Modeling of Diffusion with Partitioning in Stratum Corneum Using a Finite Element Model

Partitioning and diffusion of chemicals in skin is of interest to researchers in areas such as transdermal penetration and drug disposition, either for risk assessment or transdermal delivery. In this study a finite element method is used to model diffusion in the skin’s outermost layer, the stratum corneum (SC). The SC is considered to be a finite two-dimensional composite having different diffusivity values in each medium as well as a partition coefficient at the interfaces between media. A commercial finite element package with thermal analysis capabilities is selected due to the flexibility of this software to handle irregular geometries. Partitioning is accommodated through a change of variables technique. This technique is validated by comparison of model results with analytical solutions of steady-state flux, transient concentration profiles, and time lag for diffusion in laminates. Two applications are presented. Diffusion is solved in a two-dimensional “brick and mortar” geometry that is a simplification of human stratum corneum, with a partition coefficient between corneocyte and lipid. Results are compared to the diffusion in multiple laminates to examine effects of the partition coefficient. The second application is the modeling of diffusion with partitioning through an irregular geometry which is obtained from a micrograph of hairless mouse stratum corneum.     

透皮给药研究的新进展

透皮给药安全可控,是无创给药的新途径,有着广阔的市场前景。现有的透皮药物限于小分子和低浓度,角质层屏障使大多数药物难以通过或难以达到有效浓度和有效速率。透皮给药的关键在于促进药物渗透,使药物透皮吸收进毛细血管。促渗手段有:使用化学促渗剂;对药物进行化学修饰制成前体药物;使用物理方法;将药物载入载体。这些方法的原理大致分为三种:改变角质层结构;外力驱动药物;将药物进行修饰或包裹。简要地介绍了增强药物透皮的物理方法和载体方法研究的新进展。     

透皮给药研究的新进展

透皮给药安全可控，是无创给药的新途径，有着广阔的市场前景。现有的透皮药物限于小分子和低浓度，角质层屏障使大多数药物难以通过或难以达到有效浓度和有效速率。透皮给药的关键在于促进药物渗透，使药物透皮吸收进毛细血管。促渗手段有：使用化学促渗剂；对药物进行化学修饰制成前体药物；使用物理方法；将药物载入载体。这些方法的原理大致分为三种：改变角质层结构；外力驱动药物；将药物进行修饰或包裹。简要地介绍了增强药物透皮的物理方法和载体方法研究的新进展。     

Chemical imaging of the stratum corneum under controlled humidity with the attenuated total reflection Fourier transform infrared spectroscopy method

Attenuated total reflection Fourier transform infrared spectroscopic imaging was applied to study human stratum corneum (SC) tissue, the outermost layer of the skin. This imaging approach was combined with a controlled environment cell to demonstrate the possibility of obtaining chemical images of SC exposed to a wide range of relative humidities and diffusion of ethanol through the SC tissue with a specially designed liquid cell. The effect of water vapor sorbed into the SC on the distribution of other components in the SC was studied. Principal component analysis was applied in conjunction with univariate analysis to differentiate the distribution of different components in the SC. Swelling of the SC, a heterogeneous distribution of natural moisturizing factor and water, was detected upon the increase of relative humidity. The approach to image the penetration of liquid ethanol into the SC was also demonstrated and showed good potential and implications for studying transdermal drug delivery.     

Biophysical characterization of the stratum corneum. Relationship between structure and properties]

Studies carried out over the last ten years have determined the structure and chemical composition of the stratum corneum and their significance for biologic function; the stratum corneum (SC) is composed of flat layers linked to each other by desmosome plates. Intercellular spaces are composed mainly of double lipid layers which are responsible for the main part of the barrier function of the SC. This barrier between the body and the environment must be effective despite the deformations and various insults (chemical, physicochemical) to which the SC is subjected. The SC fulfills its barrier function through remarkable physical properties of which most originate in an ability to bind water molecules to protein sites whose presence depends on the humidity of the environment in which the SC was produced. Lastly, the SC exhibits some enzyme activities and consequently can no longer be considered as an inert membrane.     

