2.4.1. Surface Characterizations

The sample coatings were characterized in terms of wettability, through an OCA 15Pro Contact Angle Tool (DataPhysics), and surface morphology, by means of an atomic force microscope (CSI Nano-Observer AFM) in non-contact (resonant) mode, and a SEM (Helios NanoLab DualBeam, FEI). The reported values for contact angles were averaged over four independent measurements, whereas the roughness values stemmed from the averaging over a scanned area of 5 × 5 μm2.

#### 2.4.2. Exposure to Seawater

The whole devices and some coated silicon substrates (three for each coating) were submerged in a tank (440 × 265 × 240 mm3) and filled with seawater which was aimed at reproducing the marine environment (Figure 5a). The natural seawater was collected from the Ionian Sea, with the following physico-chemical parameters: salinity 3.5%, pH 8, metals (Na 10,784 ppm, K 399 ppm, Mg 1294 ppm, Ca 4120 ppm, Si 2.9 ppm), halogenides (Br−/Cl−/F− 19360/67.5/1.3 ppm), carbonates (HCO3 − 126 ppm), sulphates (SO4 2712 ppm), and dissolved gases (O2 7 ppm, N2 12.5 ppm) [52].

**Figure 5.** (**a**) Setup used to mimic the marine environment in the laboratory. (**b**) Setup used for the impedance spectroscopy (IS) measurements with the schematic of the samples adopted. (**c**) Schematic of the three-electrode cell used for the corrosion tests. (**d**) Floating platform for supporting the whole devices while being submerged in seawater for reliability tests.

A wave maker (Jebao RW4 Propeller Pump) was inserted in the tank to create sea waves and currents: in order to avoid the samples tumbling due to the water waves, a metallic grill was used as support; the coated rigid substrates were fixed on Petri dishes and placed in the grill, whereas the whole devices were made floating with a polystyrene supporting slab. A porous live rock was inserted to introduce natural bacteria, algae and invertebrates inside the artificial marine environment. Moreover, it plays the role of biological filter because it harbors aerobic and anaerobic nitrifying bacteria required for the nitrogen cycle. Furthermore, a natural seaweed was added. Its general function is to use waste products as nutrients and perform chlorophyllian photosynthesis, producing oxygen. In this study, it also served as a competing species for the development of bacteria and algae. A LED lighting system (Mini Lumina 30 Marine, Blau-aquaristic; 18 W; 10.000K white and 460 nm blue) was used to provide the solar light radiation, stimulating the growth of microorganisms. Finally, a skimmer (Scuma 0635 Int, Blau-aquaristic; 3.5 W) was installed to purify the environment keeping the water clear and limpid.

#### 2.4.3. Water Absorption Tests

The absorption of water into the devices mainly regards the polymeric layers. The conformity of the coatings is crucial because any possible asperity, uncovered area or inlet could lead to water permeation. Impedance spectroscopy (IS) was carried out to evaluate water absorption: circular samples (1.5 cm radius) of kapton coated with the selected polymers were submerged, then dried with nitrogen flow at regular time intervals. The impedances of the substrate/coating systems were collected by placing the samples between two metallic plates connected to an Agilent E4980A Precision Inductance, Capacitance, Resistance (LCR) meter (applying the same pressure for all samples) and by performing linear frequency sweeps.

## 2.4.4. Corrosion Tests

Corrosion due to seawater penetration regards all the metallic portions of the devices, thus anti-corrosion tests were performed to evaluate the corrosion resistance of the coatings. The steel panels (4 × 5 cm2) were washed with acetone and isopropanol to remove organic contaminants before the application of coatings. The corrosion tests were carried out in a three-electrode cell (Figure 5c) consisting of coated steel samples as working electrode (WE), a 5 × 5 cm<sup>2</sup> Pt mesh as counter electrode (CE) and an Ag/AgCl/sat-KCl electrode as a reference electrode (RE). Linear scanning voltammetry was performed at 1 mV/s to measure current and voltage using an AUTOLAB PGSTAT302N potentiostat, after 1h of stabilization to reach the equilibrium open circuit potential (OCP).

#### 2.4.5. Piezoelectric Generation in the Long-Term Period

The PVDF-based devices were attached to a polystyrene foam slab and submerged in the tank filled with seawater (Figure 5d): the slab served as a supporting platform to let the devices float firmly on the water. In this way, the supporting platform is always in contact with the water surface although the continuous evaporation of water, keeping the same initial conditions. The measurements were performed by displacing the samples at a proper distance, using a linear micro-actuator (Actuonix Motion Devices Inc., L16-R, 100 mm, 35:1, 6vdc) and the output signal generated by the oscillation of the devices (open circuit voltage) was detected and collected with an oscilloscope (Tektronix MDO 4104-3).

#### 2.4.6. Surface Treatments of Parylene Surface

Wettability and morphology studies were performed on treated parylene C coatings in order to evaluate qualitatively the long-term microbial adhesion. Two different treatments were applied: oxygen plasma and ultraviolet light irradiation with addition of ozone. Oxygen plasma treatment was performed by a low-pressure plasma system NANO (Diener electronic Plasma Surface Technology GmbH & Co. KG), with two different RF powers in order to compare the resulting modified morphologies, i.e., 100 W and 300 W, keeping the process time (20 min) unchanged. UV/ozone treatment was performed by a UV Ozone Cleaner ProCleaner Plus (BioForce Nanosciences) for 20 min. The two treatments were overlapped in order to evaluate the e ffects of each single treatment. As previously demonstrated [50], the etching e ffect of oxygen plasma treatment on polymeric substrates induces a structure with nanosized thread-like features with a relatively high roughness. The lower the dimensions of asperities (roughness), compared to the dimensions of air bubbles in water, the less likely these air bubbles remain trapped among the asperities, thus the wider the contact surface/water and the more favored the water absorption. For these reasons, the UV/ozone treatment was superimposed to the oxygen plasma with the intention of making the sample surfaces less rough.

Non-treated samples, samples treated with O2 plasma, and samples treated with both the O2 plasma and UV/ozone treatments, were compared. The flexible kapton/pC structures were characterized after submerging them in the tank: the characterization tests were carried out at three di fferent periods of time (15, 25, 35 days), after drying the samples in a desiccator.

The laser Doppler vibrometer (LDV-MSA-500, Polytec), equipped with HeNe laser light source, was employed to investigate the frequency response of the pC-coated kapton samples evaluating the out-of-plane deflections by a non-contact measurement. The experimental setup included also an electromechanical shaker (4819, Bruel & Kjaer) aimed at providing the mechanical vibrations in a range of 0–18kHz. The samples were previously cut and shaped in the form of cantilevers (10 × 5 mm2) by an automatic cutting plotter (Graphtec Cutting Plotter CE6000-40).
