*Article* **Reliability of Protective Coatings for Flexible Piezoelectric Transducers in Aqueous Environments**

**Massimo Mariello 1,2,\*, Francesco Guido 1, Vincenzo Mariano Mastronardi 1, Roberto Giannuzzi 1,**†**, Luciana Algieri 3, Antonio Qualteri 1, Alfonso Ma**ff**ezzoli 2 and Massimo De Vittorio 1,2**


Received: 11 October 2019; Accepted: 29 October 2019; Published: 31 October 2019

**Abstract:** Electronic devices used for marine applications suffer from several issues that can compromise their performance. In particular, water absorption and permeation can lead to the corrosion of metal parts or short-circuits. The added mass due to the absorbed water affects the inertia and durability of the devices, especially for flexible and very thin micro-systems. Furthermore, the employment of such delicate devices underwater is unavoidably subjected to the adhesion of microorganisms and formation of biofilms that limit their reliability. Thus, the demand of waterproofing solutions has increased in recent years, focusing on more conformal, flexible and insulating coatings. This work introduces an evaluation of different polymeric coatings (parylene-C, poly-dimethyl siloxane (PDMS), poly-methyl methacrylate (PMMA), and poly-(vinylidene fluoride) (PVDF)) aimed at increasing the reliability of piezoelectric flexible microdevices used for sensing water motions or for scavenging wave energy. Absorption and corrosion tests showed that Parylene-C, while susceptible to micro-cracking during prolonged oscillating cycles, exhibits the best anti-corrosive behavior. Parylene-C was then treated with oxygen plasma and UV/ozone for modifying the surface morphology in order to evaluate the biofilm formation with different surface conditions. A preliminary characterization through a laser Doppler vibrometer allowed us to detect a reduction in the biofilm mass surface density after 35 days of exposure to seawater.

**Keywords:** waterproof; coating; reliability; flexible micro-devices; piezoelectric transducers; aqueous environments; seawater
