A novel approach to the understanding of human skin barrier function

Interaction between the microbiota and the skin barrier in aging skin: a comprehensive review

The interplay between the microbes and the skin barrier holds pivotal significance in skin health and aging. The skin and gut, both of which are critical immune and neuroendocrine system, harbor microbes that are kept in balance. Microbial shifts are seen with aging and may accelerate age-related skin changes. This comprehensive review investigates the intricate connection between microbe dynamics, skin barrier, and the aging process. The gut microbe plays essential roles in the human body, safeguarding the host, modulating metabolism, and shaping immunity. Aging can perturb the gut microbiome which in turn accentuates inflammaging by further promoting senescent cell accumulation and compromising the host's immune response. Skin microbiota diligently upholds the epidermal barrier, adeptly fending off pathogens. The aging skin encompasses alterations in the stratum corneum structure and lipid content, which negatively impact the skin's barrier function with decreased moisture retention and increased vulnerability to infection. Efficacious restoration of the skin barrier and dysbiosis with strategic integration of acidic cleansers, emollients with optimal lipid composition, antioxidants, and judicious photoprotection may be a proactive approach to aging. Furthermore, modulation of the gut-skin axis through probiotics, prebiotics, and postbiotics emerges as a promising avenue to enhance skin health as studies have substantiated their efficacy in enhancing hydration, reducing wrinkles, and fortifying barrier integrity. In summary, the intricate interplay between microbes and skin barrier function is intrinsically woven into the tapestry of aging. Sound understanding of these interactions, coupled with strategic interventions aimed at recalibrating the microbiota and barrier equilibrium, holds the potential to ameliorate skin aging. Further in-depth studies are necessary to better understand skin-aging and develop targeted strategies for successful aging.

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.

Decellularised extracellular matrix decorated PCL PolyHIPE scaffolds for enhanced cellular activity, integration and angiogenesis

Wound healing involves a complex series of events where cell–cell and cell-extracellular matrix (ECM) interactions play a key role. Wounding can be simple, such as the loss of the epithelial integrity, or deeper and more complex, reaching to subcutaneous tissues, including blood vessels, muscles and nerves. Rapid neovascularisation of the wounded area is crucial for wound healing as it has a key role in supplying oxygen and nutrients during the highly demanding proliferative phase and transmigration of inflammatory cells to the wound area. One approach to circumvent delayed neovascularisation is the exogenous use of pro-angiogenic factors, which is expensive, highly dose-dependent, and the delivery of them requires a very well-controlled system to avoid leaky, highly permeable and haemorrhagic blood vessel formation. In this study, we decorated polycaprolactone (PCL)-based polymerised high internal phase emulsion (PolyHIPE) scaffolds with fibroblast-derived ECM to assess fibroblast, endothelial cell and keratinocyte activity in vitro and angiogenesis in ex ovo chick chorioallantoic membrane (CAM) assays. Our results showed that the inclusion of ECM in the scaffolds increased the metabolic activity of three types of cells that play a key role in wound healing and stimulated angiogenesis in ex ovo CAM assays over 7 days. Herein, we demonstrated that fibroblast-ECM functionalised PCL PolyHIPE scaffolds appear to have great potential to be used as an active wound dressing to promote angiogenesis and wound healing.

Decellularisation of in vitro generated extracellular matrix (ECM) provides an effective way to stimulate angiogenesis and wound healing.

Lipid composition of the stratum corneum in different regions of the body of Kuhl's pipistrelle from the Negev Desert, Israel

The most superficial epidermal layer in endotherms is the stratum corneum (SC), which is composed of dead corneocytes embedded in a lipid matrix with free fatty acids, cholesterol, ceramides, and cerebrosides; the lipid composition of the SC determines its permeability to water vapor. Lipids that are more polar, have longer hydrocarbon chains, and are less bulky are often packed in more ordered phase states to slow cutaneous evaporative water loss (CEWL); these lipids also resist transitions to more disordered phases at high ambient temperatures (T_a). In bats, wing and tail membranes (wing patagia and tail uropatagium, respectively) allow powered flight, but increase surface area, and hence CEWL, with implications for survival in arid environments. We captured Pipistrellus kuhlii from an arid habitat and measured the lipid composition of the SC of the plagiopatagium in the wing, the uropatagium, and the non-membranous region (NMR) of the body using thin layer chromatography and reversed phase high performance liquid chromatography coupled with atmospheric pressure photoionization mass spectrometry. The patagia contained more cholesterol and shorter-chained ceramides, and fewer cerebrosides than the NMR, indicating that the lipid phase transition temperature in the patagia is lower than in the NMR. Thus, at moderate T_a the lipids in the SC in all body regions will remain in an ordered phase state, allowing water conservation; but as T_a increases, the lipids in the SC of the patagia will more easily transition into a disordered phase, resulting in increased CEWL from the patagia facilitating efficient heat dissipation in hot environments.

Aestivation motifs explain hypertension and muscle mass loss in mice with psoriatic skin barrier defect

AIM

Recent evidence suggests that arterial hypertension could be alternatively explained as a physiological adaptation response to water shortage, termed aestivation, which relies on complex multi-organ metabolic adjustments to prevent dehydration. Here, we tested the hypothesis that chronic water loss across diseased skin leads to similar adaptive water conservation responses as observed in experimental renal failure or high salt diet.

METHODS

We studied mice with keratinocyte-specific overexpression of IL-17A which develop severe psoriasis-like skin disease. We measured transepidermal water loss and solute and water excretion in the urine. We quantified glomerular filtration rate (GFR) by intravital microscopy, and energy and nitrogen pathways by metabolomics. We measured skin blood flow and transepidermal water loss (TEWL) in conjunction with renal resistive indices and arterial blood pressure.

