A Four-Layer Numerical Model for Transdermal Drug Delivery: Parameter Optimization and Experimental Validation Using a Franz Diffusion Cell

The Extent of Orthorhombic Lipid Phases in the Stratum Corneum Determines the Barrier Efficiency of Human Skin In Vivo

The maturity of the corneocyte envelope (CE) provides information about the barrier functionality of the stratum corneum (SC). Corneocytes are enclosed by the CE, a protein-lipid matrix, contributing to mechanical resistance and hydrophobicity of the SC. The aim of the work was to develop a novel and robust approach to characterize CE maturity based on rigidity, hydrophobicity and surface area. This offers an alternative approach to the Nile red staining and antigenicity of involucrin to characterize the CE. The photoexposed (PE) cheek and photoprotected (PP) post-auricular sites were selected for investigation. Nine tape strips were obtained from the cheek and post-auricular sites of healthy Caucasians. CEs on the first and last tape strip were subjected to sonication to assess rigidity, and Nile red staining to determine hydrophobicity per unit surface area. In addition, the presence of involucrin and lipids was assessed to determine CE maturity by examination of the red/green pixel ratio, percentage of involucrin expressing CEs and alternatively the ratio of fluorescence density. The CE rigidity was lower in the deeper SC layers of the cheek, whereas post-auricular CEs were mechanically more resistant. Post-auricular CEs from the superficial SC had a larger surface area with a stronger fluorescence signal than those from the cheek. Interestingly, those CEs from the deeper SC layers had similar surface areas in both anatomical sites but were significantly different in hydrophobicity. These three parameters can be summarized as a relative CE maturity index that expresses CE maturity more precisely with a higher sensitivity than the conventional involucrin and Nile red staining approach. CEs of the cheek surface are more mature than CEs in the deeper SC layer, whereas CEs obtained from the post-auricular surface are more mature than those from the cheek surface. The combined method developed allows characterization of CE maturity based on hydrophobicity per unit surface area and rigidity rather than a simple ratio of lipid to involucrin. A more robust and sensitive measurement has therefore been developed addressing the limitations of earlier protocols.

In vivo real-time monitoring system of electroporation mediated control of transdermal and topical drug delivery

Barrier-disrupted skin: Quantitative analysis of tape and cyanoacrylate stripping efficiency by multiphoton tomography.

Release from a transdermal drug delivery system (TDDS) can either be controlled by diffusion in the adhesive, by diffusion processes in the stratum corneum of the skin or a combination of both. In this study, diffusion processes in monolithic type TDDS were investigated using confocal Raman microscopy. An acrylic adhesive (Duro-Tak 180-129a), a rubber adhesive (Duro-Tak H1540) and a silicone adhesive (BIO-PSA 7-4202) were used. Skin permeation of the model drug Paeonol from these adhesives was investigated. Release studies on porcine cadaver skin were carried out. Solubility of Paeonol in the different adhesives was measured. Diffusion coefficients of the drug in the TDDSs were calculated from confocal Raman depth scans, the diffusion coefficient in the stratum corneum was calculated using tape stripping. Solubility of Paeonol in the acrylic adhesive was the highest with 30 g/L among the tested systems. Paeonol had a solubility of 6 and 9 g/L in the silicone and rubber based system. Diffusion coefficient rank order was BIO-PSA 7-4204 > Duro-Tak 180-129a > Duro-Tak H1540. Release on porcine cadaver skin from the silicone was the highest followed by the rubber and the acrylic adhesive. During release studies on porcine skin with Duro-Tak H1540 no concentration gradient of Paeonol could be monitored in the Raman depth profiles, whereas in the stratum corneum an apparent diffusion gradient was detectable. Solubility of a drug in the adhesive dominated the release properties, high-diffusion coefficients of drugs in adhesives do not necessarily lead to high release rates from adhesives.