Enhancement of Transdermal Drug Delivery by ns Q-swithed Nd ：YAG Laser-induced Pressure Wave

A non-invasive laser enhancing transdermal drug delivery technique has been investigated. The second harmonic wavelength of 532 nm of a Q-Switched Nd：YAG laser with pulse duration of 15 ns was used to irradiate on a black polyethylene sheet covering on the surface of the drug solution, and hence produced pressure waves in the solution. Porcine skin and Rhodamine B were used as skin model and reagent respectively. Fluorescence microscope was employed to examine the mechanisms of drug delivery via the skin samples after laser treatment. The experiment revealed that the penetration depth of Rhodamine B under the illumination of laser increased with the energy density of the laser beam. After 20 laser shots at laser energy density of 70 m J/cm^2, the penetration depth reached 440 μm in 30 minutes, which was about three times as that without laser illumination. One possible explanation was that laser-induced pressure waves formed microchannels in the stratum corneum of the skin tissue. These microchannels provided much more effective paths for infiltration of Rhodamine B through the SC than follicular and intercellular paths. The drug solution diffused into the SC under the concentration gradient through the channels.     

Finite Element Modeling of Coupled Diffusion with Partitioning in Transdermal Drug Delivery

The finite element method is employed to simulate two-dimensional (axisymmetric) drug diffusion from a finite drug reservoir into the skin. The numerical formulation is based on a general mathematical model for multicomponent nonlinear diffusion that takes into account the coupling effects between the different components. The presence of several diffusing components is crucial, as many transdermal drug delivery formulations contain one or more permeation enhancers in addition to the drug. The coupling between the drug and permeation enhancer(s) results in nonlinear diffusion with concentration-dependent diffusivities of the various components. The framework is suitable for modeling both linear and nonlinear, single- and multicomponent diffusions, however, as it reduces to the correct formulation simply by setting the relevant parameters to zero. In addition, we show that partitioning of the penetrants from the reservoir into the skin can be treated in a straightforward manner in this framework using the mixed method. Partitioning at interface boundaries poses some difficulty with the standard finite element method as it creates a discontinuity in the concentration variable at the interface. To our knowledge, nonlinear (concentration-dependent) partitioning in diffusion problems has not been treated numerically before, and we demonstrate that nonlinear partitioning may have an important role in the effect of permeation enhancers. The mixed method that we adopt includes the flux at the interface explicitly in the formulation, allowing the modeling of concentration-dependent partitioning of the permeants between the reservoir and the skin as well as constant (linear) partitioning. The result is a versatile finite element framework suitable for modeling both linear and nonlinear diffusions in heterogeneous media where the diffusivities and partition coefficients may vary in each subregion.     

Acute Modulations in Stratum Corneum Permeability Barrier Function Affect Claudin Expression and Epidermal Tight Junction Function via Changes of Epidermal Calcium Gradient

Tight junction (TJ) is recognized as a second barrier of the skin. Altered expression of TJ proteins in various skin diseases characterized by the abnormal permeability barrier such as psoriasis suggests that TJ could be affected by stratum corneum (SC) barrier status. However, the physiological relationship between SC and TJ barrier remains to be investigated. Therefore, we examined the effect of SC barrier disruption on the expression of TJ proteins, claudin (Cldn)-1 and Cldn-4, and TJ barrier function in hairless mouse skin. We also investigated whether the alterations in epidermal Ca²⁺ affected TJ proteins expression in vivo. Repeated tape-stripping induced a sequential change of the expression and function of TJ. As early as 15-30 minutes after tape-stripping, downregulation of Cldn-1 and Cldn-4 immunoreactivity and protein level without change in mRNA level was found. This was accompanied by the abnormal leakage of lanthanum. However, by 1 hour Cldn-1 and Cldn-4 immunolocalization recovered along with normalized lanthanum permeation pattern. Moreover, the mRNA and protein levels of Cldn-1 and Cldn-4 were increased by 1 to 6 hours after tape-stripping. Inhibition of calcium loss by immersion of barrier-disrupted skin into a high Ca²⁺ solution prevented the dislocation of Cldn-1 and Cldn-4. Occlusion of barrier-disrupted skin delayed the restoration of Cldn-1 and Cldn-4. Our results suggest that the alteration of epidermal Ca²⁺ gradient caused by SC barrier perturbation affects the TJ structure and function and the faster recovery of TJ as compared to the SC barrier may imply the protective homeostatic mechanism of skin barrier.     

