LIST OF TABLES
NOMENCLATURE
INTRODUCTION
Motivation
Additionally, challenges arise in developing reliable autoinjectors for the delivery of more concentrated and viscous drug solutions. Failure started on the inner surface of the cone area due to an excessive circumferential stress.
Objectives and Structure
This setup is particularly useful for investigating the effect of the large acceleration and deceleration of the syringe during actuation. The results are particularly useful in determining whether a cavitation event within the cone region of the syringe can cause failure of the glass.
DYNAMIC EVENTS IN AUTOINJECTOR DEVICES
Actuation Sequence of a Typical Spring-Actuated Autoinjector
The needle can be pre-attached to the syringe at the factory, as shown in Figure 2.2a. Another approach is to support the syringe using the shoulder located near the tip.
Transient Events During Autoinjector Actuation
- Acoustics and Wave Dynamics
- Event 1: Syringe Acceleration
- Event 2: Syringe Deceleration
- Event 3: Impact of the Driving Rod on the Plunger-Stopper
Conversely, cavitation occurs if the change in syringe velocity is large (u < (Pvap−P0)/(ρlcl)). The deceleration of the syringe also creates a compressive axial stress wave in the container wall.
SureClick Autoinjector
Adding and increasing the size of the air gap reduces the sharpness of the pressure waves created in the fluid (sharpness of pressure waves is discussed further in Chapters 3 through 5). As explained in Chapter 3, the acceleration of the syringe is due to the friction between the plunger stopper and the syringe and the increase in pressure at the bottom of the syringe.
IN SITU MEASUREMENTS
Methodology and Material
- Step 1: Modifying the Shell
- Step 2: Modifying the Syringe Carrier
- Step 3: Removing the Needle
- Step 4: Mounting the Pressure Transducer
- Step 5: Filling the Syringe
- Step 6: Preparing the Syringe Surface
- Step 7: Installing the Strain Gauges
- Step 8: Assembling the Autoinjector
- Step 9: Mounting the Autoinjector on a Support
- Step 10: Final Preparation
- Limitations
However, there is no suitable opening in the syringe carrier to route the strain gauge leads to the outside of the autoinjector device. This problem is solved by making an elongated slot in the side wall of the syringe carrier. Maintaining firm pressure on the strain gauge is important due to the small radius of curvature of the syringe.
The leads of the strain gauges are bound to the surface of the syringe with ordinary adhesive tape. This is necessary to prevent the strain gauges from being pulled off the surface of the syringe in the remaining steps. The leads of the strain gauges attached to the surface of the syringe are wrapped around the syringe before the syringe is mounted in the carrier.
Once this is done, the strain gauge lead wires are wrapped around the syringe carrier. Unfortunately, the pressure transducer is too large to be installed in the conical portion of the syringe. This makes it impossible to attach strain gauges to the syringe surface in this area.
Results and Discussion
- Configuration 2: Glass Syringe With an Air Gap
- Configuration 3: Plastic Syringe Without an Air Gap
- Configuration 4: Plastic Syringe With an Air Gap
- Friction Estimates
The pressure transducer is located approximately halfway between the plunger clamp and the syringe cone. This result suggests that the magnitude of the acceleration due to friction between the plunger gripper and the syringe is approximately 8000 m/s2. The position and velocity of the push rod, plunger, and syringe are shown in Figure 3.12.
The impact of the driving rod on the plunger stopper is again responsible for the pressurization and acceleration of the syringe. As such, pressure rises rapidly in the syringe following the impact of the driving rod on the plunger stopper. This occurs between the impact of the drive rod on the plunger stopper and the deceleration of the syringe due to the syringe reaching its limit of travel.
This is a series of images showing the movement of the drive rod, the plunger plug and the syringe. The friction is minimal, and the relative movement of the plunger plug in the syringe is large. The acceleration of the syringe immediately after the impact of the drive rod on the plunger plug is calculated.
LARGE SCALE MODEL AUTOINJECTOR
Static Large-Scale Model Autoinjector
- Experimental Setup
Five test cases representative of the physics involved are discussed in detail. Note that the z-axis, or longitudinal axis, is defined downward as positive, and all distances are measured relative to the top end of the aluminum tube. The total mass of the test piece, including the base mounting and the plates, is over 50 kg.
The aluminum tube is placed in the base holder as shown in Figure 4.1 and it is secured in place using a crimp fit. In the first geometry (Figure 4.2a), the bottom of the aluminum tube is terminated with a flat end perpendicular to the z-axis. The half angle of the cone is 41◦.1 In both geometries there are two ports for mounting piezoelectric pressure transducers.
