55 and 0.65 M sucrose to one banding between 0.60 and 0.70 M sucrose. Whole myelin and each myelin subfraction isolated at one age contained nearly the same ratio of sterol and phospholipid to galactolipid; these ratios decreased uniformly during development suggesting enrichment with galactolipid in all myelin subfractions. Sulfatide, as percentage of total galactolipid, was relatively constant during development and appeared to be slightly enriched in the denser myelin subfractions. The findings suggest that regardless of the origin(s) of the subfractions, an age-related mechanism exists in the central nervous system which modifies myelin lipid composition relatively uniformly.