The influence of stratum corneum morphology on water permeability

Mechanisms of penetration and diffusion of drugs and cosmetic actives across the human Stratum corneum

Based on the structure of the Stratum corneum (SC) the potential penetration/diffusion pathways of drugs and cosmetic actives through the SC are presented and discussed. The well-known lipophilic pathway across the SC is presented and relevant examples are used to show that highly lipophilic molecules such as glucocorticoids, coenzyme Q10 etc. are accumulated in the SC and penetrate into the inner liquid like layer of the SC lipid bilayer by lateral diffusion. The diffusion into and across the SC of highly hydrophilic drugs and active substances such as urea, amino acids and peptides is still under discussion. Another diffusion pathway for the highly hydrophilic molecules via the corneocytes and the corneodesmosomes is presented and discussed, the corneocytary diffusion pathway.

Topically Applied Therapies for the Treatment of Skin Disease: Past, Present, and Future

The purpose of this review is to summarize essential biological, pharmaceutical, and clinical aspects in the field of topically applied medicines that may help scientists when trying to develop new topical medicines. After a brief history of topical drug delivery, a review of the structure and function of the skin and routes of drug absorption and their limitations is provided. The most prevalent diseases and current topical treatment approaches are then detailed, the organization of which reflects the key disease categories of autoimmune and inflammatory diseases, microbial infections, skin cancers, and genetic skin diseases. The complexity of topical product development through to large-scale manufacturing along with recommended risk mitigation approaches are then highlighted. As such topical treatments are applied externally, patient preferences along with the challenges they invoke are then described, and finally the future of this field of drug delivery is discussed, with an emphasis on areas that are more likely to yield significant improvements over the topical medicines in current use or would expand the range of medicines and diseases treatable by this route of administration. SIGNIFICANCE STATEMENT: This review of the key aspects of the skin and its associated diseases and current treatments along with the intricacies of topical formulation development should be helpful in making judicious decisions about the development of new or improved topical medicines. These aspects include the choices of the active ingredients, formulations, the target patient population's preferences, limitations, and the future with regard to new skin diseases and topical medicine approaches.

Enhanced in vitro and ex vivo transdermal permeation of microemulsion gel of tapentadol hydrochloride

Aim of the current study is to develop a microemulsion gel for transdermal delivery of tapentadol hydrochloride. Microemulsion was developed using phase diagram and subjected to assay, globule size, PDI, zeta potential, TEM and in vitro drug release studies. The optimized microemulsion was converted into gel using carbopol 934 NF and evaluated for viscosity, spreadability, in vitro, ex vivo, FTIR, DSC, stability and skin irritation studies. The mean globule size, PDI, zeta potential and in vitro drug release of microemulsion were found 247.3 nm, 0.298, -17.6 mV and 98.42% respectively. In vitro and ex vivo drug release of gel was found 92.2% and 88.6% in 24 h. Viscosity and spreadability results indicated ease of application and no incompatibility was observed from FTIR studies. The skin irritation studies showed absence of erythema. Key findings from the current research concluded that microemulsion gel was suitable for effective transdermal delivery.

Electromechanical convective drug delivery devices for overcoming diffusion barriers

Drug delivery systems which rely on diffusion for mass transport, such as hydrogels and nanoparticles, have enhanced drug targeting and extended delivery profiles to improve health outcomes for patients suffering from diseases including cancer and diabetes. However, diffusion-dependent systems often fail to provide >0.01-1% drug bioavailability when transporting macromolecules across poorly permeable physiological tissues such as the skin, solid tumors, the blood-brain barrier, and the gastrointestinal walls. Convection-enabling robotic ingestibles, wearables, and implantables physically interact with tissue walls to improve bioavailability in these settings by multiple orders of magnitude through convective mass transfer, the process of moving drug molecules via bulk fluid flow. In this Review, we compare diffusive and convective drug delivery systems, highlight engineering techniques that enhance the efficacy of convective devices, and provide examples of synergies between the two methods of drug transport.

Flux-Type versus Concentration-Type Sensors in Transdermal Measurements

New transdermal biosensors measure analytes that diffuse from the bloodstream through the skin, making it important to reduce the system response time and understand measurement output. While highly customized models have been created for specific sensors, a generalized model for transdermal sensor systems is lacking. Here, a simple one-dimensional diffusion model was used to characterize the measurement system and classify biosensors as either flux types or concentration types. Results showed that flux-type sensors have significantly faster response times than concentration sensors. Furthermore, flux sensors do not measure concentration, but rather have an output measurement that is proportional to skin permeability. These findings should lead to an improved understanding of transdermal measurements and their relation to blood analyte concentration. In the realm of alcohol research, where the majority of commercially available sensors are flux types, our work advocates toward moving away from transdermal alcohol concentration as a metric, and instead suggests embracing transdermal alcohol flux as a more suitable alternative.

Poroelastic behavior and water permeability of human skin at the nanoscale

Topical skin care products and hydrating compositions (moisturizers or injectable fillers) have been used for years to improve the appearance of, for example facial wrinkles, or to increase "plumpness". Most of the studies have addressed these changes based on the overall mechanical changes associated with an increase in hydration state. However, little is known about the water mobility contribution to these changes as well as the consequences to the specific skin layers. This is important as the biophysical properties and the biochemical composition of normal stratum corneum, epithelium, and dermis vary tremendously from one another. Our current studies and results reported here have focused on a novel approach (dynamic atomic force microscopy-based nanoindentation) to quantify biophysical characteristics of individual layers of ex vivo human skin. We have discovered that our new methods are highly sensitive to the mechanical properties of individual skin layers, as well as their hydration properties. Furthermore, our methods can assess the ability of these individual layers to respond to both compressive and shear deformations. In addition, since human skin is mechanically loaded over a wide range of deformation rates (frequencies), we studied the biophysical properties of skin over a wide frequency range. The poroelasticity model used helps to quantify the hydraulic permeability of the skin layers, providing an innovative method to evaluate and interpret the impact of hydrating compositions on water mobility of these different skin layers.

