Design, Verification and Validation of a Dynamic Model for an Intramuscular Autoinjector

Abstract The development of new injectable molecules for different diseases and reformation of first-generation pharmaceuticals has led to growing interest in making autoinjectors usable for different medications. The major challenge in this quest is the complexity of injecting highly viscous drugs in large volumes. This research aims to develop a dynamic model to describe the influence of drug viscosity and volume on the injection process and evaluate the sensitivity of medication fluid behaviour to variations in component dimensions of the autoinjector fluid delivery system. Using mathematical modelling, the plunger motion is characterised by the spring, fluid resistance and frictional forces between the vial and plunger. It displaces linearly and accelerates until maximum velocity is reached. Slow plunger motion is associated with increasing glide and break-loose force. Wall shear stress captured via Computational Fluid Dynamics (CFD) modelling is directly proportional to the medication viscosity and it increases linearly for Newtonian and shear-thinning medications. Separation flow is observed in the syringe for viscosities between 15 - 80 cP. This represents decreasing flow as pressure increases, increasing the chances of needle deformation and injection pain due to tissue damage. Injecting highly-viscous drugs is possible via considerations of kinematic and rheological properties.