MCR Approach Revealing Protein, Water, and Lipid Depth Profile in Atopic Dermatitis Patients’ Stratum Corneum via in Vivo Confocal Raman Spectroscopy

Effect of Penetration Enhancer on the Structure of Stratum Corneum: On-Site Study by Confocal Polarized Raman Imaging

Keratin and lipid structures in the stratum corneum (SC) are closely related to the SC barrier function. The application of penetration enhancers (PEs) disrupts the structure of SC, thereby promoting infiltration. To quantify these PE-induced structural changes in SC, we used confocal Raman imaging (CRI) and polarized Raman imaging (PRI) to explore the integrity and continuity of keratin and lipid structures in SC. The results showed that water is the safest PE and that oleic acid (OA), sodium dodecyl sulfate (SDS), and low molecular weight protamine (LMWP) disrupted the ordered structure of keratin, while azone and liposomes had less of an effect on keratin. Azone, OA, and SDS also led to significant changes in lipid structure, while LMWP and liposomes had less of an effect. Establishing this non-invasive and efficient strategy will provide new insights into transdermal drug delivery and skin health management.

Quantification of skin penetration of caffeine and propylene glycol applied topically in a mixture by tailored multivariate curve resolution-alternating least squares of depth-resolved Raman spectra

The quantitative determination of topically applied substances in the skin is severely limited and represents a challenging task. The porcine skin ex vivo was topically treated with a gel containing caffeine (CF) and propylene glycol (PG), and depth-resolved Raman spectra were recorded with two confocal Raman microscopes. We applied a novel tailored multivariate curve resolution-alternating least squares method to the selected spectral regions (512-604 and 778-1148 cm-1 ) of gel-treated skin and quantitatively determined the concentrations of CF and PG in the stratum corneum (SC). The highest concentration of CF (181 mg/cm3 ) was found at the surface, while PG (384 mg/cm3 ) was found at 10% SC depth, indicating the formation of a reservoir at the superficial SC. The concentrations of CF and PG decreased monotonically and reached the detection limit at ≈60% and ≈80% SC depth, respectively, indicating that neither permeate the SC.

Atopic Dermatitis: Molecular Alterations between Lesional and Non-Lesional Skin Determined Noninvasively by In Vivo Confocal Raman Microspectroscopy

Atopic dermatitis (AD)/atopic eczema is a chronic relapsing inflammatory skin disease affecting nearly 14% of the adult population. An important pathogenetic pillar in AD is the disrupted skin barrier function (SBF). The atopic stratum corneum (SC) has been examined using several methods, including Raman microspectroscopy, yet so far, there is no depth-dependent analysis over the entire SC thickness. Therefore, we recruited 21 AD patients (9 female, 12 male) and compared the lesional (LAS) with non-lesional atopic skin (nLAS) in vivo with confocal Raman microspectroscopy. Our results demonstrated decreased total intercellular lipid and carotenoid concentrations, as well as a shift towards decreased orthorhombic lateral lipid organisation in LAS. Further, we observed a lower concentration of natural moisturising factor (NMF) and a trend towards increased strongly bound and decreased weakly bound water in LAS. Finally, LAS showed an altered secondary and tertiary keratin structure, demonstrating a more folded keratin state than nLAS. The obtained results are discussed in comparison with healthy skin and yield detailed insights into the atopic SC structure. LAS clearly shows molecular alterations at certain SC depths compared with nLAS which imply a reduced SBF. A thorough understanding of these alterations provides useful information on the aetiology of AD and for the development/control of targeted topical therapies.

Optical Methods for Non-Invasive Determination of Skin Penetration: Current Trends, Advances, Possibilities, Prospects, and Translation into In Vivo Human Studies

Information on the penetration depth, pathways, metabolization, storage of vehicles, active pharmaceutical ingredients (APIs), and functional cosmetic ingredients (FCIs) of topically applied formulations or contaminants (substances) in skin is of great importance for understanding their interaction with skin targets, treatment efficacy, and risk assessment-a challenging task in dermatology, cosmetology, and pharmacy. Non-invasive methods for the qualitative and quantitative visualization of substances in skin in vivo are favored and limited to optical imaging and spectroscopic methods such as fluorescence/reflectance confocal laser scanning microscopy (CLSM); two-photon tomography (2PT) combined with autofluorescence (2PT-AF), fluorescence lifetime imaging (2PT-FLIM), second-harmonic generation (SHG), coherent anti-Stokes Raman scattering (CARS), and reflectance confocal microscopy (2PT-RCM); three-photon tomography (3PT); confocal Raman micro-spectroscopy (CRM); surface-enhanced Raman scattering (SERS) micro-spectroscopy; stimulated Raman scattering (SRS) microscopy; and optical coherence tomography (OCT). This review summarizes the state of the art in the use of the CLSM, 2PT, 3PT, CRM, SERS, SRS, and OCT optical methods to study skin penetration in vivo non-invasively (302 references). The advantages, limitations, possibilities, and prospects of the reviewed optical methods are comprehensively discussed. The ex vivo studies discussed are potentially translatable into in vivo measurements. The requirements for the optical properties of substances to determine their penetration into skin by certain methods are highlighted.