The polycarbonate was steam polished after processing to ensure optical clarity of the final product. The aluminum tube used with the polycarbonate base fixture is 76mm shorter to ensure that the overall distance between the top end of the tube and the entrance to the cone is 0.91m, the same as for the other two base fixtures. 1A 41◦ half-angle was chosen based on the standard tools available for making the conical tip.
TIO N A -A
TIO N B -B
LS-DYNA Numerical Model
The refined mesh is not used to perform the numerical simulations due to the increased computational cost and relatively small effect on the simulation results. The elements that form the air gap are also limited to avoid getting a very distorted mesh; they can only be deformed axially. For cases where there is no air gap, the nodes at the buffer-water interface are separated by both parts.
A built-in LS-DYNA surface-to-surface contact model (Hallquist, 2016) is used at the projectile-damper interface and at the water wall interface. In this model, when the fluid experiences stress, there is a loss of contact between the water and the wall, mimicking cavitation. The growth and collapse of these voids locally mimic the effect of bubbles during cavitation.
The gas in the air gap is modeled as an isentropically compressed perfect gas (see equation 2.22). Initially, all components are at rest except the projectile which is traveling at the impact velocity VO. Finally, all simulations are terminated shortly after the onset of cavitation due to the lack of an explicit cavitation model in the numerical simulations.
Results
- Case 1
- Case 2
- Case 3
- Case 4
- Case 5
After the projectile hits the buffer (label 1), a pressure wave forms in the liquid. The reflection of the wave in the buffer creates a second pressure wave that later reaches the bottom of the tube (label 4). The upward movement of the buffer produces voltage waves which immediately follow the second pressure wave.
The arrival of this compression wave at the bottom of the tube is detected by the pressure transducers (label 6). The sides of the straight tube and the cone are identified in the first frame of Figure 4.9. This results in the second incident pressure wave becoming sharp before it reaches the bottom of the tube.
There is no shock focusing because the transit time of the acoustic waves in the cone (25 µs). Shock focusing is possible because the rise time of the pressure wave (16 µs) is less than the acoustic transit time of the waves in the cone (25 µs). Wave steepness is observed and is responsible for the short rise time of the second pressure wave.
Conclusion on Cases 1 to 5
The bubble appears to nucleate near the tip of the cone, where the pressure transducer is located. The collapsing bubble remains close to the pressure transducer, which partially explains the larger peak pressure recorded at the tip of the cone compared to the peak pressure recorded above the cone. No shock focusing of the primary pressure wave generated upon the impact of the projectile on the buffer was observed.
The peak pressure recorded at the collapse of the cavitation bubbles was much higher at the tip of the cone than above the cone. The amplification is due to a combination of shock focusing and the effect of the cone on the collapsing bubbles. The size of the air gap drastically affects the time of multiple impacts between the projectile and the bumper.
The pressure measured at the tip of the cone is significantly greater than the pressure measured above the cone. The amplification of the pressure is due to a combination of shock focusing and the rapid collapse of a bubble in the immediate vicinity of the pressure transducer. The effect of the acceleration and deceleration of the syringe is important and this is the subject of the next section.
Dynamic Large-Scale Model Autoinjector
- Experimental Setup
- LS-DYNA Numerical Model
- Results: Syringe Acceleration and Pressurization
- Case 6
- Case 7
- Case 8
- Case 9
- Case 10
- Case 11
- Case 12
- Summary of Cases 6 to 12
- Results and Discussion: Syringe Deceleration
- Case 13
- Case 14
- Case 15
- Summary of Cases 13 to 15
The emphasis of this discussion is on the differences created by the syringe movement. First, the impact of the projectile on the buffer creates relative motion between the buffer and the syringe. The outer projectile hits the syringe buffer in the dynamic, large-scale model autoinjector.
The movement of the syringe is consistent with the development of the pressure in the tip, discussed below. The compression wave is formed at the tip of the syringe and propagates in the liquid towards the buffer.
NUMERICAL SIMULATIONS OF SHOCK FOCUSING AND SYRINGE STRESSES
Shock Focusing
- Numerical Model
- Shock Focusing in a Straight Cone
- Effect of the Half-Angle
This can affect the amount of amplification that occurs within the cone. The angular wave is the result of the reflection of the incident wave on the inclined wall of the cone. The corner and diffracted waves converge towards the symmetry axis of the syringe.
When they reach the axis of symmetry, they reflect and spread towards the wall of the cone. The pressure before the pressure wave is 0 MPa, and the pressure behind it is 1 MPa. The axial progress of the incident and angular waves towards the tip of the cone is uninterrupted.
The magnitude of the gain is expected to depend on the half angle of the cone. The amplification factorλquantifies the amplifying effect of the cone on the magnitude of the incident pressure wave. The numerical simulations indicate that the gain factor increases as the half angle of the cone decreases.
