# Controlled Release Using Gas Detonation in Needle-Free Liquid Jet Injections for Drug Delivery
## 1 Introduction  
Drug delivery without the use of hypodermic needles has been a long-term objective within the medical field [1]. Among different needle-free technology, liquid jet injectors can deliver medication to a target area by rupturing the skin through the pressure exerted by a liquid jet. The basic mechanism involves the use of a power source to compress a liquid and expel it through an orifice [2]. This technology has been in existence since the early 20 th century, and during that period the effectiveness in eliminating bio-hazardous waste and delivering a broad range of medication have made this technology ideal for mass immunization [3]. However, drawbacks such as pain, bruising, splash back, hematomas, excessive penetration and cross contamination have limited the use of needleless jet injection for both mass immunization as well as individual use [4; 5; 6].  
Recently, the technology has gained renewed interest for delivering both micro- and macromolecules and advancements in fluid dynamic research have aided in propelling this technology as an ideal platform for newly developed drug therapies, including monoclonal antibodies [7; 8]. Notably, jet injectors are capable of targeting shallow layers of the skin such as the epidermis as well as sensitive organs [9]. Furthermore, liquid jet injectors can also be used for targeting diseases which benefit from localized treatment techniques.  
Parametric studies and technological advancements throughout the years have enhanced the performance and controllability of needleless injectors. A better understanding of the driving forceand the fluid mechanics have also led to better optimization of the device in terms of delivery depth and controllability of drug flow, resulting in better drug distribution. The majority of engineering studies analyze spring- or gas-powered injectors, e.g., [10; 11; 12; 13; 14; 15; 16; 17; 18; 19], whilst experimental prototypes utilize electrically driven power sources in order to provide real-time control of jet pressure, e.g., [20; 21; 22]. Highly experimental techniques also include electrical current discharge and laser-based systems, to generate highly focused, pulsed liquid microjets from vapor bubble collapse [23; 24; 25; 26].  
However, new emerging trends in needle-free technology are creating a need for delivering higher volume as well as highly viscous injections [3]. The needle-free injector's precision as well as its ability to target deep areas of tissue such as muscle, provide good building blocks for use with new drug therapies. Nevertheless, there is a need for power sources that are sufficiently strong, and which can be accurately controlled in order to provide a liquid jet with velocities on the order of 100-200 m/s, predictable penetration depth, large-volume delivery efficiency, as well as cope with an increase in drug viscosities observed with new drug formulations used for emerging medical treatments [27; 28].  
Apart from drug delivery in humans, needle-free liquid jet injection technology also attracts significant interest for animal vaccination [29; 30; 31; 32]. It provides an efficient mean to achieve continuous injection for mass vaccination of farmed livestock. It is worth noting that different livestock such as cattle or swine have rather different skin properties and often require different vaccine doses, therefore, flexible power must be provided to the needle-free injection system.  
This study explores the use of combustion to generate the required power in order to drive the needle-free injector. Specifically, the detonative combustion mode is considered in this work. It makes use of the pressure increase across the detonation wave in order to drive the injection piston and pressurize the medication. The present study serves to highlight the feasibility of using gaseous detonation-driven power sources as a convenient and efficient means of powering liquid jet injections.