Confocal Raman microscopic investigation of the effectiveness of penetration enhancers for procaine delivery to the skin

Confocal Raman spectroscopy coupled with in vitro permeation testing to study the effects of formalin fixation on the skin barrier function of reconstructed human epidermis

Confocal Raman Spectroscopy is recognised as a potent tool for molecular characterisation of biological specimens. There is a growing demand for In Vitro Permeation Tests (IVPT) in the pharmaceutical and cosmetic areas, increasingly conducted using Reconstructed Human Epidermis (RHE) skin models. In this study, chemical fixation of RHE in 10 % Neutral Buffered Formalin for 24 h has been examined for storing RHE samples at 4 °C for up to 21 days. Confocal Raman Spectroscopy (CRS), combined with Principal Components Analysis, revealed the molecular-level effects of fixation, notably in protein and lipid conformation within the stratum corneum and viable epidermis. IVPT by means of high-performance liquid chromatography, using caffeine as a model compound, showed minimal impact of formalin fixation on the cumulative amount, flux, and permeability coefficient after 12 h. While the biochemical architecture is altered, the function of the model as a barrier to maintain rate-limiting diffusion of active molecules within skin layers remains intact. This study opens avenues for enhanced flexibility and utility in skin model research, promising insights into mitigating the limited shelf life of RHE models by preserving performance in fixed samples for up to 21 days.

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.

Rational Design of Topical Semi-Solid Dosage Forms-How Far Are We?

Specific aspects of semi-solid dosage forms for topical application include the nature of the barrier to be overcome, aspects of susceptibility to physical and chemical instability, and a greater influence of sensory perception. Advances in understanding the driving forces of skin penetration as well as the design principles and inner structure of formulations, provide a good basis for the more rational design of such dosage forms, which still often follow more traditional design approaches. This review analyses the opportunities and constraints of rational formulation design approaches in the industrial development of new topical drugs. As the selection of drug candidates with favorable physicochemical properties increases the speed and probability of success, models for drug selection based on theoretical and experimental approaches are discussed. This paper reviews how progress in the scientific understanding of mechanisms and vehicle-influence of skin penetration can be used for rational formulation design. The characterization of semi-solid formulations is discussed with a special focus on modern rheological approaches and analytical methods for investigating and optimizing the chemical stability of active ingredients in consideration of applicable guidelines. In conclusion, the combination of a good understanding of scientific principles combined with early consideration of regulatory requirements for product quality are enablers for the successful development of innovative and robust semi-solid formulations for topical application.

Topically applied lipid-containing emulsions based on PEGylated emulsifiers: Formulation, characterization, and evaluation of their impact on skin properties ex vivo and in vivo

PEGylated emulsifiers have been largely used in topical formulations for skin research. They have been a continuous study focus in our group as well. According to our previous studies, severe interruptions of the skin barrier were observed with certain types of emulsifiers. To restore the skin barrier function and counteract the effects of emulsifiers, we considered topically delivering lipids into the lipid matrix of the SC. Herein, PEG-20 cetyl ether (C20) -based oil-in-water (O/W) emulsions were developed owing to the stronger interactions of C20 with skin. The lipids containing ceramides (Cers), palmitic acids (PA), and cholesterol with different ratios and combinations were merged into the base emulsion. PEG-40 stearyl ether (S40)-based emulsion was used as a reference as S40 showed negligible impact on SC lipids. The evaluations were conducted ex vivo with confocal Raman spectroscopy (CRS) regarding the SC lipid, SC thickness, and skin penetration properties. In parallel, the in vivo irritation studies were also implemented including the transepidermal-water-loss (TEWL), skin hydration, and erythema index. The results indicated less SC lipid extraction of topically delivered lipids on ex vivo porcine skin with the addition and ratio of incorporated Cers influencing the extent of formulations counteracting the skin interruption by C20. The ex vivo penetration study showed a similar trend in drug penetration depths. In regards to the in vivo studies, TEWL was demonstrated to be suitable for differentiating the impact on skin barrier properties. The in vivo observations were generally correlated with the ex vivo results. The exact findings in this research can lead us to a better selection of applied lipid components and compositions. Future research will elucidate which type of Cer was predominantly extracted by C20, advancing future formulation development.

