In Vivo Human Skin Penetration Study of Sunscreens by Confocal Raman Spectroscopy

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.

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.

Characterization by Raman and infrared spectroscopy and fluorescence microscopy of human hair treated with cosmetic products

Analytical studies on hair structures have evolved significantly over the years and vibrational spectroscopic techniques, such as Raman and infrared, have been increasingly used for such purposes. Nowadays, there is a need to understand more and more about the action of cosmetics on the hair fiber, so this work aims to analyze the permeation of cosmetic treatments into the hair. For the molecular structural characterization, Raman and infrared spectroscopy techniques were used, being verified the efficiency in the analysis of hair samples, demonstrating the internal characteristics of the fiber and the permeation of different cosmetics. Four cosmetics were chosen for this study and, due to the techniques used, it was possible to observe the diffusion of these products inside the bleached hair. It was observed with the Raman vibrational spectroscopy that the concentration of the products is found mainly in the cuticular region, decreasing the permeate content when approaching the central region, and the infrared spectroscopy showed results compatible with the Raman spectroscopy. Therefore, vibrational spectroscopy proved to be a valuable tool for the study of cosmetic permeation into the hair fiber and for the analysis of its external and internal structure.

Confocal Raman Micro-Spectroscopy for Discrimination of Glycerol Diffusivity in Ex Vivo Porcine Dura Mater

Dura mater (DM) is a connective tissue with dense collagen, which is a protective membrane surrounding the human brain. The optical clearing (OC) method was used to make DM more transparent, thereby allowing to increase in-depth investigation by confocal Raman micro-spectroscopy and estimate the diffusivity of 50% glycerol and water migration. Glycerol concentration was obtained, and the diffusion coefficient was calculated, which ranged from 9.6 × 10^-6 to 3.0 × 10^-5 cm²/s. Collagen-related Raman band intensities were significantly increased for all depths from 50 to 200 µm after treatment. In addition, the changes in water content during OC showed that 50% glycerol induces tissue dehydration. Weakly and strongly bound water types were found to be most concentrated, playing a major role in the glycerol-induced water flux and OC. Results show that OC is an efficient method for controlling the DM optical properties, thereby enhancing the in-depth probing for laser therapy and diagnostics of the brain. DM is a comparable to various collagen-containing tissues and organs, such as sclera of eyes and skin dermis.

Novel aspects of Raman spectroscopy in skin research

The analytical technology of Raman spectroscopy has an almost 100‐year history. During this period, many modifications and developments happened in the method like discovery of laser, improvements in optical elements and sensitivity of spectrometer and also more advanced light detection systems. Many types of the innovative techniques appeared (e.g. Transmittance Raman spectroscopy, Coherent Raman Scattering microscopy, Surface‐Enhanced Raman scattering and Confocal Raman spectroscopy/microscopy). This review article gives a short description about these different Raman techniques and their possible applications. Then, a short statistical part is coming about the appearance of Raman spectroscopy in the scientific literature from the beginnings to these days. The third part of the paper shows the main application options of the technique (especially confocal Raman spectroscopy) in skin research, including skin composition analysis, drug penetration monitoring and analysis, diagnostic utilizations in dermatology and cosmeto‐scientific applications. At the end, the possible role of artificial intelligence in Raman data analysis and the regulatory aspect of these techniques in dermatology are briefly summarized. For the future of Raman Spectroscopy, increasing clinical relevance and in vivo applications can be predicted with spreading of non‐destructive methods and appearance with the most advanced instruments with rapid analysis time.

Current views on non-invasive in vivo determination of physiological parameters of the stratum corneum using confocal Raman microspectroscopy

Confocal Raman microspectroscopy is widely used in dermatology and cosmetology for analysis of the concentration of skin components (lipids, natural moisturizing factor molecules, water) and the penetration depth of cosmetic/medical formulations in the human stratum corneum (SC) in vivo. In recent years, it was shown that confocal Raman microspectroscopy can also be used for non-invasive in vivo depth-dependent determination of the physiological parameters of the SC, such as lamellar and lateral organization of intercellular lipids, folding properties of keratin, water mobility and hydrogen bonding states. The results showed that the strongest skin barrier function, which is primarily manifested by the orthorhombic organization of intercellular lipids, is provided at ≈20-40% SC depth, which is related to the maximal bonding state of water with surrounding components in the SC. The secondary and tertiary structures of keratin determine water binding in the SC, which is depth-dependent. This paper shows the technical possibility and advantage of confocal Raman microspectroscopy in non-invasive investigation of the skin and summarizes recent results on in vivo investigation of the human SC.

Revisiting techniques to evaluate drug permeation through skin

INTRODUCTION

Investigating the transportation of a drug molecule through various layers of skin and determining the amount of drug retention in skin layers is of prime importance in transdermal and topical drug delivery. The information regarding drug permeation and retention in skin layers aids in optimizing a formulation and provides insight into the therapeutic efficacy of a formulation.