Skin-interfaced electronics: A promising and intelligent paradigm for personalized healthcare

Skin-interfaced electronics (skintronics) have received considerable attention due to their thinness, skin-like mechanical softness, excellent conformability, and multifunctional integration. Current advancements in skintronics have enabled health monitoring and digital medicine. Particularly, skintronics offer a personalized platform for early-stage disease diagnosis and treatment. In this comprehensive review, we discuss (1) the state-of-the-art skintronic devices, (2) material selections and platform considerations of future skintronics toward intelligent healthcare, (3) device fabrication and system integrations of skintronics, (4) an overview of the skintronic platform for personalized healthcare applications, including biosensing as well as wound healing, sleep monitoring, the assessment of SARS-CoV-2, and the augmented reality-/virtual reality-enhanced human-machine interfaces, and (5) current challenges and future opportunities of skintronics and their potentials in clinical translation and commercialization. The field of skintronics will not only minimize physical and physiological mismatches with the skin but also shift the paradigm in intelligent and personalized healthcare and offer unprecedented promise to revolutionize conventional medical practices.

A thermodynamic and kinetic analysis of human epidermal penetration of phenolic compounds: II. Maximum flux and solute diffusion through stratum corneum lipids

Warming the skin is a key means of promoting solute permeation through the skin. Changes in solute permeation associated with variations in skin temperature also assist in understanding the mechanism by which solutes permeate the skin. However, few studies have considered the relative impact of temperature on the main determinants of the maximum flux for a solute across the skin, the solubility of a solute and its diffusivity in the stratum corneum. In this study, we quantified for the first time the thermodynamics associated with the maximum skin fluxes for a series of phenolic compounds of similar size but with varying lipophilicity (defined by the logarithms of their octanol/water partition coefficient, logP). These studies were undertaken using aqueous donor solutions (along with testosterone as a reference solute) across human epidermal membranes in vertical Franz diffusion cells at 4 °C, 24 °C and 37 °C with intermittent receptor sampling and volume replacement over 24 h. Kinetic and thermodynamic analyses included the estimation of the stratum corneum (SC) apparent SC diffusivity from the SC maximum fluxes and SC solubilities and the associated activation energies, enthalpies and entropies for diffusion. The key findings were that the differences in the maximum flux of phenolic compounds varying in lipophilicity mainly arose from differences in SC solubility at the various temperatures and that, at the highest temperature, SC permeability and SC diffusion were affected by SC lipid fluidisation and that variations in SC - water partitioning enthalpies explain some of the previously low activation energies for permeation of the more lipophilic phenols. Higher enthalpies for diffusion were seen for solutes with addition hydrogen bonding capacity and the highest negative entropy was observed with the more compact solutes. Various relationships between the derived thermodynamic parameters were explored and interpreted in a proposed model for solute partitioning into and permeation through the SC intercellular lipid lamellae.

Modeling of composite sorption isotherm for stratum corneum

Equilibrium water sorption in stratum corneum (SC) is considered by treating it as a biocomposite with two main phases, namely, corneocytes and lipids. To validate the rule of mixtures for the individual phase sorption isotherms, a new flexible fitting model is introduced by accounting for characteristic features observed in the variations of the thermodynamic correction factors corresponding to the individual sorption isotherms. The comparison of the model fitting performance with that of the five-parameter Park's model shows a remarkably good ability to fit experimental data for different types of sorption isotherms. The effect of the lipids content on the variance of the composite sorption isotherm of stratum corneum is highlighted. The sensitivity analysis reveals that for the typical water content 20-30 wt%, which corresponds to the SC in a stable condition, the sensitivity of the composite sorption isotherm to the variation of the lipids content on dry basis is predominantly positive and sufficiently small. The good agreement observed between the experimental sorption isotherm for SC and the composite isotherm, which is based on the rule of mixtures for the individual phase sorption isotherms, yields a plausible conclusion (hypothesis) that the corneocytes-lipids mechanical interaction during unconstrained swelling of the SC membrane in the in vitro laboratory experiment is negligible.

Physicochemical and biopharmaceutical aspects influencing skin permeation and role of SLN and NLC for skin drug delivery

The skin is a complex and multifunctional organ, in which the static versus dynamic balance is responsible for its constant adaptation to variations in the external environment that is continuously exposed. One of the most important functions of the skin is its ability to act as a protective barrier, against the entry of foreign substances and against the excessive loss of endogenous material. Human skin imposes physical, chemical and biological limitations on all types of permeating agents that can cross the epithelial barrier. For a molecule to be passively permeated through the skin, it must have properties, such as dimensions, molecular weight, pKa and hydrophilic-lipophilic gradient, appropriate to the anatomy and physiology of the skin. These requirements have limited the number of commercially available products for dermal and transdermal administration of drugs. To understand the mechanisms involved in the drug permeation process through the skin, the approach should be multidisciplinary in order to overcome biological and pharmacotechnical barriers. The study of the mechanisms involved in the permeation process, and the ways to control it, can make this route of drug administration cease to be a constant promise and become a reality. In this work, we address the physicochemical and biopharmaceutical aspects encountered in the pathway of drugs through the skin, and the potential added value of using solid lipid nanoparticles (SLN) and nanostructured lipid vectors (NLC) to drug permeation/penetration through this route. The technology and architecture for obtaining lipid nanoparticles are described in detail, namely the composition, production methods and the ability to release pharmacologically active substances, as well as the application of these systems in the vectorization of various pharmacologically active substances for dermal and transdermal applications. The characteristics of these systems in terms of dermal application are addressed, such as biocompatibility, occlusion, hydration, emollience and the penetration of pharmacologically active substances. The advantages of using these systems over conventional formulations are described and explored from a pharmaceutical point of view.

