Viscous Heating Assists Jet Formation During Needle-Free Jet Injection of Viscous Drugs.

OBJECTIVE: Jet injectors use a high-pressure liquid jet to pierce the skin and deliver drug into underlying tissues. This jet is formed through a short, narrow orifice; the geometry of the orifice and the properties of the fluid affect the nature of the flow. We aimed to discover information about the turbulent and viscous processes that contribute to pressure loss and flow patterns during jet injection. METHODS: We used computational fluid dynamics methods and experimental observation to investigate the effects of nozzle geometry, fluid viscosity, and viscous heating on jet production. We experimentally verified the temperature change of the jet during ejection, using an infrared camera. RESULTS: Our models accurately predict the average jet speed produced for two example nozzle geometries over two orders of magnitude of viscosity. The models reveal the previously unreported importance of viscous heating in the formation of the jet. Temperatures >65 °C were predicted at the edge of the flow as a result of viscous heating. These caused a significant local reduction in viscosity and effectively allowed the fluid to lubricate itself. Our experiments confirmed changes in mean jet temperature of up to 2.5 °C, which are similar to those predicted by our model (∼2.8 °C). CONCLUSION: These results reveal the importance of the viscous heating properties of a fluid in the formation of high-speed jets for drug delivery. SIGNIFICANCE: This property is crucial to consider when formulating new drugs for needle-free jet injection.

A compound ampoule for large-volume controllable jet injection.

We present a new design for a needle-free injector ampoule, using two concentric pistons to pressurize the fluid during the injection. The smaller, inner piston is used to provide an initial high-velocity piercing jet; it then engages the outer piston to deliver the remaining drug via a low-velocity jet. The goal of this design is to enable needle-free delivery of relatively large volumes to controlled depths in tissue, a task impractical with conventional ampoules and actuators. We demonstrate this concept by constructing a 1.2mL ampoule, measuring the jet velocity it produces in free air, and performing a set of injections into post-mortem porcine tissue. The ampoule smoothly produces the two desired phases of an injection, with a smooth transition of jet velocity as the two pistons engage. The injection is able to penetrate porcine skin to a controlled depth and deliver fluid to the subcutaneous and/or intramuscular layers, though further investigation is required to ensure that all of the fluid delivered can be retained at the desired depth.

A computational model of a controllable needle-free jet injector.

We present a mathematical model of the dynamics of a previously developed needle-free jet injector (NFJI) that is based upon a servo-controlled Lorentz-force motor. The injector creates a fluid jet that can pierce through the skin and deliver a drug to dermal, subcutaneous and muscular tissue. We use the model to predict the jet speed achieved during an injection. The model simulates the electrical response of the motor coil, the mechanical response of the drug piston and ampoule and the friction incident upon the piston during the time course of the injection. High-speed video measurements of piston movement in response to a step input show that the model predicts piston-tip position during an injection within an RMS error of 287 µm. The corresponding jet speed is predicted to be 180 m·s(-1) with a maximum overshoot to 205 m·s(-1).