RESULTS

Psoriatic animals lost large amounts of water across their defective cutaneous epithelial barrier. Metabolic adaptive water conservation included mobilization of nitrogen and energy from muscle to increase organic osmolyte production, solute-driven maximal anti-diuresis at normal GFR, increased metanephrine and angiotensin 2 levels, and cutaneous vasoconstriction to limit TEWL. Heat exposure led to cutaneous vasodilation and blood pressure normalization without parallel changes in renal resistive index, albeit at the expense of further increased TEWL.

CONCLUSION

Severe cutaneous water loss predisposes psoriatic mice to lethal dehydration. In response to this dehydration stress, the mice activate aestivation-like water conservation motifs to maintain their body hydration status. The circulatory water conservation response explains their arterial hypertension. The nitrogen-dependency of the metabolic water conservation response explains their catabolic muscle wasting.

State of the Art in Stratum Corneum Research. Part II: Hypothetical Stratum Corneum Lipid Matrix Models

This review is the second part of a series which presents the state of the art in stratum corneum (SC) lipid matrix (LM) research in depth. In this part, the various hypothetical models which were developed to describe the structure and function of the SC LM as the skin's barrier will be discussed. New as well as a cumulative assortment of older results which change the view on the different models are considered to conclude how well the different models are holding up today. As a final conclusion, a model, factoring in as much of the known data as possible, is concluded, unifying the varying different models into one which can be developed further, as new results are found in the future. So far, the model is described with a single crystalline or gel-like phase with a certain amount of nanocrystallites of concentrated ceramides (CERs) and free fatty acids and more fluid nanodomains caused by a fluidizing effect of the cholesterol. These domains are dynamically resolved and reformed and do not impair the barrier function. The chain conformation is not completely clear yet; however, an equilibrium of fully extended and hairpin-folded CERs with ratios depending on the properties of each individual CER species is proposed as most likely. An overlapping middle layer as described for the tri-layer model in part I of this series would be present for both conformations. The macroscopic broad-narrow-broad layering, observed in electron micrographs, is explained by an external templating by the lipid envelope, and an internal templating by short and long lipid chains each preferentially show a homophilic association, forming thicker and thinner bilayers, respectively. The degree of influence of the very long ω-hydroxy-CERs is discussed as well.

Permeation enhancers in transdermal drug delivery: benefits and limitations

Introduction: Transdermal drug delivery has several clinical benefits over conventional routes of drug administration. To open the transdermal route for a wider range of drugs, including macromolecules, numerous physical and chemical techniques to overcome the natural low skin permeability have been developed.Areas covered: This review focuses on permeation enhancers (penetration enhancers, percutaneous absorption promoters or accelerants), which are chemicals that increase drug flux through the skin barrier. First, skin components, drug permeation pathways, and drug properties are introduced. Next, we discuss properties of enhancers, their various classifications, structure-activity relationships, mechanisms of action, reversibility and toxicity, biodegradable enhancers, and synergistic enhancer combinations.Expert opinion: Overcoming the remarkable skin barrier properties in an efficient, temporary and safe manner remains a challenge. High permeation-enhancing potency has long been perceived to be associated with toxicity and irritation potential of such compounds, which has limited their further development. In addition, the complexity of enhancer interactions with skin, formulation and drug, along with their vast chemical diversity hampered understanding of their mechanisms of action. The recent development in the field revealed highly potent yet safe enhancers or enhancer combinations, which suggest that enhancer-aided transdermal drug delivery has yet to reach its full potential.

Clinical purpura and elastosis and their correlation with skin tears in an aged population

The previous research reported the results of a prospect cohort study that used logistic regression analysis to construct a risk prediction model for skin tears in individuals aged over 65 years. The model identified three baseline individual characteristics (male gender, history of STs, and history of falls) and two baseline skin manifestations (purpura and elastosis) that predicted the risk of dorsal forearm skin tears. This paper outlines the relationships between baseline skin manifestations and the risk of skin tears. Univariable logistic regression analysis was conducted of all the baseline data collected from the same-study participants to identify variables that significantly predicted purpura and elastosis at baseline. Amongst the 173 participants, 71 (41%) developed one or more skin tears, and in these participants, 52 (73.2%) displayed purpura, 41 (57.8%) had elastosis, and 30 (42.3%) exhibited both manifestations of the dorsal forearm at baseline. Four individual characteristics (age, history of skin tears, history of falls, and antiplatelet therapy) and three skin properties (pH, subepidermal low echogenicity band of the forearms, and skin thickness) were found to predict the risk of purpura. Conversely, three individual variables (age, gender, and smoking), three clinical skin variables (uneven skin pigmentation, cutis rhomboidalis nuchae, and history of actinic keratosis) and one skin property variable (collagen type IV) predicted the risk of skin elastosis. Progressive changes to the skin's structural and mechanical properties from the underlying effects of chronological ageing, and environmental and lifestyle-related influences increased the risk of purpura and elastotic skin manifestations and concomitantly increased risk of skin tears amongst participants.

Quantification of Lipid Phase Order of In Vivo Human Skin Using Attenuated Total Reflection Fourier Transform Infrared (ATR FT-IR) Spectroscopy and Multivariate Curve Resolution Analysis

A new analysis methodology utilizing multivariate curve resolution (MCR) has been successfully combined with Fourier transform infrared (FT-IR) measurement of in vivo human skin to resolve lipid phase constituents in the spectra relative to high and low chain ordering. A clinical study was performed to measure lipid order through different depths of stratum corneum of human subjects. Fourier transform IR spectra were collected through the top 10 layers of the skin on four sites on the left and right forearm of 12 individuals. Depth profiling was achieved by tape stripping to remove layers of skin with 10 successive tapes from each site. In vivo ATR FT-IR spectra were collected after removing each tape. Three isolated spectral regions were analyzed, centered around 2850 cm^-1, 1460-1480 cm^-1, and 730 cm^-1, corresponding to stretching, scissoring, and rocking -CH₂ vibrational modes, respectively. Both traditional lipid conformation analysis and MCR analysis were performed on the same spectral data. The lipid order ratio, expressed as the fraction of highly ordered orthorhombic (OR) lipids to the total lipids content (orthorhombic + hexagonal [HEX] + liquid crystal [LC]), was assessed as function of depth. Lipid order depth profiles (LODP) show an increase in order with the stratum corneum depth which can be adequately described by an exponential function for the data obtained in this study. The LODP derived from the three vibrational modes show very similar trends, although the absolute order ratios are somewhat different. The variance of the skin LODP across individuals is much greater than between sites within the same individual. The higher arm sites closer to the elbow on the left and right arm show no statistically significant difference and are recommended for use in comparative studies. The scissoring mode shows the highest sensitivity for determination of LODP value.

