Verification of P-Glycoprotein Function at the Dermal Barrier in Diffusion Cells and Dynamic "Skin-On-A-Chip" Microfluidic Device

In Vitro Functional and Structural Evaluation of Low-Complexity Artificial Human Epidermis for 3D Tissue Engineering

In recent times, with the need for a reduction, refinement, and replacement of in vivo animal testing, there has been an increasing demand for the use of relevant in vitro human cell systems in drug development. There is also a great demand for the replacement of skin tissue in various wounds and burns. Furthermore, human skin cell-based in vitro systems can be used to investigate the side effects (toxicity and irritation) and tissue penetration of topical preparations. In this study, exploratory experiments were performed to produce artificial epidermis using two hydrogel scaffolds, alginate and GelMA C. The amount of keratinocytes added to the matrix (10-50-100 × 106/mL) and the duration of tissue maturation (fresh, 1-3-4 weeks) were optimized in an extensive study. The behavior and structure of the two hydrogels were functionally and morphologically assessed. The permeability order for caffeine in the tested barriers was the following: alginate > GelMA C > cellulose acetate membrane > rat skin. It was concluded that GelMA C matrix provides a more favorable environment for cell survival and tissue differentiation (as demonstrated by histology and immunohistochemistry) than alginate. The 3-week incubation and 50 × 106/mL cell number proved to be the most beneficial in the given system. This study provides data for the first time on the multifactorial optimization of two potential skin substitutes for tissue manufacturing. In order to use these results in tissue engineering, the fabricated artificial epidermis preparations must also be optimized for biocompatibility and from physical and mechanical point of views.

Tape Stripping Method in Dermatological and Pharmacological Research: Evaluating Fresh and Frozen-Thawed Porcine Skin

Background The stratum corneum (SC) plays a crucial role in protecting the skin and regulating water loss. Tape stripping is a well-established method for studying skin barrier function and evaluating topical treatments. However, the behavior of fresh versus frozen-thawed skin during tape stripping has not been extensively compared. Objective This study aims to compare the removal of the stratum corneum from fresh and frozen-thawed porcine skin using tape stripping. It also aims to assess the reliability of tape weighing versus histological methods in quantifying SC removal. Methods Fresh and frozen-thawed porcine ears were obtained, cleaned, and subjected to tape stripping at varying numbers of strips from zero to 40. Tape weight and histological measurements were used to quantify SC removal. Statistical analyses were conducted to compare SC thickness and tape weight between the two types of skin. Results The study found that frozen-thawed skin exhibited a non-linear rate (r = 0.65) of SC removal per tape strip in the first five strips compared to a linear removal for fresh skin (r = 0.96). By the fifth tape strip, frozen-thawed samples had lost 80.6% of their SC, while fresh samples had only lost 33.5% (P < 0.03). Tape weighing and histological measurements showed strong correlations (r = 0.93 for fresh skin and r = 0.95 for frozen-thawed skin), indicating that tape weighing is a reliable alternative to histology for assessing SC removal on both sample types. Conclusions Fresh and frozen-thawed porcine skin respond differently to tape stripping, with frozen-thawed skin showing accelerated SC removal in the first five strips. The strong correlation between tape weighing and histological analysis supports the use of tape weighing as a practical tool for evaluating SC removal. These findings have implications for specimen selection and methodological standardization in dermatological and pharmacological research. Future research should explore alternative preservation and SC thickness measurement methods and their impact on tape stripping outcomes.

