Modeling the human skin barrier--towards a better understanding of dermal absorption

Unravelling Microarray Patch Performance: The Role of In Vitro Release Medium and Biorelevant Testing

The absence of established protocols for studying the in vitro performance of dissolvable microarray patches (MAPs) poses a significant challenge within the field. To overcome this challenge, it is essential to optimize testing methods in a way that closely mimics the skin's environment, ensuring biorelevance and enhancing the precision of assessing MAP performance. This study focuses on optimizing in vitro release testing (IVRT) and in vitro permeation testing (IVPT) methods for MAPs containing the antihistamine drugs loratadine (LOR) and chlorpheniramine maleate (CPM). Our primary objective is to investigate the impact of the composition of in vitro release media on the drug release rate, penetration through the skin, and permeation into the release medium. Artificial interstitial fluid is introduced as a biorelevant release medium and compared with commonly used media in IVRT and IVPT studies. Prior to these studies, we evaluated drug solubility in different release media and developed a method for LOR and CPM extraction from the skin using a design of experiment approach. Our findings highlight the effect of the in vitro release medium composition on both LOR and CPM release rate and their penetration through the skin. Furthermore, we identified the importance of considering the interplay between the physicochemical attributes of the drug molecules, the design of the MAP formulation, and the structural properties of the skin when designing IVRT and IVPT protocols.

Influence of co-solvents on properties of terpene-based eutectic mixtures: Implications for skin delivery

Terpene-based eutectic mixtures (EMs) are attractive platforms for transdermal delivery due to their solubilizing potential and ability to alter the barrier function of the stratum corneum (SC). Despite this, little is known about the effect of diluting EMs with co-solvents (CSs) on their solubility- and permeation-enhancing properties. Furthermore, insufficient attention has been paid to comparing these platforms with traditional solvents, such as propylene glycol (PG) or ethanol (EtOH). To address this gap, the present study investigates the impact of the CS content in EM:CS blends on the transdermal delivery of clotrimazole (CLOT). Two CSs, PG and EtOH, and two terpene-based EMs, menthol:thymol and thymol:β-citronellol, were used. Each of the EMs was investigated at two different molar ratios between the terpenes, with one being their eutectic point, to explore its potential benefit for skin permeation. At each step, properties of the blends were compared with those of pure CSs. The EM:CS blends showed a better solubilizing potential for CLOT than EMs or CSs on their own. A higher content of CSs in the blends resulted in a higher skin permeation and retention of CLOT, and a lower degree of disarrangement of the SC structure. Furthermore, the blends of EMs at their EPs led to overall poorer permeation profiles, implying that the permeation rate is more affected by the properties of the individual terpenes than by the specific ratio at the eutectic point between them. In conclusion, addition of CSs to the EMs promotes permeation and retention of CLOT, while reducing the skin impairment caused by the terpenes.

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

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

A database of chemical absorption in human skin with mechanistic modeling applications

Whether from environmental and occupational hazards or from topical pharmaceuticals, the human skin comes into contact with various chemicals every day. In vivo experiments not only require large investments of both time and money, but in vivo experiments can also be unethical due to the need to intentionally or incidentally expose humans or animals to toxic chemicals. Comparatively, in vitro experiments offer ethical and financial advantages when combined with the opportunity to selectively choose chemicals for experimentation. With in vivo experimentation being so infeasible, many scientists have chosen to make their in vitro data available publicly. Using these data, a detailed database containing 73 chemicals was created with a robust set of descriptors to be used in connection with mathematical modeling to predict diffusion, permeability, and partition coefficients. This resulting database is tailored to be easily used in various coding languages.

Lipid Biomimetic Models as Simple Yet Complex Tools to Predict Skin Permeation and Drug-Membrane Biophysical Interactions

The barrier function of the skin is primarily determined by its outermost layer, the Stratum Corneum (SC). The SC consists of corneocytes embedded in a lipid matrix composed mainly of ceramides, cholesterol, and free fatty acids in equimolar proportions and is organised in a complex lamellar structure with different periodicities and lateral packings. This matrix provides a diffusion pathway across the SC for bioactive compounds that are administered to the skin. In this regard, and as the skin administration route has grown in popularity, there has been an increase in the use of lipid mixtures that closely resemble the SC lipid matrix, either for a deeper biophysical understanding or for pharmaceutical and cosmetic purposes. This review focuses on a systematic analysis of the main outcomes of using lipid mixtures as SC lipid matrix models for pharmaceutical and cosmetic purposes. Thus, a methodical evaluation of the main outcomes based on the SC structure is performed, as well as the main recent developments in finding suitable new in vitro tools for permeation testing based on lipid models.

Cutaneous Pharmacokinetics of Topically Applied Novel Dermatological Formulations

Novel formulations are developed for dermatological applications to address a wide range of patient needs and therapeutic challenges. By pushing the limits of pharmaceutical technology, these formulations strive to provide safer, more effective, and patient-friendly solutions for dermatological concerns, ultimately improving the overall quality of dermatological care. The article explores the different types of novel dermatological formulations, including nanocarriers, transdermal patches, microsponges, and microneedles, and the techniques involved in the cutaneous pharmacokinetics of these innovative formulations. Furthermore, the significance of knowing cutaneous pharmacokinetics and the difficulties faced during pharmacokinetic assessment have been emphasized. The article examines all the methods employed for the pharmacokinetic evaluation of novel dermatological formulations. In addition to a concise overview of earlier techniques, discussions on novel methodologies, including tape stripping, in vitro permeation testing, cutaneous microdialysis, confocal Raman microscopy, and matrix-assisted laser desorption/ionization mass spectrometry have been conducted. Emerging technologies like the use of microfluidic devices for skin absorption studies and computational models for predicting drug pharmacokinetics have also been discussed. This article serves as a valuable resource for researchers, scientists, and pharmaceutical professionals determined to enhance the development and understanding of novel dermatological drug products and the complex dynamics of cutaneous pharmacokinetics.