Skin penetration/permeation success determinants of nanocarriers: Pursuit of a perfect formulation

The advent of nanocarriers in the field of pharmaceutical drug delivery, while exhibiting considerable advantages, has created challenges for researchers. Among the applications of nanocarriers, drug delivery to the skin has attracted increasing attention in recent decades due to its advantages over oral and parenteral administration. Accordingly, this work attempts to discuss the major obstacles surrounding topically applied formulations and different nanocarriers' potential to overcome these barriers to investigate whether their passive penetration through the skin is likely. Therefore, skin anatomical views and transcutaneous pathways are briefly reviewed. Factors commonly thought to influence skin penetration are discussed from the perspective of particularly penetrating nanocarriers. The formulation of these nanocarriers is outlined, and promising constituents are highlighted to help investigators optimize nanocarrier formulations.

Enhancing Permeation of Drug Molecules Across the Skin via Delivery in Nanocarriers: Novel Strategies for Effective Transdermal Applications

The transdermal route of administration provides numerous advantages over conventional routes i.e., oral or injectable for the treatment of different diseases and cosmetics applications. The skin also works as a reservoir, thus deliver the penetrated drug for more extended periods in a sustained manner. It reduces toxicity and local irritation due to multiple sites for absorption and owes the option of avoiding systemic side effects. However, the transdermal route of delivery for many drugs is limited since very few drugs can be delivered at a viable rate using this route. The stratum corneum of skin works as an effective barrier, limiting most drugs' penetration posing difficulty to cross through the skin. Fortunately, some non-invasive methods can significantly enhance the penetration of drugs through this barrier. The use of nanocarriers for increasing the range of available drugs for the transdermal delivery has emerged as a valuable and exciting alternative. Both the lipophilic and hydrophilic drugs can be delivered via a range of nanocarriers through the stratum corneum with the possibility of having local or systemic effects to treat various diseases. In this review, the skin structure and major obstacle for transdermal drug delivery, different nanocarriers used for transdermal delivery, i.e., nanoparticles, ethosomes, dendrimers, liposomes, etc., have been discussed. Some recent examples of the combination of nanocarrier and physical methods, including iontophoresis, ultrasound, laser, and microneedles, have also been discussed for improving the therapeutic efficacy of transdermal drugs. Limitations and future perspectives of nanocarriers for transdermal drug delivery have been summarized at the end of this manuscript.

Development and Qualification of a Physiologically Based Pharmacokinetic Model of Finasteride and Minoxidil Following Scalp Application

In this study, we aimed to develop and qualify a PBPK model for scalp application using two drugs with marked differences in physicochemical properties and PK profiles. The parameters related to scalp physiology, drug PK, and formulations were incorporated into a Multi-Phase and Multi-Layer (MPML) Mechanistic Dermal Absorption (MechDermA) model within the Simcyp® Simulator V17. The finasteride PBPK model was linked to its effect on dihydrotestosterone (DHT) levels in plasma and scalp using an indirect response model. Predicted PK (and PD for finasteride) profiles and parameters were compared against the clinically reported data and justified by visual predictive checks and two-fold error criteria for model verification. The PBPK/PD model for finasteride reasonably demonstrated an ability to predict its respective PK and PD profiles, and parameters following scalp application under various clinical scenarios. Using the same scalp physiological input parameters, the minoxidil PBPK model was then developed and satisfactorily qualified with independent clinical datasets. Collectively, these results suggested that the established PBPK model may have broader utility for other topical formulations intended for scalp application.

Lipid Metabolic Events Underlying the Formation of the Corneocyte Lipid Envelope

Cornified cells of the stratum corneum have a monolayer of an unusual lipid covalently attached to the outer surface. This is referred to as the corneocyte lipid envelope (CLE). It consists of a monolayer of ω-hydroxyceramides covalently attached to the outer surface of the cornified envelope. The CLE is essential for proper barrier function of the skin and is derived from linoleate-rich acylglucosylceramides synthesized in the viable epidermis. Biosynthesis of acylglucosylceramide and its conversion to the cornified envelope is complex. Acylglucosylceramide in the bounding membrane of the lamellar granule is the precursor of the CLE. The acylglucosylceramide in the limiting membrane of the lamellar granule may be oriented with the glucosyl moiety on the inside. Conversion of the acylglucosylceramide to the CLE requires removal of the glucose by action of a glucocerebrosidase. The ester-linked fatty acid may be removed by an as yet unidentified esterase, and the resulting ω-hydroxyceramide may become ester linked to the outer surface of the cornified envelope through action of transglutaminase 1. Prior to removal of ester-linked fatty acids, linoleate is oxidized to an epoxy alcohol through action of 2 lipoxygenases. This can be further oxidized to an epoxy-enone, which can spontaneously attach to the cornified envelope through Schiff's base formation. Mutations of genes coding for enzymes involved in biosynthesis of the CLE result in ichthyosis, often accompanied by neurologic dysfunction. The CLE is recognized as essential for barrier function of skin, but many questions about details of this essentiality remain. What are the relative roles of the 2 mechanisms of lipid attachment? What is the orientation of acylglucosylceramide in the bounding membrane of lamellar granules? Some evidence supports a role for CLE as a scaffold upon which intercellular lamellae unfold, but other evidence does not support this role. There is also controversial evidence for a role in stratum corneum cohesion. Evidence is presented to suggest that covalently bound ω-hydroxyceramides serve as a reservoir for free sphingosine that can serve in communicating with the viable epidermis and act as a potent broad-acting antimicrobial at the skin surface. Many questions remain.

