## 2 Materials and Methods
### Experimental  
The OCAs employed in this study were: Propylenglycol (PG), Propylenglycol combined with Oleic Acid in equal proportions (1:1, \(v/v\)) (PCOA), Vaseline (V) and Liquid Vaseline (LV). Vaseline products were purchased by a local drugstore while the other OCAs were purchased in pure form (99%) from Sigma-Aldrich. PG is an aliphatic diol, miscible in water and a common pharmaceutical excipient. It is also used as a cosolvent in formulations to enhance skin permeation due to its mild keratolytic activity. Oleic acid is an unsaturated fatty acid found abundantly in nature and it consists of a hydrophobic tail and a hydrophilic head. Fatty acids are used as intermediates to produce emulsifiers, emollients and lubricants in cosmetic formulations. When OA is applied to the skin, it induces moderate skin irritation and causes local modulation of cytokine production in the viable epidermis [28]. However, it has been widely used to enhance skin penetration in drug delivery and in polymeric micelles for sustained release. It can improve the solubility of lipophilic drugs while it boosts the delivery of both lipophilic and hydrophilic medications. The combination of OA with PG yielded an additive effect on the percutaneous absorption of the non-steroidal anti-inflammatory drug Tenoxicam by increasing the flux and decreasing the penetration lag-time [29]. Vaseline consists of a mixture of liquid and solid hydrocarbons with improved spreading and occlusive properties on the skin. Being additionally a cost-effective product, it is frequently used as a vehicle for pharmaceutical compounds or as a base for various creams and ointments. Liquid Vaseline has a similar composition to Vaseline, and it has a smoother texture as it is combined with minerals and microcrystalline wax.  
Each OCA was tested on the right polar forearm of the same healthy volunteer (Caucasian male, 50 years of age, Fitzpatrick skin type III) after informed consent was obtained. The procedure was supervised by expert dermatologists who also performed the OCT measurements via the normal clinical OCT imaging routine. OCT B-scans were recorded from the forearm skin (Figure 1). Additional images were acquired from the same anatomical site, at two time points, to estimate the percentage change in both the attenuation coefficient and epidermal thickness. Specifically, skin imaging took place just after the application of each OCA (t1 = 5 min) and at t2 = 90 min afterward.  
Epidermal thickness was inferred through OCT image analysis, by measuring the distance from the skin surface to the dermo-epidermal junction i.e., in the transition between the signal from the epidermis and the brighter signal from the dermis. Four ROIs in each calibrated OCT image were used and analyzed using Fiji software. It is important to note that the refractive index of the tissue was not taken into account. Thus, the measurements do not correspond to physical, but to optical distances.  
OCT measurements were carried out with the OCT NITID system (Dermalumics, Spain), a portable OCT system with a light source at a central wavelength of 1300 nm, axial and lateral resolution at 11 and 12 \(\upmu\)m, respectively, and penetration depth of 1.5 mm. OCT 2D scans were greyscale images with height = 512 pixels and width = 1275 pixels. Statistical analysis of \(\upmu_{\text{OCT}}\) and epidermal thickness measurements were performed with the Brown-Forsythe equality of means test (\(\alpha\) = 0.05), where variances are not assumed to be equal, and normality is not certain, due to the small size of the datasets.