*2.2. Electrospinning*

A PHB solution for electrospinning was prepared by dissolving the biopolymer at 10 wt. %. The PdNPs were added at 1 wt. % in relation to PHB to the solution. To improve the dispersion of the PdNPs, CTAB and TEOS were also added as dispersing aids at 0.5 wt. % with the PHB and PdNPs mixture.

Electrospinning was performed using a Fluidnatek® LE50 benchtop line from Bioinicia S.L. (Valencia, Spain) with a variable high-voltage 0–30 kV power supply. This device was equipped with a motorized injector that was scanning towards a metallic collector, aiming to obtain a homogeneous electrospun deposition. The different biopolymers solutions were transferred to a 30-mL plastic syringe and the syringe was connected through polytetrafluoroethylene (PTFE) tubes to a stainless-steel needle (Ø = 0.9 mm) whereas the needle tip was connected to the power supply. The solution was electrospun for 2 h under a steady flow-rate of 6 mL/h using a motorized injector, scanning horizontally towards a metallic grid. The distance between the injector and collector was optimal at 15 cm and the voltage was set at 15 kV. The biopolymer solutions were electrospun in a controlled environmental chamber at 23 ◦C and 40% relative humidity (RH), for a given processing time and in optimal conditions to achieve steady fiber formation. The processing and solution characterization parameters for the optimal electrospinning of this PHA grade were determined and optimized in a previous work [21].

Finally, the obtained electrospun mats were subjected to an annealing step below the biopolymer's melting point without applying pressure using a hydraulic press 4122-model from Carver, Inc. (Wabash, IN, USA). The annealing was found optimal at 160 ◦C, without pressure, for 5 ± 1 s, based on our previous study [21]. The resultant films were air cooled at room temperature. Prior to thermal treatment, the electrospun mats were equilibrated in a desiccator at 25 ◦C and 0% RH by using silica gel for at least 1 week.
