## 2 Materials and Methods
### Microwave Design, Operation, and Fabrication
A rigid metal body comprises two fluid ports and concentric trenches, creating the valve seat--both the width and depth of each trench amount to 100 \(\upmu\)m. The diameter of the innermost trench is 2.5 mm, and all 6 trenches are evenly spaced concentrically with a distance of 150 \(\upmu\)m from  
Figure 2: (**a**) Model of the NO valve body incorporating two fluid ports and micromachined trenches. (**b**) Light microscope picture of the micromachined valve steel body. (**c**) Model of the valve seat coated with Parylene-C. (**d**) Photograph of an assembled microvalve after laser welding and piezoelectric actuator bonding next to a Euro coin. The microvalve has a diameter of 20 mm and a height of 2.6 mm. each other. The NO microvalve is manufactured from stainless steel, which offers high resistance to plastic deformation, high machinability, and biocompatible properties [28]. Structuring of the baseplate (see Figure 2b), including valve seat trenches, is achieved by high precision milling (Kern Evo, Kern Microtechnik GmbH), resulting in valve seat trench depths of (101.8 \(\pm\) 3) \(\upmu\)m. A metal actuator foil, etched from a cold-rolled stainless steel sheet material, is joined to the valve body by laser welding using a fiber laser (1070 nm wavelength ytterbium fiber laser YLR-1000 SM, IPG Laser GmbH). We ensure hermetic sealing of the valve chamber and high strength of the weld seam by firm clamping of the foil onto the baseplate and an overflow of the welding area with shielding gas (Argon 4.6) at room temperature. During laser welding, the actuator foil experiences a temperature gradient from the welding fusion zone to zones not affected by heat, and after the molten steel solidifies, residual stresses remain in the actuator foil. These residual stresses ultimately result in an initial deflection of the actuator foil in an upwards direction and establish the NO state of the microvalve. After laser welding, the piezoelectric bending actuator is created by gluing a lead zirconate titanate (PZT) disc actuator onto the metal actuator diaphragm using a two-component epoxy glue (EPO-TEK 353 ND). We investigate three variants of the microvalve with micromachined trenches. A basic design comprises a 0.1 mm thick steel diaphragm as well as a 0.2 mm thick PZT actuator and is fabricated in a quantity of 10. In the coated design, the effect of a 15 \(\upmu\)m Parylene-C coating on the valve seat (see Figure 2c) is investigated with another 10 manufactured valves with the same bending actuator as the basic design. In a third high force design, 5 microvalves without coating on the valve seat are fabricated with a 0.15 mm thick steel diaphragm and a 0.3 mm thick PZT actuator in order to increase the contact pressure exerted to the valve seat by the actuator. Table 1 provides an overview of all three manufactured design variants of the NO microvalve. For realizing the coating of the valve seat, we use chemical vapor deposition of Parylene-C. To this end, the steel bodies are masked using a structured semiconductor wafer processing tape (Nitto SWT 10+R), limiting the coated surface to the trench area of the microcvalve. Coating via Gorham route [29] is achieved in a plasma coating system (Plasma Parylene LC 300 RW, Plasma Parylene Systems GmbH). Under vacuum conditions, the substrates are pretreated with Silane A-174 for in-situ silanization from the gas phase at room temperature. Subsequently, 25 g of Parylene-C dimer is sublimed at 130 \({}^{\circ}\)C followed by pyrolysis into reactive monomers at 740 \({}^{\circ}\)C, and final polymerization takes place in the coating chamber at room temperature. A coating process duration of approximately 4 h results in a (15 \(\pm\) 2.5) \(\upmu\)m thickness of the coating layer. Removal of the mask is enabled by laser ablation using an infrared CO\({}_{2}\) laser (10.6 \(\upmu\)m wavelength Speedy 360, TROTEC Laser GmbH), since mechanical stripping alone causes delamination of the coating.