2.4.3. Electro-Hydrodynamic Processing

The preparation of the fibrous mats was carried out using a high throughput Fluidnatek LE-500 pilot line electrospinning equipment with temperature and relative humidity control in the lab mode with a single needle injector (Bioinicia S.L., Valencia, Spain). The solutions were drawn in a 20 mL plastic syringe that was placed on a syringe pump and connected by PTFE tube to a stainless steel needle (20 Gauge). A positive electrode of a high voltage power supply was coupled to the needle. The solutions were electrospun at the constant flow rate of 3 mL/h and the voltage of 19 kV for 2 h. The fibers were homogenously deposited onto an aluminum foil sheet placed on a metallic collector using a scanning injector. The distance between the needle tip and the collector was 15 cm. The process was performed at 25 ◦C and 30% RH.

Subsequently, an annealing step was applied in order to obtain ultrathin, transparent, and continuous films by fibers coalescence. This process was carried out using a hydraulic press (4122 model, Carver Inc., Wabash, IN, USA) at temperatures 55 ◦C for 25 s without pressure. The annealing, carried out below the polymer melting point at 60 ◦C, was optimized for PCL electrospun fibers in a previous work [25]. The films were stored before physical characterization in a desiccator containing dried silica gel at 0% RH and 25 ◦C.

#### *2.5. Characterization of The PCL Based Films*

#### 2.5.1. Film Thickness

Before further analysis, the film thickness was measured at five random points of each sample by a digital micrometer with an accuracy of 0.001 mm (S00014, Mitutoyo Corporation, Kawasaki, Japan).
