## 4 Discussion  
In the previous section, we observed that the AIR-BAG prototype showed the most promising performance in terms of flow rate stability. To see how AIR-BAG compared with the existing gravity-driven method (pole-suspended drip system), we plotted the flow measurement results obtained for both methods in Figure 11. The left and right ordinates correspond to the discharge flow rate and the amount of discharged liquid, respectively, whereas the abscissa shows the measurement time. All quantities were represented in normalized and non-dimensional values to ease comparison. To avoid curves' overlapping that may cause confusion, only part of the curves (in orange and yellow) for the amounts of discharged liquid are shown in Figure 11.  
Overall, as demonstrated in Figure 11, the discharge flow rate of the AIR-BAG decreased faster over the time span of measurement, than the gravity method. As a comparison index, we chose the value 0.9 (or 90%) in terms of the amount of discharged liquid in the right ordinate and traced it to the corresponding discharge flow rates in the left ordinate (see dotted and dashed grey lines in Figure 11). When 90% of the amount of liquid was discharged from the drip bag, the flow rates infused by the AIR-BAG and the gravity decreased to 0.68 (68%) and 0.93 (93%), respectively, of the initial flow rates. In other words, during the majority (up to 90%) of the infusion process, the gravity drip method maintained a consistent level, in terms of flow rate, that was a maximum of 25% better than the AIR-BAG prototype. This prompted us to launch investigatory work into the next phase of study, to find ways to improve the AIR-BAG. Other than flow performance, much work has still to be conducted on developing the portability and ergonomics of the design. It should also be noted that the performance index described above had a scatter of approximately 10% in terms of the difference between the maximum and minimum values from the repeated measurements.  
Of note, the flow rates were adjusted by using the roller clamp and by monitoring the liquid drops in the drip chamber of the infusion tube set (see Figure 1a), which made it difficult to accurately adjust the flow rate. This resulted in certain discrepancies in flow rates between measurement runs. It may appear incomprehensible to observe that a quickly decreasing flow rate from the AIR-BAG (blue curve in Figure 11) produced a higher amount of discharged liquid (orange curve in Figure 11) than the highly consistent flow rate by the gravity method. This occurred because the actual (non-normalized) flow rate of AIR-BAG  
Figure 11: Comparison of the infusion flow rate stability between the prototype AIR-BAG and the existing gravity-driven method.  
was slightly higher than that of the gravity method, resulting in a faster accumulation of discharged liquid in the weighing vessel.  
The limitation of the infusion pump mechanism proposed in this study in general, as well as the limitation of the flow performance results discussed in the present context specifically, should also be noted. Since atmospheric pressure is the source of the driving force for the pressurization mechanism, the device performance is expected to vary with different altitudes; the difference may be especially significant between sea level and highlands or in an air ambulance or airplane. It is possible that thinner air will lower the maximum infusion flow rate of the infusion pump. This may require the need to devise 'infusion pump models for high-altitude use' in the future.