This review covers the last 20 years of research we and our collaborators have conducted on ethnic differences in facial skin moisturization placed in historical context with previous research. We have focussed particularly on the biochemical and cellular gradients of the stratum corneum (SC) with the aim of discovering new skin moisturization and SC maturation mechanisms, identifying new technologies and/or providing conceptual innovations for ingredients that will improve our understanding and treatment of dry skin. Specifically, we discuss gradients for corneodesmosomes and proteases, corneocyte phenotype-inducing enzymes, filaggrin and natural moisturizing factor (NMF), and barrier lipids. These gradients are interdependent and influence greatly corneocyte maturation. The interrelationship between corneodesmolysis and the covalent attachment of ω-hydroxy ceramides and ω-hydroxy fatty acids to the corneocyte protein envelope forming the corneocyte lipid envelope is especially relevant in our new understanding of mechanisms leading to dry skin. This process is initiated by a linoleoyl-ω-acyl ceramide transforming enzyme cascade including 12R lipoxygenase (12R-LOX), epidermal lipoxygenase-3 (eLOX3), epoxide hydrolase 3 (EPHX3), short-chain dehydrogenase/reductase family 9C member 7 (SDR9C7), ceramidase and transglutaminase 1. Our research has opened the opportunity of using novel treatment systems for dry skin based on lipids, humectants, niacinamide and inhibitors of the plasminogen system. It is clear that skin moisturization is a more complex mechanism than simple skin hydration.

The function of transdermal drug delivery (TDD) systems is complex due to the multiple layers necessary for controlling the rate of drug release and the interaction with the patient’s skin. In this work, we study a particular aspect of a TDD system, that is, the parameters that describe the drug permeation through the skin layers. Studies of the diffusion of two compounds were carried out and supported by tape stripping and numerical modeling. The experimental studies are carried out for porcine skin in a Franz diffusion cell and tape stripping is used to quantify the concentration of drug in the stratum corneum. A multi-layered numerical model, based on Fickian diffusion, is used to determine the unknown parameters that define the skin’s permeability, such as the partition between layers and the mass transfer coefficients due to the surface barrier. A significant correlation was found between the numerical modeling and experimental results, indicating that the partition and mass transfer effects at the interlayer boundary are accurately represented in the numerical model. We find that numerical modeling is essential to fully describe the diffusion characteristics.

In order to help clarify the controversially discussed dermal uptake properties of micronized titanium dioxide (TiO _ 2), we conducted extensive in vitro dermal absorption studies with 'Franz-type' diffusion cells on excised porcine skin. After biopsies and chemical fixation, the overall localization of TiO _ 2 in the skin was analyzed by means of transmission electron microscopy (TEM). The lateral and vertical distribution of TiO _ 2 within the stratum corneum (SC) was investigated by tape stripping and subsequent scanning electron microscopy (SEM) in combination with energy dispersive X-ray analysis (EDXA). TiO _ 2 was found exclusively on the outermost SC layer. The surface deposit, as displayed by TEM, featured clearly distinguishable agglomerates as well as single particles with a characteristic cubic shape and a primary particle size of about 20-50 nm. Concurrently, SEM/EDXA micrographs first showed an even distribution of TiO _ 2 on the skin surface. After 10-fold stripping, however, TiO _ 2 was found to be localized only in the furrows and not on the partially removed ridges of the skin surface. SEM/EDXA micrographs of the adhesive tape strips revealed a characteristic pattern of stripped material and free regions. This pattern was an imprint of the skin's topography. Hence, tape stripping initially removed TiO _ 2 and SC layers only from the ridges and not from the deeper furrows. Continued stripping increasingly yielded material from the deeper contours of the SC surface. TiO _ 2 was found only in traces in the upper part of the follicle without any evidence of uptake into the follicular epithelium. This indicates that there is not any relevant penetration via the follicular route. We conclude that due to the microtopography of the skin, the strip number normally does not reflect the SC layer number. Accordingly, tape stripping results should always be interpreted with care, especially in the case of topically applied particles, as even higher numbers of subsequent strips may still sample material from the outermost SC layer of the deeper furrows, which could be interpreted falsely as penetrated material. Our results clearly demonstrate that TiO _ 2 homogeneously and completely covers the outermost SC layer. It is neither delivered to the SC nor to the underlying skin layers when applied topically to porcine skin in vitro in the cosmetic vehicle used here. These findings underscore the safety of this micronized inorganic UV filter.

