Penetration monitoring of drugs and additives by ATR-FTIR spectroscopy/tape stripping and confocal Raman spectroscopy - A comparative study

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.

Structural and Functional Analysis of Excised Skins and Human Reconstructed Epidermis with Confocal Raman Spectroscopy and in Microfluidic Diffusion Chambers

Several ex vivo and in vitro skin models are available in the toolbox of dermatological and cosmetic research. Some of them are widely used in drug penetration testing. The excised skins show higher variability, while the in vitro skins provide more reproducible data. The aim of the current study was to compare the chemical composition of different skin models (excised rat skin, excised human skin and human-reconstructed epidermis) by measurement of ceramides, cholesterol, lactate, urea, protein and water at different depths of the tissues. The second goal was to compile a testing system, which includes a skin-on-a-chip diffusion setup and a confocal Raman spectroscopy for testing drug diffusion across the skin barrier and accumulation in the tissue models. A hydrophilic drug caffeine and the P-glycoprotein substrate quinidine were used in the study as topical cream formulations. The results indicate that although the transdermal diffusion of quinidine is lower, the skin accumulation was comparable for the two drugs. The various skin models showed different chemical compositions. The human skin was abundant in ceramides and cholesterol, while the reconstructed skin contained less water and more urea and protein. Based on these results, it can be concluded that skin-on-a-chip and confocal Raman microspectroscopy are suitable for testing drug penetration and distribution at different skin layers within an exposition window. Furthermore, obese human skin should be treated with caution for skin absorption testing due to its unbalanced composition.

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.

Characterization and ex vivo evaluation of excised skin samples as substitutes for human dermal barrier in pharmaceutical and dermatological studies

BACKGROUND

Excised animal and human skins are frequently used in permeability testing in pharmaceutical research. Several factors exist that may have influence on the results. In the current study some of the skin parameters that may affect drug permeability were analysed for human, mouse, rat and pig skin.

MATERIALS AND METHODS

Classic biophysical skin parameters were measured (e.g. pH, hydration, permittivity, transepidermal water loss). Physiological characteristics of the skins were also analysed by confocal Raman spectroscopy, scanning electron microscopy and two-photon microscopy.

RESULTS

Based on biophysical testing, skin barrier function was damaged in psoriatic mouse skin and in marketed pig skin. Hydration and pH values were similar among the species, but freezing and thawing reduced the water content of the skins and shifted the surface pH to acidic. Aging reduced hydration and permittivity, resulting in impaired barrier function. Mechanical sensitization used in permeability studies resulted in proportional thinning of dead epidermis.

DISCUSSION

Results indicate that depending on the scientific question it should be considered whether fresh or frozen tissue is used, and for certain purposes rodent skins are well usable. The structure of the skin tissue (ceramide, cholesterol, keratin, natural moisturizing factor or urea) is similar in rats and mice, but due to the higher skin thickness the lipid distribution is different in porcine skin. Psoriasis led to irregular chemical composition of the skin.

CONCLUSION

A comprehensive evaluation of skin samples of four species was performed. The biophysical and microscopic observations should be considered when selecting drug penetration models and experimental conditions.

In Vitro Assessment of Sunscreen Efficacy Using Fourier Transform Infrared (FTIR) Spectroscopy on Synthetic Skin

Although there are several methods for assessing the sun protection factor (SPF) of sunscreen products, there is no standard and reliable in vitro method. Each test entails limitations and drawbacks. Therefore, this study aimed to assess the employability of FTIR as an alternative and quick method to evaluate the efficacy of various sunscreen formulations, their concentrations, and the timing of their application. Infrared radiation has longer wavelengths than ultraviolet, penetrates deeply into the skin, and hence enables the understanding of sunscreens' ability to block the transmission of radiation. The FTIR transmission using synthetic skin to study the effect of sunscreen agents (oxybenzone, octyl methoxycinnamate, titanium dioxide (TiO₂), and zinc oxide (ZnO)) was conducted in the range 450-4000cm^-1. Comparison studies were made at the peak of 805cm^-1. After 2 h of sunscreen application, using the maximum concentrations, the FTIR peak at wavenumber 805cm^-1 demonstrated a significant reduction of transmission from 96.55 to 60.09%, 57.59%, 32.02%, and 37.1% for oxybenzone, octyl methoxycinnamate, TiO₂, and ZnO respectively (P<0.05). A significant reduction in transmission was observed (P<0.05) with increasing sunscreen concentrations after 2 h of application. Nevertheless, the upper limit of concentration showed no appreciable change from the middle level of concentration, and hence it is cost-effective to employ the middle concentration. Inorganic sunscreens showed a higher protection level than organic. Fixed-dose combinations of sunscreens showed an enhanced effect yet were not synergistic. In conclusion, the use of FTIR spectroscopy with synthetic skin is a quick and user-friendly technique that enables the assessment of the efficacy of sunscreen formulations.