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.

Estimating the Analytical Performance of Raman Spectroscopy for Quantification of Active Ingredients in Human Stratum Corneum

Confocal Raman microscopy (CRM) has become a versatile technique that can be applied routinely to monitor skin penetration of active molecules. In the present study, CRM coupled to multivariate analysis (namely PLSR—partial least squares regression) is used for the quantitative measurement of an active ingredient (AI) applied to isolated (ex vivo) human stratum corneum (SC), using systematically varied doses of resorcinol, as model compound, and the performance is quantified according to key figures of merit defined by regulatory bodies (ICH, FDA, and EMA). A methodology is thus demonstrated to establish the limit of detection (LOD), precision, accuracy, sensitivity (SEN), and selectivity (SEL) of the technique, and the performance according to these key figures of merit is compared to that of similar established methodologies, based on studies available in literature. First, principal components analysis (PCA) was used to examine the variability within the spectral data set collected. Second, ratios calculated from the area under the curve (AUC) of characteristic resorcinol and proteins/lipids bands (1400–1500 cm⁻¹) were used to perform linear regression analysis of the Raman spectra. Third, cross-validated PLSR analysis was applied to perform quantitative analysis in the fingerprint region. The AUC results show clearly that the intensities of Raman features in the spectra collected are linearly correlated to resorcinol concentrations in the SC (R² = 0.999) despite a heterogeneity in the distribution of the active molecule in the samples. The Root Mean Square Error of Cross-Validation (RMSECV) (0.017 mg resorcinol/mg SC), The Root Mean Square of Prediction (RMSEP) (0.015 mg resorcinol/mg SC), and R² (0.971) demonstrate the reliability of the linear regression constructed, enabling accurate quantification of resorcinol. Furthermore, the results have enabled the determination, for the first time, of numerical criteria to estimate analytical performances of CRM, including LOD, precision using bias corrected mean square error prediction (BCMSEP), sensitivity, and selectivity, for quantification of the performance of the analytical technique. This is one step further towards demonstrating that Raman spectroscopy complies with international guidelines and to establishing the technique as a reference and approved tool for permeation studies.

Thermal analysis of mammalian stratum corneum using differential scanning calorimetry for advancing skin research and drug delivery

For effective topical and transdermal drug delivery, it is necessary for most actives to penetrate and permeate through the stratum corneum (SC). Extensive investigation of the thermal behaviour of mammalian SC has been performed to understand the barrier function of the skin. However, little attention has been paid to the related experimental variables in thermal analysis of the SC using differential scanning calorimetry that may influence the results obtained from such studies. In this review, we provide a comprehensive overview of the thermal transitions of the SC of both porcine and human skin. More importantly, the selection and impact of the experimental and instrumental parameters used in thermal analysis of the SC are critically evaluated. New opportunities for the use of thermal analysis of mammalian SC in advancing skin research, particularly for elucidation of the actions of excipients employed in topical and transdermal formulations on the skin are also highlighted.

Confocal Raman spectroscopy at different laser wavelengths in analyzing stratum corneum and skin penetration properties of mixed PEGylated emulsifier systems