AREAS COVERED

This perspective covers various methods that have been explored to estimate drug/therapeutics in skin layers using in vitro, ex vivo, and in vivo conditions. In vitro methods such as diffusion techniques, ex vivo methods such as isolated perfused skin models and in vivo techniques including dermato-pharmacokinetics employing tape stripping, and microdialysis are discussed. Application of all techniques at various stages of formulation development where various local and systemic effects need to be considered.

EXPERT OPINION

The void in the existing methodologies necessitates improvement in the field of dermatologic research. Standardization of protocols, experimental setups, regulatory guidelines, and further research provides information to select an alternative for human skin to perform skin permeation experiments to increase the reliability of data generated through the available techniques. There is a need to utilize multiple techniques for appropriate dermato-pharmacokinetics evaluation and formulation's efficacy.

Image of the distribution profile of targets in skin by Raman spectroscopy-based multivariate analysis

Raman spectroscopic imaging is a label-free spectral technology to investigate the distribution of transdermal targets in skin. However, it is difficult to analyze low content of analytes in skin by direct imaging analysis. Combining Raman mapping technology with multiple linear regression algorithms, concentration contribution factor of targets in ex vivo human skin tissue at every point has been calculated. The distribution profiles are visualized as heat maps demonstrating the targets levels in different skin layers. This method has been successfully employed to investigate the vibrational imaging of distribution of hyaluronic acid and lidocaine in skin. Moreover, three dimensional (3D) images of the penetration profiles of hyaluronic acid with different molecular weight have been obtained. The results from 3D images were in good agreement with these from two-dimensional images, indicating that this method was a reliable way for monitoring the distribution of targets in skin.

Taxifolin as a Promising Ingredient of Cosmetics for Adult Skin

Active substances, effective in the reduction in or delay of skin changes caused by aging occurring in natural compounds, are desirable. Taxifolin (TXF), a flavonoid of strong antioxidant activity found in the plant Stizolophus balsamita (S. balsamita), has been tested for its biological effects on adult human skin. The aim of the study was to investigate the effects of two creams: 3% S. balsamita extract and 3% TXF on the function of adult skin. In total, 97 Caucasian women with clinical signs of skin aging were investigated. The biophysical and biomechanical skin parameters were measured before and after applying the creams, using Colorimeter CL400, Mexameter MX16, Skin-pH-Meter PH900, Skin-Thermometer ST 500, Glossymeter GL200, and Cutiscan SC100. Patch tests were performed with the investigated products to assess their potential irritant properties. The percutaneous penetration of creams was examined with the use of electrospray ionization mass spectrometry (ESI-MS) and confocal Raman spectroscopy. The 3% S. balsamita extract cream reduced hyperpigmentation, erythema, and elevated pH. All the tested preparations were proven to be nonirritant. A higher penetration rate was revealed for the 3% TXF cream than for the 3% S. balsamita extract cream. A total of 3% TXF cream improved skin viscoelasticity. The obtained results suggested that S. balsamita extract and TXF may be considered as ingredients of skincare products for adults.

A review of cosmetic skin delivery

BACKGROUND

Cosmetic Skin Delivery has a very important impact on the action of cosmetics. More and more cosmetic manufacturers are focusing on cosmetic delivery. Meanwhile, it also brings safety issues and the customization of national laws and regulations.

OBJECTIVES

This paper reviews the theoretical knowledge about cosmetic skin delivery and evaluation methods.

METHODS

An extensive literature search was conducted using PubMed, Web of Science, and other databases for articles on transdermal/skin delivery in cosmetics from 1985 to 2020.

RESULTS

The importance of skin delivery in cosmetics is outlined. The structure of the skin and the skin barrier, including delivery pathways available to cosmetic molecules in three modalities are introduced. The laws and regulations of various countries on nanomaterials for cosmetics are listed. The in vitro skin absorption test methods for cosmetics are briefly reviewed. Furthermore, the prospect of future skin penetration methods for cosmetics is presented.

CONCLUSIONS

Currently, various methods to enhance and evaluate cosmetic delivery through skin are available. However, there are no unified domestic and international laws and regulations about evaluation of transdermal delivery. This article provides a new perspective on the development of novel permeation enhancement technologies.