Dry Gel Containing Optimized Felodipine-Loaded Transferosomes: a Promising Transdermal Delivery System to Enhance Drug Bioavailability

Felodipine has a very low bioavailability due to first-pass metabolism. The aim of this study was to enhance its bioavailability by transdermal application. Felodipine-loaded transferosomes were prepared by thin-film hydration using different formulation variables. An optimized formula was designed using statistical experimental design. The independent variables were the used edge activator, its molar ratio to phosphatidylcholine, and presence or absence of cholesterol. The responses were entrapment efficiency of transferosomes, their size, polydispersity index, zeta potential, and percent drug released after 8 h. The optimized formula was subjected to differential scanning calorimetry studies and its stability on storage at 4°C for 6 months was estimated. This formula was improved by incorporation of different permeation enhancers where ex vivo drug flux through mice skin was estimated and the best improved formula was formulated in a gel and lyophilized. The prepared gel was subjected to in vivo study using Plendil® tablets as a reference. According to the calculated desirability, the optimized transferosome formula was that containing sodium deoxycholate as edge activator at 5:1 M ratio to phosphatidylcholine and no cholesterol. The thermograms of this formula indicated the incorporation of felodipine inside the prepared vesicles. None of the tested parameters differed significantly on storage. The lyophilized gel of labrasol-containing formula was chosen for in vivo study. The relative bioavailability of felodipine from the designed gel was 1.7. In conclusion, topically applied lyophilized gel containing felodipine-loaded transferosomes is a promising transdermal delivery system to enhance its bioavailability.

Presence and persistence of a highly ordered lipid phase state in the avian stratum corneum

To survive high temperatures in a terrestrial environment, animals must effectively balance evaporative heat loss and water conservation. In passerine birds, cutaneous water loss (CWL) is the primary avenue of water loss at thermoneutral temperatures, and increases slightly as ambient temperature increases, indicating a change in the permeability of the skin. In the stratum corneum (SC), the outermost layer of skin, lipids arranged in layers called lamellae serve as the primary barrier to CWL in birds. The permeability of these lamellae depends in large part on the ability of lipid molecules to pack closely together in an ordered orthorhombic phase state. However, as temperature increases, lipids of the SC become more disordered, and may pack in more permeable hexagonal or liquid crystalline phase states. In this study, we used Fourier transform infrared spectroscopy to monitor the phase state of lipids in the SC of house sparrows (Passer domesticus) at skin temperatures ranging from 25 to 50°C. As temperature increased, lipids became slightly more disordered, but remained predominantly in the orthorhombic phase, consistent with the small increase in CWL observed in house sparrows as ambient temperature increases. These results differ considerably from studies on mammalian SC, which find a predominantly hexagonal arrangement of lipids at temperatures above 37°C, and the increased order in avian SC may be explained by longer lipid chain length, scarcity of cholesterol, and the presence of cerebrosides. Our results lend further insight into the arrangement and packing of individual lipid molecules in avian SC.

Design and Evaluation of Topical Diclofenac Sodium Gel Using Hot Melt Extrusion Technology as a Continuous Manufacturing Process with Kolliphor® P407

The aim of the present context was to develop and evaluate a Kolliphor® P407-based transdermal gel formulation of diclofenac sodium by hot melt extrusion (HME) technology; central composite design was used to optimize the formulation process. In this study, we have explored first time ever HME as an industrially feasible and continuous manufacturing technology for the manufacturing of gel formulation using Kolliphor® P407 and Kollisolv® PEG400 as a gel base. Diclofenac sodium was used as a model drug. The HME parameters such as feeding rate, screw speed, and barrel temperature were crucial for the semisolid product development, and were optimized after preliminary trials. For the processing of the gel formulation by HME, a modified screw design was used to obtain a uniform product. The obtained product was evaluated for physicochemical characterization such as differential scanning calorimetry (DSC), X-ray diffraction (XRD), pH measurement, rheology, surface tension, and texture profile analysis. Moreover, it was analyzed for general appearance, spreadibility, surface morphology, and drug content. The optimized gel formulation showed homogeneity and transparent film when applied on a glass slide under microscope, pH was 7.02 and uniform drug content of 100.04 ± 2.74 (SD = 3). The DSC and XRD analysis of the HME gel formulation showed complete melting of crystalline API into an amorphous form. The Kolliphor® P407 and Kollisolv® PEG400 formed excellent gel formulation using HME with consistent viscoelastic properties of the product. An improved drug release was found for the HME gel, which showed a 100% drug release than that of a marketed product which showed only 88% of drug release at the end of 12 h. The Flux value of the HME gel was 106 than that of a marketed formulation, which showed only about 60 value, inferring a significant difference (P < 0.05) at the end of 1 h. This study demonstrates a novel application of the hot melt extrusion process for manufacturing of topical semisolid products.

Stratum corneum modulation by chemical enhancers and lipid nanostructures: implications for transdermal drug delivery

Skin is the outermost and largest protective covering of the body. The uppermost layer of the skin, stratum corneum also called the horny layer is composed of keratin-filled cells covered by a lipid matrix which shields the skin from physical and chemical entrants. The lipid lamellar structure comprises of ceramides, cholesterol, fatty acids and proteins. Chemical enhancers that mimic the lamellar chemistry, reversibly fluidize the latter can be utilized for enhancing transport of cargo across the epidermis into the dermis. This review deals with the stratum corneum chemistry, mechanisms to modulate its packing with the aid of chemical enhancers, biophysical techniques for characterization and applications in the design of nature-inspired biocompatible lipid nanostructures for transdermal delivery of drugs and bioactive agents.