Emulsifier mixtures are widely used in cosmetics and pharmaceutics and thus, brought extensive studies for their performances on skin applications. PEG-20cetyl ether (C20) is recently proposed to induce skin irritation and is of interest to study its skin interactions when mixed with other emulsifiers. PEG-2oleyl ether (O2) and PEG-20stearyl ether (S20) are selected and in specific, 50 mM of C20, O2, S20 as well as Mix1 (50 mM C20 mixed with 50 mM O2) and Mix2 (50 mM C20 mixed with 50 mM S20) solutions were applied on skin samples. Confocal Raman spectroscopy (CRS) analyses of stratum corneum (SC) thickness and SC lipid content were performed after 4 h skin treatments. In parallel, skin penetration properties were also evaluated via CRS by applying procaine solutions with/without emulsifiers on skin samples for 24 h. In terms of the CRS measurements, two excitation wavelengths of 532 nm and 785 nm are both utilized in this study and we secondly aimed to compare their results and suitability in SC and skin analyses. Based on the experimental observations, comparable results are obtained by using both excitation wavelengths of 532 nm and 785 nm demonstrating their suitability in analyzing SC and skin samples. Thereinto, 785 nm laser wavelength shows the advantage of deeper skin penetration and allows the measurements of fluorescent skin samples; 532 nm laser wavelength enables simple measurement performance without substrate and coverslip interference. With regards to the results of emulsifier mixtures, the addition of S20 and O2 reduced the skin interactions and penetration enhancing ability of C20, giving us the hint to build milder systems with emulsifier mixtures. Besides, the CRS results of stronger skin interruption were also correlated with the higher critical micelle concentration (CMC) values of emulsifiers and their mixtures, which may provide evidence in explaining the interactions between emulsifiers and skin.

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.

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.

In-Line and Off-Line Monitoring of Skin Penetration Profiles Using Confocal Raman Spectroscopy

Ex-vivo and in-vivo skin analysis has been extensively evaluated by confocal Raman spectroscopy (CRS). The off-line measurement with a CRS-suited skin-mounted device after Franz-cell incubations is the most popular choice. However, real-time monitoring of in-line measurement has clear advantages for obtaining dynamic and more timely results. In our study, a custom-built setup suitable for in-line measurements was implemented, which ensures constant skin incubation and in-situ skin detections. We aim to compare the differences between using in-line and off-line devices for monitoring skin drug penetrations. A well-assessed formulation gel with procaine-HCl as the active ingredient was used as reference. The PEG-23 lauryl ether was added to the formulation as a penetration enhancer to evaluate the enhancement effects of procaine on skin. After incubation times of 14, 20, and 24 h, skin penetration profiles were assessed. Comparable results between off-line and in-line measurements were obtained. Remarkable improvements in penetrated procaine amount and depth were observed. Based on the significant differences of their enhanced penetration amounts, fairly similar estimations were achieved from both methods. A slight difference of 14 h incubation between these two setups can still be found, which may be due to the different detection conditions and affected skin properties. Overall, in-line measurements could provide a more time- and labor-saving alternative for off-line measurements in ex-vivo study.

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.

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 Method for In-Situ Skin Penetration Analysis by Confocal Raman Microscopy

In the development of dermal drug formulations and cosmetics, understanding the penetration properties of the active ingredients is crucial. Given that widespread methods, including tape stripping, lack in spatial resolution, while being time- and labour-intensive, Confocal Raman Microscopy is a promising alternative. In optimizing topically applied formulations, or the development of generic formulations, comparative in-situ measurements have a huge potential of saving time and resources. In this work, we show our approach to in-situ skin penetration analysis by confocal Raman Microscopy. To analyse feasibility of the approach, we used caffeine solutions as model vehicles and tested the effectiveness of 1,2-pentanediol as a penetration enhancer for delivery to the skin.