Role of sunscreen formulation and photostability to protect the biomechanical barrier function of skin

The impact of sunscreen formulations on the barrier properties of human skin are often overlooked leading to formulations with components whose effects on barrier mechanical integrity are poorly understood. The aim of this study is to demonstrate the relevance of carrier selection and sunscreen photostability when designing sunscreen formulations to protect the biomechanical barrier properties of human stratum corneum (SC) from solar ultraviolet (UV) damage. Biomechanical properties of SC samples were assayed after accelerated UVB damage through measurements of the SC's mechanical stress profile and corneocyte cohesion. A narrowband UVB (305-315 nm) lamp was used to expose SC samples to 5, 30, 125, and 265 J cm-2 in order to magnify damage to the mechanical properties of the tissue and characterize the UV degradation dose response such that effects from smaller UV dosages can be extrapolated. Stresses in the SC decreased when treated with sunscreen components, highlighting their effect on the skin prior to UV exposure. Stresses increased with UVB exposure and in specimens treated with different sunscreens stresses varied dramatically at high UVB dosages. Specimens treated with sunscreen components without UVB exposure exhibited altered corneocyte cohesion. Both sunscreens studied prevented alteration of corneocyte cohesion by low UVB dosages, but differences in protection were observed at higher UVB dosages indicating UV degradation of one sunscreen. These results indicate the protection of individual sunscreen components vary over a range of UVB dosages, and components can even cause alteration of the biomechanical barrier properties of human SC before UV exposure. Therefore, detailed characterization of sunscreen formulation components is required to design robust protection from UV damage.

A Novel Technique Enables Quantifying the Molecular Interaction of Solvents with Biological Tissues

The pharmaceutical industry uses various solvents to increase drug penetrability to tissues. The solvent’s choice affects the efficacy of a drug. In this paper, we provide an unprecedented means of relating a solvent to a tissue quantitatively. We show that the solvents induce reorientation of the tissue surface molecules in a way that favors interaction and, therefore, penetrability of a solvent to a tissue. We provide, for the first time, a number for this tendency through a new physical property termed Interfacial Modulus (G_s). G_s, which so far was only predicted theoretically, is inversely proportional to such interactions. As model systems, we use HeLa and HaCaT tissue cultures with water and with an aqueous DMSO solution. The measurements are done using Centrifugal Adhesion Balance (CAB) when set to effective zero gravity. As expected, the addition of DMSO to water reduces G_s. This reduction in G_s is usually higher for HaCaT than for HeLa cells, which agrees with the common usage of DMSO in dermal medicine. We also varied the rigidities of the tissues. The tissue rigidity is not expected to relate to G_s, and indeed our results didn’t show a correlation between these two physical properties.

Pharmacotherapy: Its impact on morphofunctional characteristics of the epidermal barrier

Various pharmaceuticals used for topical and systemic therapy are capable of exerting significant impact on morphological and physiological characteristics of human epidermis, as well as its barrier properties. This may affect the course of dermatologic diseases and the efficacy of their treatment. In this literature review, the author analyzes the impact of various pharmaceutical classes on the morphofunctional characteristics of the epidermal barrier and formulates recommendations for skin disease treatment.

Enhanced Transdermal Drug Delivery by Sonophoresis and Simultaneous Application of Sonophoresis and Iontophoresis

Transdermal drug delivery has advantages of topical drug administration compared to the other conventional administration methods. However, the skin penetration of drugs is limited by the barrier properties of stratum corneum. The combinational strategy has been investigated to improve the skin permeability of the drug. For this study, we devised an improved device that can perform not only the single application of sonophoresis or iontophoresis but also the simultaneous application. The enhancement effect of sonophoresis was evaluated for various cosmeceutical drugs using a Franz diffusion cell. The enhancement ratio of niacinamide and retinol with sonophoresis was increased to 402% and 292%, respectively. The relationship was found between the enhancement effect of sonophoresis and the physicochemical properties of drugs. In particular, the simultaneous treatment of sonophoresis and iontophoresis enhanced skin penetration of glutamic acid to 240% using the fabricated device. The simultaneous application showed significantly higher enhancement ratio than application of sonophoresis or iontophoresis alone. Moreover, the improved device achieved skin penetration enhancement of various cosmeceutical drugs with lower intensity and a short application time. This combined strategy of transdermal physical enhancement methods is advantageous in terms of decline in energy density, thereby reducing the skin irritation. The miniaturized device with sonophoresis and iontophoresis is a promising approach due to enhanced transdermal drug delivery and feasibility of self-administration in cosmetic and therapeutic fields.

Wireless, Battery-Free Epidermal Electronics for Continuous, Quantitative, Multimodal Thermal Characterization of Skin

Precise, quantitative measurements of the thermal properties of human skin can yield insights into thermoregulatory function, hydration, blood perfusion, wound healing, and other parameters of clinical interest. The need for wired power supply systems and data communication hardware limits, however, practical applicability of existing devices designed for measurements of this type. Here, a set of advanced materials, mechanics designs, integration schemes, and wireless circuits is reported as the basis for wireless, battery-free sensors that softly interface to the skin to enable precise measurements of its temperature and thermal transport properties. Calibration processes connect these parameters to the hydration state of the skin, the dynamics of near-surface flow through blood vessels and implanted catheters, and to recovery processes following trauma. Systematic engineering studies yield quantitative metrics in precision and reliability in real-world conditions. Evaluations on five human subjects demonstrate the capabilities in measurements of skin hydration and injury, including examples of continuous wear and monitoring over a period of 1 week, without disrupting natural daily activities.

