*3.1. Surface Characterizations*

The AFM micrographs and the results of wettability measurements are reported in Figure 6.

Parylene C (pC) exhibited higher roughness (3.71 nm) than PMMA (0.28 nm) and PDMS-based coatings which present comparable values, i.e., 1.19 nm (PDMS), 1.70 nm (PDMS/PVDF). Concerning the wettability characteristics, parylene and PMMA showed contact angles of 89.5◦ ± 0.6◦ and 79.3◦ ± 3.0◦, respectively; PDMS-based coatings present c.a. values of 116.6◦ ± 0.9◦ (PDMS) and 117.4◦ ± 0.7◦ (PDMS/PVDF): these values indicate that PMMA has a moderately hydrophilic character, whereas the other coatings exhibit more hydrophobicity, apart from parylene which may be considered as neutral; the incorporation of PVDF into the PDMS slightly increased the contact angle.

**Figure 6.** Atomic force microscopy (AFM) topography images (scanned area: 5 × 5 μm2) for the examined coatings: (**a**) parylene-C, (**b**) poly-dimethyl siloxane (PDMS), (**c**) poly-methyl methacrylate (PMMA), (**d**) PDMS- and poly-(vinylidene fluoride) (PVDF). The vertical color bar reports the roughness in nm. The insets show the results of contact angle measurements.

#### *3.2. Water Absorption Tests*

The IS frequency-sweep analyses performed with the setup shown in Figure 5b on the submerged samples yielded the Nyquist plots reported in Figure 7a–e. The shape of the Nyquist curves is in general semi-circular and the magnitude of the vector connecting the axis origin with each point of the curves gives the values of impedance for different frequencies. Therefore, a curve stretched upward indicates that the imaginary part of impedance (reactance) of the system is higher.

**Figure 7.** *Cont*.

**Figure 7.** Nyquist plots resulting from IS measurements (frequency sweep in the range 20Hz–1MHz), for the kapton substrate (**a**) and for the coated substrates: (**b**) parylene-C, (**c**) PDMS, (**d**) PMMA, and (**e**) PDMS-PVDF. In each plot, the curves correspond to different periods of exposure to seawater, i.e., 0, 1, 5, 12 days. (**f**) The equivalent circuit model used to fit IS data: Rs is the resistance of wires and electronic parts; Rc and Cc are the resistance and capacitance of the coating. (**g**,**h**) Nyquist plots for the coated substrates, at 0 (**g**) and 12 (**h**) days of exposure to seawater. (**i**,**l**) Bode plots for the coated substrates, at 0 (**i**) and 12 (**l**) days of exposure to seawater.

The barrier behavior of the coatings is evident when comparing them with the uncoated substrate. As can be seen from the Nyquist plots, the curves in the latter case become almost linear after 5 and 12 days of exposure to seawater, whereas for all the other samples the curves keep a quasi-circular shape. In other words, with the passing of submersion time, the reactance of the system gets higher because of the absorption of water molecules with high dielectric constant, but to a much lower extent when the substrate is coated. The absorbed water molecules eventually dissociated, and ions are responsible of a change also in the real part with exposure time. The behavior of electrical resistance and reactance of the coating/substrate systems is a result of different phenomena: (1) the water molecules uptake by absorption, (2) the different swelling degree of the coatings, (3) the damage/delamination of the coatings allowing seawater permeation, (4) the dissociation of absorbed water molecules and permeation of solvated ions. The presence of salty water, or dissociated water molecules or solvated ions in the coatings induces a decrease in the electrical resistance, while the absorption of water molecules (with high dielectric constant) and the swelling behavior lead to an increase of the dielectric constant of insulating materials [53–57]. From the analysis of the IS plots it can be deduced that for the selected coatings the latter phenomenon is prevalent. Bode plots reported in Figure 7i,l highlight the frequency behavior of the impedance and phase angle for the tested substrates. The general trend is a decrease in impedance and an increase in phase with increasing frequency due to a more dominant capacitive behavior. In summary, from the Nyquist plots (Figure 7g,h) and the Bode plots (Figure 7i,l), at 0 and 12 days of exposure to seawater, it can be deduced that the protective action of the different coatings is quite comparable, and a further quantitative analysis was necessary: in Table 1, the coating resistance (Rc) and capacitance (Cc) are reported as results of the fitting of IS data in ZView®software, for which the equivalent circuit model in Figure 7f was adopted for the coating/substrate system: generally, the fitted data match the experimental, with an average error of 5.0%, and higher resistances or lower capacitances indicate a more protective ability of the coatings; after 12 days of exposure to seawater, parylene-C shows the best behavior.

**Table 1.** Impedance parameters resulting from fitting the IS data for the uncoated and coated kapton substrates (1.5 cm diameter).


## *3.3. Corrosion Tests*

The corrosion tests on the steel samples with the selected coatings yielded the current–voltage curves (Tafel plots) in the Evans diagrams, as reported in Figure 8, which provided more details about the anti-corrosive properties of the coatings: the less porosities and better conformal coverage, the more effective the coating.