Artificia Therio-Responsive Membrane Able to Control On-Off Switching Drug Release through Nude Mice Skin without Interference from Skin-Penetrating Enhancers

The influence of skin-penetrating enhancers such as Azone, ethanol and propylene glycol (PG) on the on-off switching penetration behavior of salbutamol sulfate through the thermo-responsive cholesteryl oleoyl carbonate (COC)-embedded membrane with or without application to the excised nude mice skin was examined. Without the nude mice skin, gel formulations without any enhancer and with Azone showed higher drug penetration across the COC-embedded membrane than those with ethanol and propylene glycol (PG). Moreover, the on-off switching function of COC-embedded membrane still existed. These results indicate that Azone did not alter the thermo-responsive property of COC-embedded membrane. A lower penetration behavior associated with ethanol or PG was probably due to the increased solubility of salbutamol sulfate in each gel formulation and improved drug-carbopol gel interaction. The application of COC-embedded membrane to excised nude mice skin resulted in a similar on-off switching penetration behavior to the gel, but the penetration was significantly weakened compared to the gel formulation that only penetrated through the COC-embedded membrane. However, passage of salbutamol sulfate across the COC-embedded membrane applied to the excised nude mice skin was severely restricted when an enhancer was absent from the gel. Obviously, the excised skin was the predominant barrier to drug penetration. The present study suggests that skin-penetrating enhancer does not alter the structure of the COC embedded in membrane to change the thermo-responsive on-off switching penetration behavior of salbutamol sulfate from gel formulas through COC-embedded membrane.

Human whole body cold adaptation

Reviews on whole body human cold adaptation generally do not distinguish between population studies and dedicated acclimation studies, leading to confusing results. Population studies show that indigenous black Africans have reduced shivering thermogenesis in the cold and poor cold induced vasodilation in fingers and toes compared to Caucasians and Inuit. About 40,000 y after humans left Africa, natives in cold terrestrial areas seems to have developed not only behavioral adaptations, but also physiological adaptations to cold. Dedicated studies show that repeated whole body exposure of individual volunteers, mainly Caucasians, to severe cold results in reduced cold sensation but no major physiological changes. Repeated cold water immersion seems to slightly reduce metabolic heat production, while repeated exposure to milder cold conditions shows some increase in metabolic heat production, in particular non-shivering thermogenesis. In conclusion, human cold adaptation in the form of increased metabolism and insulation seems to have occurred during recent evolution in populations, but cannot be developed during a lifetime in cold conditions as encountered in temperate and arctic regions. Therefore, we mainly depend on our behavioral skills to live in and survive the cold.

Organization of lipids in avian stratum corneum: Changes with temperature and hydration

In response to increases in ambient temperature (Ta), many animals increase total evaporative water loss (TEWL) through their skin and respiratory passages to maintain a constant body temperature, a response that compromises water balance. In birds, cutaneous water loss (CWL) accounts for approximately 65% of TEWL at thermoneutral temperatures. Although the proportion of TEWL accounted for by CWL decreases to only 25% at high Ta, the magnitude of CWL still increases, suggesting changes in the barrier function of the skin. The stratum corneum (SC) is composed of flat, dead cells called corneocytes embedded in a matrix of lipids, many of which arrange in layers called lamellae. The classes of lipids that comprise these lamellae, and their attendant physical properties, determine the rate of CWL. We measured CWL at 25, 30, 35, and 40 °C in House Sparrows (Passer domesticus) caught in the winter and summer, and in sparrows acclimated to warm and cold lab environments. We then used Fourier transform infrared spectroscopy to measure lipid-lipid and lipid-water interactions in the SC under different conditions of temperature and hydration, and correlated these results with lipid classes in the SC. As CWL increased at higher temperatures, the amount of gauche defects in lipid alkyl chains increased, indicating that lipid disorder is partially responsible for higher CWL at high temperatures. However, variation in CWL between groups could not be explained by the amount of gauche defects, and this remaining variation may be attributed to greater amounts of cerebrosides in birds with low CWL, as the sugar moieties of cerebrosides lie outside lipid lamellae and form strong hydrogen bonds with water molecules.

Breaching the skin barrier--insights from molecular simulation of model membranes

Breaching the skin's barrier function by design is an important strategy for delivering drugs and vaccines to the body. However, while there are many proposed approaches for reversibly breaching the skin barrier, our understanding of the molecular processes involved is still rudimentary. Molecular simulation offers an unprecedented molecular-level resolution with an ability to reproduce molecular and bulk level properties. We review the basis of the molecular simulation methodology and give applications of relevance to the skin lipid barrier, focusing on permeation of molecules and chemical approaches for breaching the lipid barrier by design. The bulk kinetic model based on Fick's Law describing absorption of a drug through skin has been reconciled with statistical mechanical quantities such as the local excess chemical potential and local diffusion coefficient within the membrane structure. Applications of molecular simulation reviewed include investigations of the structure and dynamics of simple models of skin lipids, calculation of the permeability of molecules in simple model membranes, and mechanisms of action of the penetration enhancers, DMSO, ethanol and oleic acid. The studies reviewed illustrate the power and potential of molecular simulation to yield important physical insights, inform and rationalize experimental studies, and to predict structural changes, and kinetic and thermodynamic quantities.

