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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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Kalia et al. found that initial tape strips removed thicker layers of the SC, relating this to a decreased number of desmosomes closer to the skin surface.8 The researchers demonstrated decreased impedance with increasing depth and thus theorized that the removal of the upper corneocyte layers and lipid matrix diminishes structural opposition to ion flow, facilitating ion transport. In addition, TEWL was found to increase disproportionally with later tape strips, and the removal of only the upper layers of the SC was insufficient to significantly enhance water loss.8
The removal of 6-8 µm of deeper layers of the SC typically resulted in significant TEWL increases.8 In contrast, removal of the outermost layers affected impedance more than TEWL, with a 40% decrease in impedance after removal of only 4 µm of SC. Nonetheless, a correlation between TEWL increase and impedance decrease was observed. Upon completion of the tape stripping experiment, the full return to basal values of impedance occurred after 3 days while TEWL recovery time took 5-6 days. External layers, more crucial in impedance, were formed prior to deep compact layers.8
These findings by Kalia et al. suggest a gradation in water-regulating ability within the SC, with the deepest layers most responsible for controlling water flux. However, via simple mathematical deduction, these results support a Fickian model.8 Though structurally heterogeneous and complex, the SC behaves as a homogenous barrier to water in vivo. The water transport route may be homogeneous throughout SC, with each layer contributing equally to the barrier.8
In fact, if the experimental TEWL values are plotted as a function of tape stripping frequency in a best-fit curve, it closely resembles a theoretical curve based on Fick’s first law of diffusion.12 The first half of the theoretical curve fits the actual curve; in the second half, experimental data shows slightly higher TEWL values than Fick’s theoretical values.12 The authors of the study offer plausible explanations for this discrepancy.12
In contrast to most studies, Schwindt et al. demonstrated that the quantity of harvested SC was constant with each strip in a given anatomical site and volunteer.9 They found a linear relationship in all anatomical sites between 1/TEWL and the total mass of removed SC, further establishing that the SC acts as a Fickian membrane for steady state water diffusion. Their work also suggests that intercellular lipids, not corneocytes, are the determining factor for SC water diffusion.9 This linear relationship was described by Pirot et al., plotting 1/TEWL as a function of SC thickness (13 subjects examined).13 Table 1 summarizes the results from the Kalia, Schwindt and Pirot studies quantifying SC thickness.