## 1 Introduction  
Transdermal drug delivery systems (TDDSs) have become an important area of pharmaceutical research due to their ability to offer sustained release of active compounds while bypassing the gastrointestinal tract and hepatic first-pass metabolism. This route of administration improves patient compliance and reduces systemic side effects commonly observed with oral or injectable formulations. Despite these advantages, the efficiency of TDDSs is severely limited by the presence of the stratum corneum, a highly organized lipid-protein matrix that constitutes the main barrier to the diffusion of many therapeutic agents [1; 2; 3; 4; 5; 6]. In order to enhance the skin permeability of active pharmaceutical ingredients (APIs), various physical and chemical enhancement techniques have been explored, including microneedles, iontophoresis, sonophoresis, and more recently, the application of electromagnetic fields (EMFs) [7; 8; 9; 10; 11; 12; 13].  
EMFs are composed of electric and magnetic components and are known to interact with biological systems at both the cellular and molecular levels. They influence membrane structure, fluidity, and ion transport mechanisms, potentially affecting the rate of passive and facilitated diffusion through biological barriers. Depending on their characteristics, EMFs are classified as static (SMFs), oscillating (OMFs), pulsed (PMFs), or rotating magnetic fields (RMFs). Among these, RMFs have gained particular attention in pharmaceutical research due to their ability to induce hydrodynamic microflows and enhance mass transport without compromising tissue integrity [14; 15; 16; 17]. Studies have demonstrated that RMFs can significantly increase the skin permeability of several NSAIDs, such as ibuprofen, naproxen, ketoprofen, caffeine, and paracetamol, by modifying physicochemical properties of the compounds and disrupting the lipid structure of the stratum corneum [13; 18; 19].  
Salicylic acid (2-hydroxybenzoic acid) is a keratolytic agent of natural origin widely used in dermatology and disentology due to its anti-inflammatory, antimicrobial, and exfoliating properties. It is effective in the treatment of acne, psoriasis, calluses, and other hyperkeratotic conditions. Despite its relatively low molecular weight (138.12 Da) and moderate lipophilicity (log P \(\approx\) 2.3), the ionization of its carboxylic group under physiological conditions leads to the formation of negatively charged species, which significantly hinders its passive transdermal absorption [20; 21; 22; 23; 24]. Formulations containing salicylic acid often require high concentrations or pH modification to enhance skin penetration, which can increase the risk of irritation or sensitization. Therefore, the development of alternative, non-invasive methods to improve its permeability is of great practical importance.  
To date, no comprehensive study has evaluated the effect of various electromagnetic field types on the transdermal delivery of salicylic acid. Thus, the present research aims to investigate how different EMFs--namely oscillating, pulsed, static (positive and negative), and rotating magnetic fields--influence the transdermal permeability and skin accumulation of salicylic acid using porcine skin as a model. In addition to permeation studies, the potential changes in the physicochemical properties of the compound (including FTIR spectra, crystalline structure, solubility in water, partition coefficient, and thermal characteristics) induced by EMF exposure will also be evaluated. This work is intended to expand our understanding of EMF-assisted transdermal drug delivery and to provide new insights into optimizing non-invasive therapeutic systems based on widely used dermatological agents.