Fluid mechanics analysis of a spring-loaded jet injector.

A syringe jet injector is a device designed to administer a drug quickly and painlessly through the skin. Though syringe jet injectors have been in use for almost 50 years, current designs still suffer from inconsistent performance. To better understand the fluid mechanics of jet injection and gain insight into how the design might influence performance, two theoretical analyses to determine the fluid pressure profile at the exit orifice were conducted. The first was a continuum analysis assuming static incompressibility. Results demonstrated that the maximum jet pressure was highly sensitive to the spring constant, initial piston velocity, and piston cross-sectional area while the time to achieve the maximum pressure was most sensitive to the injection chamber length, initial piston velocity, bulk modulus of the injectant, and the piston cross-sectional area. The second analysis was a shock wave analysis. Results demonstrated a stepwise pressure-time plot that was similar in magnitude to that for the continuum analysis assuming static incompressibility. Results from these two investigations are useful for design modification of the jet injector to achieve desired pressure-time profiles at the orifice. Control of pressure-time profiles may help to achieve a more consistent and effective injection process.