Resveratrol Nanocrystal Incorporated into Mesoporous Material: Rational Design and Screening through Quality-by-Design Approach

Bioflavonoids from grape seeds feature powerful antioxidant and immunostimulant activities, but they present problems related to solubility and bioavailability. Nanocrystal (NC) incorporated into a mesoporous carrier is a promising strategy to address these issues. However, the preparation of this formulation involves the selection of factors affecting its critical quality attributes. Hence, this study aimed to develop an NC formulation incorporating resveratrol into a soluble mesoporous carrier based on rational screening design using a systematic and continuous development process, the quality-by-design paradigm. A mesoporous soluble carrier was prepared by spray-drying mannitol and ammonium carbonate. The NC was obtained by introducing the evaporated solvent containing a drug/polymer/surfactant and mesoporous carrier to the medium. A 2^6-2 fractional factorial design (FFD) approach was carried out in the screening process to understand the main effect factors. The type and concentration of polymer and surfactant, resveratrol loading, and solvent were determined on the NC characteristics. The results indicated that drug loading, particle size, and solubility were mainly affected by RSV loading, PEG concentration, and Kolliphor EL concentration. The polymer contributed dominantly to reducing the particle size and enhancing solubility in this screening design. The presence of surfactants in this system made it possible to prolong the supersaturation process. According to the 2^6-2 FFD, the factors selected to be further developed using a statistical technique according to the quality-by design-approach, Box Behnken Design, were Kolliphor EL, PEG400, and RSV loading.

In vitro release, ex vivo penetration, and in vivo dermatokinetics of ketoconazole-loaded solid lipid nanoparticles for topical delivery

The study focused to evaluate and investigate optimized (using QbD) and novel ketoconazole (KTZ)-loaded solid lipid nanoparticles (KTZ-SLNs; 2% w/v KTZ) for enhanced permeation across skin. KTZ-SLNs were evaluated for size, distribution, zeta potential (ZP), percent entrapment efficiency (%EE), drug release, morphology (HRTEM and FESEM), thermal behaviour (DSC), spectroscopic (FTIR), and solid-state/diffraction characterization (X-ray diffraction, XRD). Moreover, ex vivo permeation and drug deposition into rat skin were conducted using Franz diffusion cell. The same was confirmed using human dermatome skin and fluorescence, confocal Raman, and vibrational ATR-FTIR microscopic methods. An in vivo dermatokinetics study was performed in rats to assess the extent of KTZ permeation into the skin. Stability including accelerated and photostability studies were conducted at different temperatures (2-8, 30, and 40 °C) for 12 months. The spherical, optimized KTZ-SLN formulation (KOF1) showed particle size of 293 nm and high EE of 88.5%. Results of FTIR, DSC, and XRD confirmed formation of KTZ-SLNs and their amorphous nature due to presence of KTZ in a dissolved state in the lipid matrix. In vitro release was slow and sustained whereas ex vivo permeation parameters were significantly high in KTZ-SLNs as compared to free drug suspension (KTZ-SUS) and marketed product (Nizral®; 2% KTZ w/v). Drug retention was 10- and five-fold higher than KTZ-SUS and marketed product, respectively. In vivo dermatokinetics parameters improved significantly with SLN formulation (410-900% enhanced). Confocal Raman spectroscopy experiment showed that KTZ-SLNs could penetrate beyond the human stratum corneum into viable epidermis. Fluorescent microscopy also indicated improved penetration of KTZ-SLNs. KTZ-SLNs were photostable and showed long-term stability over 12 months under set conditions.

