TIO N B -B
4.2 Dynamic Large-Scale Model Autoinjector
4.2.1 Experimental Setup
A schematic of the experimental setup is shown in Figure 4.17. Figure 4.17a is a view of the complete test setup, and Figure 4.17b is a zoomed-in view of the projectiles and the syringe. Note that the ζ-axis in Figure 4.17a is in a stationary frame, it is defined downward positive, and it is referenced from the top end of the syringe guide tube. The z-axis in Figure 4.17b is in an accelerated frame, it is defined downward positive, and it is referenced from the top end of the syringe barrel. The experimental apparatus consists of five main components: the projectile guide tube, the projectile assembly, the test specimen or syringe, the syringe guide tube, and the decelerator.
The projectile guide tube is only partially shown inFigure 4.17a. This is the same guide tube as the one used in the static, large-scale model autoinjector experiments reported inSection 4.1. The purpose of this tube is to guide the projectiles while they are vertically accelerated to velocities up to 6.4 m/s using gravity alone.
The projectile assembly consists of two concentric projectiles: the inner and the outer projectiles. The inner projectile is a 0.33 kg aluminum cylinder – diameter = 39.2 mm and length = 102 mm – which can slide freely within the outer projectile. The outer projectile is a 0.44 kg aluminum cylinder – inner diameter = 38.5 mm, outer diameter = 50.1 mm, and length = 204 mm – which can slide freely within the guide tube. A #00 nylon screw (about 1.2 mm diameter) is used to fix the inner projectile inside the outer projectile at an adjustable position λ0 prior to a test (see Figure 4.17b). The nylon screw breaks as soon as one of the two projectiles impacts on the syringe barrel and/or the buffer, resulting in both projectiles traveling independently thereafter.
(a) View of the entire test setup (b) Zoomed-in view of the syringe, the buffer and the projectile assembly Figure 4.17: Schematic of the dynamic, large-scale model autoinjector experimental setup.
115 The test specimen consists of a thick-wall aluminum or polycarbonate tube with an outer diameter of 49.5 mm and an inner diameter of 38.1 mm. The polycarbonate tube is clear and vapor polished, making it possible to observe the inside of the syringe. As shown inFigure 4.17b, the tube is either terminated with a flat wall or a cone. The half-angle of the cone is 41◦, identical to the half-angle of the cone used inSection 4.1. The tip, which contains the flat wall or the cone, is a separate part that is glued to the bottom end of the barrel. The tip with a flat wall is fabricated using aluminum. Two conical tips were fabricated: one with aluminum, and another one with clear, vapor polished polycarbonate. The length of the barrel is 337 mm.
The overall syringe length, including the tip, is 353 mm for the syringe terminated with a flat wall, and 372 mm for the syringe terminated with a cone. The mass of the aluminum and polycarbonate syringes are respectively 0.81 kg and 0.36 kg.
The liquid inside the syringe during an experiment is de-ionized water. Degassed, de-ionized water was also tested, but this did not create significant differences in the measured pressure or strains as compared to tests without degassing (seeAppendix B for a list of all tests performed). Apparently, there are sufficient nucleation sites in both the water and the wall of the sample that the cavitation behavior is not prohibited by degassing. The results obtained with degassed water are not reported.
The syringe guide tube is an aluminum cylinder with a length of 813 mm, an inner diameter of 50.5 mm, and an outer diameter of 76.1 mm. The guide tube ensures proper alignment and motion of the different components during a test. A #00 nylon screw (about 1.2 mm diameter) is used to hold the syringe inside the guide tube, well above the decelerator, prior to an experiment. As soon as one of the two projectiles impacts on the syringe and/or buffer, the nylon screw breaks, and the syringe is accelerated downward. The syringe guide tube is mounted into an aluminum base fixture which is bolted to heavy stainless steal plates that are bolted and epoxied to the floor. A tight fit is used between the guide tube and the base fixture, and no adhesive is used to secure it in place. There are two oblong axial slots 180 degrees apart on the syringe guide tube used for syringe visualization and routing of the wires connected to the pressure and strain gauges.
The top end of the syringe barrel is initially located atζ= 203 mm. After completion of a test, the top end of the syringe is located atζ= 343 mm. The travel distance of the syringe is therefore 140 mm.