Drug delivery through the skin: molecular simulations of barrier lipids to design more effective noninvasive dermal and transdermal delivery systems for small molecules, biologics, and cosmetics

The delivery of drugs through the skin provides a convenient route of administration that is often preferable to injection because it is noninvasive and can typically be self-administered. These two factors alone result in a significant reduction of medical complications and improvement in patient compliance. Unfortunately, a significant obstacle to dermal and transdermal drug delivery alike is the resilient barrier that the epidermal layers of the skin, primarily the stratum corneum, presents for the diffusion of exogenous chemical agents. Further advancement of transdermal drug delivery requires the development of novel delivery systems that are suitable for modern, macromolecular protein and nucleotide therapeutic agents. Significant effort has already been devoted to obtain a functional understanding of the physical barrier properties imparted by the epidermis, specifically the membrane structures of the stratum corneum. However, structural observations of membrane systems are often hindered by low resolutions, making it difficult to resolve the molecular mechanisms related to interactions between lipids found within the stratum corneum. Several models describing the molecular diffusion of drug molecules through the stratum corneum have now been postulated, where chemical permeation enhancers are thought to disrupt the underlying lipid structure, resulting in enhanced permeability. Recent investigations using biphasic vesicles also suggested a possibility for novel mechanisms involving the formation of complex polymorphic lipid phases. In this review, we discuss the advantages and limitations of permeation-enhancing strategies and how computational simulations, at the atomic scale, coupled with physical observations can provide insight into the mechanisms of diffusion through the stratum corneum.

Dermal and transdermal delivery: prodrugs

Attempts to deliver drugs into and through the skin (dermal and transdermal delivery) have not been very successful because the physicochemical properties of drugs are often not optimal. Prodrugs can be used to optimize those physicochemical properties of drugs and optimize their delivery by transiently masking their polar functional groups. For a drug to cross the rate-limiting barrier to delivery (the stratum corneum) it must dissolve in and cross multiple lipid and aqueous phases within the stratum corneum. Prodrugs can be designed to exhibit increased lipid and aqueous solubilities resulting in increased delivery. In order to identify the optimal prodrugs, they must be evaluated as saturated solutions where their thermodynamic activities are maximal in the solution and in the skin. If prodrugs are evaluated at concentrations less than at saturation, inaccurate conclusions about the optimal physicochemical properties may result. Prodrugs must be designed to optimize both their lipid and aqueous solubilities to optimize their delivery into and through the skin.

Evaluation of glycofurol-based gel as a new vehicle for topical application of naproxen

In view of the good skin tolerability, glycofurol was used as a vehicle-based gel, and its effect in the topical penetration of Naproxen (NAP) was investigated. The aims of this study were to develop a suitable gel with bioadhesive property, spreadability, and viscosity for topical anti-inflammatory effect. Three gelling and adhesive agents were examined: Carbopol 974P, Gantrez AN 119, and polyvinylpyrollidone K30. Skin permeation rates and lag times of NAP were evaluated using the Franz-type diffusion cell in order to optimize the gel formulation. The permeation rate of NAP-based gel across the excised rat skin was investigated. A significant increase in permeability parameters such as steady-state flux (J(ss)), permeability coefficient (K(p)), and penetration index (PI) was observed in optimized formulation containing 2% Transcutol as an permeation enhancer. From skin irritation test, it was concluded that the optimized novel glycofurol-based gel formulation was safe to be used for topical drug delivery. The developed glycofurol-based gel appeared promising for dermal and transdermal delivery of naproxen and could be applicable with water-insoluble drugs, which would circumvent most of the problems associated with drug therapy.

Conformational changes in stratum corneum lipids by effect of bicellar systems

Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy was applied to study the effects of the bicelles formed by dimyristoyl-glycero-phosphocholine (DMPC) and dihexanoyl-glycero-phosphocholine (DHPC) in porcine stratum corneum (SC) in vitro. A comparison of skin samples treated and untreated with bicelles at different temperatures was carried out. The analysis of variations after treatment in the position of the symmetric CH2 stretching, CH2 scissoring, and CH2 rocking vibrations reported important information about the effect of bicelles on the skin. Bicellar systems caused a phase transition from the gel or solid state to the liquid crystalline state in the lipid conformation of SC, reflecting the major order-disorder transition from hexagonally packed to disordered chains. Grazing incidence small and wide X-ray scattering (GISAXS and GIWAXS) techniques confirmed this effect of bicelles on the SC. These results are probably related to with the permeabilizing effect previously described for the DMPC/DHPC bicelles.

Molecular organization of the lipid matrix in intact Stratum corneum using ATR-FTIR spectroscopy

ATR-FTIR spectroscopy is useful in investigating the lateral organization of Stratum corneum (SC) lipids in full-thickness skin. Based on studies of the thermotropic phase transitions in n-tricosane and in excised human skin, the temperature dependence of the CH2 scissoring bandwidth emerged as a measure of the extent of orthorhombic and hexagonal phases. This dependence provides a simpler measure of the lateral order in lipid assemblies than the common spectroscopic approaches based on difference spectra, curve fitting of the CH2 scissoring region, and the position of the CH2 stretching vibrations. It has the advantages of ease of determination, relatively low variability, and high discriminative power for the type of lateral intermolecular chain packing. A comparison of the lateral organization of the lipids at the SC surface of mammalian skin using the scissoring bandwidth revealed considerable differences between human abdominal skin (containing mostly orthorhombic phases), porcine ear skin (containing mostly hexagonal phases), and reconstructed human epidermis (containing mostly disordered phases). This parameter also correctly described the different effects of propylene glycol (minimally disturbing) and oleic acid (formation of a highly disordered phase) on the SC lipids in excised human skin. The procedure described here is applicable to in vivo studies in the areas of dermatology, transdermal drug delivery, and skin biophysics.

