**1. Introduction**

One of the most crucial engineering issues regarding the application of micro and nanoscale electronic devices in liquid environments is the need of being protected from the external harsh surroundings. This is mainly due to the risk of short-circuit caused by the absorption and permeation of water, which is generally also responsible of degradation of devices made of layered functional structures [1–3]. In this respect, delamination is one of the major modes of failure of organic and inorganic layered systems and consists of the weakening or loss of adherence between the different layers, resulting from mechanical strain mismatches or electrochemical reactions at the interfaces [4]. Moreover, the contact between water (especially salty water when dealing with marine applications) and the metal parts inside the micro-systems leads in most cases to spread or localized corrosion of electrodes and wires, degrading the electronic conduction and the device intrinsic performance, as a consequence [5–7]. In addition, an unavoidable phenomenon related to the submersion of devices underwater, is the accumulation of micro-organisms, such as bacteria or unicellular algae, on their surface, namely the biofouling, which is characterized by a gradual and persistent formation of slimy biofilms [8–14].

For applications in aqueous environments, electronic micro-systems must therefore be protected with watertight electrically insulating coatings. Nowadays, the miniaturization of devices, the discovery of new lightweight materials as substrates and the adoption of advanced microfabrication techniques, have led to the design and fabrication of novel flexible devices of cm-size with components of the order of 0.1 mm. They are generally based on stacking sequences of several functional thin-film layers. In this work we focused in particular on the reliability of micro-transducers for harvesting mechanical energy from fluid flows. Two sample categories were selected. The first one embodies transducers made of kapton as substrate, covered by a piezoelectric stack deposited by reactive sputtering and patterned by lithography and etching techniques. The expanded view of the layered structure is reported in Figure 1: here, the active region consists of an aluminum nitride (AlN) piezoelectric layer sandwiched between two thin molybdenum (Mo) electrodes.

**Figure 1.** Photo and expanded view of the layered structure of the first group of piezoelectric micro-devices employed for energy harvesting in fluid environments. The thin films were deposited by reactive sputtering, as described in [15,16].

The second sample group includes unimorph devices with a simpler structure made of a poly-(vinylidene fluoride), (PVDF), bi-axially oriented piezoelectric foil sandwiched between two thin-film aluminum (Al) electrodes, as illustrated in Figure 2.

**Figure 2.** Photo and schematic of the second group of micro-devices. The thin electrodes were deposited by thermal evaporation using shadow masks.

These devices are aimed at undergoing low-frequency fluid-induced oscillations in a single-cantilever-beam configuration. The purpose is to scavenge energy which would be otherwise lost (harvesters) or to sense water motions (sensors), thus they should continuously work for long-lasting

periods without decrease in performances [17]. As an example, Figure 3a,b report scanning electron microscope (SEM) images of some defects generated during a long working period of an AlN-based device at the points of crimped regions, whereas Figure 3c shows the general decreasing in output signal after the sudden exposure to seawater: it is worth noticing that after removing and drying the devices, their performances come back unaltered although the presence of defects, such as delaminations grown in short-testing period, was found.

**Figure 3.** Scanning electron microscope (SEM) images showing crack growth before (**a**) and after (**b**) long-term utilization in correspondence of local defect points, i.e., the areas where the electrical crimp terminals are inserted. The output signals (peak-to-peak voltage) show a decrease in performance due to crack propagation; scale bars: 1.0 s (horizontal), 200 mV (vertical). In short-lasting periods, i.e., after submersion, the decreasing signal amplitude is due to sudden defects, such as delaminations (**c**).

Due to continuous oscillatory movement of the flexible devices, some cracks, growing from surface defects, propagate inside the film and they reach eventually the active region of the device, therefore affecting its performance. Figure 4 shows SEM images of a parylene-coated kapton substrate after a bending deformation. The surface crack may be clearly observed and, although it does not go deeper inside up to the substrate, it could propagate during the submersion period due to fatigue damage, as shown in the cross section inset achieved by focused ion beam (FIB).

**Figure 4.** SEM micrograph: flexible kapton substrate coated with thin-film parylene after oscillatory movements in long-term period. The inset shows a cross section made by focused ion beam (FIB) in correspondence of the surface crack. The yellow circle indicates that the crack does not go deep inside up to the substrate.

In this respect, the external coatings must have specific characteristics in order to protect the devices without a ffecting their flexibility: they should be conformal, lightweight, not fragile, non-porous, insulating and anti-biofouling [18–22].

Materials employed for insulating coatings of microscale electronic devices are mostly polymeric. The most frequently used materials are: (1) elastomers, (2) polyacrylates, (3) fluoropolymers, (4) poly-para-xylylenes (also known as parylenes).