A compartment model to predict in vitro finite dose absorption of chemicals by human skin

Dermal uptake is an important and complex exposure route for a wide range of chemicals. Dermal exposure can occur due to occupational settings, pharmaceutical applications, environmental contamination, or consumer product use. The large range of both chemicals and scenarios of interest makes it difficult to perform generalizable experiments, creating a need for a generic model to simulate various scenarios. In this study, a model consisting of a series of four well-mixed compartments, representing the source solution (vehicle), stratum corneum, viable tissue, and receptor fluid, was developed for predicting dermal absorption. The model considers experimental conditions including small applied doses as well as evaporation of the vehicle and chemical. To evaluate the model assumptions, we compare model predictions for a set of 26 chemicals to finite dose in-vitro experiments from a single laboratory using steady-state permeability coefficient and equilibrium partition coefficient data derived from in-vitro experiments of infinite dose exposures to these same chemicals from a different laboratory. We find that the model accurately predicts, to within an order of magnitude, total absorption after 24 h for 19 of these chemicals. In combination with key information on experimental conditions, the model is generalizable and can advance efficient assessment of dermal exposure for chemical risk assessment.

Nanotechnology-Based Approaches for Cosmeceutical and Skin Care: A Systematic Review

Cosmeceuticals have gained great importance and are among the top-selling products used for skin care. Because of changing lifestyles, climate, and increasing pollution, cosmeceuticals are utilized by every individual, thereby making cosmeceuticals a fruitful field for research and the economy. Cosmeceuticals provide incredibly pleasing aesthetic results by fusing the qualities of both cosmetics and medicinal substances. Cosmeceuticals are primarily utilized to improve the appearance of skin by making it smoother, moisturized, and wrinkle-free, in addition to treating dermatological conditions, including photoaging, burns, dandruff, acne, eczema, and erythema. Nanocosmeceuticals are cosmetic products that combine therapeutic effects utilizing nanotechnology, allowing for more precise and effective target-specific delivery of active ingredients, and improving bioavailability.

The Progress in the Application of Dissolving Microneedles in Biomedicine

In recent years, microneedle technology has been widely used for the transdermal delivery of substances, showing improvements in drug delivery effects with the advantages of minimally invasive, painless, and convenient operation. With the development of nano- and electrochemical technology, different types of microneedles are increasingly being used in other biomedical fields. Recent research progress shows that dissolving microneedles have achieved remarkable results in the fields of dermatological treatment, disease diagnosis and monitoring, and vaccine delivery, and they have a wide range of application prospects in various biomedical fields, showing their great potential as a form of clinical treatment. This review mainly focuses on dissolving microneedles, summarizing the latest research progress in various biomedical fields, providing inspiration for the subsequent intelligent and commercial development of dissolving microneedles, and providing better solutions for clinical treatment.

Utilization of 3D bioprinting technology in creating human tissue and organoid models for preclinical drug research - State-of-the-art

Rapid development of tissue engineering in recent years has increased the importance of three-dimensional (3D) bioprinting technology as novel strategy for fabrication functional 3D tissue and organoid models for pharmaceutical research. 3D bioprinting technology gives hope for eliminating many problems associated with traditional cell culture methods during drug screening. However, there is a still long way to wider clinical application of this technology due to the numerous difficulties associated with development of bioinks, advanced printers and in-depth understanding of human tissue architecture. In this review, the work associated with relatively well-known extrusion-based bioprinting (EBB), jetting-based bioprinting (JBB), and vat photopolymerization bioprinting (VPB) is presented and discussed with the latest advances and limitations in this field. Next we discuss state-of-the-art research of 3D bioprinted in vitro models including liver, kidney, lung, heart, intestines, eye, skin as well as neural and bone tissue that have potential applications in the development of new drugs.

Poroelastic behavior and water permeability of human skin at the nanoscale

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

A regression analysis using simple descriptors for multiple dermal datasets: Going from individual membranes to the full skin

In silico methods to estimate and/or quantify skin absorption of chemicals as a function of chemistry are needed to realistically predict pharmacological, occupational, and environmental exposures. The Potts-Guy equation is a well-established approach, using multi-linear regression analysis describing skin permeability (Kp) in terms of the octanol/water partition coefficient (logP) and molecular weight (MW). In this work, we obtained regression equations for different human datasets relevant to environmental and cosmetic chemicals. Since the Potts-Guy equation was published in 1992, we explored recent datasets that include different skin layers, such as dermatomed (including dermis to a defined thickness) and full skin. Our work was consistent with others who have observed that fits to the Potts-Guy equation are stronger for experiments focused on the epidermis. Permeability estimates for dermatomed skin and full skin resulted in low regression coefficients when compared to epidermis datasets. An updated regression equation uses a combination of fitted permeability values obtained with a published 2D compartmental model previously evaluated. The resulting regression equation was: logKp = -2.55 + 0.65logP - 0.0085MW, R2  = 0.91 (applicability domain for all datasets: MW ranges from 18 to >584 g/mol and -4 to >5 for logP). This approach demonstrates the advantage of combining mechanistic with structural activity relationships in a single modeling approach. This combination approach results in an improved regression fit when compared to permeability estimates obtained using the Potts-Guy approach alone. The analysis presented in this work assumes a one-compartment skin absorption route; future modeling work will consider adding multiple compartments.

Current Advances in Lipid Nanosystems Intended for Topical and Transdermal Drug Delivery Applications

Skin delivery is an exciting and challenging field. It is a promising approach for effective drug delivery due to its ease of administration, ease of handling, high flexibility, controlled release, prolonged therapeutic effect, adaptability, and many other advantages. The main associated challenge, however, is low skin permeability. The skin is a healthy barrier that serves as the body's primary defence mechanism against foreign particles. New advances in skin delivery (both topical and transdermal) depend on overcoming the challenges associated with drug molecule permeation and skin irritation. These limitations can be overcome by employing new approaches such as lipid nanosystems. Due to their advantages (such as easy scaling, low cost, and remarkable stability) these systems have attracted interest from the scientific community. However, for a successful formulation, several factors including particle size, surface charge, components, etc. have to be understood and controlled. This review provided a brief overview of the structure of the skin as well as the different pathways of nanoparticle penetration. In addition, the main factors influencing the penetration of nanoparticles have been highlighted. Applications of lipid nanosystems for dermal and transdermal delivery, as well as regulatory aspects, were critically discussed.

Geometry design for a fully insertable glucose biosensor with multimodal optical readout

Significance

Insertable optical continuous glucose monitors (CGMs) with wearable readers are a strong option for monitoring individuals with diabetes. However, a fully insertable CGM requires a small form factor while still delivering sufficient signal to be read through tissue by an external device. Previous work has suggested that a multimodal repeating unit (barcode) approach may meet these requirements, but the biosensor geometry must be optimized to meet performance criteria.

Aim

This work details in silico trials conducted to evaluate the geometry of a fully insertable multimodal optical biosensor with respect to both optical output and species diffusion in vivo.

