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Mechanisms of Tape Stripping and Protein Quantification
By: Ali Alikhan, MD, and Howard I. Maibach, MD
Posted: February 26, 2010, from the March 2010 issue of Cosmetics & Toiletries.
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Tape Stripping and TEWL
The SC is composed of annucleated, keratin-rich corneocytes embedded in an extracellular multilamellar lipid matrix organized into membrane-like bilayers, in which inter-corneocyte communication occurs through desmosomes.7 Ceramides, cholesterol and free fatty acids comprise the lipid matrix of the SC, providing invaluable roles in barrier structure and function.7 However, lipid levels decrease in aged human skin, possibly due to an increase in pH of the SC and subsequent lipid processing impairment.7
The SC provides the rate determining step for the passage of most molecules across skin.8 Therefore, the concentration of topical agents within the SC is directly related to that in the epidermis and dermis, the typical target sites. Additionally, corneocytes and intercellular lipids are responsible for preventing excess transepidermal water loss (TEWL),9 which can be measured with an evaporimeter and is frequently used to assess skin barrier integrity.9
Anatomically, regional SC variations in percutaneous drug absorption, lipid composition, TEWL measurements, mean thickness, and number of cell layers have been described. Yet, despite this structural heterogeneity, each layer of the SC equally contributes to the prevention of water loss.9 In doing so, the SC behaves as a membrane that is compatible with Fick’s laws of passive diffusion. Therefore, it is possible to calculate water diffusivity across the SC.9
TEWL increase as a function of tape strip number depends on factors such as anatomical site, pressure, pressure duration and tape removal rate.10 Loffler et al. demonstrated that TEWL increased fastest on the forehead, followed by the back and the forearm.10 These findings may be explained by variations in SC thickness, differences in spontaneous desquamation (SC cohesion), and pressure resistance due to inherent viscoelasticity and type of tissue underlying the skin.10 Rapid removal, shorter pressure duration (2 sec vs. 10 sec) and higher pressure (330 g cm-2 vs. 165 cm-2) all more readily produced TEWL increases.10
A similar study by Breternitz et al. revealed the greatest increase in TEWL on the cheek, compared with the back, upper arm and forearm.11 Interestingly, the cheek also demonstrated the greatest increase in SC hydration after stripping.11 The researchers further established a greater, earlier TEWL increase with higher pressure (7 N stamp vs. 2 N) and longer application (10 sec vs. 2 sec).11 Moreover, using the thumb, stretching the skin and utilizing a roller or stamp all resulted in varying quantities of harvested SC.11 Use of the thumb removed the greatest amount of SC and produced the highest TEWL, even when compared with the use of a roller or stretching the skin. Occlusion of the test site prior to the stripping procedure also resulted in higher TEWL values.11 In conclusion, reliable, reproducible results depend upon the standardization of the aforementioned variables.