Raman and AFM-IR chemical imaging of stratum corneum model membranes

Abstract

Stratum corneum (SC), the outermost layer of the epidermis, is the primary barrier to percutaneous absorption. The diffusion of substances through the skin occurs through the SC lipid fraction, which is essentially constituted of an equimolar mixture of ceramides, free fatty acids, and cholesterol. The lipid constituents of SC are mainly forming continuous multilamellar membranes in the solid/crystalline state. However, recent findings suggest the presence of a highly disordered (liquid) phase formed by the unsaturated C18 chain of ceramide EOS, surrounded by a highly ordered lipid environment. The aim of the present work was to study the lipid spatial distribution of model SC membranes composed of ceramide EOS, ceramide NS, a mixture of free fatty acids, and cholesterol, using Raman microspectroscopy and AFM-IR spectroscopy techniques. The enhanced spatial resolution at the tens of nanometers scale of the AFM-IR technique revealed that the lipid matrix is overall homogeneous, with the presence of small, slightly enriched, and depleted regions in a lipid component. No liquid domains of ceramide EOS were observed at this scale, a result that is consistent with the model proposing that the oleate nanodrops are concentrated in the central layer of the three-layer organization of the SC membranes forming the long periodicity phase. In addition, both Raman microspectroscopy and AFM-IR techniques confirmed the fluid nature of the unsaturated chain of ceramide EOS while the rest of the lipid matrix was found highly ordered.

Very Long Chain Lipids Favor the Formation of a Homogeneous Phase in Stratum Corneum Model Membranes

“…All the images were obtained over a 30 × 30 μm 2 area, with a lateral resolution of 0.67 μm/pixel and an integration time of 5 s per spectrum, leading to a total acquisition time of ∼3 h per map. The spectra were treated to obtain the FFA molar proportion following a previously reported procedure . In short, the areas of the νCD (2000–2250 cm –1 ) and νCH (2780–3100 cm –1 ) regions were considered proportional to, respectively, the CD and CH local concentration.…”

“…The spectra were treated to obtain the FFA molar proportion following a previously reported procedure. 38 In short, the areas of the νCD (2000−2250 cm −1 ) and νCH (2780−3100 cm −1 ) regions were considered proportional to, respectively, the CD and CH local concentration. The deuterated FFA molar proportion of each pixel, X CD pix , was estimated using:…”

AFM-IR for Nanoscale Chemical Characterization in Life Sciences: Recent Developments and Future Directions

“…80−83 This includes the hair medulla, 81 cuticle, 80 and cortex. 82 Other tissues analyzed include the anterior cruciate ligament, 84 stratum corneum 85 (as well as drug penetration on the skin), 86 and breast cancer, 87 which further expand the application of this technique to the histological context.…”

“…We would also like to highlight recent studies focusing on characterizing human hair samples at the nanoscale using AFM-IR. − This includes the hair medulla, cuticle, and cortex . Other tissues analyzed include the anterior cruciate ligament, stratum corneum (as well as drug penetration on the skin), and breast cancer, which further expand the application of this technique to the histological context.…”