Thermotropic behavior of ceramides and their isolation from wool

The composition of internal wool lipids (IWL) resembles that of lipids present in other keratinic tissues such as human hair or the stratum corneum. Advances in the isolation of ceramides from wool and in the characterization of their thermotropic properties could facilitate their application in human skin care treatments. IWL are solvents extracted from wool fibers. Ceramide isolation is carried out by medium-pressure liquid chromatography. The different fractions obtained were analyzed quantitatively by thin layer chromatography coupled to an automated flame ionization detector and by high-performance thin layer chromatography using a densitometric detector. Two important fractions were isolated: one was a mixture of different ceramides and the other was exclusively made of ceramide 2 (nonhydroxy acid sphingosine [NS]). The thermotropic behavior of IWL and their isolated fractions were studied by thermogravimetric analysis, differential scanning calorimetry (DSC), and by attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) methodologies. The transition temperature (Tm) obtained was compared with the results of the IWL extract, stratum corneum lipids, and the values found in the bibliography for isolated ceramides. The Tm obtained for IWL (48 degrees C) was lower than that achieved for SCL (65 degrees C). This discrepancy could be due to the different ceramide pattern and to the larger amounts of free fatty acids present in the IWL extract. Although the isolated ceramides had higher Tm values, they resembled the values reported in the bibliography. The suitability of the fraction composed exclusively of ceramide 2 [NS] for skin applications was confirmed by ATR-FTIR and DSC.

Low frequency changes in skin surface potentials by skin compression: experimental results and theories

Human living skin generates an increase in the skin potential when compressed. This was measured on eight subjects with a matrix of nine Ag/AgCl electrodes. The potential increased with the pressure until it reached a maximum. When the pressure was increased stepwise, the response showed an overshoot at each step. Human cadaver skin did not show these potential increments. Neither did pads of collagen, paper tissue soaked in a KCl solution, nor layers of cultured keratinocytes. Three theories are described that may explain the origin of the measured skin potentials. The first is based on the piezoelectric characteristics of proteins in the skin. The second theory assumes that the skin is a charged membrane which generates a streaming potential when deformed. A third theory is proposed in which deformation of absorbed charged protein layers on structures in the skin change the alignment of Donnan potentials in the surrounding tissue.

Ceramides and skin function

Ceramides are the major lipid constituent of lamellar sheets present in the intercellular spaces of the stratum corneum. These lamellar sheets are thought to provide the barrier property of the epidermis. It is generally accepted that the intercellular lipid domain is composed of approximately equimolar concentrations of free fatty acids, cholesterol, and ceramides. Ceramides are a structurally heterogeneous and complex group of sphingolipids containing derivatives of sphingosine bases in amide linkage with a variety of fatty acids. Differences in chain length, type and extent of hydroxylation, saturation etc. are responsible for the heterogeneity of the epidermal sphingolipids. It is well known that ceramides play an essential role in structuring and maintaining the water permeability barrier function of the skin. In conjunction with the other stratum corneum lipids, they form ordered structures. An essential factor is the physical state of the lipid chains in the nonpolar regions of the bilayers. The stratum corneum intercellular lipid lamellae, the aliphatic chains in the ceramides and the fatty acids are mostly straight long-chain saturated compounds with a high melting point and a small polar head group. This means that at physiological temperatures, the lipid chains are mostly in a solid crystalline or gel state, which exhibits low lateral diffusional properties and is less permeable than the state of liquid crystalline membranes, which are present at higher temperatures. The link between skin disorders and changes in barrier lipid composition, especially in ceramides, is difficult to prove because of the many variables involved. However, most skin disorders that have a diminished barrier function present a decrease in total ceramide content with some differences in the ceramide pattern. Formulations containing lipids identical to those in skin and, in particular, some ceramide supplementation could improve disturbed skin conditions. Incomplete lipid mixtures yield abnormal lamellar body contents, and disorder intercellular lamellae, whereas complete lipid mixtures result in normal lamellar bodies and intercellular bilayers. The utilization of physiological lipids according to these parameters have potential as new forms of topical therapy for dermatoses. An alternative strategy to improving barrier function by topical application of the various mature lipid species is to enhance the natural lipid-synthetic capability of the epidermis through the topical delivery of lipid precursors.

Description of transdermal transport of hydrophilic solutes during low-frequency sonophoresis based on a modified porous pathway model

Application of low-frequency ultrasound has been shown to increase skin permeability, thereby facilitating delivery of macromolecules (low-frequency sonophoresis). In this study, we sought to determine a theoretical description of transdermal transport of hydrophilic permeants induced by low-frequency sonophoresis. Parameters such as pore size distribution, absolute porosity, and dependence of effective tortuosity on solute characteristics were investigated. Pig skin was exposed to low-frequency ultrasound at 58 kHz to achieve different skin resistivities. Transdermal delivery of four permeants [mannitol, luteinizing hormone releasing hormone (LHRH), inulin, dextran] in the presence and absence of ultrasound was measured. The porous pathway model was modified to incorporate the permeant characteristics into the model and to achieve a detailed understanding of the pathways responsible for hydrophilic permeant delivery. The slopes of the log kp(p) versus log R graphs for individual solutes changed with solute molecular area, suggesting that the permeability-resistivity correlation for each permeant is related to its size. The tortuosity that a permeant experiences within the skin also depends on its size, where larger molecules experience a less tortuous path. With the modified porous pathway model, the effective tortuosities and skin porosity were calculated independently. The results of this study show that low-frequency sonophoresis creates pathways for permeant delivery with a wide range of pore sizes. The optimum pore size utilized by solutes is related to their molecular radii.

A theoretical analysis of low-frequency sonophoresis: dependence of transdermal transport pathways on frequency and energy density

PURPOSE

Application of low-frequency ultrasound has been shown to increase skin permeability, thereby facilitating delivery of macromolecules (low-frequency sonophoresis). In this study, we seek to determine the dependence of transport pathways during low-frequency sonophoresis on ultrasound parameters.

METHODS

Pig skin is exposed to low-frequency ultrasound over a range of frequencies to achieve different skin resistivities. The porous pathway model is used to study the dependence of average pore size, porosity, and tortuosity on ultrasound parameters. Imaging experiments are also carried out to visualize the transport pathways created by ultrasound.

RESULTS

The data show that the average pore size, determined from the porous pathway model, does not depend on application frequency. Both in the presence and absence of ultrasound the average pore size determined from mannitol delivery is the same (28 +/- 12 A). With the application of ultrasound the skin porosity could be increased by up to 1700-fold. The effect of ultrasound on skin is heterogeneous thereby creating localized transport pathways (LTP). The porosity of these transport pathways is of the same order of magnitude as that of the dermis.