Systematic Investigation of the Effect of Non-Ionic Emulsifiers on Skin by Confocal Raman Spectroscopy-A Comprehensive Lipid Analysis

Non-ionic emulsifiers are commonly found in existing pharmaceutical and cosmetic formulations and have been widely employed to enhance the penetration and permeation of active ingredients into the skin. With the potential of disrupting skin barrier function and increasing fluidity of stratum corneum (SC) lipids, we herein examined the effects of two kinds of non-ionic emulsifiers on intercellular lipids of skin, using confocal Raman spectroscopy (CRS) with lipid signals on skin CRS spectrum. Non-ionic emulsifiers of polyethylene glycol alkyl ethers and sorbitan fatty acid esters were studied to obtain a deep understanding of the mechanism between non-ionic emulsifiers and SC lipids. Emulsifier solutions and dispersions were prepared and applied onto excised porcine skin. Water and sodium laureth sulfate solution (SLS) served as controls. SC lipid signals were analysed by CRS regarding lipid content, conformation and lateral packing order. Polyethylene glycol (PEG) sorbitan esters revealed no alteration of intercellular lipid properties while PEG-20 ethers appeared to have the most significant effects on reducing lipid content and interrupting lipid organization. In general, the polyoxyethylene chain and alkyl chain of PEG derivative emulsifiers might indicate their ability of interaction with SC components. HLB values remained critical for complete explanation of emulsifier effects on skin lipids. With this study, it is possible to characterize the molecular effects of non-ionic emulsifiers on skin lipids and further deepen the understanding of enhancing substance penetration with reduced skin barrier properties and increased lipid fluidity.

Permeation Efficacy of a Transdermal Vehicle with Steroidal Hormones and Nonsteroidal Anti-inflammatory Agents as Model Drugs

BACKGROUND

Transdermal delivery is an alternative route for the administration of drugs. However, it requires the development of vehicles that allow the drugs to cross the layers of the skin and reach the systemic circulation.

OBJECTIVE

In this study, a new transdermal vehicle was evaluated using progesterone, estradiol, estradiol + estriol (Biest) and ketoprofen administered as model drugs.

METHODS

To evaluate the ex vivo permeation of the drugs, the Franz vertical diffusion cell with human skin was used.

RESULTS

After 24 h, the vehicle was able to deliver 18.32 µg/cm2 of progesterone and 92.07 µg/cm2 of ketoprofen through the skin to the receptor medium. The permeation percentages were 91%, 78.8%, 48.5%, 73.2%, and 63.6%, respectively, for estradiol, estradiol (Biest), estriol (Biest), progesterone and ketoprofen. For all drugs, sufficient amounts were delivered to achieve a systemic effect, and it was also possible to decrease the amount of emulsion applied.

CONCLUSION

Thus, the vehicle demonstrated a high performance and the possibility of it being used for drugs that present difficulties in regards to administration by the transdermal route.

A comparative study of ex vivo skin optical clearing using two-photon microscopy

Multiphoton tomography (MPT) is a prospective tool for imaging the skin structure. Aiming to increase the probing depth, a comparative ex vivo study of optical clearing of porcine ear skin was performed by using two optical clearing agents (OCAs), i.e., glycerol and iohexol (Omnipaque^TM ) at different concentrations, which exhibit different osmotic properties. The results show that a topical application of glycerol or Omnipaque^TM solutions onto the skin for 60 min significantly improved the depth and contrast of the MPT signals. By utilizing 40%, 60% and 100% glycerol, and 60% and 100% Omnipaque^TM it was demonstrated that both agents improve autofluorescence and SHG (second harmonic generation) signals from the skin. At the applied concentrations and agent time exposure, glycerol is more effective than Omnipaque^TM . However, tissue shrinkage and cell morphology changes were found for highly concentrated glycerol solutions. Omnipaque^TM , on the contrary, increases the safety and has no or minimal tissue shrinkage during the optical clearing process. Moreover Omnipaque^TM allows for robust multimodal optical/X-ray imaging with automatically matched optically cleared and X-ray contrasted tissue volumes. These findings make Omnipaque^TM more prospective than glycerol for some particular application.

Ex vivo skin permeation and penetration of nonivamide from and in vivo skin tolerability of film-forming formulations containing porous silica

AIM

The purpose of this study was to evaluate skin permeation and penetration of nonivamide which has been formulated in novel film-forming formulations (FFFs). These formulations aim to prolong the availability of capsaicinoids which are used in long-term treatment of chronic pruritus.

