# Piezoelectric Silicon Micropump for Drug Delivery Applications
## 3 Results and Discussion
### Standard Characterization  
The characteristics and fluidic performance of the ten micropumps used for the evaluation of the dosing precision are summarized in Table 1.  
The first indication for the functionality of a diaphragm pump is the achievable actuator stroke, since it directly determines the volume displaced within one pump cycle and, hence, achievable flow rates. Furthermore, the optical actuator stroke measurement allows to detect a touchdown, i.e., the mechanical blocking of further movement when the diaphragm reaches the chamber bottom. It causes a characteristic kink in the displacement curve and limits the stroke volume. None of the tested pumps shows a touchdown. The average stroke (\(\Delta z\)) of all ten microactuators achieved with \(-20\) to 100 V quasi-static actuation is \(\Delta z=15.0\) um \(\pm\) 1.1 um leading to an approximate stroke volume (\(\Delta V~{}=~{}\pi\cdot R^{2}/3\cdot\Delta z\)) of \(\Delta V=79\) nL. This volume corresponds well to the characterization of similar pumps by [PERSON] et al. [27]. All pumps are characterized regarding their air transport, e.g., backpressure capability as well as frequency dependent air flow rate. The air flow achieved with an actuation frequency of 65 Hz and a peak to peak voltage of 120 V, which is the signal used for later bolus dosing, is 0.26 mL/min \(\pm\) 0.07 mL/min. With an actuation optimized for air transport (\(-50/100\) V; 1.6 kHz), the pumps achieve higher flow rates of 6.5 mL/min \(\pm\) 0.5 mL/min. The pressure capability when transporting air depends strongly on the compression ratio, which is approximately 0.15 for the 5 \(\times\) 5 silicon pump and is sufficient for gas transport [14; 30]. Sufficient backpressure capability with air is a condition for bubble tolerance and therefore evaluated with the same actuation signal used for fluid transport, even though this actuation is not optimized for air transport. From a linear fit of the pressure dependent mean air flow of all samples we calculate a backpressure capability of 14.3 kPa \(\pm\) 0.3 kPa (Figure 4). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Fluid** & **Characteristics** & **Actuation** & **Mean Value** \\ \hline \multirow{2}{*}{-} & Actuator stroke & \(-20\)/+100 V; & \multirow{2}{*}{15.0 \(\pm\) 1.1 μm} \\  & & quasi-static & \\ \hline \multirow{2}{*}{Air} & \multirow{2}{*}{Flow rate} & \(-50\)/100 V; 1.6 kHz & \multirow{2}{*}{6.5 \(\pm\) 0.5 min} \\  & & \(-30\)/90 V; 65 Hz & 0.26 \(\pm\) 0.07 min \\ \hline \multirow{2}{*}{Air} & \multirow{2}{*}{Backpressure} & \(-30\)/90 V; 65 Hz & \multirow{2}{*}{14.3 \(\pm\) 0.3 kPa 1  \\ \hline DI water & Flow rate at 0 kPa & \(-30\)/90 V; 15 Hz & 0.074 \(\pm\) 0.006 min \\ \hline DI water & Blocking pressure & \(-30\)/90 V; 15 Hz & 51.2 \(\pm\) 0.9 kPa 1  \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Average mechanical and fluidic characteristics of the ten evaluated piezoelectric silicon micropumps at a given actuation signal (amplitude; frequency). The water flow rate with and without applied backpressure is analyzed for all pumps (Figure 4). Without additional backpressure, the mean water flow rate of the ten pumps is Qwater0 kPa = 74 uL/min \(\pm\) 6 uL/min. With U100 insulin, this allows to deliver a bolus of 10 U in approximately 90 s and puts our micropump in the same range of delivery speed as currently available delivery systems [24]. A linear regression of the water flow rate at increasing backpressures allows to determine the blocking pressure, where the flow reaches zero. At 40 kPa the average achieved flow rate is Qwater40 kPa = 16 uL/min \(\pm\) 7 uL/min and the linear regression shows an extrapolated blocking pressure of \(p_{\text{block}}\) = 51.2 kPa \(\pm\) 0.9 kPa. These results are in good agreement with previous research [27]. The high achievable pressure is necessary to overcome counterforces in a delivery system.