# Design and Analysis: Servo-Tube-Powered Liquid Jet Injector for Drug Delivery Applications
## 3 Results and Discussions
### Reproducing Standard Injector Performance  
One of the key elements in illustrating the prototype's feasibility is the ability to shape a pressure output and consistently maintain that result with respect to changing nozzle size and fluid viscosity. Therefore, it was decided to reproduce a similar pressure output in line with commercially available units. Figure 11 depicts a stagnation pressure wave, with a peak of 15 MPa occurring at 5 ms and an average delivery pressure of 6 MPa reached at approximately 7.5 ms and utilizing the servo amplifier's maximum rates of accel/decel.  
This velocity profile was tested under seven different viscosities and four different nozzle sizes. The fluid viscosity was varied by utilizing glycerol and making aqueous solutions of different weight percentages. Figure 12 shows the variation of both density and viscosity as a function of weight %; the different test conditions correspond to 0, 20, 30, 40, 60, 70 and 80 wt.% glycerol, respectively. Each nozzle size and viscosity combination were run with a minimum of 12 repetitions to ensure meaningful conclusions could be drawn from the data sets. It should also be noted that 90 and 100 wt.% were also considered, as the test device could create a liquid jet without attaining the stalling current of the linear motor; however, after approximately three injections, the nozzles would show signs of blockage. It was impossible to dislodge the blockage without heating the tip of the nozzle with a propane torch, which would cause deformation of the orifice. Consequently, it was decided to test only up to 80 wt.% in order to extend the longevity of the nozzles.  
Figure 13 depicts an initial pressure trace, which illustrates the system's ability to dynamically shape a pressure profile. One of the positive aspects of the gathered results shows that the system accurately reached the desired peak and mean injection pressures. Discrepancies were found in the time required to reach the peak, which was targeted at 5 ms, but occurred at approximately 2.5 ms, a trend that is consistent with all the profiles that were tested; further investigation into this phenomenon needs to be conducted. Oscillations about the mean injection pressure were also observed; this is in line with pressure plots from other needle-free devices. Although the oscillations were present, they settled toward the tail end of the injection. The oscillation can partly be attributed to the rapid loading and unloading of the piezoelectric force sensor.  
Figure 11: Pressure Profile representing typical behavior of commercially available injectors.  
Comparisons between the experimental values displayed in Figure 14 and the theoretical values, corrected for change in density of the fluid and nozzle size, yielded maximum deviations of approximately 5.5 MPa when comparing pressure peaks and 1.37 MPa when comparing mean stagnation pressures.  
Figure 14: _Cont._  
Figure 12: Density and viscosity as a function of weight % glycerol.  
Figure 13: A sample showing the experimental and target pressure profile to illustrate the system’s ability to dynamically shape a pressure profile.  
Figure 14: Experimental and targeted pressure profiles tested under various viscosities and nozzle sizes: (**a**) Nozzle 6, (**b**) 8, (**c**) 10 and (**d**) 12.