During the last decades, several technologies were developed for testing drug delivery through the dermal barrier. Investigation of drug penetration across the skin can be important in topical pharmaceutical formulations and also in cosmeto-science. The state-of- the-art in the field of skin diffusion measurements, different devices, and diffusion platforms used, are summarized in the introductory part of this review. Then the methodologies applied at Pázmány Péter Catholic University are shown in detail. The main testing platforms (Franz diffusion cells, skin-on-a-chip devices) and the major scientific projects (P-glycoprotein interaction in the skin; new skin equivalents for diffusion purposes) are also presented in one section. The main achievements of our research are briefly summarized: (1) new skin-on-a-chip microfluidic devices were validated as tools for drug penetration studies for the skin; (2) P-glycoprotein transport has an absorptive orientation in the skin; (3) skin samples cannot be used for transporter interaction studies after freezing and thawing; (4) penetration of hydrophilic model drugs is lower in aged than in young skin; (5) mechanical sensitization is needed for excised rodent and pig skins for drug absorption measurements. Our validated skin-on-a-chip platform is available for other research groups to use for testing and for utilizing it for different purposes.

Psoriasis is a chronic inflammatory disease with unmet medical needs. To clarify potential therapeutic targets, different animal models have been developed. In the current study, imiquimod-induced psoriasiform dermatitis was used for monitoring the changes in skin thickness, transepidermal water loss, body weight, blood perfusion and drug permeability for a topical cream formulation of caffeine, both in wild type and in knock out mice. Morphological characterization of control and diseased tissues was performed by scanning electron microscopy and two-photon microscopy. The chemically induced psoriatic group showed increased skin permeability for the model drug during disease progression. In wild type and TRPA1 KO mice, however, enhanced skin thickness and hyperkeratosis blocked further increase of drug penetration at the late phase (96 h). These results indicate that topical drug therapy can be more effective in early phases of plaque development, when skin thickness is lower. Although paracellular connections (tight junctions) are looser in the advanced phase, hyperkeratosis blocks drug delivery through the transappendageal routes. Novel drug formulations may have the potency for effective drug delivery across the epidermal barrier even in the advanced phase. For development of more effective topical drugs, further research is proposed to explore drug penetration both in healthy and diseased conditions.

Organ-on-a-chip technologies show exponential growth driven by the need to reduce the number of experimental animals and develop physiologically relevant human models for testing drugs. In vitro, microfluidic devices should be carefully designed and fabricated to provide reliable tools for modeling physiological or pathological conditions and assessing, for example, drug delivery through biological barriers. The aim of the current study was to optimize the utilization of three existing skin-on-a-chip microfluidic diffusion chambers with various designs. For this, different perfusion flow rates were compared using cellulose acetate membrane, polyester membrane, excised rat skin, and acellular alginate scaffold in the chips. These diffusion platforms were integrated into a single-channel microfluidic diffusion chamber, a multi-channel chamber, and the LiveBox2 system. The experimental results revealed that the 40 µL/min flow rate resulted in the highest diffusion of the hydrophilic model formulation (2% caffeine cream) in each system. The single-channel setup was used for further analysis by computational fluid dynamics simulation. The visualization of shear stress and fluid velocity within the microchannel and the presentation of caffeine progression with the perfusion fluid were consistent with the measured data. These findings contribute to the development and effective application of microfluidic systems for penetration testing.

Mathematical models of epidermal and dermal transport are essential for optimization and development of products for percutaneous delivery both for local and systemic indication and for evaluation of dermal exposure to chemicals for assessing their toxicity. These models often help directly by providing information on the rate of drug penetration through the skin and thus on the dermal or systemic concentration of drugs which is the base of their pharmacological effect. The simulations are also helpful in analyzing experimental data, reducing the number of experiments and translating the in vitro investigations to an in-vivo setting. In this study skin penetration of topically administered caffeine cream was investigated in a skin-on-a-chip microfluidic diffusion chamber at room temperature and at 32°C. Also the transdermal penetration of caffeine in healthy and diseased conditions was compared in mouse skins from intact, psoriatic and allergic animals. In the last experimental setup dexamethasone, indomethacin, piroxicam and diclofenac were examined as a cream formulation for absorption across the dermal barrier. All the measured data were used for making mathematical simulation in a three-compartmental model. The calculated and measured results showed a good match, which findings indicate that our mathematical model might be applied for prediction of drug delivery through the skin under different circumstances and for various drugs in the novel, miniaturized diffusion chamber.

In recent times, with the need for a reduction, refinement, and replacement of in vivo animal testing, there has been an increasing demand for the use of relevant in vitro human cell systems in drug development. There is also a great demand for the replacement of skin tissue in various wounds and burns. Furthermore, human skin cell-based in vitro systems can be used to investigate the side effects (toxicity and irritation) and tissue penetration of topical preparations. In this study, exploratory experiments were performed to produce artificial epidermis using two hydrogel scaffolds, alginate and GelMA C. The amount of keratinocytes added to the matrix (10–50–100 × 10⁶/mL) and the duration of tissue maturation (fresh, 1–3–4 weeks) were optimized in an extensive study. The behavior and structure of the two hydrogels were functionally and morphologically assessed. The permeability order for caffeine in the tested barriers was the following: alginate > GelMA C > cellulose acetate membrane > rat skin. It was concluded that GelMA C matrix provides a more favorable environment for cell survival and tissue differentiation (as demonstrated by histology and immunohistochemistry) than alginate. The 3-week incubation and 50 × 10⁶/mL cell number proved to be the most beneficial in the given system. This study provides data for the first time on the multifactorial optimization of two potential skin substitutes for tissue manufacturing. In order to use these results in tissue engineering, the fabricated artificial epidermis preparations must also be optimized for biocompatibility and from physical and mechanical point of views.

In recent times, with the decline of in vivo animal experiments, there has been an increasing demand for the use of relevant in vitro human cell systems in the development of new drugs. Furthermore, the production of artificial tissues and tissue engineering is also gaining an increasingly important role in regenerative medicine. For example, there is a great demand for the replacement of skin tissue in various acute and chronic wounds and burns. On the other hand, these human skin cell-based in vitro systems can also be used to investigate the side effects (toxicity and irritation) and tissue penetration of topical preparations. In this study, exploratory experiments were performed to produce artificial epidermis using two different hydrogel scaffolds, alginate and GelMA C. Amount of keratinocytes added to the matrix and the duration of tissue maturation were optimized. In addition, the behavior and structure of the two epithelial cell-containing hydrogels were functionally and morphologically tested. Finally, it was concluded that the GelMA C matrix provides a more favorable biological environment for cell survival and tissue differentiation. The 3-week incubation and 50M/mL cell number proved to be the most favorable in the given system. In order to use these results in 3D bioprinting, the fabricated artificial epidermis preparations must be optimized also for biocompatibility and from a physical and mechanical point of view.