Topical dexpanthenol effects on physiological parameters of the stratum corneum by Confocal Raman Microspectroscopy

BACKGROUND

Topical use of dexpanthenol presents well-established moisturizing properties and maintenance and repair of the skin barrier function, however, its exact action mechanisms are not completely elucidated. In this context, Confocal Raman Microspectroscopy is an optical method that enables non-invasive and non-destructive in vivo analysis with the sensitive acquisition of molecular changes in different skin layers. Herein, the aim was to evaluate the effects of topical dexpanthenol on the components and physiological parameters of the stratum corneum (SC).

MATERIALS AND METHODS

Ten healthy female subjects underwent skin evaluation by means of a Confocal Raman Spectrometer Skin Analyzer 3510. Spectral data were obtained from the skin of the anterior forearm region, before and 2 h after applying a cosmetic formulation containing or not containing 5% dexpanthenol.

RESULTS

Semiquantitative analysis of the natural moisturizing factor showed a significant decrease in content after 2 h of topical dexpanthenol application, while the analysis of the lamellar organization of intercellular lipids and the secondary structure of keratin showed a significant increase in hexagonal organization of lipids at the first half of the SC and a significant increase in β-pleated sheet conformation of keratin.

CONCLUSION

Effects of topical dexpanthenol on SC suggest a contribution in increasing fluidity of both lipidic and protein components of the SC and are compatible with dexpanthenol activity in maintaining adequate physiological conditions and preventing transepidermal water loss. This study also contributes to the elucidation of action mechanisms and other concurrent biochemical processes.

Continuous Removal of Single Cell Layers by Tape Stripping the Stratum Corneum - a Histological Study

Studies on the penetration of toxicologically or pharmaceutically relevant substances through the skin and, more specifically, through the stratum corneum (s.c.) often rely on the well-established method of tape stripping. Tape stripping involves the removal of skin layers by means of adhesive tape, which is usually followed by quantification of dermally applied substances in these layers. However, the amount of s.c. removed by each individual tape strip is still a matter of scientific debate. While some studies imply that the amount of s.c. adhering to each tape strip decreases with increasing depth into the s.c., others observed a constant removal rate. All these studies rely on the quantification of the amount of s.c. captured on individual or pooled tape strips. Here, we present an approach whereby we measured the amount of s.c. remaining on excised porcine skin in the process of tape stripping. Staining and bloating of the s.c. allowed to measure its thickness and to count individual s.c. layers, respectively. Histologically, we show that the s.c. remaining on the skin decreased linearly as a function of strips taken. We found that each tape strip removes about 0.4µm of s.c., which corresponds to approximately one cellular layer. With a high coefficient of determination (r²>0.95), we were able to linearly correlate the thickness of the remaining s.c., the number of remaining cell layers and the number of tape strips applied. Furthermore, we elaborate on possible reasons for the discrepancies reported in the scientific literature regarding the amount of s.c. removed by each tape strip.

Quantitative determination of concentration profiles of skin components and topically applied oils by tailored multivariate curve resolution-alternating least squares using in vivo confocal Raman micro-spectroscopy

The main components of the stratum corneum (SC), water, lipids, and proteins, are non-homogeneously distributed throughout the depth. The quantitative determination of their concentration profiles and penetration depth of topically applied substances are urgent topics of dermatological and cosmetic research. Confocal Raman micro-spectroscopy has distinct advantages when determining semi-quantitative concentrations of SC components and topically applied substances non-invasively and in vivo. In this work, we applied a tailored multivariate curve resolution-alternating least squares (tMCR-ALS) method to analyze Raman spectra of the SC in the 2000-4000 cm-1 region for quantitatively determining the concentrations of water, lipids, proteins, and topically applied oils using substance-related spectral loadings which were allowed to change depth-dependently from the SC's surface toward its bottom. tMCR-ALS makes matching of depth-dependent signal attenuation, that is, the normalization on keratin, unnecessary and requires only a few additional experiments for calibration - Raman spectra of the pure materials and their densities.