The rise of transcutaneous drug delivery for the management of alopecia: a review of existing literature and an eye towards the future

Introduction: Fractional lasers and microneedling devices are increasingly used with topical drugs to treat various conditions, including alopecia, as they grant access to dermal structures such as hair follicles and cutaneous vasculature. Objective: To perform a comprehensive review on transcutaneous drug delivery for the management of alopecia. Methods: PubMed, Embase, and Ovid Medline databases were searched using terms including: alopecia, microneedling, lasers, androgenetic alopecia (AGA), alopecia areata (AA), drug delivery. Articles were examined for inclusion criteria: diagnosis of alopecia regardless of type, use of fractional laser or microneedling devices, and subsequent administration of topical medication. Results: 8 studies, 6 prospective clinical trials and 2 case series, examining either AA or AGA were identified. For AA, five studies examined microneedling together with topical triamcinolone in three of these, while two studies used photodynamic therapy. Regarding AGA, two studies used topical minoxidil plus microneedling, and one examined topical finasteride with fractional erbium glass laser. Improvement was seen in 6 of the 8 studies. Discussion: Transcutaneous drug delivery via fractional laser and microneedling is a promising modality with preliminary evidence for increased hair regrowth over topical therapy alone. Further studies are needed to elucidate treatment parameters and appropriate device selection for drug delivery.

Chemical Enhancer: A Simplistic Way to Modulate Barrier Function of the Stratum Corneum

Human skin could be a prime target to deliver drugs into the human body as it is the largest organ of human body. However, the main challenge of delivering drug into the skin is the stratum corneum (SC), the outer layer of epidermis, which performs the main barrier function of the skin. Scientists have developed several techniques to overcome the barrier properties of the skin, which include other physical and chemical techniques. The most common and convenient technique is to use special formulation additives (chemical enhancers, CEs) which either drags the drug molecule along with it or make changes in the SC structure, thereby allowing the drug molecule to penetrate in to the SC. The main focus is to deliver drugs in the certain layers of the skin (for topical delivery) or ensuring proper percutaneous absorption (for transdermal delivery). However, skin drug delivery is still very challenging as different CEs act in different ways on the skin and they have different types of interaction with different drugs. Therefore, proper understanding on the mechanism of action of CE is mandatory. In this article, the effect of several CEs on skin has been reviewed based on the published articles. The main aim is to compile the recent knowledge on skin-CE interaction in order to design a topical and transdermal formulation efficiently. A properly designed formulation would help the drug either to deposit into the target layer or to cross the barrier membrane to reach the systemic circulation.

Transdermal Delivery of Ibuprofen Utilizing a Novel Solvent-Free Pressure-sensitive Adhesive (PSA): TEPI® Technology

PURPOSE

The main objective of this present study was the investigation of potential novel transdermal patch technology (TEPI®) delivering ibuprofen as the active pharmaceutical ingredient (API) using a novel poly(ether-urethane)-silicone crosslinked pressure-sensitive adhesive (PSA) as the drug reservoir in a solvent-free manufacturing process.

METHODS

The patch was synthesized utilizing the hot-melt crosslinking technique without the addition of solvents at 80 °C in 100% relative humidity. Dissolution and permeation studies performed utilizing diffusion cells and subsequently HPLC validated methods were employed to determine the API content in the acceptor solution. Accelerated stability studies were also performed at 40 °C and 70% relative humidity. The adhesive performance of the fabricated patch was evaluated utilizing loop tack adhesion tests.

RESULTS

In vitro permeation experiments across both Strat-M® and human skin demonstrated that ibuprofen can easily be released from the adhesive matrix and penetrate through the studied membrane. A comparison on the permeation rates of the API across the two membranes indicated that there is not a strong correlation between the obtained data. The presence of chemical enhancers facilitated an increased flux of the API higher than observed in the basic formulation. Initial stability studies of the optimized formulation showed no degradation with respect to the drug content. Adhesion studies were also performed indicating higher values when compared with commercially available products.

CONCLUSIONS

The present study demonstrated the fabrication of an ibuprofen patch utilizing a versatile, solvent-free drug delivery platform. Upon optimization of the final system, the resulting patch offers many advantages compared to commercially available formulations including high drug loading (up to 25 wt%), good adhesion, and painless removal leaving no residues on the skin. This PSA offers many advantages over existing adhesive technology. Graphical Abstractᅟ.

The relationship between skin function, barrier properties, and body-dependent factors

BACKGROUND

Skin is a multilayer interface between the body and the environment, responsible for many important functions, such as temperature regulation, water transport, sensation, and protection from external triggers.

OBJECTIVES

This paper provides an overview of principal factors that influence human skin and describes the diversity of skin characteristics, its causes and possible consequences. It also discusses limitations in the barrier function of the skin, describing mechanisms of absorption.

METHODS

There are a number of in vivo investigations focusing on the diversity of human skin characteristics with reference to barrier properties and body-dependent factors.

RESULTS

Skin properties vary among individuals of different age, gender, ethnicity, and skin types. In addition, skin characteristics differ depending on the body site and can be influenced by the body-mass index and lifestyle. Although one of the main functions of the skin is to act as a barrier, absorption of some substances remains possible.

CONCLUSIONS

Various factors can alter human skin properties, which can be reflected in skin function and the quality of everyday life. Skin properties and function are strongly interlinked.