CONCLUSIONS

With this study it is shown that low-frequency ultrasound increases skin permeability by increasing skin porosity rather than by increasing the size of the pores that are responsible for permeant delivery.

Synergistic effect of low-frequency ultrasound and surfactants on skin permeability

Low-frequency ultrasound (20 kHz) and surfactants have been individually shown to enhance transdermal drug transport. In this study, we investigated the synergistic effect of ultrasound and surfactants on transdermal drug delivery. Surfactants with different head group chemistries including anionic, cationic, and nonionic with varying tail lengths (8-16-carbon atoms) were studied. We found that surfactants possessing anionic and cationic head groups were more potent than those possessing nonionic head groups in increasing skin conductivity in the presence of ultrasound. Furthermore, for surfactants possessing the same head group, those with a 14-carbon tail length were found to be most effective in enhancing skin permeability. The data presented in this report show that ultrasound and surfactants synergistically enhance skin permeability. Two mechanisms are shown to play a role in this synergistic effect. First, ultrasound enhances surfactant delivery (enhanced delivery) into the skin and, second, ultrasound disperses surfactant (enhanced dispersion) within the skin. In general, surfactants that are potent enhancers by themselves are potent enhancers in the presence of ultrasound as well. We performed imaging experiments to assess the effect of ultrasound on delivery of a model permeant, sulforhodamine B, into the skin. These experiments show that ultrasound enhances surfactant delivery and dispersion in the skin.

Frequency dependence of sonophoresis

PURPOSE

Application of low-frequency ultrasound has been shown to increase skin permeability, thereby facilitating delivery of macromolecules (low-frequency sonophoresis). In this study, we sought to determine the dependence of low-frequency sonophoresis on ultrasound frequency, intensity and energy density.

METHODS

Pig skin was exposed to low-frequency ultrasound over a range of ultrasound frequency and intensity conditions. The degree of skin permeabilization was measured using its conductivity. Imaging experiments were also carried out to visualize the transport pathways created by ultrasound.

RESULTS

The data showed that for each frequency (in the range of 19.6-93.4 kHz), there exists a threshold intensity below which no detectable conductivity enhancement was observed. The threshold intensity increased with frequency. It is feasible to achieve the desired conductivity (permeability) enhancement regardless of the choice of frequency, although the necessary energy density is higher at higher frequencies. Low frequencies (approximately 20 kHz) induced localized transport compared to a more dispersed effect seen with higher frequencies (approximately 58.9 kHz).

CONCLUSIONS

This study provides a quantitative understanding of the effects of low-frequency ultrasound on skin permeability.

Localization of ceramide and glucosylceramide in human epidermis by immunogold electron microscopy

Ceramides and glucosylceramides are pivotal molecules in multiple biologic processes such as apoptosis, signal transduction, and mitogenesis. In addition, ceramides are major structural components of the epidermal permeability barrier. The barrier ceramides derive mainly from the enzymatic hydrolysis of glucosylceramides. Recently, anti-ceramide and anti-glucosylceramide anti-sera have become available that react specifically with several epidermal ceramides and glucosylceramides, respectively. Here we demonstrate the detection of two epidermal covalently bound omega-hydroxy ceramides and one covalently bound omega-hydroxy glucosylceramide species by thin-layer chromatography immunostaining. Moreover, we show the ultrastructural distribution of ceramides and glucosylceramides in human epidermis by immunoelectron microscopy on cryoprocessed skin samples. In basal epidermal cells and dermal fibroblasts ceramide was found: (i) at the nuclear envelope; (ii) at the inner and outer mitochondrial membrane; (iii) at the Golgi apparatus and the endoplasmic reticulum; and (iv) at the plasma membrane. The labeling density was highest in mitochondria and at the inner nuclear membrane, suggesting an important role for ceramides at these sites. In the upper epidermis, ceramides were localized: (i) in lamellar bodies; (ii) in trans-Golgi network-like structures; (iii) at the cornified envelope; and (viii) within the intercellular space of the stratum corneum, which is in line with the known analytical data. Glucosylceramides were detected within lamellar bodies and in trans-Golgi network-like structures of the stratum granulosum. The localization of glucosylceramides at the cornified envelope of the first corneocyte layer provides further proof for the existence of covalently bound glucosylceramides in normal human epidermis.

Mouse anti-ceramide antiserum: a specific tool for the detection of endogenous ceramide

Ceramide is a pivotal molecule in signal transduction and an essential structural component of the epidermal permeability barrier. The epidermis is marked by a high concentration of ceramide and by a unique spectrum of ceramide species: Besides the two ceramide structures commonly found in mammalian tissue, N-acylsphingosine and N-2-hydroxyacyl-sphingosine, six additional ceramides differing in the grade of hydroxylation of either the sphingosine base or the fatty acid have been identified in the epidermis. Here we report on the characterization of an IgM-enriched polyclonal mouse serum against ceramide. In dot blot assays with purified epidermal lipids the antiserum bound to a similar extent to N-acyl-sphingosine (ceramide 2), N-acyl-4-hydroxysphinganine (ceramide 3), and N-(2-hydroxyacyl)-sphingosine (ceramide 5), whereas no specific reaction was detected with glycosylceramides, sphingomyelin, free sphingosine, phospholipids, or cholesterol. In contrast, a monoclonal IgM antibody, also claimed to be specific for ceramide, was shown to bind specifically to sphingomyelin and therefore was not further investigated. In thin-layer chromatography immunostaining with purified lipids a strong and highly reproducible reaction of the antiserum with ceramide 2 and ceramide 5 was observed, whereas the reaction with ceramide 1 and ceramide 3 was weaker and more variable. Ceramide 2 and ceramide 5 were detected in the nanomolar range at serum dilutions of up to 1:100 by dot blot and thin-layer immunostaining. In thin-layer chromatography immunostaining of crude lipid extracts from human epidermis, the antiserum also reacted with N-(2-hydroxyacyl)-4-hydroxysphinganine (ceramide 6) and N-(2-hydroxyacyl)-6-hydroxysphingosine (ceramide 7). Furthermore, the suitability of the antiserum for the detection of endogenous ceramide by immunolight microscopy was demonstrated on cryoprocessed human skin tissue. Double immunofluorescence labeling experiments with the anti-ceramide antiserum and the recently described anti-glucosylceramide antiserum (Brade et al., 2000, Glycobiology 10, 629) showed that both lipids are concentrated in separate epidermal sites. Whereas anti-ceramide stained the dermal and basal epidermal cells as well as the corneocytes, anti-glucosylceramide staining was concentrated in the stratum granulosum. In conclusion, the specificity and sensitivity of the reagent will enable studies on the subcellular distribution and biological functions of endogenous ceramide.

