## 5 Conclusions  
The results of this study confirm the potential of electromagnetic field (EMF) application as a physical method for enhancing the transdermal delivery of salicylic acid. Among the tested configurations, pulsed magnetic fields, particularly the five/five on/off cycle, and rotating magnetic fields at frequencies of 30 Hz and 50 Hz, exhibited the most pronounced enhancement in drug permeation. These field types significantly increased the cumulative amount permeated, steady-state flux, and permeability coefficient, while also reducing the lag time, indicating their capacity to disrupt the skin barrier and facilitate drug transport transiently. This study also highlights the role of EMF configuration in modulating not only the rate of transdermal permeation but also the extent of skin accumulation. Static magnetic fields of negative polarity were associated with an increased retention of salicylic acid in the skin, suggesting a mechanism that favors local drug deposition. This dual effect--the ability to either promote systemic delivery or encourage localized retention--demonstrates the versatility of EMFs in tailoring release profiles according to therapeutic needs. Physicochemical analyses, including FTIR, XRD, TGA, and DSC, confirmed that exposure to electromagnetic fields did not alter the molecular structure or significantly affect the thermal stability of salicylic acid. The absence of chemical degradation ensures the safety and integrity of the active compound, which is critical for pharmaceutical and cosmetic applications.  
Collectively, these findings support the use of electromagnetic stimulation as a non-invasive, tunable method for optimizing transdermal drug delivery. By selecting appropriate EMF parameters, it is possible to enhance drug permeation through the skin or increase drug retention within the skin tissue, thereby enabling both fast-acting and sustained-release topical therapies. This approach offers a promising platform for the development of next-generation transdermal systems, particularly for moderately lipophilic and ionizable active pharmaceutical ingredients.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app15137606/s1](https://www.mdpi.com/article/10.3390/app15137606/s1)]([https://www.mdpi.com/article/10.3390/app15137606/s1](https://www.mdpi.com/article/10.3390/app15137606/s1)), Figure S1: FTIR spectra of salicylic acid exposed to different EMFs; Figure S2: XRD patterns of salicylic acid exposed to different EMFs; Figure S3: The TG, DTG, and c-DTA curves of salicylic acid exposed to different EMFs; Figure S4: The DSC curve of salicylic acid exposed to different EMFs; Table S1: Enhancement ratios (ERs) calculated for different electromagnetic field (EMF) configurations based on the steady-state flux (JSS) relative to the control sample. ER values illustrate the relative increase or decrease in salicylic acid permeability under EMF exposure.  
Conceptualization, P.O.-R.; methodology, A.N. and P.O.-R.; formal analysis, K.Z.-C., A.N., A.M.-S., L.K., M.K., and P.O.-R.; investigation, P.O.-R.; data curation, P.O.-R. and R.R.; writing--original draft preparation, K.Z.-C., A.N., and P.O.-R.; writing--review and editing, P.O.-R.; visualization, K.Z.-C. and P.O.-R.; supervision, P.O.-R.; project administration, R.R.; funding acquisition, R.R. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Science Centre, grant number NCN OPUS 25 NR UMO-2023/49/B/ST8/00605.  
Not applicable.  
**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.  
**Conflicts of Interest:** The authors declare no conflicts of interest.