An array of porous microneedles for transdermal monitoring of intercellular swelling

An array of porous microneedles was developed for minimally-invasive transdermal electrolytic connection through the human skin barrier, the stratum corneum. The length of microneedle was designed to be 100 μm so that it penetrates into the epidermis layer without pain. Each microneedle was supported by a thicker cylindrical post protruding from a planar substrate to realize its effective penetration even into elastic human skin. Since this support (post and substrate) was equally porous as the needles, the needle chip was entirely permeable for electrolyte. This ion-conductive porous microneedle array was applied to the transdermal conductometry with small direct current for local monitoring of intercellular swelling, edema. The porous needle-based electrode system could be a platform for various transdermal electrical diagnosis and treatments.     

Comparison of ovalbumin and ovalbumin epitope peptide for transdermal delivery and vaccination mediated by the photothermal effect of gold nanorods

Transdermal protein delivery is a powerful and attractive method for protein therapy and dermal vaccination compared with other administrations. However, this delivery method is restricted by the low permeability of the stratum corneum (SC), a hydrophobic barrier that restricts the entry of hydrophilic molecules such as proteins. In this study, we developed an improved gel patch system carrying ovalbumin and ovalbumin epitope peptide, and then compared their permeability into the skin. First, the gel patch was placed on mouse skin to allow contact with the polymer coated gold nanorods and then irradiated by a continuous-wave laser. Thermal ablation of the SC improved the permeability and translocation of ovalbumin and the peptide. Fluorescence images showed the translocation was enhanced when the skin was treated with the FITC-modified ovalbumin epitope peptide. However, induction of anti-OVA IgG production after treatment with the FITC-modified ovalbumin epitope peptide was lower than that with FITC-OVA.     

Modelling formation of a drug reservoir in the stratum corneum and its impact on drug monitoring using reverse iontophoresis

Reverse iontophoresis is a relatively new technique for non-invasive drug monitoring in the body. It involves a small electrical current being passed through the skin to facilitate the movement of small charged ions and polar molecules on the skin's surface where the amount of drug can then be measured and hence an accurate estimate of the blood concentration can be made. In vivo studies for several molecules show that initially large amounts of drug are extracted from the body, which are unrelated to the magnitude of the blood concentration; over time the fluxes of extraction decrease to a level proportional to the steady state blood concentration. This suggests that, at first, the drug is being extracted from some source other than the blood; one such candidate for this source is the dead cells which form the stratum corneum. In this paper, we construct two related mathematical models; the first describes the formation of the drug reservoir in the stratum corneum as a consequence of repeated drug intake and natural death of skin cells in the body. The output from this model provides initial conditions for the model of reverse iontophoresis in which charged ions from both the blood and the stratum corneum reservoir compete for the electric current. Model parameters are estimated from data collected for lithium monitoring. Our models will improve interpretation of reverse iontophoretic data by discriminating the subdermal from the skin contribution to the fluxes of extraction. They also suggest that analysis of the skin reservoir might be a valuable tool to investigate patients' exposure to chemicals including therapeutic drugs.     

Rapid fabrication method of a microneedle mold with controllable needle height and width

The main issue of transdermal drug delivery is that macromolecular drugs cannot diffuse through the stratum corneum of skin. Many studies have pursued micro-sized needles encapsulated with drugs to overcome this problem, as these needles can pierce the stratum corneum and allow drugs to enter the circulatory system of the human body. However, most microneedle fabrication processes are time-consuming and require expensive equipment. In this study, we demonstrate a rapid method for fabricating a microneedle mold using drawing lithography and a UV-cured resin. The mold was filled with a water-soluble material, polyvinylpyrrolidone (PVP), which was then demolded to produce a water-soluble microneedle array. The results of an in vitro skin insertion test using PVP microneedles and pig ear skin demonstrated the feasibility of the microneedle mold. In addition, by controlling the viscosity of the UV-cured resin through various heat treatments, microneedles with different heights and aspect ratios were produced. Compared with other methods, this technology significantly simplifies and accelerates the mold fabrication process. In addition, the required equipment is relatively simple and inexpensive. Through this technology, we can rapidly fabricate microneedle molds with controllable dimensions for various applications.     

