*2.1. Materials*

Parylene-C was provided by Specialty Coating Systems in form of dimer powders. PDMS (Sylgard 184 Silicone Elastomer) was supplied by Dow Corning Corporation in two compounds: a viscous uncured pre-polymer and a curing agent. PMMA 950 in Anisole e-beam resist (anisole 80–100%, PMMA 1–20%) was purchased from MicroChem Corp. Granular PVDF powder (average Mw ~ 534,000 by GPC) and 2-butanone (MEK) solvent were supplied by Sigma Aldrich.

The piezoelectric devices employed in the present work were grouped into two categories: (i) AlN-based, and (ii) PVDF-based seaweed-shape transducers. The AlN flags were made using kapton HN (by Dupont in the form of 25 μm-thick foils) as substrate for thin film deposition. The commercial PVDF foils of the second groups of harvesters were provided by TE Connectivity's Measurement Specialties (MEAS).

AISI304 steel samples were selected as substrates for corrosion tests and were provided by RS Components.

#### *2.2. Fabrication of Flexible Transducers*

The fabrication of the adopted transducers is reported, as for previous works [15,16].

## 2.2.1. AlN-Based Transducers

The kapton substrate was attached to a silicon wafer using polydimethyl siloxane (PDMS) spin-coated at 1000 rpm for 30 s and then cured at 90 ◦C for 15 min. A vacuum step was needed before curing to remove any residual air bubbles. The overlaying layers were deposited by reactive sputtering in a single run in order to minimize contaminations, with the following parameters: an AlN oriented growth-favoring interlayer (120 nm) and the first Mo layer (200 nm) were deposited in a single step and patterned by optical lithography and chemical etching. The AlN interlayer stemmed from a high-purity (99.9995%) Al target in a mixture of Ar (20 sccm) and N2 (20 sccm) gases with a total pressure of 2.8 × 10−<sup>3</sup> mbar and with DC pulsed power supply of 750 W. The Mo electrode was deposited from a pure (99.95%) Mo target at room temperature, with a total pressure of 5 × 10−<sup>3</sup> mbar in an Ar atmosphere (66 sccm) and with DC power of 400 W. The pattering of Mo bottom electrode and of AlN interlayer was performed by dry etching with inductively-coupled plasma-reactive ion etching (ICP-RIE) system: the gas mixture was made of BCl3 (45 sccm) and N2 (25 sccm) for Mo, and of BCl3 (100 sccm) and Ar (25 sccm) for the AlN. The applied power was 250 W to the platen and 600 W to the coil. The AlN piezoelectric layer (~1μm) and the second top Mo electrode (200 nm) were deposited in the same run: for the AlN film a high-purity Al target (99.9995%) was used with a gas mixture of N2 (20 sccm) and Ar (20 sccm) at a pressure of 2.8 × 10−<sup>3</sup> mbar in DC pulsed mode with a frequency of 100 kHz and a power of 1000 W; for the Mo layer the same conditions as for the bottom electrode were applied. The chemical dry etching was performed by the ICP-RIE system under the same conditions as before. The chamber temperature during sputtering increased to ~70 ◦C and to ~165 ◦C for the Mo and AlN steps, respectively. Finally, the polymeric substrate was peeled o ff from the rigid wafer and the wire connections to the electrodes were made with a crimping tool.

## 2.2.2. PVDF-Based Transducers

The PVDF foil was first cleaned with acetone and isopropanol before depositing the aluminum thin-film electrodes (200 nm) by thermal evaporation. The electrodes were patterned by using shadow masks made of adhesive kapton tape, whose shape was designed properly. Thermal evaporation allows to deposit the electrodes at a temperature of ~60 ◦C, which is lower than the Curie temperature of PVDF (~100–110 ◦C) and it doesn't a ffect the piezoelectric feature of the functional layer. Finally, the wire connections were made with a crimping tool.

#### *2.3. Preparation of Coatings*

The methods for preparing the coatings are also reported in [52]. Moreover, in this work, the temperature was kept inside the range allowed by the device materials.

PDMS coating was prepared by mixing the pre-polymer with the cross-linker (10:1 wt), degassed for 30 min and applied on the whole devices by dip-coating. Finally, PDMS was cured in oven at 90 ◦C for 15 min.

PMMA-based coating was prepared by dip-coating the devices in the PMMA solution, then they were suspended in a heated oven at 90 ◦C for 1h for solvent evaporation.

Coatings made of blends of PDMS and PVDF were obtained first mixing PDMS uncured pre-polymer with PVDF in a 3:1 (wt) ratio, then the mixture was heated in oven at 200 ◦C, above the PVDF melting point (177 ◦C), and stirred every 5 min. After cooling down at room temperature, the blend was mixed adding the curing agen<sup>t</sup> (weight ratio 10:1 with respect to the PDMS). Finally, it was applied onto the devices by dip-coating and cured in oven at 90 ◦C for 15 min.

Parylene-C deposition process was performed by a RT-CVD equipment (Specialty Coating Systems, PDS 2010 Labcoater system model). The powdered dimer vaporized at a temperature of ~100–150 ◦C and at a pressure of 1 torr to undergo a pyrolysis and be reduced in monomers; then, the polymerization of the gaseous monomers occurred at ~650–700 ◦C and 0.5 torr; the gas entered the deposition chamber at 20–25 ◦C and 0.1 torr and a conformal polymeric coating deposits on the substrate. An amount of 1 g of dimer powder yielded a deposited layer with thickness of 1μm and the process lasted approximately one hour.

#### *2.4. Characterization and Reliability Tests*

With the exception of parylene, the other coatings were applied on the devices by dip-coating so the control of thickness was subjected to uncertainties and non-uniformity. Di fferent portions of the device were selected and the coating thickness was measured by means of a profilometer (Bruker Dektak Xt).
