Impact of the mechanical properties of penetrated media on the injection characteristics of needle-free jet injection

Abstract

Needle-free jet injection systems are characterized by the increased drug delivery efficacy over conventional needle-based injection systems and have become a viable alternative. The majority of previous studies have investigated underlying mechanisms of needle-free jet injection systems with an emphasis on the fluid dynamics of the propelled microjet without considering the influence of an injected medium on the penetration and dispersion of the microjet. Unlike the injected fluid introduced directly into a target region of the skin tissue through a hollow needle, the fluid microjet produced by a needle-free injector has to work through multiple skin tissue layers and interfaces having different mechanical properties. Here we evaluated injection characteristics in two hydrogel media having similar stiffness controlled precisely but different pore size and hence different fracture toughness. High-speed imaging data showed that the fracture toughness of the medium influenced the penetration mechanism of the injected microjet, whereas viscoelastic and poroelastic characteristics of the medium determined the final attainable injection profiles. The injection profiles were also dependent on the type and depth of the interface between the layers of two media, where tight and loose interfaces can have different effects on the distribution of injected drug as the injected fluid prefers the path having the least resistance. Our findings would improve the present understanding of needle-free jet injection systems and could help to develop more effective drug delivery systems.