Modified TEWL in vitro measurements on transdermal patches with different additives with regard to water vapour permeability kinetics

Systematic investigation of factors, such as the impact of emulsifiers, which influence the measurement of skin barrier integrity by in-vitro trans-epidermal water loss (TEWL)

Trans-epidermal water loss (TEWL) has been the most widely used method to assess the integrity of the skin barrier and evaluate the irritation potential or the protective properties of topical products for many years. It detects the amount of water that diffuses across the stratum corneum (SC) to the external environment. As one of the most important functions of the skin is to keep water inside the body, an increase in TEWL is used to indicate the skin's impaired barrier function. So far, a variety of commercial instruments are available to measure the TEWL. Their applications mainly focus on the in-vivo TEWL measurements for dermatological examinations or formulation development. Recently, an in-vitro TEWL probe has also been commercially released enabling preliminary tests with excised skin samples. In our study, we first aimed to optimize the experimental procedures for detecting the in-vitro TEWL of porcine skin. Secondly, different kinds of emulsifiers were applied to the skin, including polyethylene glycol-containing emulsifiers (PEG-ylated emulsifiers), sorbitan esters, cholesterol, and lecithin. Sodium lauryl sulfate (SLS) was used as a positive control, and water as a negative control. Based on the findings, we established a protocol for accurately measuring the in-vitro TEWL values, emphasizing that the temperature of the skin sample should be constantly maintained at 32℃. Subsequently, the influences of emulsifiers on the in-vitro TEWL were analyzed. They indicated a significant skin barrier impairment of PEG-20 cetyl ether, PEG-20 stearyl ether, and SLS on in-vitro skin. Furthermore, we interestingly found that there consistently was an alteration of the TEWL values, even after the application of water to the skin. Our findings are of special interest, as the European Medicines Agency (EMA) recommends the use of in-vitro TEWL to determine skin barrier intactness during Franz cell experiments. Thus, this study provides a validated protocol for measuring the in-vitro TEWL and elucidates the impact of emulsifiers on the skin barrier. It also improves the understanding of tolerable variations of in-vitro TEWL and offers recommendations for its use in research.

Effects of Ceramide and Dihydroceramide Stereochemistry at C-3 on the Phase Behavior and Permeability of Skin Lipid Membranes

Ceramides (Cer) are key components of the skin permeability barrier. Sphingosine-based CerNS and dihydrosphingosine-based CerNdS (dihydroCer) have two chiral centers; however, the importance of the correct stereochemistry in the skin barrier Cer is unknown. We investigated the role of the configuration at C-3 of CerNS and CerNdS in the organization and permeability of model skin lipid membranes. Unnatural l-threo-CerNS and l-threo-CerNdS with 24-C acyl chains were synthesized and, along with their natural d-erythro-isomers, incorporated into membranes composed of major stratum corneum lipids (Cer, free fatty acids, cholesterol, and cholesteryl sulfate). The membrane microstructure was investigated by X-ray powder diffraction and infrared spectroscopy, including deuterated free fatty acids. Inversion of the C-3 configuration in CerNS and CerNdS increased phase transition temperatures, had no significant effects on lamellar phases, but also decreased the proportion of orthorhombic packing and decreased lipid mixing in the model membranes. These changes in membrane organization resulted in membrane permeabilities that ranged from unchanged to 5-fold higher (depending on the permeability markers, namely, water loss, electrical impedance, flux of theophylline, and flux of indomethacin) compared to membranes with natural CerNS/NdS isomers. Thus, the physiological d-erythro stereochemistry of skin Cer and dihydroCer appears to be essential for their correct barrier function.

Effects of sphingomyelin/ceramide ratio on the permeability and microstructure of model stratum corneum lipid membranes

The conversion of sphingomyelin (SM) to a ceramide (Cer) by acid sphingomyelinase (aSMase) is an important event in skin barrier development. A deficiency in aSMase in diseases such as Niemann-Pick disease and atopic dermatitis coincides with impaired skin barrier recovery after disruption. We studied how an increased SM/Cer ratio influences the barrier function and microstructure of model stratum corneum (SC) lipid membranes. In the membranes composed of isolated human SC Cer (hCer)/cholesterol/free fatty acids/cholesteryl sulfate, partial or full replacement of hCer by SM increased water loss. Partial replacement of 25% and 50% of hCer by SM also increased the membrane permeability to theophylline and alternating electric current, while a higher SM content either did not alter or even decreased the membrane permeability. In contrast, in a simple membrane model with only one type of Cer (nonhydroxyacyl sphingosine, CerNS), an increased SM/Cer ratio provided a similar or better barrier against the permeation of various markers. X-ray powder diffraction revealed that the replacement of hCer by SM interferes with the formation of the long periodicity lamellar phase with a repeat distance of d=12.7nm. Our results suggest that SM-to-Cer processing in the human epidermis is essential for preventing excessive water loss, while the permeability barrier to exogenous compounds is less sensitive to the presence of sphingomyelin.