This study presents a novel approach combining stimulated Raman scattering (SRS) microscopy with mechanistic modelling to quantify the disposition and permeation of topically applied compounds in skin. The dermal delivery of 4-cyanophenol and caffeine was investigated using SRS microscopy, generating high-resolution images resolving their distribution within the stratum corneum (SC) lipid matrix and corneocytes. The imaging data revealed distinct permeation pathways, with 4-cyanophenol preferentially localising within the intercellular lipid matrix and caffeine within the corneocyte-rich regions. Depth-resolved SRS imaging provided concentration profiles across the SC, showing a monotonic decrease in 4-cyanophenol concentration, whereas caffeine exhibited depth-dependent transport behaviour. These experimental concentration profiles were modelled using a two-dimensional mechanistic model with calibration to reveal the diffusion, partition and deposition behaviour within the lipid and corneocyte domains. For 4-cyanophenol, the optimised permeability was consistent with literature-reported values and baseline model predictions, while for caffeine, optimisation substantially improved permeability predictions. Simulations further captured formulation-dependent effects predicting 1.3-fold and 1.5-fold increases in SC uptake of 4-cyanophenol and caffeine, respectively, in propylene glycol relative to aqueous vehicles. Overall, this work demonstrates the potential of combining SRS microscopy with inverse mechanistic modelling to derive transport parameters directly from skin imaging data and reveal dermal drug disposition and permeation. To our knowledge, this represents the first application of SRS-derived depth profiles to inform mechanistic modelling of skin permeation.

Improving the aqueous solubility of moderately soluble drugs using safe and sustainable solvent systems remains an important challenge in pharmaceutical formulation. In this study, a series of aqueous natural deep eutectic solvents (NADESs) were systematically designed and evaluated to enhance the solubility, stability, and biocompatibility of acetaminophen (ACP). The effects of hydrogen bond donor type, hydrogen bond acceptor type, molar ratio, and NADES concentration were investigated to identify the optimal formulation. Among the tested systems, the aqueous betaine chloride-acetic acid NADES (BeCl-Ace) at a molar ratio of 1 : 4 and a concentration of 70% exhibited the highest solubilization capacity, increasing ACP solubility by 25.28 and 22.14-fold at 25 and 37 °C, respectively, compared with water. Computational analyses based on electrostatic potential mapping, binding energy calculations, and interaction region indicator analysis revealed that the enhanced solubility originates from cooperative hydrogen bonding, electrostatic attraction, and van der Waals interactions between ACP and the BeCl-Ace components, particularly BeCl. In addition, the selected NADES formulation significantly improved ACP stability under thermal stress, retaining 89.13% of the initial drug content after 30 days at 37 °C, compared with 70.20% in water. Cytotoxicity evaluation on HEK-293A cells further confirmed good biocompatibility, with cell viability remaining above 92% across the tested concentration range. These findings indicate that aqueous BeCl-Ace NADESs represent a promising and sustainable solvent system for improving ACP formulation performance.