Critical parameters for accurate monitoring of caffeine penetration in porcine skin using confocal Raman spectroscopy

In this research, we addressed a challenge while measuring the penetration performance of caffeine (CAF) using confocal Raman spectroscopy (CRS). Normally in the process of CRS analysis, skin sample was moved from an incubation setup to a specified CRS-measuring sample holder. Accurate data collection may be questioned due to the variation of the environment the skin placed in. Therefore, two critical parameters including the CRS measuring temperature and proper skin hydration were focused; accordingly, four different conditions were designed. First, the skin was incubated in a real-time device with the skin placing onto PBS-filled chamber where the temperature was adjusted to 32℃. This device can be fixed under the CRS microscope, enabling simultaneous skin incubation and dynamic CRS measurements (condition i, reference). The other conditions referred to skins incubated in Franz diffusion cells for simulating the common experimental procedures. In order to control variables of CRS measuring condition, skins were transferred from cells to real-time device and open device. In real-time device, proper skin hydration was maintained and the skin temperature was adjusted to 32℃ (condition ii) and room temperature (condition iii). In open device, the skin was in a less hydrated state by moving onto a PBS-soaked filter paper and wrapped with aluminum foil at room temperature (condition iv). The skin penetration performances measured in these conditions were compared with reference. Caffeine solution and gel formulation were separately applied to the skin. The results showed in both cases that the decrease of skin temperature and hydration in condition iii and iv would apparently induce the decrease of detected caffeine signal, resulting in the inaccurate data collection. To this point, it indicates the reduction of solubilized caffeine in skin layer. We suggest the forming of caffeine crystallization at varied skin conditions to be the factor. Achieved video image, CRS spectrum collection and surface scan demonstrated the caffeine crystallization process on superficial skin layer. Polarized microscopic images exemplified the crystalline drug on tape stripped skin layers. It can be a potential support of caffeine crystallization inside skin. In short, we suggest the consideration of these parameters during CRS measurements for accurate monitoring of topical drug delivery. Meanwhile, the use of real-time device for dynamic skin incubation and data collection provides advantages in saving time and efforts in this study.

Physiologically-Based Pharmacokinetic Modeling to Support Determination of Bioequivalence for Dermatological Drug Products: Scientific and Regulatory Considerations

Physiologically-based pharmacokinetic (PBPK) modeling and simulation provides mechanism-based predictions of the pharmacokinetics of an active ingredient following its administration in humans. Dermal PBPK models describe the skin permeation and disposition of the active ingredient following the application of a dermatological product on the skin of virtual healthy and diseased human subjects. These models take into account information on product quality attributes, physicochemical properties of the active ingredient and skin (patho)physiology, and their interplay with each other. Regulatory and product development decision makers can leverage these quantitative tools to identify factors impacting local and systemic exposure. In the realm of generic drug products, the number of US Food and Drug Administratioin (FDA) interactions that use dermal PBPK modeling to support alternative bioequivalence (BE) approaches is increasing. In this report, we share scientific considerations on the development, verification and validation (V&V), and application of PBPK models within the context of a virtual BE assessment for dermatological drug products. We discuss the challenges associated with model V&V for these drug products stemming from the fact that target-site active ingredient concentrations are typically not measurable. Additionally, there are no established relationships between local and systemic PK profiles, when the latter are quantifiable. To that end, we detail a multilevel model V&V approach involving validation for the model of the drug product of interest coupled with the overall assessment of the modeling platform in use while leveraging in vitro and in vivo data related to local and systemic bioavailability.

