High-Speed Radiographic Analysis of Subcutaneous Injection Depots: Dispersion, Morphology, and Diffusion in Autoinjector Delivery

Autoinjectors facilitate self-administration of subcutaneous (SC) medications. Despite their benefits for patients and healthcare systems, experimental research on how they impact drug dispersion and absorption is limited. This study investigates how autoinjector injection parameters influence plume growth, morphology, and diffusion during and post injection. We investigated three commercial autoinjector models of a similar design with varying delivered volumes (0.5 mL, 1 mL, and 2 mL) injected into excised pork belly tissue. Synchrotron radiography provided high-resolution, real-time 2 D visualization of plume dynamics during injection, while the 3 D morphology of the depot postinjection was captured with a synchrotron CT. Our results show that plume growth is nonlinear, with an initial rapid phase slowing over time. The final plume volume exceeds the delivered dose by 25%, reflecting depot spread within the tissue. Regardless of autoinjector model or injection volume, the plume predominantly expands horizontally (parallel to the tissue), with the aspect ratio increasing throughout the injection and reaching a final value of approximately 4. Post-injection diffusion appears to be driven more by tissue properties than by the autoinjector design parameters. These findings provide a foundation for refining autoinjector designs, developing more accurate computational models to predict drug absorption, and optimizing SC delivery systems.