In vitro characterization of anti-glucosylceramide rabbit antisera

Glucosylceramides (GlcCer) are biosynthetic precursors of glycosphingolipids. They are widely distributed in biological systems where they exhibit numerous biological functions. Studies on the localization of glucosylceramides in different tissues have used biochemical methods only since specific antibodies against GlcCer were not previously available. We have characterized two commercially available rabbit antisera which were prepared against GlcCer of plant origin (1-O-(beta-D-glucopyranosyl)-N-acyl-4-hydroxysphinganine; GlcCer-3) or human origin (1-O-(beta-D-glucopyranosyl)-N-acyl-sphingosine; GlcCer-2) and claimed to be specific for GlcCer. The antisera were also able to detect specifically GlcCer species in crude lipid extracts from human epidermis after separation by thin-layer chromatography. The reagents are sensitive since both antisera reacted at dilutions higher than 1:500 with their homologous antigen in the nanogram range in thin layer immunostaining or dot-blot assays. The antisera are specific for GlcCer although they did not differentiate between GlcCer-2 and GlcCer-3 containing sphingosine or 4-hydroxysphinganine. The antisera also reacted with N-stearoyl-DL-dihydroglucocere-broside indicating that the naturally occurring structural variations in the amino alcohol moiety are not determining the specificity. No crossreactivity was observed with other mono- or diglycosylceramides (galactosylceramides, lactosyl-ceramide), free ceramides or structurally unrelated lipids (cholesterol, sphingomyelin, or phospholipids). Therefore, the glycosylmoiety seems to represent the major antigenic determinant. Finally, the antisera also proved to be useful for the immunohistochemical localization of GlcCer in human epidermis by which earlier biochemical data on the distribution of GlcCer in the various epidermal layers were confirmed.

Evaluation of transcutol as a clonazepam transdermal permeation enhancer from hydrophilic gel formulations

The influence of diethyleneglycol monoethyl ether (transcutol), alone or in combination with propylene glycol, on clonazepam permeation through an artificial membrane and excised rabbit ear skin from Carbopol hydrogels was investigated. Drug kinetic permeation parameters were determined for both series of experiments and compared. Rheological characteristics, drug solubility and membrane/vehicle partition coefficient for each gel formulation were also determined, and their role in the formulation performance was investigated. Both series of experiments showed an increase of drug permeation as a function of transcutol content in the formulation. The combination of transcutol and propylene glycol resulted in a synergistic enhancement of clonazepam flux. A different trend was found in experiments with gels containing mixtures of the two enhancers, where an increase (in the case of artificial membrane) or a decrease (in the case of rabbit ear skin) of drug permeation was found by increasing the transcutol/propylene glycol ratio in the mixture. Such a result is explained on the basis of the particular mechanism of action demonstrated for transcutol which associates the increase of drug solubility to the potent effect of a depot in the skin.

A new computer-based evaporimeter system for rapid and precise measurements of water diffusion through stratum corneum in vitro

It is important to have reliable methods for evaluation of skin barrier function when questions such as barrier perturbing effects of different agents and occlusive effects of different formulations are to be elucidated. A wealth of clinical work relates to measurements of transepidermal water loss in vivo, a method much affected by ambient air relative humidity, temperature, skin irritation processes, psychologic status of the subject, etc., factors that cause the method to suffer from low precision (i.e., high random error). Relating to these obstacles, we have developed a closed in vitro system for measurements of water diffusion rate through pieces of isolated stratum corneum at steady-state conditions, where the relative humidity and temperature is held constant and data can be collected continuously. Our evaporimeter-based in vitro system has a more than 3-fold higher precision (lower random error) ( approximately 10%) than measurements of transepidermal water loss in vivo ( approximately 35%). The results of our study show that: (i) the corneocyte envelopes contribute to the barrier capacity of stratum corneum; (ii) removal of the lipid intercellular matrix results in approximately a 3-fold increase in the water diffusion rate through the isolated stratum corneum (n = 20; p < 0.05), not a 100-fold as has previously been suggested; (iii) exposure to sodium dodecyl sulfate in water does neither alter the water diffusion rate (n = 10; p > 0.05) nor the water holding capacity (n = 10; p > 0.05) of stratum corneum; (iv) exposure to 1 M CaCl2 in water yields an increased water diffusion rate through stratum corneum (n = 10; p < 0.05); and (v) when applied to the stratum corneum in excess concentrations, the penetration enhancer Azone has occlusive effects on water diffusion through the stratum corneum (n = 6; p < 0.05).

Fragrance compounds and amphiphilic association structures

Fragrance formulations have traditionally been based on alcohol as the solvent, but the recent legal restrictions on volatile organic solvents have prompted the industry to change to aqueous solubilized systems. The article reviews the fundamental factors in the application of such systems evaluating the influence by different amphiphilic association structures on the vapor pressure of fragrance compounds. This information is subsequently used to estimate the variation of fragrance compound vapor pressures during evaporation. The results reveal that the vapor pressure versus time variation is improved compared to solvent-based formulations.