Profiling skin penetration using PEGylated emulsifiers as penetration enhancers via confocal Raman spectroscopy and fluorescence spectroscopy

Non-ionic emulsifiers have been continuous research focus in skin analysis. With the aim of finding their role as penetration enhancers in dermal drug delivery systems, PEGylated emulsifiers of polyethylene glycol (PEG) ethers were targeted to be investigated ex-vivo. The effectiveness of them in the enhancement of skin penetration was examined by conventional tape stripping method and confocal Raman spectroscopy (CRS). Fluorescein sodium salt (Fluo-Na) and procaine HCl were respectively used as model drugs. The drug delivery performances were compared in the aspects of penetration amount and depth. Based on the results from both analyses, all investigated emulsifiers have the ability to enhance the amount of drug penetration. PEG-20 ethers showed higher ability than PEG-2 oleyl ether (O2) in promoting drug distribution by depth, especially PEG-20 cetyl ether (C20) showed a distinct effect. According to this study, their penetration enhancing performances seem to be linked to their interruption of intercellular lipids, which can be considered as the underlying mechanism for governing the ability of PEGylated emulsifiers as penetration enhancers. Further instrumental comparison highlighted the benefits of using CRS as an alternative in skin penetration analysis.

Dermal Delivery of Niacinamide-In Vivo Studies

In vivo human studies are considered to be the “gold standard” when investigating (trans)dermal delivery of actives. Previously, we reported the effects of a range of vehicles on the delivery of niacinamide (NIA) using conventional Franz cell studies. In the present work, dermal delivery of NIA was investigated in vivo in human subjects using confocal Raman spectroscopy (CRS) and tape stripping (TS). The vehicles investigated included propylene glycol (PG), Transcutol^® P (TC), binary combinations of PG with oleic acid (OA) or linolenic acid (LA) and a ternary system comprising of TC, caprylic/capric triglyceride (CCT) and dimethyl isosorbide (DMI). For the CRS studies, higher area under curve (AUC) values for NIA were observed for the PG:LA binary system compared with PG, TC and TC:CCT:DMI (p < 0.05). A very good correlation was found between the in vitro cumulative permeation of NIA and the AUC values from Raman intensity depth profiles, with a Pearson correlation coefficient (R²) of 0.84. In addition, an excellent correlation (R² = 0.97) was evident for the signal of the solvent PG and the active. CRS was also shown to discriminate between NIA in solution versus crystalline NIA. The findings confirm that CRS is emerging as a powerful approach for dermatopharmacokinetic studies of both actives and excipients in human.

X-Ray Photoelectron Spectroscopy on Microbial Cell Surfaces: A Forgotten Method for the Characterization of Microorganisms Encapsulated With Surface-Engineered Shells

Encapsulation of single microbial cells by surface-engineered shells has great potential for the protection of yeasts and bacteria against harsh environmental conditions, such as elevated temperatures, UV light, extreme pH values, and antimicrobials. Encapsulation with functionalized shells can also alter the surface characteristics of cells in a way that can make them more suitable to perform their function in complex environments, including bio-reactors, bio-fuel production, biosensors, and the human body. Surface-engineered shells bear as an advantage above genetically-engineered microorganisms that the protection and functionalization added are temporary and disappear upon microbial growth, ultimately breaking a shell. Therewith, the danger of creating a "super-bug," resistant to all known antimicrobial measures does not exist for surface-engineered shells. Encapsulating shells around single microorganisms are predominantly characterized by electron microscopy, energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, particulate micro-electrophoresis, nitrogen adsorption-desorption isotherms, and X-ray diffraction. It is amazing that X-ray Photoelectron Spectroscopy (XPS) is forgotten as a method to characterize encapsulated yeasts and bacteria. XPS was introduced several decades ago to characterize the elemental composition of microbial cell surfaces. Microbial sample preparation requires freeze-drying which leaves microorganisms intact. Freeze-dried microorganisms form a powder that can be easily pressed in small cups, suitable for insertion in the high vacuum of an XPS machine and obtaining high resolution spectra. Typically, XPS measures carbon, nitrogen, oxygen and phosphorus as the most common elements in microbial cell surfaces. Models exist to transform these compositions into well-known, biochemical cell surface components, including proteins, polysaccharides, chitin, glucan, teichoic acid, peptidoglycan, and hydrocarbon like components. Moreover, elemental surface compositions of many different microbial strains and species in freeze-dried conditions, related with zeta potentials of microbial cells, measured in a hydrated state. Relationships between elemental surface compositions measured using XPS in vacuum with characteristics measured in a hydrated state have been taken as a validation of microbial cell surface XPS. Despite the merits of microbial cell surface XPS, XPS has seldom been applied to characterize the many different types of surface-engineered shells around yeasts and bacteria currently described in the literature. In this review, we aim to advocate the use of XPS as a forgotten method for microbial cell surface characterization, for use on surface-engineered shells encapsulating microorganisms.