Direct intracellular detection of biomolecule specific bound-water with Raman spectroscopy

Lipids, proteins, and nucleic acids have closely associated water molecules (Bound water), which exhibit considerably different physical properties compared to bulk water. Here we investigate the possibility of resolving Raman spectra of the specific hydration shell of these biomolecules in intracellular regions using Raman imaging. Lipids and proteins + nucleic acids Raman spectral components resolved in the analysis showed associated water spectral features, which are uniquely different from that of bulk water. These spectral profiles agree with water spectral profile observed in the case of corresponding hydrated pure biomolecules. The results show the prospects of Raman imaging in examining intracellular hydration in biomolecules and its functional relation.

Quantitative insights into tightly and loosely bound water in hydration shells of amino acids

The hydration of amino acids closely correlates the hydration of peptides and proteins and is critical to their biological functions. However, complete and quantitative understanding about the hydration of amino acids is lacking. Here, tightly and loosely bound water of 20 zwitterionic amino acids are quantitatively distinguished and determined by Raman spectroscopy with multivariate curve resolution (Raman-MCR) and differential scanning calorimetry (DSC). The total hydration water obtained from Raman-MCR and the tightly bound water determined by DSC have certain relevance, but they do not exactly correspond. In particular, Pro, Arg and Lys exhibit larger number of tightly bound water molecules (4.02-6.59), showing a significant influence on the onset transition temperature and the melting enthalpy values of water molecules, which provides direct evidence for their unique functions associated with biological water. Asn, Ser, Thr, Met, His and Glu have a smaller number of tightly bound water molecules (0.30-1.31), whilst the other remaining 11 amino acids only contain loosely bound water molecules. Four exceptional amino acids Ile, Leu, Phe and Val show fewer tightly bound water molecules but a higher number of loosely bound water molecules. As for the hydration shell structure, most amino acids except Pro and Trp enhance tetrahedral water structure and H-bonds relative to pure water and at least 1.9% of the hydration water molecules associated with the amino acids show non-hydrogen-bonded OH defects. This work combines two effective experimental techniques to reveal the hydration water structure and quantitatively analyze two kinds of bound water molecules of 20 amino acids.

Blind source separation of molecular components of the human skin in vivo: non-negative matrix factorization of Raman microspectroscopy data

Determination of the molecular composition of the skin is crucial for numerous tasks in medicine, pharmacology, dermatology and cosmetology. Confocal Raman microspectroscopy is a sensitive method for the evaluation of molecular depth profiles in the skin in vivo. Since the Raman spectra of most of the skin constituents significantly superimpose, a spectral decomposition by a set of predefined library components is usually performed to disentangle their contributions. However, the incorrect choice of the number and type of components or differences between the spectra of the basic components measured in vitro and in vivo can lead to incorrect results of the decomposition procedure. Here, we investigate an alternative data-driven approach based on a non-negative matrix factorization (NNMF) algorithm of depth-resolved Raman spectra of skin that does not require a priori information of spectral data for the analysis. Using the model and experimentally measured depth-resolved Raman spectra of the upper epidermis in vivo, we show that NNMF provides depth profiles of endogenous molecular components and exogenous agents penetrating through the upper epidermis for the spectra and concentration. Moreover, we demonstrate that this approach is capable of providing new information on the molecular profiles of the skin.

Tracking heavy-water-incorporated confocal Raman spectroscopy for evaluating the effects of PEGylated emulsifiers on skin barrier

The class of PEGylated emulsifiers finds broad application in the pharmaceutical and cosmetic industry. We target on one of the categories of polyethylene glycol (PEG) alkyl ethers with different lipophilic and hydrophilic chain length and aim to examine their effects on the skin comprehensively. In this study, we employed confocal Raman spectroscopy for skin depth profiling and imaging. A unique probe of heavy water (D₂ O) was incorporated, which can be tracked percutaneously and simultaneously monitor the effects caused by emulsifiers. According to the results, most of the PEGylated emulsifiers caused changes in skin lipid content/organization and induced the alteration in relative water content/hydrogen bonding structure. The results obtained from the depth profiling analysis provided the possibility to estimate the least penetration depth of emulsifiers. Among them, PEG-20 ethers displayed the most penetration ability. Meanwhile, it is interesting to find that the treatment of emulsifiers also affected the spatial distribution of D₂ O whose differences were in line with the molecular skin variations. In particular, the isotopic H/D substitution in the skin was highlighted in detail. This result supports the possibility to use D₂ O as an excellent and cost-effective probe to evaluate the skin barrier function.

