Inertial liquid loading on the nozzle of a needle-free injection device

Abstract

Needle-free injection devices allow delivery of liquid drugs without a hypodermic needle. Advantages include ease of use, patient preference, dose accuracy and elimination of the risk of ‘needle-stick’ injuries. All liquid needle-free devices use the same principle: the liquid drug is pressurized and delivered at high velocity through a small orifice in close proximity to the skin. The liquid jet penetrates the skin and the drug is delivered into the skin tissues. The high pressures within the drug capsule, together with pharmaceutical requirements that restrict material choice, provide a challenging design problem. This problem is further complicated by the possibility of bubbles within the drug liquid. Bubbles may significantly increase peak pressures and lead to capsule failure. In this paper a model is proposed for the inertial response of the liquid in an open conical nozzle. An equation of motion is derived for the liquid displacement, which is used to predict the pressure distribution. The equation for liquid flow in the nozzle is incorporated into a mathematical model of a needle-free injection device; this model is used in numerical simulations to predict the effect of a bubble at the orifice on the pressure in the nozzle.