**4. Conclusions**

In this study protective, barrier and anti-corrosive properties of different polymeric coatings for applications in marine environments, were compared. The final specific use of these thin films consists of employing them as conformal coatings for insulating flexible micro-devices adopted for harvesting mechanical energy from fluid-induced oscillations.

The desired application poses several challenges and issues to be addressed, i.e., the risk of short-circuit, the corrosion of metal electrodes, the water absorption and permeation into the devices leading to delamination or damages, the adhesion of microorganisms on the device surfaces. Therefore, the selection of the coatings to be tested was made in order to satisfy several requirements, such as flexibility, conformability, ease of application, insulating and barrier properties.

IS analyses, corrosion tests and the measurements of the piezoelectric signal for coated submerged devices revealed that parylene-C provides the best protecting, insulating and adhesion properties among the other coatings, even though it is susceptible to surface crack formation more than elastomers (PDMS-based coatings), because of its thermoplastic semi-crystalline character. Definitely, PMMA is the worst alternative mainly because it easily undergoes exfoliation during exposure to seawater.

Therefore, parylene was also used to observe qualitatively the accumulation of microorganisms and formation of biofilm on the surface of submerged devices: different surface treatments were adopted as well to modify the morphology and wettability of parylene in order to correlate microbial adhesion changes in the long-term period. The surface treatments selected for this purpose comprised an oxygen plasma treatment and a UV/ozone treatment. The reduction in the surface mass density of the grown biofilm, for both treated and non-treated substrates, revealed a loss of adherence for the microorganisms which is ascribed to the local scarcity of nutrients and to the protective action of parylene-C thin film: thus, together with its strong chemical and moisture resistance, this further property of parylene-C makes it more attractive as a conformal passivation coating on polymeric substrate materials and flexible micro-devices.

**Author Contributions:** M.M. conceived and performed the experiments; F.G., V.M.M. and A.Q. contributed for observation of surface defects; R.G. contributed for corrosion tests; L.A. contributed for AFM imaging of adhered

microorganisms; M.M., F.G. and R.G. analyzed the data; F.G., R.G., V.M.M., and A.Q. contributed analysis tools; M.M. wrote the paper; A.M. contributed to mass transport issues, paper writing and supervised the experiments; M.D.V. supervised the experiments.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