Approach

Monte Carlo modeling is used to evaluate the luminescent output of three presupposed biosensor designs based on size constraints for an injectable and logical placement of the bar code compartments. Specifically, the sensitivity of the luminescent output to displacement of the biosensor in the X and Y directions, overall size of the selected design, and size of an individual repeating unit are analyzed. Further, an experimentally validated multiphysics model is used to evaluate the diffusion and reaction of glucose and oxygen within the biosensor to estimate the occurrence of chemical crosstalk between the assay components.

Results

A stacked cylinder multimodal biosensor 4.4 mm in length with repeating units 0.36 mm in length was found to yield a greater luminescent output than the current "barcode" biosensor design. In addition, it was found that a biosensor with enzymatic elements does not significantly deplete glucose locally and thus does not impact the diffusion profile of glucose in adjacent compartments containing nonenzymatic assays.

Conclusions

Computational modeling was used to design the geometry of a multimodal, insertable, and optical CGM to ensure that the optical output and chemical diffusion profile are sufficient for this device to function in vivo.

Film-trigger applicator (FTA) for improved skin penetration of microneedle using punching force of carboxymethyl cellulose film acting as a microneedle applicator

Background

Dissolving microneedle (DMN) is a transdermal drug delivery system that creates pore in the skin and directly deliver drug through the pore channel. DMN is considered as one of the promising system alternatives to injection because it is minimally invasive and free from needle-related issues. However, traditional DMN patch system has limitations of incomplete insertion and need of complex external devices. Here, we designed film-trigger applicator (FTA) system that successfully delivered DMN inside the skin layers using fracture energy of carboxymethyl cellulose (CMC) film via micropillars. We highlighted advantages of FTA system in DMN delivery compared with DMN patch, including that the film itself can act as DMN applicator.

Methods

FTA system consists of DMNs fabricated on the CMC film, DMN array holder having holes aligned to DMN array, and micropillars prepared using general purpose polystyrene. We analyzed punching force on the film by micropillars until the film puncture point at different CMC film concentrations and micropillar diameters. We also compared drug delivery efficiency using rhodamine B fluorescence diffusion and skin penetration using optical coherence tomography (OCT) of FTA with those of conventional DMN patch. In vivo experiments were conducted to evaluate DMN delivery efficiency using C57BL/6 mice and insulin as a model drug.

Results

FTA system showed enhanced delivery efficiency compared with that of the existing DMN patch system. We concluded CMC film as a successful DMN applicator as it showed enhanced DMN penetration in OCT and rhodamine B diffusion studies. Further, we applied FTA on shaved mouse dorsal skin and observed successful skin penetration. The FTA group showed higher level of plasma insulin in vivo than that of the DMN patch group.

Conclusions

FTA system consisting of simple polymer film and micropillars showed enhanced DMN delivery than that of the existing DMN patch system. Because FTA works with simple finger force without sticky patch and external devices, FTA is a novel and promising platform to overcome the limitations of conventional microneedle patch delivery system; we suggest FTA as a next generation applicator for microneedle application in the future.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40824-022-00302-5.

Transdermal Delivery of Metformin Utilizing Ionic Liquid Technology: Insight Into the Relationship Between Counterion Structures and Properties

PURPOSE

The purpose of the present study was to explore the feasibility of transdermal delivery of metformin, a commonly used oral antidiabetic drug, by ionic liquid (IL) technology.

METHODS

Metformin hydrochloride (MetHCl) was first transformed into three kinds of ILs with different counterions. The physicochemical properties of the obtained ILs were characterized in depth. The simulation of stable configuration and calculation of interaction energies were conducted based on density functional theory (DFT). Skin-PAMPA was used to evaluate the intrinsic transdermal permeation properties. The cytotoxicity assay of these ILs was conducted using HaCaT cells to evaluate the toxicity to skin. These metformin ILs were then formulated into transdermal patch, and the transdermal potential was further evaluated using in vitro dissolution test and skin permeation assay. Finally, the pharmacokinetic profiles of these metformin IL-containing patches were determined.

RESULTS

Among all the three Met ILs, metformin dihexyl sulfosuccinate (MetDH) with proper overall physiochemical and biological properties demonstrated the highest relative bioavailability. Metformin docusate (MetD) with the highest lipophilicity and intrinsic transdermal permeability exhibited the most significant sustained release profile in vivo. Both MetDH and MetD were the promising candidates for further clinical investigations.

CONCLUSIONS

Overall, the properties of ILs were closely related to the structures of counterion. IL technology provided the opportunities to finely tune the solid-state and biological properties of Metformin and facilitated the successful delivery by transdermal route.

Permeation of albumin through the skin depending on its concentration and the substrate used in simulated conditions in vivo

OBJECTIVE

Many drugs applied to the skin with a systemic effect do not have a therapeutic effect, due to the barrier posed by the complex structure of the skin. To counteract this, absorption promoters are often added to the drug formulation. The use of albumin as an effective drug carrier is increasingly being addressed. Albumin, a natural, non-toxic polymer, can target drugs to specific cells and extend their biological half-life. This study was designed to trace the permeation of albumin after topical administration to the skin as a potential carrier of therapeutic substances.

MATERIALS AND METHODS

Four dermal formulations based on different polymers were prepared: methyl cellulose, sodium alginate, hypromellose and chitosan with methyl cellulose, obtaining final concentrations of albumin of 2%, 1.5% and 1%. The permeation of albumin through the skin was examined under simulated in vivo conditions.

RESULTS

Most albumin permeated from the methylcellulose-based hydrogel. Depending on the concentration of albumin, permeation profiles were plotted and permeation rate constant and AUC_(0-24 h) were calculated.

CONCLUSION

Methylcellulose was the optimal polymer for albumin release, whereas hypromellose was the least favorable. The concentration of albumin influences the amount and rate of permeation of this protein. The optimal concentration was 10 mg/g, from which the most albumin penetrated and the fastest. Human skin appeared to be more permeable to albumin than pig skin. However, the similar permeation profile through both membranes successfully allows the use of pig skin to track and evaluate the permeation of therapeutic substances with systemic effects.