Skin-on-a-chip technologies towards clinical translation and commercialization

Skin is the largest organ of the human body which plays a critical role in thermoregulation, metabolism (e.g. synthesis of vitamin D), and protection of other organs from environmental threats, such as infections, microorganisms, ultraviolet radiation, and physical damage. Even though skin diseases are considered to be less fatal, the ubiquity of skin diseases and irritation caused by them highlights the importance of skin studies. Furthermore, skin is a promising means for transdermal drug delivery, which requires a thorough understanding of human skin structure. Current animal andin vitrotwo/three-dimensional skin models provide a platform for disease studies and drug testing, whereas they face challenges in the complete recapitulation of the dynamic and complex structure of actual skin tissue. One of the most effective methods for testing pharmaceuticals and modeling skin diseases are skin-on-a-chip (SoC) platforms. SoC technologies provide a non-invasive approach for examining 3D skin layers and artificially creating disease models in order to develop diagnostic or therapeutic methods. In addition, SoC models enable dynamic perfusion of culture medium with nutrients and facilitate the continuous removal of cellular waste to further mimic thein vivocondition. Here, the article reviews the most recent advances in the design and applications of SoC platforms for disease modeling as well as the analysis of drugs and cosmetics. By examining the contributions of different patents to the physiological relevance of skin models, the review underscores the significant shift towards more ethical and efficient alternatives to animal testing. Furthermore, it explores the market dynamics ofin vitroskin models and organ-on-a-chip platforms, discussing the impact of legislative changes and market demand on the development and adoption of these advanced research tools. This article also identifies the existing obstacles that hinder the advancement of SoC platforms, proposing directions for future improvements, particularly focusing on the journey towards clinical adoption.

Models for barrier understanding in health and disease in lab-on-a-chips

The maintenance of body homeostasis relies heavily on physiological barriers. Dysfunction of these barriers can lead to various pathological processes, including increased exposure to toxic materials and microorganisms. Various methods exist to investigate barrier function in vivo and in vitro. To investigate barrier function in a highly reproducible manner, ethically, and high throughput, researchers have turned to non-animal techniques and micro-scale technologies. In this comprehensive review, the authors summarize the current applications of organ-on-a-chip microfluidic devices in the study of physiological barriers. The review covers the blood-brain barrier, ocular barriers, dermal barrier, respiratory barriers, intestinal, hepatobiliary, and renal/bladder barriers under both healthy and pathological conditions. The article then briefly presents placental/vaginal, and tumour/multi-organ barriers in organ-on-a-chip devices. Finally, the review discusses Computational Fluid Dynamics in microfluidic systems that integrate biological barriers. This article provides a concise yet informative overview of the current state-of-the-art in barrier studies using microfluidic devices.

Topical drug delivery strategies for enhancing drug effectiveness by skin barriers, drug delivery systems and individualized dosing

Topical drug delivery is widely used in various diseases because of the advantages of not passing through the gastrointestinal tract, avoiding gastrointestinal irritation and hepatic first-pass effect, and reaching the lesion directly to reduce unnecessary adverse reactions. The skin helps the organism to defend itself against a huge majority of external aggressions and is one of the most important lines of defense of the body. However, the skin's strong barrier ability is also a huge obstacle to the effectiveness of topical medications. Allowing the bioactive, composition in a drug to pass through the stratum corneum barrier as needed to reach the target site is the most essential need for the bioactive, composition to exert its therapeutic effect. The state of the skin barrier, the choice of delivery system for the bioactive, composition, and individualized disease detection and dosing planning influence the effectiveness of topical medications. Nowadays, enhancing transdermal absorption of topically applied drugs is the hottest research area. However, enhancing transdermal absorption of drugs is not the first choice to improve the effectiveness of all drugs. Excessive transdermal absorption enhances topical drug accumulation at non-target sites and the occurrence of adverse reactions. This paper introduces topical drug delivery strategies to improve drug effectiveness from three perspectives: skin barrier, drug delivery system and individualized drug delivery, describes the current status and shortcomings of topical drug research, and provides new directions and ideas for topical drug research.