Figure 8 indicates that the current passing through the seawater solution, (when the WE is pristine steel), is higher whereas it is much lower when steel is coated. As matter of fact, a coating is more effective if the allowed current level is lower.

As illustrated in Figure 8f, the reason for a non-zero current passing through the coating regards the presence of defects across its thickness: these defects, which include porosities, delamination and cracks, allows the direct contact of seawater with the underlying steel, closing the circuit.

The sparks on the anodic polarization curves are due to noisy current fluctuations which are unavoidable and cannot be eliminated [58]. Since the medium in the electrochemical three-electrode cell consists of seawater (basically water and NaCl) with pH slightly above 8, steels show passive behavior but Cl− ions continuously disrupt the passivation film which tries to re-form. Another mechanism could involve the formation of OH\* radicals at the metal surface, which attack the polymer leading to deterioration [35]. Therefore, the alternating process of localized formation and breaking of the passive film results in current fluctuations in the Tafel plots: this behavior is more evident for uncoated steel samples whose curves are overall quite noisy. In addition, the sparks at the edge of the curves (i.e., in potentials far from the corrosion potential), even for coated samples, may be caused by bubbling of gases produced by electrolysis of water.

The Tafel plots for the studied coatings were overlapped and compared for the same period of time, i.e., at 0 and 30 days of submersion, as shown in Figure 8g,h. It is worth noticing that:


Electrochemical kinetic values (corrosion potential Ecorr and corrosion current density jcorr) were calculated from the intersection of coordinates of Tafel plot [59] and are presented in Table 2. The protection efficiency (ηj%) of the coatings are also reported according to calculations made using the following formula [60,61]:

$$\text{tr}\_{\text{l}}\text{\textsuperscript{\text{\textquotesingle}}}\text{\textsuperscript{\text{\textquotesingle}}} = [1 - (\text{j}\_{\text{corr}}/\text{j}^{0}\text{\textsuperscript{\text{\textquotesingle}}})] \times 100\tag{1}$$

where j0corr and jcorr are the corrosion current densities in the absence and presence of coatings, respectively.

**Figure 8.** Evan diagrams displaying the Tafel plots resulting from corrosion tests for the pristine steel samples (**a**) and for the coated samples: (**b**) PDMS, (**c**) PMMA, (**d**) PDMS-PVDF, (**e**) parylene-C. The curves in each plot correspond to different periods of submersion, i.e., 0, 7, 14, and 30 days. The *y*-axis in the plots is in logarithmic scale. (**f**) Corrosion mechanism of steel samples due to seawater absorption and permeation. (**g**,**h**) Tafel plots for the differently coated steel samples, at 0 (**g**) and 30 (**h**) days of exposure to seawater.


**Table 2.** Electrochemical parameters and the corresponding inhibition efficiency for steel coated with the selected coatings.

The time variation of corrosion potential and current density during submersion is shown in Figure 9 for the pristine steel and the coated samples.

**Figure 9.** Corrosion potential (**a**) and corrosion current density (**b**) vs. submersion time, for the pristine steel samples and the selected coatings.

From these results it is evident that initially the protection efficiency increases in the following order: PMMA < PDMS < PDMS-PVDF < pC, whereas, for long-term periods: PMMA < PDMS-PVDF < PDMS < pC.

#### *3.4. Piezoelectric Generation in the Long-Term Period*

The measurements of the open circuit voltage generated by the submerged devices yielded the plot reported in Figure 10a, as percentage decrease of the output signal. All curves are characterized by a plateau indicating that the piezoelectric performance remains constant in continuous use applications, even if with remarkable differences among the coatings. These results confirm those obtained in corrosion tests, i.e., a complete loss of signal is observed after 7 days with PMMA coating, exhibiting the worst performances (exfoliation can be observed in Figure 10b). PDMS and PDMS/PVDF coatings are substantially equivalent showing a stable voltage loss of 70%, while a voltage loss of less than 20% is observed when pC is used. These results provide a direct correlation among device performance and mass transport properties of the coatings.

**Figure 10.** (**a**) Decrease (%) in output voltage of piezoelectric PVDF-based transducers due to prolonged exposure to seawater. (**b**) Exfoliation of PMMA coatings after exposure to seawater.

#### *3.5. Observation of Microbial Adhesion on Pristine and Treated Parylene*

This study was also focused on evaluating the possibility of modifying the surface properties of parylene-C thin films in order to use them as protecting materials against marine biofouling in electronic devices. Both the treatments adopted on the polymeric surfaces bring about two kinds of effects due to the simultaneous occurring of different processes [62]: (1) a physical effect, strictly related to the etching and sputtering of the polymer owing to the reactions of oxygen atoms with the surface carbon atoms and to the impingement of plasma ions onto the surface, leading to a change in morphology; (2) a chemical effect, which is provided through the incorporation of hydrophilic and oxygen-rich functional groups on the surface, and through the breakage of organic bonds promoted by the UV radiation emitted by the plasma or by the UV source. Plasma treatments, as well as UV/ozone, are usually used for cleaning, surface activation, deposition and etching, and are typically employed for modifying the chemical and physical surface properties of polymers [63].

