IN SITU MEASUREMENTS
3.2 Results and Discussion
3.2.3 Configuration 3: Plastic Syringe Without an Air Gap
Figure 3.16: Growth and collapse of bubbles along the barrel of a glass syringe when an air gap is present (test SC-0137).
cavitation occurs. The collapse of the cavitation bubbles can create significant, but highly localized, pressures within the liquid. The collapse of a bubble close to a wall can result in substantial wall stresses, likely capable of causing failure of the syringe. The pressure and stress transients generated upon deceleration of the moving components occurring when the syringe reaches its travel limit are not significantly affected by the presence of an air gap.
75 Note that the syringe material is not the only difference between the BD HyPack and the Daikyo Crystal Zenith syringes. The lubrication of the syringes and the plunger-stoppers is also different. The plunger-stopper and the inner surface of the BD HyPack syringe are coated with a thin layer of silicone oil. As mentioned in Chapter 2, siliconization of the components is the typical approach used to lubricate pre-filled syringes and plunger-stoppers. In the contrary, the inner surface of the Daikyo syringe and the plunger-stopper are not siliconized. In fact, no lubrication at all is applied on the syringe, and the plunger-stopper is coated with the Daikyo Flurotec film (West Pharmaceutical Services, n.d.), a dry lubricant. The break-loose force and the dynamic friction are therefore expected to be significantly different between the two syringe systems. In particular, the relation between friction and relative velocity of the plunger-stopper into the syringe is expected to be different.
Results obtained with the Daikyo plastic syringe are shown in Figures3.17to3.19.
The motion of the internal components, the liquid pressure and the strains are not qualitatively different from what was recorded using a glass syringe without an air gap (i.e., configuration 1) during the time period which immediately follows the impact of the driving rod on the plunger-stopper (i.e., 0 ms to 1 ms). The impact of the driving rod on the plunger-stopper is again responsible for the pressurization and the acceleration of the syringe. Quantitatively, the acceleration of the syringe immediately after the impact event is approximately 22,000 m/s2, a value which is significantly larger than the 15,000 m/s2 recorded in configuration 1 with a glass syringe. The substantial difference is explained by the smaller mass of the plastic syringe compared to the glass syringe. The total mass of the liquid content, syringe, and syringe carrier is 5.2 g for the plastic syringe, compared to 7.0 g for the glass syringe.
The reader is reminded about the absence of an air gap in the present configuration.
As such, the pressure rises rapidly in the syringe following the impact of the driving rod on the plunger-stopper. The results indicate the acceleration and the pressur- ization of the syringe occur almost simultaneously, and no cavitation occurs in the syringe. This behavior is identical to the one observed in configuration 1 with a glass syringe.
The deceleration of the syringe upon reaching its travel limit occurs between 2.5 ms and 4.0 ms. The magnitude of the deceleration is 34,000 m/s2, comparable to the 36,000 m/s2 recorded in configuration 1. There is however a significant difference between the behavior of the plastic and the glass syringe near the end of the rapid
(a) Position
(b) Velocity
Figure 3.17: Position and velocity of the moving components in a SureClick autoin- jector – plastic syringe without an air gap (test SC-200).
77
Figure 3.18: Liquid pressure in a SureClick autoinjector – plastic syringe without an air gap (test SC-200).
Figure 3.19: Hoop and axial strains on the barrel of the syringe in a SureClick autoinjector – plastic syringe without an air gap (test SC-200).
deceleration. There is more compliance in the system when a plastic syringe is used, and the increased compliance has a measurable effect on the motion of the internal components (seeFigure 3.17a). There is a noticeable overshoot in the displacement of the components past the equilibrium, or extrusion position, as the syringe reaches its travel limit. Furthermore, the rebound of the syringe is not as important.
The current hypothesis is that the overshoot is caused by a substantial deformation of the syringe and/or the plastic syringe-carrier in the vicinity of the point-of-contact between those two components. This could not be confirmed visually as the point- of-contact is virtually impossible to observe. The effect of this increased compliance is also visible inFigure 3.17b: the syringe does not travel at the same velocity as the plunger-stopper and the driving rod during the rebound, which indicates there is a momentary decrease in the force applied on the top surface of the plunger-stopper.
This, in turn, causes a momentary decrease in liquid pressure during the syringe rebound (seeFigure 3.18).
Another difference between the results obtained in configurations 1 and 3 is the weak, momentarily deceleration of the plastic syringe at around 2.0 ms. This occurs between the impact of the driving rod on the plunger-stopper and the syringe deceleration resulting from the syringe reaching its travel limit. This is not a difference which is believed to result from using a plastic syringe, but instead results from test-to-test variation in the results.
The weak, momentarily deceleration of the plastic syringe at around 2.0 ms is noticeable on the velocity plot (see Figure 3.17b), and it also causes a pressure increase (seeFigure 3.18). This deceleration of the syringe could be caused by an interaction between some of the components inside the autoinjector. This hypothesis is supported visually using the high-speed video for this test. It is possible to observe the shell of the device deform in the circumferential direction, and this appears to apply a force on the syringe carrier in the radial direction. This pushes the syringe and the syringe carrier sideways. The root cause of this interaction between the components has not been identified.
The hoop and axial strains measured with a plastic syringe (see Figure 3.19) are qualitatively similar to the strains measured with a glass syringe (seeFigure 3.11).
Quantitatively, the strains measured with a plastic syringe are one order of magnitude larger than the strains measured with a glass syringe. This is explained by the different Young’s modulus of elasticity of the materials, as indicated inTable 3.1.
79 The larger strains measured with a plastic syringe do not necessarily indicate that the maximum principal stress σ1 is larger with a plastic syringe than with a glass syringe. Equations3.4and3.5are used to estimate the wall stresses. The maximum principal stressσ1≈ σθalong the barrel is estimated to be 14 MPa, a value which is less than the 22 MPa previously obtained with a glass syringe. The peak compressive axial stress in the plastic syringe is estimated to be -4.4 MPa, a magnitude which is less than the -8.3 MPa previously obtained with a glass syringe. This difference is explained usingEquation 2.19, which suggests the peak axial stress depends linearly on the density of the syringe material.
To summarize, the results for a glass and a plastic syringe without an air gap are qualitatively similar. In both cases, pressurization of the liquid content and acceleration of the syringe occur nearly simultaneously. As a result, cavitation is suppressed. The system formed by the plastic syringe and the syringe carrier is more compliant than the system formed by the glass syringe and the syringe carrier.
The increased compliance alters the behavior of the system during the final stage of the rebound. The increased compliance of the plastic syringe also results in larger strains, not to be mistakenly associated with larger stresses. Because the Young’s modulus of glass is approximately 26.5 times larger than the Young’s modulus of plastic (seeTable 3.1), the strains are expected to be approximately 26.5 times larger in the plastic syringe than in the glass syringe for the same stress σapplied on the syringe wall ( ∼σ/E).