Quantitative Analysis of a Pilot Transwell Barrier Model with Automated Sampling and Mathematical Modeling

In the preclinical phase of drug development, it is necessary to determine how the active compound can pass through the biological barriers surrounding the target tissue. In vitro barrier models provide a reliable, low-cost, high-throughput solution for screening substances early in the drug candidate development process, thus reducing more complex and costly animal studies. In this pilot study, the transport properties of TB501, an antimycobacterial drug candidate, were characterized using an in vitro barrier model of VERO E6 kidney cells. The compound was delivered into the apical chamber of the transwell insert, and its concentration passing through the barrier layer was measured through the automated sampling of the basolateral compartment, where media were replaced every 30 min for 6 h, and the collected samples were stored for further spectroscopic analysis. The kinetics of TB501 concentration obtained from VERO E6 transwell cultures and transwell membranes saturated with serum proteins reveal the extent to which the cell layer functions as a diffusion barrier. The large number of samples collected allows us to fit a detailed mathematical model of the passive diffusive currents to the measured concentration profiles. This approach enables the determination of the diffusive permeability, the diffusivity of the compound in the cell layer, the affinity of the compound binding to the cell membrane as well as the rate by which the cells metabolize the compound. The proposed approach goes beyond the determination of the permeability coefficient and offers a more detailed pharmacokinetic characterization of the transwell barrier model. We expect the presented method to be fruitful in evaluating other compounds with different chemical features on simple in vitro barrier models. The proposed mathematical model can also be extended to include various forms of active transport.

Human Skin Drug Metabolism: Relationships between Methyl Salicylate Metabolism and Esterase Activities in IVPT Skin Membranes

The presence of esterase enzymes in human skin and their role in drug metabolism has been reported, but their distribution in the various skin layers and the relative contributions of those layers to metabolism is poorly defined. To gain further insight into esterase distribution, we performed in vitro skin permeation of a commercial 28.3% methyl salicylate (MeSA) cream (Metsal™) in Franz diffusion cells, using a range of human skin membranes, all from the same donor. The membranes were viable epidermis separated by a dispase II enzymatic method, heat separated epidermis, dermatomed skin, and dermis separated by a dispase II enzymatic method. Methyl salicylate and its metabolite, salicylic acid (SA), were measured by high-performance liquid chromatography. Alpha naphthyl acetate and Hematoxylin and Eosin staining provided qualitative estimations of esterase distribution in these membranes. The permeation of methyl salicylate after 24 h was similar across all membranes. Salicylic acid formation and permeation were found to be similar in dermatomed skin and dermis, suggesting dermal esterase activity. These results were supported by the staining studies, which showed strong esterase activity in the dermal-epidermal junction region of the dermis. In contrast with high staining of esterase activity in the stratum corneum and viable epidermis, minimal stained and functional esterase activity was found in heat-separated and dispase II-prepared epidermal membranes. The results are consistent with dispase II digesting hemidesmosomes, penetrating the epidermis, and affecting epidermal esterases but not those in the dermis. Accordingly, whilst the resulting dispase II-generated dermal membranes may be used for in vitro permeation tests (IVPT) involving esterase-based metabolic studies, the dispase II-generated epidermal membranes are not suitable for this purpose.

Development of Organs-on-Chips and Their Impact on Precision Medicine and Advanced System Simulation

Drugs may undergo costly preclinical studies but still fail to demonstrate their efficacy in clinical trials, which makes it challenging to discover new drugs. Both in vitro and in vivo models are essential for disease research and therapeutic development. However, these models cannot simulate the physiological and pathological environment in the human body, resulting in limited drug detection and inaccurate disease modelling, failing to provide valid guidance for clinical application. Organs-on-chips (OCs) are devices that serve as a micro-physiological system or a tissue-on-a-chip; they provide accurate insights into certain functions and the pathophysiology of organs to precisely predict the safety and efficiency of drugs in the body. OCs are faster, more economical, and more precise. Thus, they are projected to become a crucial addition to, and a long-term replacement for, traditional preclinical cell cultures, animal studies, and even human clinical trials. This paper first outlines the nature of OCs and their significance, and then details their manufacturing-related materials and methodology. It also discusses applications of OCs in drug screening and disease modelling and treatment, and presents the future perspective of OCs.