Development of aceclofenac nanovesicular system using biomaterial for transdermal delivery: physical characterization,ex vivo,in vivo,and anti-inflammatory studies

Context: Aceclofenac is an important NSAID; however, it causes GI disturbances whereas employing transdermal route would require permeation enhancer for systemic application, thereby causing skin damage. Ceramide 2 is a natural lipid having an important role in the maintenance of skin. Objective: Aceclofenac-loaded nanovesicles of ceramide-2, cholesterol, palmitic acid, and cholesteryl sulfate were formulated and analyzed for physical and biological properties. Materials and method: Film hydration method was used to prepare the vesicles and physical parameters, in vitro drug release and stability were evaluated. Then, they were formulated into gel and evaluated against a commercial formulation (CF) and gel-containing plain drug (CPG) for ex vivo, in vivo drug permeation, and anti-inflammatory activity. Results: The developed formulations showed best physical profile and ACV-1 gave 92.89% drug release in in vitro studies. Ex vivo studies showed drug permeation between 15.32–31.12?μg/cm², whereas CPG and CF released 0.47 and 2.81?μg/cm², respectively. ACVG-1 and CF showed C_max of 8.1 and 1.2?μg/ml at 8 and 4?h, respectively. ACVG-1 showed 11.6 times AUC than CF. ACVG-1 inhibited edema by 44% in first hour itself. Discussion: Ceramide 2 and palmitic acid played an important role in the formulation and promotes the drug permeation through stratum corneum and dermis. Ceramide content of the formulation also contributes towards stability and skin protection. Conclusion: The composition of the vesicle formulation performs an important role in physical properties and drug permeation, thereby producing an optimum formulation.     

Innovative drug delivery strategies for topical photodynamic therapy using porphyrin precursors.

Photodynamic therapy (PDT) has been extensively investigated as a treatment for tumors and neoplasias of the skin, bladder, mouth, and female reproductive tract. The most convenient drug delivery route, when focusing on the photodynamic treatment of such tumors and neoplasia, is the transdermal. However, with the inherent "barrier function" of the stratum corneum of the skin, drugs with high molecular weight (> 500 Daltons) or extremes of polarity will find it difficult to successfully cross the skin. Therefore, preformed photosensitizers, which are generally large, highly conjugated molecules, are not commonly used in topical PDT. This has led to the idea of endogenous photosensitization using the small (167.8 Daltons), although hydrophilic, 5-aminolevulinic acid (ALA) being the most frequently employed agent in modern topical PDT. Although clinical application of ALA and its bimolecular effects within target cells remain as primary research themes, the design and evaluation of delivery systems required for effective photosensitizer administration have been less well addressed. This paper briefly reviews traditional approaches to topical delivery of ALA and its esters, and highlights several innovative strategies recently employed to increase the efficacy of ALA-PDT.     

Optical clearing of human skin: comparative study of permeability and dehydration of intact and photothermally perforated skin

Accelerated diffusion of water and hyperosmotic optical clearing agents is studied as a result of enhanced epidermal permeability. A lattice of microzones (islets) of damage in stratum corneum is induced using a flash-lamp applique system. An optical clearing agent composed of 88% glycerol in aqueous solution is used for all experiments. Research of skin dehydration and glycerol delivery through epidermis at both intact and perforated stratum corneum is presented. The dehydration process induced by both stimuli of evaporation and osmotic agent action is studied by weight measurements. Dynamics of refractive index alteration of both glycerol solution and water during their interaction with skin samples is monitored. The amounts of water escaping from skin through the stratum corneum, due to hyperosmotic-agent action, and glycerol penetrating through the skin sample, are estimated. The results show that the proposed method allows for effective transepidermal water loss and delivery of optical clearing agents.     

Dynamic cooperation between epidermal barriers and Langerhans cells

Skin is the structure that covers our body and protects it from not only the entry of pathogens or allergens but also from the leakage of water, solutes or nutrients. These outside-in and inside-out skin barrier functions are dependent on the epidermis, a stratified epithelial cellular sheet. While mucus covers the epidermis in fish and amphibian tadpoles, terminally differentiated cornified cellular sheets called stratum corneum (SC) constitute the outermost epidermal barrier in amphibian adults, reptiles, birds and mammals. Beneath the mucus or SC, apical paracellular spaces of epidermal cells are sealed with tight junctions (TJs) that might limit paracellular leakage of water and electrolytes to maintain fluid homeostasis. We have recently reported in mice that Langerhans cells (LCs) elongate their dendrites to penetrate through epidermal TJs upon activation and uptake antigens from extra-TJ environment. During antigen uptake, new TJs are formed between keratinocytes and LC dendrites to maintain the integrity of epidermal TJ barriers. To understand the epidermal barrier system and its deficiency observed in human skin diseases, we need to re-evaluate human epidermal barrier as a composite barrier consisting of SC and TJs and to investigate the molecular mechanism and immunological consequences of the extra-TJ antigen uptake activity of LCs.     

