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Development of a Self-Injection Syringe Testing System
Event Type
Poster Presentation
TimeThursday, April 152:00pm - 3:00pm EDT
LocationMedical and Drug Delivery Devices
DescriptionIntroduction
An increasingly wider range of the population is self-injecting medication using prefilled syringes (PFS) on a regular basis. This includes individuals with diabetes, cancer, and other medical conditions. Often these individuals are elderly, potentially with comorbidities, and often without formal healthcare training. Current literature on injectability tends to focus on needle and syringe design to optimize fluid flow, but largely ignores the human factors and ergonomics involved in performing the injection (Rathore, Pranay et al. 2011). Another limitation is that much of the PFS laboratory research investigates the force required to inject a fluid at a fixed, predetermined velocity. However, the few studies that have recorded data from manual injections found large amounts of variability in velocity and force application (Cilurzo, Selmin et al. 2011, Verwulgen, Beyers et al. 2018).
The magnitude of forces required to perform a self-injection with a PFS from populations with diminished hand and finger strength is also limited. One possible reason for this is the absence of reliable data collection methods and instruments designed to collect force and velocity data during self-injections.
The goal of this project was to address these limitations by developing an integrated system to simulate a variety of common PFS types and medications to measure force and velocity during self-injections. The development of this system will allow the systematic assessment of force requirements in clinical populations and the ability to control backpressure to address surrogate tissue limitations.

Methods
A self-injection testing system was designed using a syringe connected to a fluid reservoir, force sensing resistor (FSR), flow meter, and control valve. Injection characteristics for Newtonian fluids are controlled with a microcontroller to simulate different syringe, needle, fluid viscosity, and back-pressure characteristics as a simulated self-injection is performed in a realistic posture.
A 2.25ml glass syringe was used and a 3D printed interface with integrated FSR measured the force applied to inject a water-glycerol mixture with ratios representing 5, 10, 15, 20, 25 cP. Three known loads were applied, and 4 replicates were recorded for a total of 60 trials. Measured results were compared to model results to determine agreement.

Results
A model for Newtonian fluid flow through a syringe was programmed (Rathore, Pranay et al. 2011) and experimentally verified using a testing fixture. This model predicts the relationship between input force and plunger velocity as a function of barrel and needle size and fluid properties (e.g. viscosity). The model also provides predictions for frictional forces. Tissue back pressure is highly variable, but averages can be used in the model (Thomsen, Hernandez-Garcia et al. 2014). Agreement between model predicted force and measured force across all trials was high with an r2 = 0.970.

Future Plans
We anticipate using the testing system for a variety of purposes to gain knowledge about the user experience of self-injection when using PFS across medical indications and patient capabilities. For example, knowledge of the force capabilities of users can inform syringe design to improve the patient experience and reduce medication delivery errors. Additionally, users can be trained on how best to perform a self-injection without the need of multiple needle sticks. The availability of self-injection testing systems will encourage additional research to improve drug delivery from self-injection.

References
Cilurzo, F., F. Selmin, P. Minghetti, M. Adami, E. Bertoni, S. Lauria and L. Montanari (2011). "Injectability evaluation: an open issue." AAPS PharmSciTech 12(2): 604-609.
Rathore, N., P. Pranay, B. Eu, W. Ji and E. Walls (2011). "Variability in syringe components and its impact on functionality of delivery systems." PDA J Pharm Sci Technol 65(5): 468-480.
Thomsen, M., A. Hernandez-Garcia, J. Mathiesen, M. Poulsen, D. N. Sørensen, L. Tarnow and R. Feidenhans'l (2014). "Model Study of the Pressure Build-Up during Subcutaneous Injection." PLOS ONE 9(8): e104054.
Verwulgen, S., K. Beyers, T. Van Mulder, T. Peeters, S. Truijen, F. Dams and V. Vankerckhoven (2018). "Assessment of Forces in Intradermal Injection Devices: Hydrodynamic Versus Human Factors." Pharm Res 35(6): 120.