Elastomers are thermosetting polymers with rubber-like properties, such as high stretchability and softness. The most widely used silicone-based elastomer is poly(dimethyl siloxane), PDMS, which has remarkable rheological, optical and non-toxic properties [23]. Generally, it is formed as a viscous bi-component thermosetting mixture between a siloxane-based oligomer and a curing agent, and applied by spin-coating [24], dip-coating [25] or spray-coating [26]. PDMS could also be incorporated, at di fferent contents, into other polymeric coatings, such as polyurethane (PU), in order to enhance their antibiofouling properties, as reported in [27], or to make new composite materials [28].

Poly-(methyl methacrylate), PMMA, is one of the most common polyacrylates: it is a transparent thermoplastic polymer with noticeable mechanical and optical properties. It is generally used as a lightweight alternative to glass, but also for other various applications, such as inks and coatings, or for microfabrication processes as sacrificial layer. Coan et al [29] reported the development of composite coatings of PMMA with hexagonal boron nitride (hBN) as filler, for metal surface protection against corrosion.

Fluoropolymers contain plenty of carbon-fluorine bonds which confer to the backbone a high chemical inertness against many solvents, acids or bases [30]. Some well-known examples are PVDF, or poly(tetrafluoroethylene), (PTFE). In particular, PVDF can be found commercially as extruded foils, pellets and micrometric powder as well, which can be incorporated in other polymeric matrices or dissolved in solvents.

Poly(-para-xylylenes) are thermoplastic semicrystalline polymers discovered by Michael Szwarc in the late 1940s and commercialized in 1965. These polymers are synthesized by chemical vapor deposition (CVD) and have very attractive properties among which low-adhesion coe fficient at room temperature, and conformability to di fferent types of substrates. The current method adopted to synthesize parylene is called the Gorham route and is a very e fficient polymerization process, in fact it allows complete control of the deposition parameters: the process basically consists of pyrolizing the precursor dimer and polymerizing the resulting monomers during deposition onto the substrate [31–33]. Several di fferent kinds of parylene may be synthesized, depending on the functional groups bonded to the backbone of the precursor (2,2-para-cyclophane): these substituents are not modified during the CVD process, making it possible to tailor chemical, mechanical, electrical and optical properties of parylene thin films and, therefore, to introduce diverse functionalities into the coated surfaces [31]. The range of application fields for parylene is wide: in particular, its insulating and moisture barrier properties make it suitable for protecting implanted biomedical micro-systems or devices in contact with water or wet environments [34–36].

Several works have been published about external polymeric coatings for insulation and barrier purposes [37–39]. Lewis and Weaver provide in [40] a review of thin-film permeation-barrier technologies for flexible organic light-emitting devices. Fredj et al. [41] studied the natural and artificial ageing of marine organic coatings. Deyab et al. [42] prepared a wax coating using waste materials (isolated microcrystalline waxes) to protect petroleum pipelines against corrosion in 0.6 M NaCl solution. The same author analyzed the effect of carbon nanotubes (CNTs) [43], newly synthesized titanium phosphates [44] or M-porphyrins [45] on corrosion protection of carbon steel coated by alkyd resin and tested after immersion in sodium chloride solution. Li et al. [46] performed accelerated soak tests to study the corrosion behavior and failure mechanisms of parylene-metal-parylene thin films. Davies and Evrard [47] studied polyurethanes for marine applications through accelerated tests at high temperatures.

However, the application of protective coatings onto flexible micro-devices continuously moving underwater is a tricky issue, thus finding an optimal solution for guaranteeing at the same time (1) protection from the environment and (2) the best device performances is still an ongoing challenge.

In this work, we have investigated the barrier behavior and surface properties of different polymeric coatings of the flexible piezoelectric transducers described above: parylene-C, poly-methyl methacrylate (PMMA) and poly-dimethyl siloxane (PDMS). These choices were made for the ease of applicability, conformability, low cost and compatibility with the flexible piezoelectric energy harvesters, since the external layers is crucial for the reliability of the device itself. The PDMS coating was used both in the neat form and mixed with a powder of PVDF. The combination of the conformability of the elastomer and the hydrophobicity and chemical inertness of the fluoropolymer was expected to confer higher water repellence to the coating, limiting water permeation in the device [48,49].

Furthermore, parylene was also designated as a suitable surface platform for evaluating the accumulation of microorganisms in a long-term period. Since previous articles in literature reported the surface functionalization of parylene by physico/chemical treatments [19,50,51], besides the pristine parylene-C (pC) coating deposited by CVD, surface-treated pC was taken into account: in particular, two surface treatments were adopted based on oxygen plasma etching and UV/ozonization.

The seawater absorption of coatings was analyzed by impedance spectroscopy (IS) measurements. The anti-corrosion properties of the coatings were tested by dynamic linear scanning voltammetry (LSV) measurements. Additionally, atomic force microscopy (AFM) measurements provided further information in terms of surface morphology. The barrier properties of the coatings and their reliability were therefore correlated to the higher or lower capability of retaining the formed biofilms: this evaluation was possible by estimating the amount of added microbial mass on the exposed surface through a laser Doppler vibrometer (LDV).

#### **2. Materials and Methods**