Recent advances in in vitro skin-on-a-chip models for drug testing

INTRODUCTION

The skin is an organ that has the largest surface area and provides a barrier against external environment. While providing protection, it also interacts with other organs in the body and has implications in various diseases. Development of physiologically realistic in vitro models of the skin in the context of the whole body is important for studying these diseases, and will be a valuable tool for pharmaceutical, cosmetics, and food industry.

AREA COVERED

This article covers the basic background in skin structure, physiology, as well as drug metabolism in the skin, and dermatological diseases. We summarize various in vitro skin models currently available, and novel in vitro models based on organ-on-a-chip technology. We also explain the concept of multi-organ-on-a-chip and describe recent developments in this field aimed at recapitulating the interaction of the skin with other organs in the body.

EXPERT OPINION

Recent development in the organ-on-a-chip field has enabled the development of in vitro model systems that resemble human skin more closely than conventional models. In near future, we will be seeing various model systems that allow researchers to study complex diseases in a more mechanistic manner, which will help the development of new pharmaceuticals for such diseases.

Influence of cryopreservation methods of ex vivo rat and pig skin on the results of in vitro permeation test

In vitro permeation test (IVPT) is a frequently used method for in vitro assessment of topical preparations and transdermal drug delivery systems. However, the storage of ex vivo skin for IVPT remains a challenge. Here, two cryopreservation media were chosen to preserve rat and pig skin at -20 °C and -80 °C for further IVPT, namely, 10% DMSO and 10% GLY. The skin viability test confirmed that the skin protective capacity of 10% DMSO and 10% GLY was almost equal. The results of skin viability and IVPT showed that the skin viability and permeability of rat skin in 10%DMSO or 10% GLY were maintained for at least 7 and 30 days at -20 °C and -80 °C compared to fresh skin, respectively; in contrast, those of porcine skin were just maintained for less than 7 days at -20 °C and -80 °C. These results indicated that ex vivo skin for IVPT preserved at -80 °C in 10% DMSO or 10% GLY was optimal. Furthermore, skin permeability was independent of skin barrier integrity. Our study provides reference conditions for preserving IVPT skin, and skin viability can be a potential indicator of IVPT skin.

Transdermal Delivery of α-Aminophosphonates as Semisolid Formulations-An In Vitro-Ex Vivo Study

α-Aminophosphonates are organophosphorus compounds with an obvious similarity with α-amino acids. Owing to their biological and pharmacological characteristics, they have attracted the attention of many medicinal chemists. α-Aminophosphonates are known to exhibit antiviral, antitumor, antimicrobial, antioxidant and antibacterial activities, which can all be important in pathological dermatological conditions. However, their ADMET properties are not well studied. The aim of the current study was to provide preliminary information about the skin penetration of three preselected α-aminophosphonates when applying them as topical cream formulations in static and dynamic diffusion chambers. The results indicate that aminophosphonate 1a, without any substituent in the para position, shows the best release from the formulation and the highest absorption through the excised skin. However, based on our previous study, the in vitro pharmacological potency was higher in the case of para-substituted molecules 1b and 1c. The particle size and rheological studies revealed that the 2% cream of aminophosphonate 1a was the most homogenous formulation. In conclusion, the most promising molecule was 1a, but further experiments are proposed to uncover the possible transporter interactions in the skin, optimize the topical formulations and improve PK/PD profiles in case of transdermal delivery.