In Vitro-In Vivo Correlation in Dermal Delivery: The Role of Excipients

The composition of topical and transdermal formulations is known to determine the rate and the extent of drug delivery to and through the skin. However, to date, the role of excipients in these formulations on skin delivery of actives has received little attention from scientists in the field. Monitoring skin absorption of both drug and vehicle may provide insights into the mechanism by which excipients promote permeation and may facilitate the design of effective and safer products. Previously, we have investigated the use of quantitative Confocal Raman Spectroscopy (CRS) to investigate the delivery of an active to the skin, and we also reported the first fully quantitative study that compared this method with the well-established in vitro permeation test (IVPT) model. To further explore the potential of quantitative CRS in assessing topical delivery, the present work investigated the effects of commonly used excipients on the percutaneous absorption of a model drug, ibuprofen (IBU). Permeation of IBU and selected solvents following finite dose applications to human skin was determined in vitro and in vivo by Franz diffusion studies and quantitative CRS, respectively. The solvents used were propylene glycol (PG), dipropylene glycol (DPG), tripropylene glycol (TPG), and polyethylene glycol 300 (PEG 300). Overall, the cumulative amounts of IBU that permeated at 24 h in vitro were similar for PG, DPG, and TPG (p > 0.05). These three vehicles outperformed PEG 300 (p < 0.05) in terms of drug delivery. Concerning the vehicles, the rank order for in vitro skin permeation was DPG ≥ PG > TPG, while PEG 300 did not permeate the skin. A linear relationship between maximum vehicle and IBU flux in vitro was found, with a correlation coefficient (R²) of 0.95. When comparing in vitro with in vivo data, a positive in vitro–in vivo (IVIV) correlation between the cumulative permeation of IBU in vitro and the total amount of IBU that penetrated the stratum corneum (SC) in vivo was observed, with a Pearson correlation coefficient (R²) of 0.90. A strong IVIV correlation, R² = 0.82, was found following the linear regression of the cumulative number of solvents permeated in vitro and the corresponding skin uptake in vivo measured with CRS. This is the first study to correlate in vivo permeation of solvents measured by CRS with data obtained by in vitro diffusion studies. The IVIV correlations suggest that CRS is a powerful tool for profiling drug and vehicle delivery from dermal formulations. Future studies will examine additional excipients with varying physicochemical properties. Ultimately, these findings are expected to lead to new approaches for the design, evaluation, and optimization of formulations that target actives to and through the skin.

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.

A new perspective of membrane fouling control by ultraviolet synergic ferrous iron catalytic persulfate (UV/Fe(II)/PS) as pretreatment prior to ultrafiltration

The main purpose of this study was to control membrane fouling by degrading natural organic matter, mainly based on free radicals, with a dead-end ultrafiltration system integrated with pretreatment. Four advanced oxidation processes, namely, ultraviolet (UV)/Fe(II), UV/persulfate (PS), Fe(II)/PS and UV/Fe(II)/PS, were used to pretreat raw water prior to ultrafiltration. The priority of the pretreatment effect followed the order of UV/Fe(II)/PS > Fe(II)/PS > UV/PS > UV/Fe(II). In the UV/Fe(II)/PS pretreatment (Fe(II) = 100 μM and PS = 400 μM), the removal rates of UV₂₅₄ with a UV irradiation time of 60 min reached 93.07%. Degradation experiments of free-radical probes (carbamazepine) and free-radical scavenger addition (sodium hyposulfite or tert-butanol) showed that the sulfate radical (SO₄^-) was dominant in degrading organic compounds. The specific flow rate (J/J₀) increased by 139.13% and the irreversible fouling resistance was reduced by 69.94%. The total interfacial energy of the colloid-membrane interaction decreased by 84.42% and the separation distance was shortened to ~2 nm. The release of Fe(III) from water under UV radiation and its possible conversion to Fe(II) were observed on the surface of the fouled membranes. After UV/Fe(II)/PS pretreatment, bulky and rough pollutant particles were transformed into a slew of sheet-contaminated layer with the appearance of numerous permeable holes, and the average surface roughness was reduced to 38.1 nm according to atomic-force-microscopy characterization.

