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.

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