Textile-Based Wearable Sensor for Skin Hydration Monitoring

This research describes a wearable skin hydration sensor based on cotton textile to determine the state of hydration within the skin via impedance analysis. The sensor structure comprises a textile substrate, thermoplastic over-layer, conductive patterns, and encapsulant, designed for stable and reliable monitoring of the skin's impedance change in relation to hydration level. The porcine skin with different hydration levels was prepared as a model system of the skin, and the textile-based sensor carefully investigated the porcine skin samples' impedance characteristics. The impedance study reveals that (1) the total impedance of skin decreases as its hydration level increases, and (2) the impedance of the stratum corneum and epidermis layers are more dominantly affected by the hydration level of the skin than the dermis layer. Even after repetitive bending cycles, the impedance data of skin measured by the sensor exhibit a reliable dependence on the skin hydration level, which validates the flexibility and durability of the sensor. Finally, it is shown that the textile-based skin hydration sensor can detect various body parts' different hydration levels of human skin while maintaining a stable conformal contact with the skin. The resulting data are well-matched with the readings from a commercial skin hydration sensor.

A review on microplastics and nanoplastics in the environment: Their occurrence, exposure routes, toxic studies, and potential effects on human health

Microplastics (MPs) and nanoplastics (NPs) are emerging environmental pollutants, having a major ecotoxicological concern to humans and many other biotas, especially aquatic animals. The physical and chemical compositions of MPs majorly determine their ecotoxicological risks. However, comprehensive knowledge about the exposure routes and toxic effects of MPs/NPs on animals and human health is not fully known. Here this review focuses on the potential exposure routes, human health impacts, and toxicity response of MPs/NPs on human health, through reviewing the literature on studies conducted in different in vitro and in vivo experiments on organisms, human cells, and the human experimental exposure models. The current literature review has highlighted ingestion, inhalation, and dermal contacts as major exposure routes of MPs/NPs. Further, oxidative stress, cytotoxicity, DNA damage, inflammation, immune response, neurotoxicity, metabolic disruption, and ultimately affecting digestive systems, immunology, respiratory systems, reproductive systems, and nervous systems, as serious health consequences.

Dermal Penetration Studies of Potential Phenolic Compounds Ex Vivo and Their Antioxidant Activity In Vitro

Phenolic compounds with miscellaneous biological activities are an interesting component in dermatology and cosmetology practices. The aim of our study was to determine the phenolic compounds released from emulsion, emulgel, gel, ointment, and oleogel formulations penetration into human skin layers, both the epidermis and dermis, and estimate their antioxidant activity. The ex vivo penetration study was performed using Bronaugh type flow-through diffusion cells. Penetration studies revealed that, within 24 h, the chlorogenic acid released from the oleogel penetrated into skin layers to a depth of 2.0 ± 0.1 µg/mL in the epidermis and 1.5 ± 0.07 µg/mL in the dermis. The oleogel-released complex of phenolic compounds penetrating into epidermis showed the strongest DPPH free radical scavenging activity (281.8 ± 14.1 µM TE/L). The study estimated a strong positive correlation (r = 0.729) between the amount of quercetin penetrated into epidermis and the antioxidant activity detected in the epidermis extract. Plant based phenolic compounds demonstrated antioxidant activity and showed great permeability properties through the skin.

Adapting Clofazimine for Treatment of Cutaneous Tuberculosis by Using Self-Double-Emulsifying Drug Delivery Systems

Although chemotherapeutic treatment regimens are currently available, and considerable effort has been lavished on the development of new drugs for the treatment of tuberculosis (TB), the disease remains deeply intractable and widespread. This is due not only to the nature of the life cycle and extraordinarily disseminated habitat of the causative pathogen, principally Mycobacterium tuberculosis (Mtb), in humans and the multi-drug resistance of Mtb to current drugs, but especially also to the difficulty of enabling universal treatment of individuals, immunocompromised or otherwise, in widely differing socio-economic environments. For the purpose of globally eliminating TB by 2035, the World Health Organization (WHO) introduced the "End-TB" initiative by employing interventions focusing on high impact, integrated and patient-centered approaches, such as individualized therapy. However, the extraordinary shortfall in stipulated aims, for example in actual treatment and in TB preventative treatments during the period 2018-2022, latterly and greatly exacerbated by the COVID-19 pandemic, means that even greater pressure is now placed on enhancing our scientific understanding of the disease, repurposing or repositioning old drugs and developing new drugs as well as evolving innovative treatment methods. In the specific context of multidrug resistant Mtb, it is furthermore noted that the incidence of extra-pulmonary TB (EPTB) has significantly increased. This review focusses on the potential of utilizing self-double-emulsifying drug delivery systems (SDEDDSs) as topical drug delivery systems for the dermal route of administration to aid in treatment of cutaneous TB (CTB) and other mycobacterial infections as a prelude to evaluating related systems for more effective treatment of CTB and other mycobacterial infections at large. As a starting point, we consider here the possibility of adapting the highly lipophilic riminophenazine clofazimine, with its potential for treatment of multi-drug resistant TB, for this purpose. Additionally, recently reported synergism achieved by adding clofazimine to first-line TB regimens signifies the need to consider clofazimine. Thus, the biological effects and pharmacology of clofazimine are reviewed. The potential of plant-based oils acting as emulsifiers, skin penetration enhancers as well as these materials behaving as anti-microbial components for transporting the incorporated drug are also discussed.

Comparison Between Franz Diffusion Cell and a novel Micro-physiological System for In Vitro Penetration Assay Using Different Skin Models

In vitro diffusive models are an important tool to screen the penetration ability of active ingredients in various formulations. A reliable assessment of skin penetration enhancing properties, mechanism of action of carrier systems, and an estimation of a bioavailability are essential for transdermal delivery. Given the importance of testing the penetration kinetics of different compounds across the skin barrier, several in vitro models have been developedThe aim of this study was to compare the Franz Diffusion Cell (FDC) with a novel fluid-dynamic platform (MIVO) by evaluating penetration ability of caffeine, a widely used reference substance, and LIP1, a testing molecule having the same molecular weight but a different lipophilicity in the two diffusion chamber systems. A 0.7% caffeine or LIP1 formulation in either water or propylene glycol (PG) containing oleic acid (OA) was topically applied on the Strat-M® membrane or pig ear skin, according to the infinite-dose experimental condition (780 ul/cm²). The profile of the penetration kinetics was determined by quantify the amount of molecule absorbed at different time-points (1, 2, 4, 6, 8 hours), by means of HPLC analysis. Both diffusive systems show a similar trend for caffeine and LIP1 penetration kinetics. The Strat-M® skin model shows a lower barrier function than the pig skin biopsies, whereby the PGOA vehicle exhibits a higher penetration, enhancing the effect for both diffusive chambers and skin surrogates. Most interestingly, MIVO diffusive system better predicts the lipophilic molecules (i.e. LIP1) permeation through highly physiological fluid flows resembled below the skin models.