Transdermal drug administration has been attracting attention for its variety of advantages, such as elimination of pain and its minimal invasiveness. However, the efficacy of transdermal drug delivery is limited by the stratum corneum, the outermost layer of the skin. The permeable molecular size through the stratum corneum is known to be roughly below 500 Da, which is known as the 500-Da rule in the area of transdermal drug delivery (1). Various techniques have been explored to enhance transdermal permeation, including, microneedles, thermal ablation, microdermabrasion, electroporation and cavitational ultrasound (2). Among them, microneedles, in particular, have been extensively studied and tested for clinical use.In this work, we discovered that the blunt frustoconical-shaped microneedles or microposts can increase the permeability of skin by locally stretching the stratum corneum without penetration (Figure 1a). Through testing with various molecular sizes, we successfully demonstrated that molecules with a size of approximately up to 10 kDa can be injected. Due to its minimally invasive nature, we propose this as an alternative noninvasive approach to conventional microneedle transdermal drug delivery.Porous frustoconical microneedle arrays (PMN) were fabricated using methacrylic-based materials with microscopic porous network structures (Figure 1b), based on our group’s protocol (3). Ions and molecules can pass through the network by applying weak electric field to introduce electrophoresis and electroosmosis. Using the developed micropost arrays, we conducted the following experiments. Firstly, we measured the transdermal electrical resistance of a pig skin with the PMN array pressed onto the skin. A non-penetrating frustoconical PMN array was found to reduce the transdermal resistance to a similar level as the sharp PMN array that physically penetrates stratum corneum, indicating that local stretching of the stratum corneum can enhance ion and molecular permeability of the skin even without penetration. Secondly, we applied a weak electric field to the PMN array to induce electroosmotic flow (EOF) inside the porous network and measured the drug permeabilities through pig skin using fluorescent-labeled drug molecules with different molecular sizes (Figure 1c). The amount of the permeated drug was evaluated by enzymatically degrading the skin samples and measuring its fluorescent intensity. We found that drug molecules could permeate only when the skin was stretched with the PMN array and EOF was present simultaneously. The synergy between the extension of the stratum corneum and EOF promotion enables the minimally invasive transdermal drug delivery that is even less invasive than the conventional sharp microneedles. We quantitatively confirmed that at least 10-kDa molecules can permeate through the stratum corneum with our developed frustoconical PMN array.In summary, we developed a porous micropost array that can locally stretch stratum corneum to enhance chemical and drug transdermal permeability. By measuring the electric resistance and using fluorescent imaging, we confirmed that drug molecules with a size of approximately 10 kDa can be delivered using a porous micropost array without penetrating the stratum corneum. Our approach of using micropost array significantly widens the window of possible drug molecular sizes for transdermal applications.Figure (a) Frustoconical microposts locally stretch the stratum corneum and increase drug permeability through the skin. Electroosmotic flow pumps out the drug molecules loaded in the porous structure. (b) Fabricated porous micropost array. (c) Fluorescent image of 10-kDa FITC-Dextran molecules permeating through the stratum corneum.References Bos JD & Meinardi MMHM (2000) The 500 Dalton rule for the skin penetration of chemical compounds and drugs. Exp Dermatol 9(3):165-169. Prausnitz MR & Langer R (2008) Transdermal drug delivery. Nat Biotechnol 26(11):1261-1268. Terutsuki D, Segawa R, Kusama S, Abe H, & Nishizawa M (2023) Frustoconical porous microneedle for electroosmotic transdermal drug delivery. J Control Release 354:694-700. Figure 1