How Predictable Are Human Stratum Corneum Lipid/Water Partition Coefficients? Assessment and Useful Correlations for Dermal Absorption

Partition coefficients between human stratum corneum lipids and water (K^sclip/w) are collected or deduced from a variety of sources in a manner that approximately doubles the available data compared to the current state-of-the-art model (Hansen et al., Adv Drug Deliv Rev. 2013;65(2):251-264). An additional datum for water itself in porcine SC that considerably extends the molecular size and lipophilicity range of the data set is considered. The data are analyzed in terms of an extended linear free energy relationship involving octanol/water partition coefficients, Abraham solvation parameters, and a secondary, power law molecular weight dependence. The optimum fit to log K^sclip/w for the full data set reduces the standard error of prediction from 0.50 for a Hansen-like model to 0.39; corresponding multiplicative errors in K^sclip/w are reduced from a factor of 3.1 to one of 2.5. The difference in performance is driven by the water datum, which requires a more complex dependence on molecular size than that afforded by Abraham parameters. In the absence of the water value, the Hansen-like model, which does not include a dependence on molecular size, is essentially optimum. A comparison is presented to fluid-phase phospholipid-water systems, which have a demonstrably different structure-property relationship.

Multimodal epidermal devices for hydration monitoring

Precise, quantitative in vivo monitoring of hydration levels in the near surface regions of the skin can be useful in preventing skin-based pathologies, and regulating external appearance. Here we introduce multimodal sensors with important capabilities in this context, rendered in soft, ultrathin, 'skin-like' formats with numerous advantages over alternative technologies, including the ability to establish intimate, conformal contact without applied pressure, and to provide spatiotemporally resolved data on both electrical and thermal transport properties from sensitive regions of the skin. Systematic in vitro studies and computational models establish the underlying measurement principles and associated approaches for determination of temperature, thermal conductivity, thermal diffusivity, volumetric heat capacity, and electrical impedance using simple analysis algorithms. Clinical studies on 20 patients subjected to a variety of external stimuli validate the device operation and allow quantitative comparisons of measurement capabilities to those of existing state-of-the-art tools.

Screening sunscreens: protecting the biomechanical barrier function of skin from solar ultraviolet radiation damage

OBJECTIVE

Solar ultraviolet (UV) radiation is ubiquitous in human life and well known to cause skin damage that can lead to harmful conditions such as erythema. Although sunscreen is a popular form of protection for some of these conditions, it is unclear whether sunscreen can maintain the mechanical barrier properties of skin. The objective of this study was to determine whether in vitro thin-film mechanical analysis techniques adapted for biological tissue are able to characterize the efficacy of commonly used UV inhibitors and commercial sunscreens to protect the biomechanical barrier properties of stratum corneum (SC) from UV exposure.

METHODS

The biomechanical properties of SC samples were assayed through measurements of the SC's drying stress profile and delamination energy. The drying stresses within SC were characterized from the curvature of a borosilicate glass substrate onto which SC had been adhered. Delamination energies were characterized using a double-cantilever beam (DCB) cohesion testing method. Successive DCB specimens were prepared from previously separated specimens by adhering new substrates onto each side of the already tested specimen to probe delamination energies deeper into the SC. These properties of the SC were measured before and after UV exposure, both with and without sunscreens applied, to determine the role of sunscreen in preserving the barrier function of SC.

RESULTS

The drying stress in SC starts increasing sooner and rises to a higher plateau stress value after UVA exposure as compared to non-UV-exposed control specimens. For specimens that had sunscreen applied, the UVA-exposed and non-UV-exposed SC had similar drying stress profiles. Additionally, specimens exposed to UVB without protection from sunscreen exhibited significantly lower delamination energies than non-UV-exposed controls. With commercial sunscreen applied, the delamination energy for UV-exposed and non-UV-exposed tissue was consistent, even up to large doses of UVB.

CONCLUSION

In vitro thin-film mechanical analysis techniques can readily characterize the effects of SC's exposure to UV radiation. The methods used in this study demonstrated commercial sunscreens were able to preserve the biomechanical properties of SC during UV exposure, thus indicating the barrier function of SC was also maintained.

Transdermal and Skin-Targeted Drug Delivery

Background:

The application of therapeutic agents to the skin addresses three general objectives: (a) the treatment of a variety of dermatologic diseases; (b) the “targeted” delivery of drugs to deeper subcutaneous tissues, with a concomitant reduction in systemic exposure; and (c) socalled transdermal administration to elicit a systemic pharmacologic effect.

Objective:

Recently, significant progress towards all three goals has been recorded and the level of research and development activity remains high. We aim to discuss these advances from mechanistic and clinical standpoints.

Results:

For the topical treatment of skin disease, novel vehicles (e.g., stabilized, supersaturated systems and liposomal formulations) have led to dramatic improvements in local drug bioavailability. Transdermal delivery of drugs for systemic effect, though limited in terms of the number of compounds, is perhaps the most commercially successful (in terms of the number of products) of the controlled release technologies. Considerable activity continues to enhance drug delivery (and hence to extend the range of drugs for which transdermal delivery can be used). Existing patches use formulations that contain solvents and adjuvants capable of reducing the barrier function of the skin. Much effort is directed at iontophoresis (electrically enhanced transport), particularly for small peptides that are difficult to administer by other routes. “Reverse iontophoresis” may allow the extraction of glucose (without skin puncture) so that continuous, noninvasive monitoring of blood sugar in diabetics approaches realization.

Conclusion:

In the not too distant future, the skin may also play a role not only in drug delivery, but also with respect to measurements in clinical chemistry.

