Composition and structure analysis of different depths in the stratum corneum using confocal Raman microscopy combined with two-dimensional correlation spectroscopy

The chemical composition and structure of the stratum corneum (SC) play a crucial role in the skin barrier function. Therefore, accurately determining the SC thickness and studying the changes in lipid and keratin structure and distribution within it are key aspects of skin barrier research. Currently, there are limited analytical tools and data analysis methods available for real-time and online studies of SC composition and structural changes. In this study, we focus on depth as a perturbation and employ confocal Raman microscopy combined with moving-window two-dimensional correlation spectroscopy (MW2D) technique to investigate the SC thickness. Additionally, we employ confocal Raman microscopy combined with perturbation-correlation moving-window two-dimensional correlation spectroscopy (PCMW2D) to precisely characterize the stratification of the SC. Furthermore, the two-dimensional correlation spectroscopy (2DCOS) method is utilized to examine the content of various conformations in the keratin secondary structure within the SC, as well as the subtle interrelationships between lipid and keratin structures.

Optimal configuration of confocal Raman spectroscopy for precisely determining stratum corneum thickness: Evaluation of the effects of polyoxyethylene stearyl ethers on skin

Properties regarding stratum corneum (SC), the outermost membrane of the skin, remain an active area in dermatologic and cosmetic research. The reduced thickness of SC is associated with varied adverse statuses such as skin lipid deficiency, skin barrier dysfunctions and skin deceases, etc. Emulsifiers with existing irritative effects on skin components also face the risk of decreasing SC thickness. We have been focusing on the effects of PEGylated emulsifiers on the skin and have an interest in finding the role of their polyethylene glycol (PEG)-chain length in tuning skin irritations. With this aim, PEG-stearyl ethers with different numbers of hydrophilic chains were applied on the skin, and their influence on skin thickness was discovered to determine their skin barrier effect. Confocal Raman spectroscopy (CRS) with extensive application in skin research was used here. To obtain the precise determination of skin thickness, our secondary aim was to find the optimal CRS configuration referring to varied objectives and pinhole sizes where further study is still in demand. Therefore, SC thickness measured via eddy current approach served as reference. The applied PEG-stearyl ethers formed the system to achieve varied thicknesses. Results confirmed that the skin interactions rose with increasing PEG-chain length, however only up to a certain limit, with decreasing effects recorded from PEG-40 stearyl ether and no effects observed from PEG-100 stearyl ether. Simultaneously, CRS combined with water immersion objective and 50 μm pinhole presented the most consistent values to the references and exhibited better spectral intensity and signal-to-noise ratio. Correlation plots involving different cases of configurations were calculated for error corrections. Taken together, this work helps to identify the potential mechanisms governing the interactions between PEG-stearyl ethers and skin and offers powerful evidence of using CRS as a reliable alternative to obtain accurate thickness values.

Investigation of TEMPO partitioning in different skin models as measured by EPR spectroscopy - Insight into the stratum corneum

Electron paramagnetic resonance (EPR) spectroscopy represents an established tool to study properties of microenvironments, e.g. to investigate the structure and dynamics of biological and artificial membranes. In this study, the partitioning of the spin probe 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) in ex vivo human abdominal and breast skin, ex vivo porcine abdominal and ear skin as well as normal and inflammatory in vitro skin equivalents was investigated by EPR spectroscopy. Furthermore, the stratum corneum (SC) lipid composition (as determined by high-performance thin-layer chromatography), SC lipid chain order (probed by infrared spectroscopy) and the SC thickness (investigated by histology) were determined in the skin models. X-band EPR measurements have shown that TEMPO partitions in the lipophilic and hydrophilic microenvironment in varying ratios in different ex vivo and in vitro skin models. Ex vivo human abdominal skin exhibited the highest amount of TEMPO in the lipophilic microenvironment. In contrast, the lowest amount of TEMPO in the lipophilic microenvironment was determined in ex vivo human breast skin and the inflammatory in vitro skin equivalents. Individual EPR spectra of epidermis including SC and dermis indicated that the lipophilic microenvironment of TEMPO mainly corresponds to the most lipophilic part of the epidermis, the SC. The amount of TEMPO in the lipophilic microenvironment was independent of the SC lipid composition and the SC lipid chain order but correlated with the SC thickness. In conclusion, EPR spectroscopy could be a novel technique to determine differences in the SC thickness, thus suitably complementing existing methods.

Confocal Raman microspectroscopy as an alternative method to investigate the extraction of lipids from stratum corneum by emulsifiers and formulations

The purpose of this study was to investigate the impact of emulsifiers and formulations on intercellular lipids of porcine stratum corneum (SC) and evaluate confocal Raman microscopy (CRM) as an alternative method in this research context. To this end, four different formulations were used: three conventional creams that contained ionic and/or non-ionic emulsifiers and one surfactants-free emulsion stabilized by a polymeric emulsifier. Additionally, all emulsifiers were tested in aqueous solution/dispersion in the respective concentrations as present in the formulations. CRM and HPTLC were used to analyse changes in SC lipid content after treatment. Furthermore, lipid extraction was visualized by fluorescence staining and SC thickness was measured by CRM and light microscopy. Various emulsifiers and emulsifier mixtures showed different impact on SC lipid content and SC thickness, while none of the tested formulations had any effect on SC lipids. Emulsifiers and their mixtures that reduced the lipids content also reduced SC thickness, indicating lipid extraction is the reason for SC thinning. Results from CRM and conventional methods showed a strong positive correlation for both lipid content and SC thickness measurements. With easy sample preparation and fast analytical readout, CRM has the potential to be a standardized analytical method for skin lipids investigation.

Assessment of Raman spectroscopy as a fast and non-invasive method for total stratum corneum thickness determination of pig skin

Determination of total stratum corneum (SC) thickness is necessary to construct accurate SC drug concentration depth profiles that are used to evaluate the skin absorption of locally acting active components. Currently, different established methods such as the microscopic or gravimetric approach, estimation via transepidermal water loss or NIR densitometry are used. However, some of them represent time consuming strategies. In the present study, Raman spectroscopy was assessed as a non-invasive and fast method for total SC thickness estimation. All techniques employed in this study yielded comparable results with SC values of 11.15 ± 1.52 μm derived from Raman experiments, 10.22 ± 2.64 μm from NIR densitometry measurements and 10.91 ± 2.03 μm from light microscopy studies suggesting Raman spectroscopy as an appropriate and rapid method for total SC thickness determination. As a further objective of the study, the storage conditions of the skin samples during Raman measurements and the impact of keeping the skin on the cartilage during NIR densitometry measurements were investigated. Skin samples can be stored dry during Raman measurements, if immediate measurement is not feasible. Furthermore, skin samples for NIR densitometry studies should be kept on the cartilage during the stripping procedure to avoid SC thickness underestimation.