Overcoming skin barriers through advanced transdermal drug delivery approaches

Upon exhaustive research, the transdermal drug delivery system (TDDS) has appeared as a potential, well-accepted, and popular approach to a novel drug delivery system. Ease of administration, easy handling, minimum systemic exposure, least discomfort, broad flexibility and tunability, controlled release, prolonged therapeutic effect, and many more perks make it a promising approach for effective drug delivery. Although, the primary challenge associated is poor skin permeability. Skin is an intact barrier that serves as a primary defense mechanism to preclude any foreign particle's entry into the body. Owing to the unique anatomical framework, i.e., compact packing of stratum corneum with tight junction and fast anti-inflammatory responses, etc., emerged as a critical physiological barrier for TDDS. Fusion with other novel approaches like nanocarriers, specially designed transdermal delivery devices, permeation enhancers, etc., can overcome the limitations. Utilizing such strategies, some of the products are under clinical trials, and many are under investigation. This review explores all dimensions that overcome poor permeability and allows the drug to attain maximum potential. The article initially compiles fundamental features, components, and design of TDDS, followed by critical aspects and various methods, including in vitro, ex vivo, and in vivo methods of assessing skin permeability. The work primarily aimed to highlight the recent advancement in novel strategies for effective transdermal drug delivery utilizing active methods like iontophoresis, electroporation, sonophoresis, microneedle, needleless jet injection, etc., and passive methods such as the use of liposomes, SLN, NLC, micro/nanoemulsions, dendrimers, transferosomes, and many more nanocarriers. In all, this compilation will provide a recent insight on the novel updates along with basic concepts, the current status of clinical development, and challenges for the clinical translation of TDDS.

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.

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.

Journey of organ on a chip technology and its role in future healthcare scenario

Organ on a chip refers to microengineered biomimetic system which reflects structural and functional characteristics of human tissue. It involves biomaterial technology, cell biology and engineering combined together in a miniaturized platform. Several models using different organs such as lungs on a chip, liver on a chip, kidney on a chip, heart on a chip, intestine on a chip and skin on a chip have been successfully developed. Food and Drug administration (FDA) has also shown confidence in this technology and has partnered with industries/institutes which are working with this technology. In this review, the concepts and applications of Organ on a chip model in different scientific domains including disease model development, drug screening, toxicology, pathogenesis study, efficacy testing and virology is discussed. It is envisaged that amalgamation of various organs on chip modules into a unified body on chip device is of utmost importance for diagnosis and treatment, especially considering the complications due to the ongoing COVID-19 pandemic. It is expected that the market demand for developing organ on chip devices to skyrocket in the near future.

In vitro blood brain barrier models: An overview

Understanding the composition and function of the blood brain barrier (BBB) enables the development of novel, innovative techniques for administering central nervous system (CNS) medications and technologies for improving the existing models. Scientific and methodological interest in the pathology of the BBB resulted in the formation of numerous in vitro BBB models. Once successfully studied and modelled, it would be a valuable tool for elucidating the mechanism of action of the CNS disorders prior to their manifestation and the pathogenic factors. Understanding the rationale behind the selection of the models as well as their working may enable the development of state-of-the-art drugs for treating and managing neurological diseases. Hence, to have realistic simulation of the BBB and test its drug permeability the microfluidics-based BBB-on-Chip model has been developed. To summarise, we aim to evaluate the advanced, newly developed and frequently used in vitro BBB models, thereby providing a brief overview of the components essential for in vitro BBB formation, the methods of chip fabrication and cell culturing, its applications and the recent advances in this technological field. This will be critical for developing CNS treatments with improved BBB penetrability and pharmacokinetic properties.

Skin-on-a-Chip Technology for Testing Transdermal Drug Delivery-Starting Points and Recent Developments

During the last decades, several technologies were developed for testing drug delivery through the dermal barrier. Investigation of drug penetration across the skin can be important in topical pharmaceutical formulations and also in cosmeto-science. The state-of- the-art in the field of skin diffusion measurements, different devices, and diffusion platforms used, are summarized in the introductory part of this review. Then the methodologies applied at Pázmány Péter Catholic University are shown in detail. The main testing platforms (Franz diffusion cells, skin-on-a-chip devices) and the major scientific projects (P-glycoprotein interaction in the skin; new skin equivalents for diffusion purposes) are also presented in one section. The main achievements of our research are briefly summarized: (1) new skin-on-a-chip microfluidic devices were validated as tools for drug penetration studies for the skin; (2) P-glycoprotein transport has an absorptive orientation in the skin; (3) skin samples cannot be used for transporter interaction studies after freezing and thawing; (4) penetration of hydrophilic model drugs is lower in aged than in young skin; (5) mechanical sensitization is needed for excised rodent and pig skins for drug absorption measurements. Our validated skin-on-a-chip platform is available for other research groups to use for testing and for utilizing it for different purposes.

