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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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Later studies utilizing wavelengths of 430 nm established optical spectroscopy in the visible range as a sensitive and reproducible method for protein quantification.2 Absorbance in this range depends exclusively on the quantity of corneocyte aggregates and adequately reflects SC mass.2 Corneocyte aggregates adhering to tape strips decrease the transmission of visible light by scattering, reflecting and diffracting it. The resulting pseudo-absorption has been successfully correlated with the mass of removed SC particles.2
Absorbance measurements allow for the determination of absolute mass from corneocyte aggregates that are harvested via tape stripping. In addition, spectroscopic measurements are unaffected by topically applied substances, unlike gravimetric measurements—which explains the differences in mass detected using gravimetry in the most superficial strips.20
Practically comparing spectrally measured quantity (absorbance) with corneocyte aggregate weight requires correcting for any topical applications in the upper SC layers, interstitial fluids in the deeper SC layers, the “stack effect” that decreases absorbance, and the tape stripping procedure itself (e.g., nonhomogeneous removal of tape or incomplete tape contact with skin).2 Once these factors are corrected, primarily by excluding analysis of the most superficial and deep strips, R2 is equal to 0.93, demonstrating proportionality between quantification methods.2
A multicenter study involving 24 subjects found a correlation coefficient of R2 = 0.94 when comparing UV/VIS spectroscopy (430 nm) with conventional weight determination.20 Superficial (first five) and deep (19–23) strips were excluded on the basis of weight-enhancement. Application of an o/w emulsion, as a part of the study, inflated superficial strip weight, and intrinsic interstitial fluid increased deep strip weight.20 Nonhomogeneous strips and those subject to handling errors were excluded.20 Only 66% of total strips were utilized to determine the correlation coefficient.20 Weigmann et al. explain that pseudo-absorption/weight correlation can be extrapolated to the deepest layers of the SC.20
A recent study demonstrated a strong correlation (R2 = 0.92 and R2 = 0.95) between pseudo-absorption at 430 nm, and both protein absorption at 278 nm and absorption of Trypan blue-stained proteins at 652 nm.17 However, protein absorption at 278 nm was characterized by a weak band, implying its application was limited to tape strips with high amounts of corneocytes.17 Mass determination based on UV absorption was further limited by the potential super-positioning of strong absorption bands from exogenous substances and/or tape components in the same spectral range. Unlike the previous study, correlation was described using all tape strips (superficial and deep), regardless of adherent exogenous or endogenous components.17