Integration of Engineered Delivery with the Pharmacokinetics of Medical Candidates via Physiology-Based Pharmacokinetics

Physiologically based pharmacokinetic (PBPK) modeling is a mechanistic computational model that can be used to predict a drug product's ADME (absorption, distribution, metabolism, and excretion) and pharmacokinetics (PK). In recent years, PBPK modeling and simulation has been used increasingly to address many biopharmaceutics and clinical pharmacology questions, such as the effect of formulations, intrinsic factors (age, organ dysfunction, etc.), and extrinsic factors (comedications, food) on the PK of an investigational drug product. In this chapter, we will briefly introduce various PBPK models for ADME prediction and general procedures for PBPK modeling and simulations. The readers are encouraged to read updated literature on new applications of PBPK modeling and simulation which is still an emerging area in pharmaceutical development.

Design and Development of OECT Logic Circuits for Electrical Stimulation Applications

This paper presents the first successful implementation of fully printed electronics for flexible and wearable smart multi-pad stimulation electrodes intended for use in medical, sports and lifestyle applications. The smart multi-pad electrodes with the electronic circuits based on organic electrochemical transistor (OECT)-based electronic circuits comprising the 3–8 decoder for active pad selection and high current throughput transistors for switching were produced by multi-layer screen printing. Devices with different architectures of switching transistors were tested in relevant conditions for electrical stimulation applications. An automated testbed with a configurable stimulation source and an adjustable human model equivalent circuit was developed for this purpose. Three of the proposed architectures successfully routed electrical currents of up to 15 mA at an output voltage of 30 V, while one was reliably performing even at 40 V. The presented results demonstrate feasibility of the concept in a range of conditions relevant to several applications of electrical stimulation.

Optimization of topical formulations using a combination of in vitro methods to quantify the transdermal passive diffusion of drugs

This paper describes a new approach to the early-stage optimization of topical products and selection of lead formulation candidates. It demonstrates the application of open flow microperfusion in vitro in conjunction with the Franz diffusion cell to compare time-resolved, 24-hour profiles of diclofenac passive diffusion through all skin layers (including the skin barrier, dermis, and subcutis) resulting from nine topical formulations of different composition. The technique was successfully validated for in vitro sampling of diclofenac in interstitial fluid. A multi-compartmental model integrating the two datasets was analyzed and revealed that the passive diffusion of diclofenac through the dermis and subcutis does not correlate with its diffusion through the skin barrier and cannot be predicted using Franz diffusion cell data alone. The combined application of the two techniques provides a new, convenient tool for product development and selection enabling the comparison of topical formulation candidates and their impact on drug delivery through all skin layers. This approach can also generate the experimental data required to improve the robustness of mechanistic PBPK models, and when combined with clinical sampling via open flow microperfusion - for the development of better in vivo-in vitro correlative models.

Plant-Derived Nanoscale-Encapsulated Antioxidants for Oral and Topical Uses: A Brief Review

Several plant-based nanoscale-encapsulated antioxidant compounds (rutin, myricetin, β-carotene, fisetin, lycopene, quercetin, genkwanin, lutein, resveratrol, eucalyptol, kaempferol, glabridin, pinene, and whole-plant bio-active compounds) are briefly introduced in this paper, along with their characteristics. Antioxidants’ bioavailability has become one of the main research topics in bio-nanomedicine. Two low patient compliance drug delivery pathways (namely, the oral and topical delivery routes), are described in detail in this paper, for nanoscale colloidal systems and gel formulations. Both routes and/or formulations seek to improve bioavailability and maximize the drug agents’ efficiency. Some well-known compounds have been robustly studied, but many remain elusive. The objective of this review is to discuss recent studies and advantages of nanoscale formulations of plant-derived antioxidant compounds.

Skin interaction, permeation, and toxicity of silica nanoparticles: Challenges and recent therapeutic and cosmetic advances

Silica nanoparticles (SNPs) received more attention with the emergence of nanotechnology with the aim and promise of becoming innovative drug delivery systems. They have been fulfilling this objective with excellence and nowadays they play a central role in biomedical applications. New SNPs application routes are being explored such as the epidermal, dermal, and transdermal routes. With that, novel models of synthesis, functionalization, and applications constantly appear. However, it is essential that such innovations are accompanied by in-depth studies on permeation, biodistribution, metabolization, and elimination of the generated by-products. Such studies are still incipient, if not rare. This article reviews significant findings on SNPs and their skin interactions. An extensive literature review on SNPs synthesis and functionalization methodologies was performed, as well as on the skin characteristics, skin permeation mechanisms, and in vivo toxicity assessments. Furthermore, studies of the past 5 years on the main therapeutic and cosmetic products employing SNPs, with greater emphasis on in vivo and ex vivo studies were included.

Development and Evaluation of an In Silico Dermal Absorption Model Relevant for Children

The higher skin surface area to body weight ratio in children and the prematurity of skin in neonates may lead to higher chemical exposure as compared to adults. The objectives of this study were: (i) to provide a comprehensive review of the age-dependent anatomical and physiological changes in pediatric skin, and (ii) to construct and evaluate an age-dependent pediatric dermal absorption model. A comprehensive review was conducted to gather data quantifying the differences in the anatomy and physiology of child and adult skin. Maturation functions were developed for model parameters that were found to be age-dependent. A pediatric dermal absorption model was constructed by updating a MoBi implementation of the Dancik et al. 2013 skin permeation model with these maturation functions. Using a workflow for adult-to-child model extrapolation, the predictive performance of the model was evaluated by comparing its predicted rates of flux of diamorphine, phenobarbital and buprenorphine against experimental observations using neonatal skin. For diamorphine and phenobarbital, the model provided reasonable predictions. The ratios of predicted:observed flux in neonates for diamorphine ranged from 0.55 to 1.40. For phenobarbital, the ratios ranged from 0.93 to 1.26. For buprenorphine, the model showed acceptable predictive performance. Overall, the physiologically based pediatric dermal absorption model demonstrated satisfactory prediction accuracy. The prediction of dermal absorption in neonates using a model-based approach will be useful for both drug development and human health risk assessment.