Microneedles facilitate transdermal drug delivery by piercing microscale pores through the stratum corneum. They usually penetrate only thought the stratum corneum thus the nociceptors of the skin will not be stimulated. Therefore, as an alternative approach, microneedles provide a minimally invasive method for drug delivery. Additive manufacturing which is called threedimensional (3D) printing revolutionized the field of pharmaceutical and biomedical sciences due to their capabilities for fast and cost-effective prototyping of complex structures. Biodegradable 3D printed PLA microneedles are an emerging class of novel transdermal drug delivery systems. Aims of this study were to fabricate 3D printed microneedles and investigate for the first time the ability to coat different drug formulations on 3D printed microneedles. We demonstrated that 3D printing combined with the post-fabrication etching step could make ideally sized and shaped microneedles. Dip coating method revealed to be the best coating method for 3D printed microneedles because of its simplicity and ability to create a uniform load over the printed microneedles. We have also shown that 3D printed microneedles could successfully penetrate and break off into porcine skin.

Transdermal Drug Delivery System (TDDS) is described as a self-contained or discrete dosage form that is applied to the intact skin. This rout of drug administration of drugs through the skin for therapeutic use is an alternative approach to oral, intravascular, subcutaneous, and transmucosal routes. The delivery of drugs through the skin to the systemic circulation provides a convenient route of administration for a variety of clinical indications. Transdermal Drug Delivery System allows continuous drug administration, use of drugs with short biological half lives, avoids increases hepatic first pass elimination and rapid termination of medication by removing the transdermal drug delivery system from the skin. Various transdermal technologies may be applied for different categories of pharmaceuticals used for the treatment of disorders of the skin or for systemic effects to treat diseases of other organs. Several transdermal products and applications include hormone replacement therapy, contraception, pain management, angina pectoris, smoking cessation, and neurological disorders such as Parkinson's disease. The most commonly used transdermal system is the skin patch using various types of technologies. Stratum corneum is the outermost layer of the skin and it is the main barrier layer for permeation of drug in transdermal delivery of drugs. So, to circumvent the barrier properties of stratum corneum and to increase the flux of drug through skin membrane various penetration enhancement techniques are used in transdermal drug delivery system. The review presents different physical and chemical methods in penetration enhancement approaches and to optimize the transdermal delivery system.

To date, the transdermal delivery study mainly focused on the drug delivery systems' design and efficacy evaluation. Few studies reported the structure-affinity relationship of the drug with the skin, further revealing the action sites of the drugs for enhanced permeation. Flavonoids attained a considerable interest in transdermal administration. The aim is to develop a systematic approach to evaluate the substructures that were favorable for flavonoid delivery into the skin and understand how these action sites interacted with lipids and bound to multidrug resistance protein 1 (MRP1) for enhanced transdermal delivery. First, we investigated the permeation properties of various flavonoids on the porcine skin or rat skin. We found that 4'-OH (hydroxyl group on the carbon 4' position) rather than 7-OH on the flavonoids was the key group for flavonoid permeation and retention, while 4'-OCH3 and -CH2═CH2-CH-(CH3)2 were unfavorable for drug delivery. 4'-OH could decrease flavonoids' lipophilicity to an appropriate log P and polarizability for better transdermal drug delivery. In the stratum corneum, flavonoids used 4'-OH as a hand to specifically grab the C═O group of the ceramide NS (Cer), which increased the miscibility of flavonoids and Cer and then disturbed the lipid arrangement of Cer, thereby facilitating their penetration. Subsequently, we constructed overexpressed MRP1 HaCaT/MRP1 cells by permanent transfection of human MRP1 cDNA in wild HaCaT cells. In the dermis, we observed that 4'-OH, 7-OH, and 6-OCH3 substructures were involved in H-bond formation within MRP1, which increased the flavonoid affinity with MRP1 and flavonoid efflux transport. Moreover, the expression of MRP1 was significantly enhanced after the treatment of flavonoids on the rat skin. Collectively, 4'-OH served as the action site for increased lipid disruption and enhanced affinity for MRP1, which facilitate the transdermal delivery of flavonoids, providing valuable guidelines for molecular modification and drug design of flavonoids.

Chapter 15 - Cutaneous and Transdermal Drug Delivery: Techniques and Delivery Systems

Quantification of porcine skin permeability in transdermal diffusion with a numerical model

Umbilical therapy for promoting transdermal delivery of topical formulations: Enhanced effect and underlying mechanism