## 2 Materials and Methods
### Electromagnetic Field Exposure  
Salicylic acid samples were subjected to exposure under different electromagnetic field conditions for a total duration of 8 h. The control group consisted of samples not exposed to any field. Five types of magnetic field configurations were applied in this study:  
* Oscillating magnetic field (OMF): generated using alternating current (AC) with a frequency range of 45-65 Hz and magnetic induction values between 26 and 35 mT.
* Pulsed magnetic field (PMF): characterized by a 65 Hz current with magnetic induction up to 26 mT, operating in a cyclic on/off mode with pulse durations of 1-10 s.
* Static magnetic field--positive polarity (SMF+): produced using direct current (DC) with a supply voltage of 5-25 V and magnetic induction of 4-21 mT.
* Static magnetic field--negative polarity (SMF--): DC-generated with reverse polarity, voltage of 5-25 V, and induction of 3-15 mT.
* Rotating magnetic field (RMF): formed by the superposition of three AC fields (each 120\({}^{\circ}\) out of phase), delivering a rotating magnetic vector at 10-50 Hz and 33-45 mT magnetic induction.  
The electromagnetic field parameters used in this study were selected to reflect commonly studied extremely low-frequency electromagnetic field (ELF-EMF) conditions, while also taking into account the technical constraints of the available instrumentation. Specifically, frequency ranges of 45-65 Hz for oscillating magnetic fields (OMFs) and 10-50 Hz for rotating magnetic fields (RMFs) were chosen to align with the ELF-EMF spectrum, typically defined as 3-300 Hz. This frequency range has been extensively investigated in biomedical research due to its potential biological relevance, including applications in bone regeneration, tissue repair, neurological modulation, and anti-inflammatory therapies [25; 26; 27].  
Magnetic induction values between 3 and 45 mT were selected to ensure non-invasive exposure conditions that do not exceed internationally accepted safety thresholds and do not induce thermal or structural degradation of either the active compound or the biological membrane. The pulse durations applied in the pulsed magnetic field (PMF) configurations (1-10 s) were intended to mimic short, intermittent modulations of the skin barrier, allowing for reversible effects on permeability without causing sustained mechanical or thermal stress to the tissue. These experimental parameters were also supported by preliminary optimization trials conducted in-house, reflecting the operational capabilities of the electromagnetic field generator used in the present study.  
All field exposures were performed in a custom-built reactor system equipped with integrated electromagnetic field generators. Each reactor chamber was maintained at a constant temperature (37.0 \(\pm\) 0.1 \({}^{\circ}\)C) using a water-jacketed circulation system connected to a thermostatic controller. This setup permitted the simultaneous exposure of samples under different EMF types as well as a control sample.  
Both pure salicylic acid (as the active pharmaceutical ingredient) and its 70% ethanol (EtOH) solutions were subjected to electromagnetic field exposure. In the case of the EtOH-based solutions, transdermal permeability studies were additionally carried out to evaluate the influence of EMF conditions on skin penetration efficiency.