In silico predictions of absorption of MDI substances after dermal or inhalation exposures to support a category based read-across assessment

Methylenediphenyl diisocyanate (MDI) substances used polyurethane production can range from their simplest monomeric forms (e.g., 4,4'-MDI) to mixtures of the monomers with various homologues, homopolymer, and prepolymer derivatives. The relative dermal or inhalation absorption of 39 constituents of these substances in human were predicted using the GastroPlus® program. Predicted dermal uptake and absorption of the three MDI monomers from an acetone vehicle was 84-86% and 1.4-1.5%, respectively, with lower uptake and absorption predicted for the higher MW analogs. Lower absorption was predicted from exposures in a more lipophilic vehicle (1-octanol). Modeled inhalation exposures afforded the highest pulmonary absorption for the MDI monomers (38-54%), with 3-27% for the MW range of 381-751, and <0.1% for the remaining, higher MW derivatives. Predicted oral absorption, representing mucociliary transport, ranged from 5 to 10% for the MDI monomers, 10-25% for constituents of MW 381-751, and ≤3% for constituents with MW > 900. These in silico evaluations should be useful in category-based, worst-case, Read-Across assessments for MDI monomers and modified MDI substances for potential systemic effects. Predictions of appreciable mucociliary transport may also be useful to address data gaps in oral toxicity testing for this category of compounds.

Ion-Pair Compounds of Strychnine for Enhancing Skin Permeability: Influencing the Transdermal Processes In Vitro Based on Molecular Simulation

This research aimed to explore how Strychnine (Str) ion-pair compounds affect the in vitro transdermal process. In order to prevent the influence of different functional groups on skin permeation, seven homologous fatty acids were selected to form ion-pair compounds with Str. The in vitro permeation fluxes of the Str ion-pair compounds were 2.2 to 8.4 times that of Str, and Str-C₁₀ had the highest permeation fluxes of 42.79 ± 19.86 µg/cm²/h. The hydrogen bond of the Str ion-pair compounds was also confirmed by Fourier Transform Infrared (FTIR) Spectroscopy, Nuclear Magnetic Resonance (NMR) Spectroscopy and molecular simulation. In the process of molecular simulation, the intercellular lipid and the viable skin were represented by ceramide, cholesterol and free fatty acid of equal molar ratios and water, respectively. It was found by the binding energy curve that the Str ion-pair compounds had better compatibility with the intercellular lipid and water than Str, which indicated that the affinity of Str ion-pair compounds and skin was better than that of Str and skin. Therefore, it was concluded that Str ion-pair compounds can be distributed from the vehicle to the intercellular lipid and viable skin more easily than Str. These findings broadened our knowledge about how Str ion-pair compounds affect the transdermal process.

High-Payload Nanosuspension of Centella asiatica Extract for Improved Skin Delivery with No Irritation

Background

The titrated extract of Centella asiatica (CA) has received much attention as a cosmeceutical ingredient owing to its anti-wrinkle effect. However, due to the low solubility and high molecular weight of pharmacologically active constituents, including asiatic acid (AA), madecassic acid (MA), and asiaticoside (AS), it is challenging to fabricate high-payload topical preparations of CA with satisfactory skin absorption profiles.

Purpose

This study aimed to design a high-payload topical preparation of CA using nanocrystallization technique and to evaluate its skin absorption profile and local tolerability.

Methods

High-payload nanocrystal suspensions (NSs) were prepared using lab-scale bead-milling technology, by adjusting the type and amount of suspending agent, CA content, type of vehicle, and milling speed. CA-loaded NSs were characterized in terms of morphology, particle size, crystallinity, and in vitro dissolution pattern. Skin absorption of CA nanocrystals was evaluated using a vertical Franz diffusion cell mounted with porcine skin. In vivo skin irritation following topical application of high-payload NS was assessed in normal rats.

Results

The optimized NS system, composed of 10% (w/v) CA, 0.5% polyvinylpyrrolidone (PVP) K30 as steric stabilizer, and 89.5% of distilled water, was characterized as follows: spherical or elliptical in shape, 200 nm in size, with low crystallinity. The in vitro dissolution of AA or MA from NSs was markedly faster compared to raw material, under sink condition. Penetration of AA, MA, and AS in the porcine skin was markedly elevated using the high-payload NS formula, providing 5-, 4-, and 4.5-fold higher accumulation in skin layer, compared to that of the marketed cream formula (CA 1%, Madeca cream). Moreover, topical application of high-payload NS was tolerable, showing neither erythema nor oedema in normal rats.

Conclusion

The novel NS system is expected to be a virtuous approach for offering a better skin absorption of CA, without using an excess quantity of solubilizers.

Repurposing pentosan polysulfate sodium as hyaluronic acid linked polyion complex nanoparticles for the management of osteoarthritis: A potential approach

Osteoarthritis is still a disease burden for pharmaceutical scientists and strategy makers. It is associated with the chronic inflammation of joints especially weight-bearing joints like knee, hip, backbone, and phalanges. NSAIDs that are used for the management of inflammation associated with osteoarthritis have high side effects related to gastric upset, gastric ulcer, and long term treatment associated with liver and kidney damage. Nanotechnology has gained a huge scope for the management of arthritis as it can reach out to the deep inside the cell and alter cellular physiology as desired. The present study hypothesizes the use of polyion complex nanoparticles of hyaluronic acid linked Pentosan polysulfate sodium, a disease-modifying agent for the treatment of osteoarthritis administered through transdermal route. The hypothesis involves the use of drug repurposing as the drug was initially approved for interstitial cystitis, a condition of the urinary bladder associated with pain and swelling. Being very low oral bioavailability and gastric irritation profile, the transdermal route would be beneficial. To overcome the problem associated with the oral route, there is a need for the targeted approach that will particularly reach at inflammatory sites. Thereby transdermal delivery of hyaluronic acid linked Pentosan polysulfate sodium through polyion complex nanoparticle therapy will be a novel therapeutic approach to combat osteoarthritis.

Transfersomes improved delivery of ascorbic palmitate into the viable epidermis for enhanced treatment of melasma

Ascorbic palmitate (AP) is widely used in the topical pharmaceutical or cosmetic formulations for melasma treatment. However, the presence of the skin barriers makes it difficult for the highly lipophilic drug molecules to traverse the stratum corneum (SC) and diffuse into the viable epidermis (EP) to reach the melanocytes, thereby exerting suboptimal antimelasma effects. Herein, AP was encapsulated into the transfersomes (TFs), yielding AP-TFs. AP-TFs utilized the deformability of TFs to squeeze through the skin pores in the SC under the transepidermal hydration gradient forces, leading to 14.1-fold increase in AP accumulation to the EP. AP-TFs could slowly release the encapsulated AP, while whether the released AP or transfersomal AP showed comparable uptake into the melanocytes, thereby exerting similar inhibitory effects on tyrosinase activity and melanogenesis. Ultimately, in the rat melasma model, AP-TFs showed superior antimelasma efficacy to free AP, with effective relief of oxidative stress and inflammation in the skin. Moreover, AP-TFs did not induce skin irritation. Therefore, the study provides a safe and effective approach to elevating the delivery of highly lipophilic drugs to the EP for enhanced treatment of melasma.