Seawater contact angle (WCA) measurements, as shown in Figure 11 (compared to Figure 6), confirmed that the oxygen plasma surface treatment increases the hydrophilic character of the sample surface with respect to non-treated surface (94.7◦ ± 2.8◦), and this also occurs to a larger extent when a higher generator power is set: in fact the recorded WCA is 11.3◦ ± 1.4◦ for 100 W, and 3.8◦ ± 0.6◦ for 300 W. On the other hand, the UV/ozone treatment, performed subsequently to the oxygen plasma treatment, counter-intuitively results in a slight decrease in hydrophilicity (maintaining however a hydrophilic character, with WCA values of 13.4◦ ± 1.2◦ and 19.9◦ ± 0.6◦ for 100 W and 300 W powers of oxygen plasma treatment, respectively), revealing a counteraction between the two treatments.

The wettability properties are strictly correlated to the surface roughness (Rms) measured by AFM and reported in Figure 11. The recorded roughness of non-treated samples is 6.17 nm, whereas the oxygen plasma treatment increases the roughness, with the formation of thin threadlike structures, at a higher extent for lower powers (15.82 nm for 100 W, 10.84 nm for 300 W), due to the stronger physical etching provoked by plasma, with a resulting higher etching rate.

The blurry aspect of AFM images of samples treated with oxygen plasma and UV/ozone can be correlated to the chemical modification caused by the UV/ozone treatment. This process increases the amount of oxygen-containing reactive species on the surface, which are very likely to electrostatically interact with the surrounding water vapor molecules. Furthermore, the UV/ozone treatment causes a reduction of roughness (6.00 nm and 5.39 nm for 100 W and 300 W of oxygen plasma treatment, respectively) with less sharp asperities. This result can be explained by a more homogeneous levelling action than that of oxygen plasma. The decrease in roughness is also consistent with the decrease in hydrophilicity, according to Wenzel's wettability model [64].

**Figure 11. Seawater contact angle** (WCA) measurements and AFM topography micrographs for non-submerged samples: non-treated (**a**), treated with oxygen plasma at 100 W (**b**) and 300 W (**c**), treated with oxygen plasma at 100 W, 300 W and UV/ozone (**d,e**). The scanned area for all AFM images was 5 × 5 μm2. (**f**) Seawater contact angle, WCA, and (**g**) roughness vs. submersion time: each curve corresponds to one specific treatment; each point in the roughness plot is the result of averaging on the scanned area.

During submersion the non-treated sample became more hydrophilic (54.8◦ ± 7.3◦ at 35 days) and the others became more hydrophobic (in the range between 57◦ and 72◦). However, the final values of water contact angle were quite similar, as shown in Figure 11f. This is an indirect confirmation that the long-term growth of biofilm induces a common wetting behavior. AFM results for the roughness evolution of the submerged samples are plotted in Figure 11g. As can be seen, microorganisms colonize randomly the devices surface, producing extracellular matrix, thus the roughness values provide information on compactness, homogeneity and regularity of the biofilm. After an initial increase in roughness (up to 25 days), the biofilm becomes more and more populated and composed of microbial aggregates rather than isolated cells, whose effect is a self-levelling action, with a reduction in roughness (after 30 days) [65]. The only exception is given by the non-treated sample, allegedly because the microorganisms attach on the surface but, with the passing of time and the progressive scarcity of nutrients, their bioadhesive strength weakens. Since there are much less asperities than for the other samples, they are washed off more easily, leaving a much more non-homogeneous (thus rougher) surface after 30 days.

AFM and SEM micrographs in Figure 12 highlight the presence of microorganisms on the sample surface, in particular a large number of bacteria (with dimensions in the range of 1 ÷ 2 μm), and diatoms Bacillariophyceae (6 ÷ 10 μm).

**Figure 12.** (**a**) AFM image with bacteria (~1 μm). (**b**) SEM micrograph with diatoms (~10 μm).

Finally, as a further proof of the microbial adhesion, LDV analyses highlighted the change of the resonance frequency of the submerged samples with respect to the non-submerged ones (Figure 13a), caused by the growth of biofilm on their surfaces. Finite element method (FEM) simulations (Eigenfrequency analysis, COMSOL multiphysics) were needed to derive the mass surface density of the grown biofilm from the experimental data on resonance frequencies. A simple 3D model was built on (a bi-layer pC/kapton substrate with a variable added biofilm mass on the top of it; see the inset in Figure 13a), showing that the experimental frequencies are in good agreemen<sup>t</sup> with the computational values allowing to derive and calculate the biofouling mass added on the sample surfaces: Table 3 summarizes the experimental and computational resonance frequencies, with the corresponding biofilm added mass, for the differently treated samples.

**Figure 13.** (**a**) Shift in resonance frequency due to the growth of biofilm; the inset shows the model used for the simulations. (**b**) Biofilm surface density vs. submersion days: each curve corresponds to a specific surface treatment.


**Table 3.** Results of finite element method (FEM) simulations.

At longer times, a common general decrease of the surface mass density, associated with an increase of resonance frequency, was detected on non-treated and treated substrates due to the degradation of biofilm and the death of microorganisms (Figure 13b).