Response to comment by Puppels et al. on "A modification for the calculation of water depth profiles in oil-treated skin by in vivo Raman microscopy"

The presence of penetrated oils in the stratum corneum (SC), oil-induced occlusion of the SC and formation of occluding homogeneous film on the skin surface are discussed in relation to their influence on results of water profile calculations using conventional and newly proposed extended methods. It is shown that the conventional method does not determine the water profiles in treated skin correctly due to the superposition of Raman bands of SC's proteins and penetrated and remnant oils.

A modification for the calculation of water depth profiles in oil-treated skin by in vivo confocal Raman microscopy

In this study, an extended calculation method for the determination of the water profiles in oil-treated skin is proposed, which is based on the calculation of the ratio between the Raman band intensities of water (3350-3550 cm^-1 ) and keratin Amide I at 1650 cm^-1 . The proposed method is compared with the conventional method based on the ratio of the Raman band intensities of water (3350-3550 cm^-1 ) and keratin at 2930 cm^-1 . The conventional method creates artifacts in the depth profiles of the water concentration in oil-treated skin, showing a lower amount of water in the upper and intermediate layers of the stratum corneum, which is due to the superposition of oil- and keratin-related Raman bands at 2930 cm^-1 . The proposed extended method shows no artifacts and has the potential to determine the water depth profiles after topical application of formulations on the skin.

In vivo Tracking of DNA for Precise Determination of the Stratum Corneum Thickness and Superficial Microbiome Using Confocal Raman Microscopy

The skin barrier function is mostly provided by the stratum corneum (SC), the uppermost layer of the epidermis. To noninvasively analyze the physiological properties of the skin barrier functionin vivo, it is important to determine the SC thickness. Confocal Raman microscopy (CRM) is widely used for this task. In the present in vivo study, a new method based on the determination of the DNA concentration profile using CRM is introduced for determining the SC thickness. The obtained SC thickness values are compared with those obtained using other CRM-based methods determining the water and lipid depth profiles. The obtained results show almost no significant differences in SC thickness for the utilized methods. Therefore, the results indicate that it is possible to calculate the SC thickness by using the DNA profile in the fingerprint region, which is comparable with the SC thickness calculated by the water depth profiles (ANOVA test p = 0.77) and the lipid depth profile (ANOVA test p = 0.74). This provides the possibility to measure the SC thickness by using the DNA profile, in case the water or lipid profile analyses are influenced by a topically applied formulation. The increase in DNA concentration in the superficial SC (0-2 µm) is related to the DNA presence in the microbiome of the skin, which was not present in the SC depth below 4 µm.

Non-invasive depth profiling of the stratum corneum in vivo using confocal Raman microscopy considering the non-homogeneous distribution of keratin

Confocal Raman microscopy has a number of advantages in investigating the human stratum corneum (SC) in vivo and ex vivo. The penetration profiles of xenobiotics in the SC, as well as depth profiles of the physiological parameters of the SC, such as the concentration of water depending on the strength of hydrogen bonds, total water concentration, the hydrogen bonding state of water molecules, concentration of intercellular lipids, the lamellar and lateral packing order of intercellular lipids, the concentration of natural moisturizing factor molecules, carotenoids, and the secondary and tertiary structure properties of keratin are well investigated. To consider the depth-dependent Raman signal attenuation, in most cases a normalization procedure is needed, which uses the main SC's protein keratin-related Raman peaks, based on the assumption that keratin is homogeneously distributed in the SC. We found that this assumption is not accurate for the bottom part of the SC, where the water concentration is considerably increased, thus, reducing the presence of keratin. Our results demonstrate that the bottom part of the SC depth profile should be multiplied by 0.94 in average in order to match this non-homogeneity, which result in a decrease of the uncorrected values in these depths. The correctly normalized depth profiles of the concentration of lipids, water, natural moisturizing factor and carotenoids are presented in this work. The obtained results should be taken into consideration in future skin research using confocal Raman microscopy.

Review of Modern Techniques for the Assessment of Skin Hydration

Skin hydration is a complex process that influences the physical and mechanical properties of skin. Various technologies have emerged over the years to assess this parameter, with the current standard being electrical probe-based instruments. Nevertheless, their inability to provide detailed information has prompted the use of sophisticated spectroscopic and imaging methodologies, which are capable of in-depth skin analysis that includes structural and composition details. Modern imaging and spectroscopic techniques have transformed skin research in the dermatological and cosmetics disciplines, and are now commonly employed in conjunction with traditional methods for comprehensive assessment of both healthy and pathological skin. This article reviews current techniques employed in measuring skin hydration, and gives an account on their principle of operation and applications in skin-related research.

An initial estimation method using cosine similarity for multivariate curve resolution: application to NMR spectra of chemical mixtures

Mixture spectra is decomposed into pure spectra without prior knowledge, and the MCR calculation refines the spectra and provides the concentrations.