Dissolving Microneedles Developed in Association with Nanosystems: A Scoping Review on the Quality Parameters of These Emerging Systems for Drug or Protein Transdermal Delivery

The largest organ of the body provides the main challenge for the transdermal delivery of lipophilic or high molecular weight drugs. To cross the main barrier of the skin, the stratum corneum, many techniques have been developed and improved. In the last 20 years, the association of microneedles with nanostructured systems has gained prominence for its versatility and for enabling targeted drug delivery. Currently, the combination of these mechanisms is pointed to as an emerging technology; however, some gaps need to be answered to transcend the development of these devices from the laboratory scale to the pharmaceutical market. It is known that the lack of regulatory guidelines for quality control is a hindrance to market conquest. In this context, this study undertakes a scoping review of original papers concerning methods applied to evaluate both the quality and drug/protein delivery of dissolving and hydrogel-forming microneedles developed in association with nanostructured systems.

Topical drug delivery: History, percutaneous absorption, and product development

Topical products, widely used to manage skin conditions, have evolved from simple potions to sophisticated delivery systems. Their development has been facilitated by advances in percutaneous absorption and product design based on an increasingly mechanistic understanding of drug-product-skin interactions, associated experiments, and a quality-by-design framework. Topical drug delivery involves drug transport from a product on the skin to a local target site and then clearance by diffusion, metabolism, and the dermal circulation to the rest of the body and deeper tissues. Insights have been provided by Quantitative Structure Permeability Relationships (QSPR), molecular dynamics simulations, and dermal Physiologically Based PharmacoKinetics (PBPK). Currently, generic product equivalents of reference-listed products dominate the topical delivery market. There is an increasing regulatory interest in understanding topical product delivery behavior under 'in use' conditions and predicting in vivo response for population variations in skin barrier function and response using in silico and in vitro findings.

Critical parameters for accurate monitoring of caffeine penetration in porcine skin using confocal Raman spectroscopy

In this research, we addressed a challenge while measuring the penetration performance of caffeine (CAF) using confocal Raman spectroscopy (CRS). Normally in the process of CRS analysis, skin sample was moved from an incubation setup to a specified CRS-measuring sample holder. Accurate data collection may be questioned due to the variation of the environment the skin placed in. Therefore, two critical parameters including the CRS measuring temperature and proper skin hydration were focused; accordingly, four different conditions were designed. First, the skin was incubated in a real-time device with the skin placing onto PBS-filled chamber where the temperature was adjusted to 32℃. This device can be fixed under the CRS microscope, enabling simultaneous skin incubation and dynamic CRS measurements (condition i, reference). The other conditions referred to skins incubated in Franz diffusion cells for simulating the common experimental procedures. In order to control variables of CRS measuring condition, skins were transferred from cells to real-time device and open device. In real-time device, proper skin hydration was maintained and the skin temperature was adjusted to 32℃ (condition ii) and room temperature (condition iii). In open device, the skin was in a less hydrated state by moving onto a PBS-soaked filter paper and wrapped with aluminum foil at room temperature (condition iv). The skin penetration performances measured in these conditions were compared with reference. Caffeine solution and gel formulation were separately applied to the skin. The results showed in both cases that the decrease of skin temperature and hydration in condition iii and iv would apparently induce the decrease of detected caffeine signal, resulting in the inaccurate data collection. To this point, it indicates the reduction of solubilized caffeine in skin layer. We suggest the forming of caffeine crystallization at varied skin conditions to be the factor. Achieved video image, CRS spectrum collection and surface scan demonstrated the caffeine crystallization process on superficial skin layer. Polarized microscopic images exemplified the crystalline drug on tape stripped skin layers. It can be a potential support of caffeine crystallization inside skin. In short, we suggest the consideration of these parameters during CRS measurements for accurate monitoring of topical drug delivery. Meanwhile, the use of real-time device for dynamic skin incubation and data collection provides advantages in saving time and efforts in this study.

Elastic and Ultradeformable Liposomes for Transdermal Delivery of Active Pharmaceutical Ingredients (APIs)

Administration of active pharmaceutical ingredients (APIs) through the skin, by means of topical drug delivery systems, is an advanced therapeutic approach. As the skin is the largest organ of the human body, primarily acting as a natural protective barrier against permeation of xenobiotics, specific strategies to overcome this barrier are needed. Liposomes are nanometric-sized delivery systems composed of phospholipids, which are key components of cell membranes, making liposomes well tolerated and devoid of toxicity. As their lipid compositions are similar to those of the skin, liposomes are used as topical, dermal, and transdermal delivery systems. However, permeation of the first generation of liposomes through the skin posed some limitations; thus, a second generation of liposomes has emerged, overcoming permeability problems. Various mechanisms of permeation/penetration of elastic/ultra-deformable liposomes into the skin have been proposed; however, debate continues on their extent/mechanisms of permeation/penetration. In vivo bioavailability of an API administered in the form of ultra-deformable liposomes is similar to the bioavailability achieved when the same API is administered in the form of a solution by subcutaneous or epi-cutaneous injection, which demonstrates their applicability in transdermal drug delivery.

Skin cancer therapeutics: nano-drug delivery vectors—present and beyond

Background

Skin cancers are among the widely prevalent forms of cancer worldwide. The increasing industrialization and accompanied environmental changes have further worsened the skin cancer statistics. The stern topical barrier although difficult to breach is a little compromised in pathologies like skin cancer. The therapeutic management of skin cancers has moved beyond chemotherapy and surgery.

Main body of the abstract

The quest for a magic bullet still prevails, but topical drug delivery has emerged as a perfect modality for localized self-application with minimal systemic ingress for the management of skin cancers. Advances in topical drug delivery as evidenced by the exploration of nanocarriers and newer technologies like microneedle-assisted/mediated therapeutics have revolutionized the paradigms of topical treatment. The engineered nanovectors have not only been given the liberty to experiment with a wide-array of drug carriers with very distinguishing characteristics but also endowed them with target specificity. The biologicals like nucleic acid-based approaches or skin penetrating peptide vectors are another promising area of skin cancer therapeutics which has demonstrated potential in research studies. In this review, a panoramic view is presented on the etiology, therapeutic options, and emerging drug delivery modalities for skin cancer.

