# Piezoelectric Silicon Micropump for Drug Delivery Applications
## 3 Results and Discussion
### Bolus Precision Investigation
Nevertheless, four out of ten pumps  
Figure 5: Repeatability of package dosing for each individual micropump (\(\mu\)P_01 to \(\mu\)P_10) with water for small (**a**), medium (**b**) and large (**c**) packages of approximately 0.5 mg, 5 mg and 50 mg, respectively. The overview (**d**) of the mean dosed weight and the mean standard deviation of all tested pumps shows an overall high dosing precision. For 0.5 U and 5 U packages, the standard deviation varies between 0.3 and 6.5%, however only two pumps, \(\mu\)P_6 and \(\mu\)P_10, show standard deviations over 5%. The slight decrease in precision for the insulin dosing compared to water dosing is probably due to poorer priming of the fluid path, e.g., fluidic connectors and filter chamber of the test housing. The sample volume was limited and less fluid could passively be pushed through the setup to flush all air from the system. Furthermore, a high surface tension causes the liquid-air interface to be more stable for the insulin solution compared to DI water. Thus, the transport of bubbles through the valves, the pump chamber or fluidic connectors requires more pressure and has an increased impact on the fluid transport. Hence, an optimization of the pump towards higher bubble tolerance and bubble independent dosing is of even higher relevance when changing the dosed medium from DI water to insulin solution. Overall, the experiments show a high dosing precision of the 5 \(\times\) 5 mm\({}^{2}\) silicon pump. For 0.5 U and 5 U, only one pump shows a higher standard deviation than the generally required 5%. This high achieved precision stands out in comparison to insulin delivery units on the market. For example, [PERSON] et al. [10] analyze two commercially available units and show that only 20% of 0.5 U bolus are delivered within \(\pm\)5% accuracy [10]. The  
Figure 6: Repeatability of package dosing for each individual micropump (\(\mu\)P_01 to \(\mu\)P_10) with NovoRapid U100 insulin injection solution for small (**a**), medium (**b**) and large (**c**) packages of approximately 0.05 U (\(\sim\)0.5 mg), 0.5 U (\(\sim\)5 mg) and 5 U (\(\sim\)50 mg), respectively. The mean dosed weight and the mean standard deviation (**d**) are similar to experiments conducted with water. high precision possible with this piezoelectric micro diaphragm pump for 0.5 U (5 mg packages) can therefore enable a dosing unit with higher precision than current insulin delivery systems available on the market. The smallest package size tested in this work is not commonly tested for insulin dosing systems and most studies start their experiments with 0.5 U or 1 U [10,24]. Only [PERSON] et al. [25] consider small bolus sizes of 0.05 U, however, for accurate measurements, they calculate the accuracy from series of ten to twenty doses. The detection of outliers is, therefore, not possible. The analysis of 0.05 U packages shows that most of the tested pumps are able to dose these small units precisely with seven out of ten pumps showing a standard deviation of 1.9 to 3.5%. Only two pumps dose packages with a standard deviation of more than 10%. For those pumps, a time dependent decrease in package size is visible. However, this decrease in performance is not permanent, since the pump regains the expected package size for the consecutive measurement of 0.5 and 5 U packages. The decrease can be caused by insufficient priming: enclosed air in the outlet capillary is transported to the outlet reservoir on the balance and impairs the correct detection of transported volume. Additionally, the introduced silicon pumps are bubble tolerant due to their compression ratio. However, dosing is influenced by air bubbles passing through the pump chamber. That means that the pump is able to transport the liquid-air interfaces, but the dosed package volume decreases until the chamber is filled with liquid. To achieve high accuracy even with bubbles in the system therefore requires active control. Closed-loop control systems can detect undelivered strokes and trigger compensating strokes.