Assessment of Transdermal Delivery of Topical Compounds in Skin Scarring Using a Novel Combined Approach of Raman Spectroscopy and High-Performance Liquid Chromatography

Objective: The goal of any topical formulation is efficient transdermal delivery of its active components. However, delivery of compounds can be problematic with penetration through tough layers of fibrotic dermal scar tissue. Approach: We propose a new approach combining high-performance liquid chromatography (HPLC) and Raman spectroscopy (RS) using a topical of unknown composition against a well-known antiscar topical (as control). Results: Positive detection of compounds within the treatment topical using both techniques was validated with mass spectrometry. RS detected conformational structural changes; the 1,655/1,446 cm^-1 ratio estimating collagen content significantly decreased (p < 0.05) over weeks 4, 12, and 16 compared with day 0. The amide I band, known to represent collagen and protein in skin, shifted from 1,667 to 1,656 cm^-1, which may represent a change from β-sheets in elastin to α-helices in collagen. Confirmatory elastin immunohistochemistry decreased compared with day 0, conversely the collagen I/III ratio increased in the same samples by week 12 (p < 0.05, and p < 0.0001, respectively), in keeping with normal scar formation. Optical coherence tomography attenuation coefficient representing collagen deposition was significantly decreased at week 4 compared with day 0 and increased at week 16 (p < 0.05). Innovation: This study provides a platform for further research on the simultaneous evaluation of the effects of compounds in cutaneous scarring by RS and HPLC, and identifies a role for RS in the therapeutic evaluation and theranostic management of skin scarring. Conclusions: RS can provide noninvasive information on the effects of topicals on scar pathogenesis and structural composition, validated by other analytical techniques.

Unplanned absorption of sunscreen ingredients: Impact of formulation and evaluation methods

Permeation of sunscreens agents reduces its effectiveness and safety, leading to systemic circulation and causing unknown adverse effects. In order to maintain the sunscreen efficacy and safety, the filters must stay on the skin surface, with minimum penetration through dermis. Even facing the possibility of filters permeation, the use of sunscreen is important to avoid skin damage as erythema, free-radicals formation, skin ageing and skin cancer, caused by ultraviolet radiation. Aiming potential side effects caused by topical absorption of sunscreens, studies are carried to improve formulation characteristics and stability, reduce skin permeation and evaluate sun protections factor (SPF). Current assays to detect the permeation of sunscreens involve in vivo or in vitro studies, to simulate physiological conditions of use. The aim of this review is to revisit sunscreen skin permeation data over the last decade and the factors that can enhance skin permeation or improve the sunscreen efficacy.

Raman mapping coupled to self-modelling MCR-ALS analysis to estimate active cosmetic ingredient penetration profile in skin

Confocal Raman mapping (CRM) is a powerful, label free, non-destructive tool, enabling molecular characterization of human skin with applications in the dermo-cosmetic field. Coupling CRM to multivariate analysis can be used to monitor the penetration and permeation of active cosmetic ingredients (ACI) after topical application. It is presently illustrated how multivariate curve resolution alternating least squares (MCR-ALS) can be applied to detect and semi-quantitatively describe the diffusion profile of Delipidol, a commercially available slimming ACI, from Raman spectral maps. Although the analysis outcome can be critically dependent on the a priori selection of the number of regression components, it is demonstrated that profiling of the kinetics of diffusion into the skin can be established with or without additionnal spectral equality constraints in the multivariate analysis, with similar results. Ultimately, MCR-ALS, applied without spectral equality contraints, specifically identifies the ACI as one of main spectral components enabling to investigate its distribution and penetration into the stratum corneum and underlying epidermis layers.

2-Hydroxy-4-methoxybenzophenone Enhances the Suppression of Superoxide Anion Radicals Generated via UVA-induced Photosensitizing by t-Butyl Methoxydibenzoylmethane

4-tert-Butyl-4'-methoxydibenzoylmethane (BMDM) is widely used throughout the world as a highly effective UVA absorber that can prevent the progression of photoaging in skin. However, due to its low photostability, BMDM is also known for the disadvantage of having a reduced capability to absorb UVA during prolonged exposure to sunlight. Although many studies have been carried out to overcome this disadvantage of BMDM, little attention has been paid to how the radicals generated from BMDM during UV exposure influence the skin. Therefore, the purpose of this study was twofold: One goal was to clarify the influence of radicals on human skin using cytotoxicity as a parameter. The second was to propose a solution that could reduce the radical formation while taking photostability into consideration. Using ESR spin trapping and superoxide dismutase (SOD) treatment, the radicals produced by the UV exposure of BMDM were shown to be superoxide anion radicals (•O₂^-). HaCaT keratinocytes exposed to UVA in the presence of BMDM showed a significant reduction in cell viability, indicating that the radicals produced from BMDM have a harmful influence on the skin. UVA exposure coincidently led to a reduction of UVA absorbance by BMDM. Interestingly, 2-hydroxy-4-methoxybenzophenone (Benzophenone-3; BP3) reduced both the radical formation and the cytotoxicity resulting from the UVA-exposure of BMDM, while also restoring its UVA absorbance. In conclusion, the results show that BMDM and BP3 is an effective combination to reduce the influence of UVA-exposed BMDM on the skin and to prevent the loss of UVA absorbance by BMDM during UV exposure.