Short conclusion

Nanocarriers have presented innumerable opportunities for interventions in skin cancer therapeutics. Challenge persists for the bench to bedside translation of these highly potential upcoming therapeutic strategies.

Graphic abstract

Nanofiber as a novel vehicle for transdermal delivery of therapeutic agents: challenges and opportunities

Background

Transdermal delivery of drugs is a quite challenging task for pharmaceutical scientists. The transdermal route is preferred over the oral route due to various advantages like avoidance of the first-pass effect, non-invasiveness, and high patient compliance. Therefore, it is necessary to develop an effective carrier system that enables the effective passage of the drug through the dermal barrier.

Main body of abstract

Various novel drug delivery systems are used to enhance the permeation of a variety of drugs through the skin barrier. Researchers around the globe have explored nanofibers for the transdermal delivery of various therapeutic agents. Nanofibers are designed to have a high concentration of therapeutic agents in them promoting their flux through various skin layers. Polymeric nanofibers can be explored for the loading of both hydrophilic and lipophilic drugs. Biopolymer-based nanofibers have been also explored for transdermal delivery. They are capable of controlling the release of therapeutic agents for a prolonged time.

Short conclusion

The literature presented in this review paper provides significant proof that nanofibers will have an intense impact on the transdermal delivery of different bioactive molecules in the future.

Graphic abstract

Development of Polylactic Acid and Bovine Serum Albumin-layered-coated Chitosan Microneedles Using Novel Bees Wax Mould

Objectives

This work illustrates a novel method of fabrication of polymeric microneedle (MN) construct using bees wax as mould and development of coated polymeric MNs for drug delivery.

Materials and Methods

A novel method of MN fabrication using bees wax as mould was established. The porous chitosan MN arrays were fabricated and coated with polylactic acid (PLA). The optimized MN arrays were coated with bovine serum albumin (BSA). The MNs were subjected to physiochemical and tensile strength characterization, followed by drug release study. The skin penetration and irritation study were performed in vivo in Wistar Albino rats.

Results

The constructed MN arrays contain MNs with 0.9 mm length, 600 μm width at the base, 30-60 μm diameter at the tip, and 1.5 mm distance between 2 needles. These MNs patch was having good mechanical strength (0.72 N/needle) and tensile strength 15.23 Mpa. The MN array patch had 6.26% swelling index and 98.5% drug release was observed on the 50th hr. Good penetration and no skin irritation was observed for optimized MN batch.

Conclusion

Polymeric MN arrays were successfully developed using bees wax mould and were successfully coated with PLA to deliver the BSA through skin epidermis layer.

Comparison of Membrane Depth Determination Techniques for Active Ingredient Skin Penetration Studies Using Microdialysis

INTRODUCTION

The skin is a major physical barrier to the environment, and thus, percutaneous delivery of active ingredients to the dermal target site faces a unique set of hurdles. The efficacy of these active ingredients is governed by their release into the underlying epidermal and dermal tissue, especially when administered topically.

OBJECTIVE

The aim of this study was to understand if different physicochemical properties influence the skin penetration of active ingredients and the depth to which they penetrate into the dermis.

METHODS

A microdialysis (MD) setup was used to compare the percutaneous penetration in superficial and deep implanted MD membranes in porcine skin. The precise MD membrane depth was determined using histological sectioning paired with microscopy, ultrasound, and a novel computed tomographic approach.

RESULTS

In study A, the measured depth of the superficial and deep implanted MD membranes was compared using histological sectioning, ultrasound, and computed tomography. Experimental determination of the depth up to which penetration occurs was found to be crucial to percutaneous penetration studies. In study B, the lipophilic differences of the active ingredients and its influences on the penetration was tested using hydrophilic caffeine and lipophilic LIP1 as model compounds, which have an identical molecular weight with different lipophilic characteristics. It is assumed that the lipophilic characteristics of active ingredients influence their penetration and thus governs the concentration of these molecules reaching their target site.

CONCLUSION

The transdermal penetration of caffeine was found to exceed that of LIP1 through the hydrophilic environment of the dermis. Thus, the findings of this study show that the precise MD dermis localization and the physicochemical properties, such as lipophilicity, influence the penetration rate of active ingredients and lay the foundation for creating optimized transdermal delivery systems.

Recent Advances in Nanomaterials for Dermal and Transdermal Applications

The stratum corneum, the most superficial layer of the skin, protects the body against environmental hazards and presents a highly selective barrier for the passage of drugs and cosmetic products deeper into the skin and across the skin. Nanomaterials can effectively increase the permeation of active molecules across the stratum corneum and enable their penetration into deeper skin layers, often by interacting with the skin and creating the distinct sites with elevated local concentration, acting as reservoirs. The flux of the molecules from these reservoirs can be either limited to the underlying skin layers (for topical drug and cosmeceutical delivery) or extended across all the sublayers of the epidermis to the blood vessels of the dermis (for transdermal delivery). The type of the nanocarrier and the physicochemical nature of the active substance are among the factors that determine the final skin permeation pattern and the stability of the penetrant in the cutaneous environment. The most widely employed types of nanomaterials for dermal and transdermal applications include solid lipid nanoparticles, nanovesicular carriers, microemulsions, nanoemulsions, and polymeric nanoparticles. The recent advances in the area of nanomaterial-assisted dermal and transdermal delivery are highlighted in this review.

Microneedle-mediated transdermal drug delivery for treating diverse skin diseases

Transdermal drug delivery is an attractive route for dermatological disease therapy because it can directly target the lesion site on the skin, reduce adverse reactions associated with systemic administration, and improve patient compliance. However, the stratum corneum, as the main skin barrier, severely limits transdermal drug penetration, with compromised bioavailability. Microneedles (MNs), which are leveraged to markedly improve the penetration of therapeutic agents by piercing the stratum corneum and creating hundreds of reversible microchannels in a minimally invasive manner, have been envisioned as a milestone for effective transdermal drug delivery, especially for superficial disease therapy. Here, the emergence of versatile MNs for the transdermal delivery of various drugs is reviewed, particularly focusing on the application of MNs for the treatment of diverse skin diseases, including superficial tumors, scars, psoriasis, herpes, acne, and alopecia. Additionally, the promises and challenges of the widespread translation of MN-mediated transdermal drug delivery in the dermatology field are summarized.