METHODS

An oily solution of nonivamide was loaded into porous silica particles which then were suspended in an aqueous dispersion of a sustained release polymer. Permeation and penetration experiments were performed ex vivo with postauricular porcine skin using modified Franz diffusion cells. The penetrated drug amount was assessed ex vivo by skin surface biopsy followed by cryo-sectioning. Furthermore, in vivo skin irritation experiments were performed to compare the potential skin irritation caused by the FFFs to conventionally used semi-solid formulations.

RESULTS

Permeation rates of nonivamide from FFF through the skin are comparable to that from clinically used immediate release formulations. This elucidates the therapeutic safety profile of the novel FFF. Penetration studies confirmed the prolonged drug availability at the site of action. FFFs were found not to irritate the skin of healthy volunteers.

CONCLUSION

FFFs with sustained nonivamide penetration represent safe and easy-to-use formulations. They therefore may improve the treatment of chronic pruritus with capsaicinoids by enhancing patient compliance through a sustained release regime.

Confocal Raman microscopy and multivariate statistical analysis for determination of different penetration abilities of caffeine and propylene glycol applied simultaneously in a mixture on porcine skin ex vivo

Propylene glycol is one of the known substances added in cosmetic formulations as a penetration enhancer. Recently, nanocrystals have been employed also to increase the skin penetration of active components. Caffeine is a component with many applications and its penetration into the epidermis is controversially discussed in the literature. In the present study, the penetration ability of two components - caffeine nanocrystals and propylene glycol, applied topically on porcine ear skin in the form of a gel, was investigated ex vivo using two confocal Raman microscopes operated at different excitation wavelengths (785nm and 633nm). Several depth profiles were acquired in the fingerprint region and different spectral ranges, i.e., 526-600cm(-1) and 810-880cm(-1) were chosen for independent analysis of caffeine and propylene glycol penetration into the skin, respectively. Multivariate statistical methods such as principal component analysis (PCA) and linear discriminant analysis (LDA) combined with Student's t-test were employed to calculate the maximum penetration depths of each substance (caffeine and propylene glycol). The results show that propylene glycol penetrates significantly deeper than caffeine (20.7-22.0μm versus 12.3-13.0μm) without any penetration enhancement effect on caffeine. The results confirm that different substances, even if applied onto the skin as a mixture, can penetrate differently. The penetration depths of caffeine and propylene glycol obtained using two different confocal Raman microscopes are comparable showing that both types of microscopes are well suited for such investigations and that multivariate statistical PCA-LDA methods combined with Student's t-test are very useful for analyzing the penetration of different substances into the skin.

How Confocal Is Confocal Raman Microspectroscopy on the Skin? Impact of Microscope Configuration and Sample Preparation on Penetration Depth Profiles

BACKGROUND/AIMS

The aim of the study was to elucidate the effect of sample preparation and microscope configuration on the results of confocal Raman microspectroscopic evaluation of the penetration of a pharmaceutical active into the skin (depth profiling).

METHODS

Pig ear skin and a hydrophilic formulation containing procaine HCl were used as a model system. The formulation was either left on the skin during the measurement, or was wiped off or washed off prior to the analysis. The microscope configuration was varied with respect to objectives and pinholes used.

RESULTS

Sample preparation and microscope configuration had a tremendous effect on the results of depth profiling. Regarding sample preparation, the best results could be observed when the formulation was washed off the skin prior to the analysis. Concerning microscope configuration, the use of a 40 × 0.6 numerical aperture (NA) objective in combination with a 25-µm pinhole or a 100 × 1.25 NA objective in combination with a 50-µm pinhole was found to be advantageous.

CONCLUSION

Complete removal of the sample from the skin before the analysis was found to be crucial. A thorough analysis of the suitability of the chosen microscope configuration should be performed before acquiring concentration depth profiles.