## 2 Materials and Methods
### Microwave Design, Operation, and Fabrication  
The design of the active diaphragm microvalve is depicted in Figure 1. It consists of a metal valve body (I) with an inlet (II) and an outlet (III), as well as a structured valve seat (IV). A metal diaphragm (V) is spanned over the valve seat with a piezoelectric ceramic layer (VI) bonded on top to create a bending actuator. In the initial state, the microvalve is normally open, as the diaphragm is bent in upward direction due to stresses imprinted by the manufacturing process described later. In non-actuated state, the NO microvalve allows for fluid flow from the inlet to the outlet. Using the inverse piezoelectric effect, the valve opens further when a negative electric field is applied. Under a positive electric field, closing of the valve is achieved when the actuator diaphragm is pressed tightly onto the valve seat. In this closed state, a low leakage rate of the microvalve is crucial for medical applications and can be drastically reduced by optimizing its sealing properties. To this end, we discuss impact parameters on sealing properties and introduce selected measures for low leakage in closed state: a concentric trench design, a high force actuator diaphragm, and additional coating on the valve seat. Figure 1: Working principle of the diaphragm-type piezoelectric NO valve. In non-actuated state, the valve is open and allows for fluid flow. By piezoelectric actuation, a negative electric field opens the valve further, resulting in reduced fluidic resistance, and a high positive field closes the fluid path by exerting a high contact pressure on the valve seat. In principle, tight sealing of a diaphragm-type microvalve is achieved by high contact pressure exerted by the actuator pressing the diaphragm onto a valve seat designed for low leakage. Therefore, both the piezoelectric bending actuator and the geometric design of the valve seat can be adapted to achieve optimal performance. Analogously to microvalves presented in research for low leakage gas applications [22; 23; 24], a valve seat design for low leakage can be realized by a concentric trench design (see Figure 2). In comparison to a flat valve seat, this design exhibits multiple beneficial effects towards leakage reduction. Firstly, a reduction in the contact area of the valve diaphragm and valve seat increases the contact pressure at the interface. A second effect results from increased surface friction of the fluid over the increased valve seat surface, eventually causing lower leakage flow. Finally, the fluid flowing over the valve seat exerts shear stress relative to the confined, resting fluid in the trenches, leading to a cascade of pressure losses across each trench. To further enhance sealing, the contact pressure on the valve seat exerted by the valve diaphragm can be increased with a piezoelectric actuator that exhibits higher forces. For piezoelectric bulk material, this can be achieved by increasing the thickness of the actuator, resulting in a higher force when actuated by the same electric field. Another method of modifying the leakage of two surfaces in contact is to use coatings to enhance their surface properties with a view to increased sealing behavior. A good candidate is Parylene-C, a chemically inert, biocompatible polymer with beneficial mechanical properties frequently used in medical applications [25]. As the [PERSON]'s modulus of Parylene-C is two orders of magnitude smaller than the [PERSON]'s modulus of steel [26; 27], the polymer's elasticity can be used to create a tight sealing in microvalves [22]. In order to evaluate the potential of each of the mentioned design measures and their impact on the performance of the microvalve, we design and manufacture them in different variants with respect to geometrical dimensions, structuring of the valve seat, and additional coating. Figure 2a shows the basic design of the presented NO microvalve.