Insights and controversies on sunscreen safety

Although sunlight provides several benefits, ultraviolet (UV) radiation plays an important role in the development of various skin damages such as erythema, photoaging, and photocarcinogenesis. Despite cells having endogenous defense systems, damaged DNA may not be efficiently repaired at chronic exposure. In this sense, it is necessary to use artificial defense strategies such as sunscreen formulations. UV filters should scatter, reflect, or absorb solar UV radiation in order to prevent direct or indirect DNA lesions. However, the safety of UV filters is a matter of concern due to several controversies reported in literature, such as endocrine alterations, allergies, increased oxidative stress, phototoxic events, among others. Despite these controversies, the way in which sunscreens are tested is essential to ensure safety. Sunscreen regulation includes mandatory test for phototoxicity, but photogenotoxicity testing is not recommended as a part of the standard photosafety testing program. Although available photobiological tests are still the first approach to assess photosafety, they are limited. Some existing tests do not always provide reliable results, mainly due to limitations regarding the nature of the assessed phototoxic effect, cell UV sensitivity, and the irradiation protocols. These aspects bring queries regarding the safety of sunscreen wide use and suggest the demand for the development of robust and efficient in vitro screening tests to overcome the existing limitations. In this way, Saccharomyces cerevisiae has stood out as a promising model to fill the gaps in photobiology and to complete the mandatory tests enabling a more extensive and robust photosafety assessment.

The application of label-free imaging technologies in transdermal research for deeper mechanism revealing

The penetration behavior of topical substances in the skin not only relates to the transdermal delivery efficiency but also involves the safety and therapeutic effect of topical products, such as sunscreen and hair growth products. Researchers have tried to illustrate the transdermal process with diversified theories and technologies. Directly observing the distribution of topical substances on skin by characteristic imaging is the most convincing approach. Unfortunately, fluorescence labeling imaging, which is commonly used in biochemical research, is limited for transdermal research for most topical substances with a molecular mass less than 500 Da. Label-free imaging technologies possess the advantages of not requiring any macromolecular dyes, no tissue destruction and an extensive substance detection capability, which has enabled rapid development of such technologies in recent years and their introduction to biological tissue analysis, such as skin samples. Through the specific identification of topical substances and endogenous tissue components, label-free imaging technologies can provide abundant tissue distribution information, enrich theoretical and practical guidance for transdermal drug delivery systems. In this review, we expound the mechanisms and applications of the most popular label-free imaging technologies in transdermal research at present, compare their advantages and disadvantages, and forecast development prospects.

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.

Caffeine delivery in porcine skin: a confocal Raman study

Confocal Raman microscopy is a novel optical method for studies of pro-drug and drug delivery. This method is a promising technique that enables non-destructive measurement of the permeation profile through skin layers. Peaks of compounds are usually normalised to skin peaks (amino-acid and amide I) for semi-quantitative evaluation. The present study seeks to optimise a methodology for complete quantitative measurement of the amount of an active compound at different depths. Caffeine was used as a tracer to evaluate compound's skin penetration using confocal Raman microscopy. A semi-quantitative depth profile of caffeine was obtained with normalisation of the Raman intensities. These ratios of Raman intensities were correlated with the caffeine concentration using an external calibration curve. The calibration curve was carried out with porcine skin incubated in different concentrations of caffeine; afterwards, each skin sample was analysed by confocal Raman microscopy and HPLC to determine the relation between the Raman signal intensity and the caffeine concentration per skin mass and to create a depth profile. These correlation curves allow the full quantification of the caffeine in skin from Raman intensity ratios at different depths.

Human skin in vivo has a higher skin barrier function than porcine skin ex vivo-comprehensive Raman microscopic study of the stratum corneum

Porcine skin is widely used as a human skin model in dermatology. For both, porcine stratum corneum (SC) ex vivo and human SC in vivo, the hydrogen bonding states of water, the secondary and tertiary structures of keratin, the natural moisturizing factor (NMF) concentrations and the intercellular lipids' (ICL) lateral organization are investigated depth-dependently using confocal Raman microscopy. The SC depth profiles show that porcine SC ex vivo is characterized by lower hydrogen bonding states of water (10%-30% SC depth), lower NMF concentration in the whole SC, more β-sheet form of keratin (10%-90% SC depth), more folded tertiary keratin structures (30%-70% SC depth) and higher hexagonal lateral packing order of ICL (10%-50% SC depth) compared to human SC in vivo. The results clearly show a higher value of skin barrier function of human SC in vivo than of porcine SC ex vivo. Thus, the human SC in vivo is less permeable for lipophilic and hydrophilic substances than porcine SC ex vivo. Considering the porcine SC as an ex vivo model of human SC in vivo, these findings should be taken into consideration.