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.

Mechanism investigation of ethosomes transdermal permeation

Ethosomes are widely used to promote transdermal permeation of both lipophilic and hydrophilic drugs, but the mechanism of interaction between the ethosomes and the skin remains unclear. In this work, it was exploded with several technologies and facilities. Firstly, physical techniques such as attenuated total reflectance fourier-transform infrared and laser confocal Raman were used and the results indicated that the phospholipids configuration of stratum corneum changes from steady state to unstable state with the treatment of ethosomes. Differential scanning calorimetry reflected the thermodynamics change in stratum corneum after treatment with ethosomes. The results revealed that the skin of Bama mini-pigs, which is similar to human skin, treated by ethosomes had a relatively low T_m and enthalpy. Scanning electron microscopy and transmission electron microscopy showed that the microstructure and ultrastructure of stratum corneum was not damaged by ethosomes treatment. Furthermore, confocal laser scanning microscopy revealed that lipid labeled ethosomes could penetrate the skin via stratum corneum mainly through intercellular route, while during the process of penetration, phospholipids were retained in the upper epidermis. Cell experiments confirmed that ethosomes were distributed mainly on the cell membrane. Further study showed that only the drug-loaded ethosomes increased the amount of permeated drug. The current study, for the first time, elucidated the mechanistic behavior of ethosomes in transdermal application from molecular configuration, thermodynamic properties, ultrastructure, fluorescent labeling and cellular study. It is anticipated that the approaches and results described in the present study will benefit for better design of drug-loaded ethosomes.

Methods for Optical Skin Clearing in Molecular Optical Imaging in Dermatology

This short review describes recent progress in using optical clearing (OC) technique in skin studies. Optical clearing is an efficient tool for enhancing the probing depth and data quality in multiphoton microscopy and Raman spectroscopy. Here, we discuss the main mechanisms of OC, its safety, advantages, and limitations. The data on the OC effect on the skin water content are presented. It was demonstrated that 70% glycerol and 100% Omnipaque^TM 300 reduce the water content in the skin. Both OC agents (OCAs) significantly affect the strongly bound and weakly bound water. However, Omnipaque^TM 300 causes considerably less skin dehydration than glycerol. In addition, the results of examination of the OC effect on autofluorescence in two-photon excitation and background fluorescence in Raman scattering at different skin depths are presented. It is shown that Omnipaque^TM 300 is a promising OCA due to its ability to reduce background fluorescence in the upper skin layers. The possibility of multimodal imaging combining optical methods and OC technique is discussed.

The role of viscosity on skin penetration from cellulose ether-based hydrogels

Background

The rheological properties of dermal drug delivery systems are of importance when designing new formulations. Viscosity not only affects features such as spreadability and skin feel, but may also affect the skin penetration of incorporated actives. Data on the latter aspect are controversial. Our objective was to elucidate the relation between viscosity and drug delivery performance of different model hydrogels assuming that enhanced microviscosity might delay drug release and penetration.

Materials and Methods

Hydrogels covering a broad viscosity range were prepared by adding either HPMC or HEC as gelling agents in different concentrations. To investigate the ability of the gels to deliver a model drug into the skin, sulphadiazine sodium was incorporated and its in vitro skin penetration was monitored using tape stripping/HPLC analysis and non‐invasive confocal Raman spectroscopy.

Results

The trends observed with the two different experimental setups were comparable. Drug penetration depths decreased slightly with increasing viscosity, suggesting slower drug release due to the increasingly dense gel networks. However, the total penetrated drug amounts were independent of the exact formulation viscosity.

Conclusion

Drug penetration was largely unaffected by hydrogel viscosity. Moderately enhanced viscosity is advisable when designing cellulose ether hydrogels to allow for convenient application.

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.