Molecular Dynamics Simulation Study of Permeation of Molecules through Skin Lipid Bilayer

Stratum Corneum (SC), the outermost layer of skin, is mainly responsible for skin's barrier function. The complex lipid matrix of SC determines these barrier properties. In this study, the lipid matrix is modeled as an equimolar mixture of ceramide (CER), cholesterol (CHOL), and free fatty acid (FFA). The permeation of water, oxygen, ethanol, acetic acid, urea, butanol, benzene, dimethyl sulfoxide (DMSO), toluene, phenol, styrene, and ethylbenzene across this layer is studied using a constrained MD simulations technique. Several long constrained simulations are performed at a skin temperature of 310 K under NPT conditions. The free energy profiles and diffusion coefficients along the bilayer normal have been calculated for each molecule. Permeability coefficients are also calculated and compared with experimental data. The main resistance for the permeation of hydrophilic and hydrophobic permeants has been found to be in the interior of the lipid bilayer and near the lipid-water interface, respectively. The obtained permeability is found to be a few orders of magnitude higher than experimental values for hydrophilic molecules while for hydrophobic molecules more discrepancy was observed. Overall, the qualitative ranking is consistent with the experiments.

Membrane transport of naphthalene and dodecane in jet fuel mixtures

Jet fuels are formulated with numerous aliphatic and aromatic components that are thought to cause dermal irritation in air force personnel. However, diffusion of these components in such a complex mixture is not well understood. The purpose of this study is to evaluate the physicochemical properties of these mixtures in the context of how they influence partitioning, diffusion, and absorption of aromatic (14C-naphthalene) and aliphatic (14C-dodecane) markers in porcine skin and silastic membranes in vitro. In these 5-h flowthrough diffusion studies, Jet-A, JP-8, and JP-8(100) fuels, and weathered JP-8 (JP-8 (Puddle)) were tested. In both membrane systems and across all jet fuels tested, naphthalene absorption (1.29-1.84% dose) was significantly greater than dodecane absorption (0.14-0.28% dose). However, significantly more dodecane than naphthalene was observed in the stratum corneum (SC; 4.23-7.23% dose vs. 1.88-4.08% dose) and silastic membranes (59.2-81.7% dose vs. 30.5-36.7% dose). Naphthalene was least likely to be retained on the skin surface compared to dodecane, while this trend was reversed in silastic membranes. In porcine skin. weathered JP-8 significantly increased dodecane absorption, permeability (0.19×10−4 cm/h), and diffusivity, and also naphthalene deposition in the SC compared to other jet fuels. In contrast, weathered JP-8 appears to decrease naphthalene flux (1.56 μg/cm2/h) and permeability (1.14×10−4 cm/h) in skin. There were no differences among the three jet fuels in terms of their ability to influence naphthalene or dodecane disposition in skin and, generally, no significant differences among the four jet fuel mixtures were observed in silastic membranes. In conclusion, these transport studies suggest that absorption and deposition of naphthalene and dodecane are different when dosed in various jet fuel mixtures, and disposition in weathered jet fuel is significantly different from that in commercial and military fuels. These interactions may not only be related to the unique chemistry of these components, but also specific membrane interactions in the SC and viable epidermis.

Skin barrier modification with organic solvents

The primary barrier to body water loss and influx of exogenous substances resides in the stratum corneum (SC). The barrier function of the SC is provided by patterned lipid lamellae localized to the extracellular spaces between corneocytes. SC lipids are intimately involved in maintaining the barrier function. It is generally accepted that solvents induce cutaneous barrier disruption. The main aim of this work is the evaluation of the different capability of two solvent systems on inducing changes in the SC barrier function. SC lipid modifications will be evaluated by lipid analysis, water sorption/desorption experiments, confocal-Raman visualization and FSTEM images. The amount of SC lipids extracted by chloroform/methanol was significantly higher than those extracted by acetone. DSC results indicate that acetone extract has lower temperature phase transitions than chloroform/methanol extract. The evaluation of the kinetics of the moisture uptake and loss demonstrated that when SC is treated with chloroform/methanol the resultant sample reach equilibrium in shorter times indicating a deterioration of the SC tissue with higher permeability. Instead, acetone treatment led to a SC sample with a decreased permeability thus with an improved SC barrier function. Confocal-Raman and FSTEM images demonstrated the absence of the lipids on SC previously treated with chloroform/methanol. However, they were still present when the SC was treated with acetone. Results obtained with all the different techniques used were consistent. The results obtained increases the knowledge of the interaction lipid-solvent, being this useful for understanding the mechanism of reparation of damaged skin.

Adaption of a dermal in vitro method to investigate the uptake of chemicals across amphibian skin

BACKGROUND

Literature data indicate that terrestrial life stages of amphibians may be more sensitive to xenobiotics than birds or mammals. It is hypothesized that dermal exposure could potentially be a significant route of exposure for amphibians, as there is evidence that their skin is more permeable than the skin of other vertebrate species. Thus, higher amounts of xenobiotics might enter systemic circulation by dermal uptake resulting in adverse effects. Heretofore, no guidelines exist to investigate dermal toxicity of chemicals to amphibians. In order to minimize vertebrate testing, this work was targeted to develop an in vitro test system as a possible model to assess the dermal uptake of chemicals across amphibian skin.

RESULTS

The dermal absorption in vitro method (OECD guideline 428), an established toxicological (mammal) test procedure, was adapted to amphibian skin, in a first approach using the laboratory model organism Xenopus laevis and reference compounds (caffeine and testosterone). Skin permeability to both reference substances was significantly higher compared to published mammalian data. Caffeine permeated faster across the skin than testosterone, with ventral skin tending to be more permeable than dorsal skin. As usage of frozen mammalian skin is accepted, frozen skin of X. laevis was tested in parallel. To the freshly excised skin, however, freezing led to increased skin permeability, in particular to caffeine, indicating a loss of skin integrity due to freezing (without additional preservation measures).