A decelerator is located at the bottom end of the guide tube to stop the syringe motion upon reaching its travel limit. It is a piece of urethane (Shore 60A hardness)
Figure 4.18: Schematic of the decelerators used to stop the syringe in the dynamic, large-scale model autoinjector experimental setup.
prepared using a home-made mold. The elastomer is from Frosch Polymer Corp (URS-5160). The decelerator is supported with an aluminum mount. The shape of the decelerator is different depending on the tip geometry: the two different shapes and the dimensions of the decelerators are shown inFigure 4.18.
The test specimen is sealed at its top end using a 104 mm long polycarbonate cylinder used as a buffer between the projectile and fluid. This is the same 134 g buffer as the one used inSection 4.1. There is a small hole along the longitudinal axis of the buffer which is closed using a socket screw before a test. This opening allows for the introduction of an air gap of controlled size δ0 between the bottom end of the buffer and the water contained in the tube, as shown in Figure 4.17. For all cases reported in this section, the bottom end of the buffer is located atz= 53 mm.
The syringe is instrumented with a piezoelectric pressure transducer PCB 113B23 mounted into the tip. The outer surface of the aluminum syringe is instrumented with 12 to 14 strain gauges to measure the hoop and axial strains. The strain gauges are a combination of Vishay CEA-06-125UN-350/P2 and HBM K-LY4-3-05-350- 3-2. The nominal location of each strain gauge is indicated inTable 4.5. The exact location of the strain gauges is reported on the plots that contain the strain signals.
The same electronics as inSection 4.1is used to power the sensors and acquire the data.
Two high-speed video cameras – a combination of Vision Research Phantom V7.0G, V711, and/or V1612 – are used to visualize the projectiles, the buffer and the syringe, making it possible to track the components. Quantitative image analysis makes it possible to obtain the velocity and acceleration of each component. When a clear, polycarbonate syringe is used, it is possible to visualize the cavitation events.
117 Table 4.5: Nominal axial location of the strain gauges mounted on the outer surface of the syringe used in the dynamic, large-scale model autoinjector experimental setup.
Station S1 S2 S3 S4 S5 S6 S7
z(mm) 73 121 175 232 279 327 345
The analogy between the test setup and an actual autoinjector is as follows: the projectile assembly corresponds to the spring-actuated power pack, the buffer cor- responds to the plunger-stopper, the aluminum/polycarbonate tube corresponds to the syringe, and the water corresponds to the drug solution. The decelerator corre- sponds to the autoinjector feature which is responsible for decelerating the syringe upon reaching the right penetration depth for the needle.
In the tests reported herein, only the outer projectile is used. The outer projectile can be oriented as shown inFigure 4.17to impact on the syringe wall. The orientation of the outer projectile can also be reversed to impact only on the buffer. This approach was used for simplicity, making it possible to use the same projectile for all tests.
Another possibility (not reported in this thesis) is to have the outer projectile impact on the syringe wall, causing the syringe to accelerate (event 1 inSection 2.2), and the inner projectile impact on the syringe buffer, pressurizing the air gap and the liquid inside the syringe (event 3 inSection 2.2). The timing between the impact on the buffer and the syringe wall is controlled by varying distanceλ0(seeFigure 4.17b). A large value ofλ0results in the impact of the inner projectile on the buffer occurring before the outer projectile impacts on the syringe wall, causing pressurization of the liquid prior to acceleration of the syringe. Figure 4.17bshows two different initial configuration of the projectiles: smallλ0on the left-hand side, and large λ0on the right-hand side.
Note that damping material in the form of two stacked O-rings is introduced between the syringe wall and the outer projectile. This prevents the creation of large axial stress waves in the syringe due to the impact event. A metal-to-metal impact creates large amplitude stress waves which take a long time to dissipate, and this pollutes the measurements. The presence of damping material reduces the magnitude of the syringe acceleration, but the acceleration remains large enough to create substantial
Figure 4.19: LS-DYNA model used to simulate the transient events created by the impact of the reversed outer projectile on the buffer of the syringe in the dynamic, large-scale model autoinjector.
pressure and stress transients. No damping material is used when the reversed outer projectile is used to impact on the buffer.
The travel distance of the syringe is relatively large, resulting in a temporal separation of 50-60 ms between the syringe acceleration and deceleration. This is sufficient time for the transient events created by the impact of the outer projectile on the syringe wall or buffer to dissipate before the syringe is decelerated. This means that events 1 and 3 (i.e., pressurization and acceleration) are decoupled from event 2 (i.e., deceleration). For this reason, events 1 and 3 are studied separately from event 2, both experimentally and numerically.