3.8. In Vitro Diffusion and Skin Permeation Studies

In vitro CA diffusion and skin permeation from the P123 micelles was evaluated by Franz cells at two different temperatures, 32 and 40 °C. Particularly, the diffusion barrier was made with Visking tubing (Spectra/Por^®, cut-off 12–14 kDa), while, for the permeation studies, rabbit ear skin was used. The artificial membrane and skin were clamped between the donor and the receptor compartment of the Franz diffusion cell. The effective penetration area was 0.416 cm² and the receptor compartment capacity was 5.5 mL. The receptor compartment was filled with medium and, at various time points, the medium in the receptor compartment was collected and replenished immediately with an equal amount to maintain the sink conditions. Then, the CA concentrations released were quantified using UV–Vis spectroscopy. The cumulative amount of CA permeated per unit area was calculated, and the permeation profile was plotted as a function of time. In order to determine the amount of CA retained on the skin, the skin was carefully separated from the diffusion cell at the end of the skin permeation experiments (24 h). Then, it was rinsed with deionised water to remove any residual formulation. Then, the retained CA on the skin was extracted by soaking it in 10 mL of ethanol for 1 h with constant stirring in the dark. The solution was then filtered by a membrane (0.22 μm), and the content of CA was analysed by UV–Vis spectroscopy.