Novel polyion complex micelles for liver-targeted delivery of diammonium glycyrrhizinate: in vitro and in vivo characterization

This study reports the effect of liposome particle size at the nanoscale and bilayer deformability on the permeation through MatTek human skin equivalents and provides a comparative quantitative measure through calculation of diffusion coefficients. Exploring DOPC and DPPC fluorescent liposomes, our results demonstrate the faster diffusion of 50 nm liposomes compared with 100 and 200 nm liposomes when the lipid bilayer remains the same. Diffusion kinetics of the 50 nm particles appear not to depend on the rigidity of the lipid layer, whereas diffusion of particles larger than 100 nm is significantly affected by the rigidity of the bilayer, and DOPC liposomes diffuse faster than their DDPC equivalents. Our results suggest that liposomes composed of a rigid bilayer can be expected to remain intact after passing through the stratum corneum.     

Electromechanical method coupling non-invasive skin impedance probing and in vivo subcutaneous liquid microinjection: controlling the diffusion pattern of nanoparticles within living soft tissues

Transdermal drug delivery is the way to transport drug carriers, such as nanoparticles, across the skin barrier to the dermal and/or subcutaneous layer. In order to control the transdermal drug delivery process, based on the heterogeneous and nonlinear structures of the skin tissues, we developed a novel electromechanical method combining in vivo local skin impedance probing, subcutaneous micro-injection of colloidal nanoparticles, and transcutaneous electrical stimulation. Experiments on the nude mice using in vivo fluorescence imaging exhibited significantly different apparent diffusion patterns of the nanoparticles depending on the skin impedance: Anisotropic and isotropic patterns were observed upon injection into low and high impedance points, respectively. This result implies that the physical complexity in living tissues may cause anisotropic diffusion of drug carriers, and can be used as a parameter for controlling drug delivery process. This method also can be combined with microneedle-based drug release systems, micro-fabricated needle-electrodes, and/or advanced in vivo targeting/imaging technologies using nanoparticles.     

Non-melanoma skin cancer: The hygiene hypothesis

Protection against ultra violet radiation-induced DNA-damage in the skin is not only provided by the pigmentary system. The epidermal barrier consisting of stratum corneum keratinocytes, filaggrin and other proteins is an additional component of the UV-shield. Disruption of the epidermal barrier through frequent body cleansing with soaps and cosmetics may increase the risk of non-melanoma skin cancer.     

Unraveling barrier properties of three different in-house human skin equivalents

Human skin equivalents (HSEs) are three-dimensional culture models that are used as a model for native human skin. In this study the barrier properties of two novel HSEs, the fibroblast-derived matrix model (FDM) and the Leiden epidermal model (LEM), were compared with the full-thickness collagen model (FTM) and human skin. Since the main skin barrier is located in the lipid regions of the upper layer of the skin, the stratum corneum (SC), we investigated the epidermal morphology, expression of differentiation markers, SC permeability, lipid composition, and lipid organization of all HSEs and native human skin. Our results demonstrate that the barrier function of the FDM and LEM improved compared with that of the FTM, but all HSEs are more permeable than human skin. Further, the FDM and LEM have a relatively lower free fatty acid content than the FTM and human skin. Several similarities between the FDM, LEM and FTM were observed: (1) the morphology and the expression of the investigated differentiation markers were similar to those observed in native human skin, except for the observed expression of keratin 16 and premature expression of involucrin that were detected in all HSEs, (2) the lipids in the SC of all HSEs were arranged in lipid lamellae, similar to human skin, but show an increase in the number of lipid lamellae in the intercellular regions and (3) the SC lipids of all HSEs show a less densely packed lateral lipid organization compared with human SC. These findings indicate that the HSEs mimic many aspects of native human skin, but differ in their barrier properties.