Pressure and stress transients in autoinjector devices.

Spring-actuated autoinjectors delivering viscous drug solutions resulting from large drug concentrations require large spring forces which can create high peak pressures and stresses within syringes. The high peak pressures and stresses can lead to device failure. Measurements with a suite of novel instrumentation and analysis using numerical simulation explain the peak pressures and peak stresses as originating from mechanical impacts between moving components, the large acceleration of the components, and surprisingly, the production of tension waves in the liquid resulting in cavitation. The presence and intensity of cavitation depend on relative timing between the pressurization and the acceleration of the syringe, which, in turn, depend on the size and location of an air gap inside the syringe. We show that production of localized but very high pressures can result from shock wave focusing in the conical section of the syringe.