# Piezoelectric Silicon Micropump for Drug Delivery Applications
## 2 Materials and Methods  
All investigations in this work are conducted with Fraunhofer EMFT's 5 \(\times\) 5 \(\times\) 0.6 mm\({}^{3}\) micropumps [27]. Compared to former versions of the pump, the chamber height is increased to 20 \(\upmu\)m and the valve seat circumference as well as flap area are increased. Therewith, the fluidic resistance is reduced, which enables higher flow rates and the transport of higher viscous fluids. A set of ten pumps is used to establish statistical significance. All wetted surfaces are composed of silicon and respective native oxides. The pump consists of two valve layers and the actuator diaphragm with the glued on piezoelectric disc actuator (Figure 1). Fluid transport bases on the indirect piezoelectric effect: an alternating high voltage signal causes the piezoelectric ceramic to expand and contract and thus induces an oscillating upwards and downwards movement of the diaphragm. The resulting expansion and contraction of the pump chamber leads, in combination with the passive flap valves, to a directed fluid flow. Figure 1: Schematic cross-section (**a**) of a 5 \(\times\) 5 \(\times\) 0.6 mm\({}^{3}\) silicon micropump (**b**) with a 4.5 mm pump chamber diameter. The three grey layers represent the silicon pump body with the lower valve unit (I), the upper valve unit (II), and the actuator diaphragm (III) with the glued on piezoceramic (IV). For the fluidic characterization, the pumps (IX) are mounted into a test housing (**c**) that consists of a polyether ether ketone body (V) with drilled fluidic paths (X), inlet and outlet capillaries (VI), a stainless-steel filter (VII), and a soft sealing (VIII). The pumps are manufactured based on standard silicon processes (Figure 2). For the valve structure, two n-doped silicon (100) wafers are covered with a silicon dioxide/silicon nitride hard mask that is subsequently structured by a lithography and dry etching process step (1). After the first KOH wet etching (2), one wafer is flipped and the wafer are bonded to form the valve unit. After oxidation and bonding the valve wafer stack is finalized with a grind and polishing step (3). A third silicon wafer (100) with a pre-etched micropump chamber is bonded on top of the valve wafer stack (4) and a grind and a spin etching process step finalize the actuator diaphragm of the micropump (5). The piezoelectric disc actuator is glued on the actuator diaphragm using a two-component epoxy adhesive (6). Table 1 (Appendix 1) gives an overview of the used material. Bubble tolerance of the pumps is achieved with a specific adhesion process for the piezoelectric actuator [28]. During the curing phase, we apply an electric field that puts the ceramic in its contracted state. Once the glue is hardened, the voltage is disconnected and the ceramic expands and therewith bulges up the pump chamber. Due to this mounting technique, the compression ratio (ratio of displaced volume to dead volume in the chamber), is high, which is a condition for a high backpressure capability with compressive fluids as well as robust bubble tolerance. The compression ratio of the silicon micropump is estimated from its design: due to exact silicon manufacturing techniques, the volume of the pump chamber can be calculated easily. The stroke volume is determined analytically and confirmed during the fluidic characterisation of the pumps. Based on the dead volume and the single stroke volume, we calculate the compression ratio. The power consumption of the pump itself can be estimated considering the piezoelectric actuator as ideal capacitor. The used disc actuators have a capacitance of 2.45 nF. An actuation optimized for liquid transport (120 V\({}_{\text{pp}}\) and 65 Hz) results in approximately 1.2 mW. To achieve maximal air flow, a higher voltage amplitude and a high frequency (150 V\({}_{\text{pp}}\) and 1.6 kHz) are necessary, which requires approximately 440 mW, though an operation with a lower frequency is possible to limit the power consumption.