Development and Evaluation of a Human Skin Equivalent in a Semiautomatic Microfluidic Diffusion Chamber

There is an increasing demand for transdermal transport measurements to optimize topical drug formulations and to achieve proper penetration profile of cosmetic ingredients. Reflecting ethical concerns the use of both human and animal tissues is becoming more restricted. Therefore, the focus of dermal research is shifting towards in vitro assays. In the current proof-of-concept study a three-layer skin equivalent using human HaCaT keratinocytes, an electrospun polycaprolactone mesh and a collagen-I gel was compared to human excised skin samples. We measured the permeability of the samples for 2% caffeine cream using a miniaturized dynamic diffusion cell (“skin-on-a-chip” microfluidic device). Caffeine delivery exhibits similar transport kinetics through the artificial skin and the human tissue: after a rapid rise, a long-lasting high concentration steady state develops. This is markedly distinct from the kinetics measured when using cell-free constructs, where a shorter release was observable. These results imply that both the established skin equivalent and the microfluidic diffusion chamber can serve as a suitable base for further development of more complex tissue substitutes.

Development of Skin-On-A-Chip Platforms for Different Utilizations: Factors to Be Considered

There is increasing interest in miniaturized technologies in diagnostics, therapeutic testing, and biomedicinal fundamental research. The same is true for the dermal studies in topical drug development, dermatological disease pathology testing, and cosmetic science. This review aims to collect the recent scientific literature and knowledge about the application of skin-on-a-chip technology in drug diffusion studies, in pharmacological and toxicological experiments, in wound healing, and in fields of cosmetic science (ageing or repair). The basic mathematical models are also presented in the article to predict physical phenomena, such as fluid movement, drug diffusion, and heat transfer taking place across the dermal layers in the chip using Computational Fluid Dynamics techniques. Soon, it can be envisioned that animal studies might be at least in part replaced with skin-on-a-chip technology leading to more reliable results close to study on humans. The new technology is a cost-effective alternative to traditional methods used in research institutes, university labs, and industry. With this article, the authors would like to call attention to a new investigational family of platforms to refresh the researchers’ theranostics and preclinical, experimental toolbox.

Freezing Weakens the Barrier Function of Reconstructed Human Epidermis as Evidenced by Raman Spectroscopy and Percutaneous Permeation

The development and characterization of reconstructed human epidermis (RHE) is an active area of R&D. RHE can replace animal tissues in pharmaceutical, toxicological and cosmetic sciences, yielding scientific and ethical advantages. RHEs remain costly, however, due to consumables and time required for their culture and a short shelf-life. Storing, i.e., freezing RHE could help reduce costs but to date, little is known on the effects of freezing on the barrier function of RHE. We studied such effects using commercial EpiSkin™ RHE stored at -20, -80 and -150 °C for 1 and 10 weeks. We acquired intrinsic Raman spectra in the stratum corneum (SC) of the RHEs as well as spectra obtained following topical application of resorcinol in an aqueous solution. In parallel, we quantified the effects of freezing on the permeation kinetics of resorcinol from time-dependent permeation experiments. Principal component analyses discriminated the intrinsic SC spectra and the spectra of resorcinol-containing RHEs, in each case on the basis of the freezing conditions. Permeation of resorcinol through the frozen RHE increased 3- to 6-fold compared to fresh RHE, with the strongest effect obtained from freezing at -20 °C for 10 weeks. Due to the extensive optimization and standardization of EpiSkin™ RHE, the effects observed in our work may be expected to be more pronounced with other RHEs.