CONCLUSIONS

This work has demonstrated that the chosen method can be applied successfully to amphibian skin, providing the basis for further investigations. In future, well-established in vitro test systems and a broad dataset for many chemicals may help assess potential amphibian risk from xenobiotics without the need for extensive vertebrate testing.

Topical therapies for skin cancer and actinic keratosis

The global incidence of skin cancer and actinic keratosis (AK) has increased dramatically in recent years. Although many tumours are treated with surgery or radiotherapy topical therapy has a place in the management of certain superficial skin neoplasms and AK. This review considers skin physiology, non-melanoma skin cancer (NMSC), the relationship between AK and skin cancer and drugs administered topically for these conditions. The dermal preparations for management of NMSC and AK are discussed in detail. Notably few studies have examined drug disposition in cancerous skin or in AK. Finally, recent novel approaches for targeting of drugs to skin neoplasms and AK are discussed.

Effects of anionic surfactants on the water permeability of a model stratum corneum lipid membrane

The stratum corneum (SC) is the ourtermost layer of the epidermis and has a brick-and-mortar-like structure, in which multilamellar lipid bilayers surround flattened dead cells known as corneocytes. The SC lipid membranes provide the main pathway for the transport of water and other substances through the SC. While the physicochemical properties of the SC can be affected by exogenous materials such as surfactants, little is known about how the water barrier function of the SC lipid membranes is compromised by common surfactants. Here, we study the effect of common anionic surfactants on the water permeability of a model SC lipid membrane using a quartz crystal microbalance with dissipation monitoring (QCM-D). Particularly, the effect of sodium dodecyl sulfate (SDS) and sodium lauryl ether sulfate (SLES) is compared. These two surfactants share commonality in their molecular structure: sulfate in the polar headgroup and the same apolar tail. The mass of the lipid membranes increases after the surfactant treatment at or above the critical micelle concentration (CMC) of the surfactants due to their absorption into the membranes. The incorporation of the surfactants into the lipid membranes is also accompanied by partial dissolution of the lipids from the model SC lipid membranes as confirmed by Fourier-transform infrared (FT-IR) spectroscopy. Although the water sorption of pure SDS is much lower than that of pure SLES, the water sorption of SDS-treated membranes increases significantly similar to that of SLES-treated membranes. By combining QCM-D and FT-IR spectroscopy, we find that the chain conformational order and stiffness of the lipid membranes decrease after SDS treatment, resulting in the increased water sorption and diffusivity. In contrast, the conformational order and stiffness of the SLES-treated lipid membranes increase, suggesting that the increased water sorption capacity of SLES-treated lipid membranes is due to the hygroscopic nature of SLES.

Modulation of electroosmosis and flux through skin: effect of propylene glycol

The effect of propylene glycol (PG) on transdermal flux under current was investigated using conventional in vitro iontophoresis methodology. The results were evaluated to explain how PG affects the electroosmotic volume flow (EVF) and electromigrational flux through skin. As a marker molecule for the direction and magnitude of EVF, a non-charged neutral molecule, acetaminophen (AAP), was used. At pH 7.4, the direction of EVF was from anode to cathode. During anodal and cathodal current application, PG decreased AAP flux and this decrease was proportional to the concentration of PG, indicating that the presence of PG in the medium decreased the EVF. This decrease is likely due to the decrease in dielectric constant of the medium and the increases in medium viscosity by the addition of PG. The increase in AAP solubility and the viscosity of the medium by PG may also contribute to the decrease in diffusional flux. The magnitude of EVF was estimated to be about 4.2 μl/cm(2 )h. The effect of PG on the flux of a positively charged drug, donepezil hydrochloride (DH), was further investigated using pH 4.6 phosphate buffer solution. The permselectivity of skin in this solution was also investigated and revealed that the isoelectric point of hairless mouse skin is higher than pH 4.6. Anodal delivery showed much higher flux than cathodal and passive flux, indicating that electromigration is playing the major role for DH flux. As the concentration of PG increased, anodal flux of DH decreased. The main reason for this decrease in electromigration is likely due to the increase in medium viscosity. These results and discussions clearly suggest that the incorporation of frequently used organic cosolvents and penetration enhancers into the iontophoretic formulation should be carefully chosen with a thorough investigation for their effect on flux. Overall, these results provided further mechanistic insights into the role of electroosmosis and electromigration in flux across skin, and how they can be modulated by organic cosolvent, PG.

Penetration and haptenation of p-phenylenediamine

Although p-phenylenediamine (PPD) has been recognized as an extreme sensitizer for many years, the exact mechanism of sensitization has not been elucidated yet. Penetration and the ability to bind to proteins are the first two hurdles that an allergen has to overcome to be able to sensitize. This review is an overview of studies regarding PPD penetration through skin (analogues) and studies on the amino acids that are targeted by PPD. To complete this review, the auto-oxidation and N-acetylation steps involved in PPD metabolism are described. In summary, under normal hair dyeing exposure conditions, <1% of the applied PPD dose penetrates the skin. The majority (>80%) of PPD that penetrates will be converted into the detoxification products monoacetyl-PPD and diacetyl-PPD by the N-acetyltransferase enzymes. The small amount of PPD that does not become N-acetylated is susceptible to auto-oxidation reactions, yielding protein-reactive PPD derivatives. These derivatives may bind to specific amino acids, and some of the formed adducts might be the complexes responsible for sensitization. However, true in vivo evidence is lacking, and further research to unravel the definite mechanism of sensitization is needed.