# Design and Analysis: Servo-Tube-Powered Liquid Jet Injector for Drug Delivery Applications
###### Abstract
This is due to the nature of the device; when the spring is released from its compressed state, the energy decreases nonlinearly. Thus, the highest forces are produced at the start of the injection, which is necessary to breach the epidermis; however, the subsequent jet power decays exponentially. The advent of gas-powered needle-free injectors, e.g., [20; 21; 22; 23; 24; 25; 26; 27], has helped deliver medication to target areas of the skin more accurately. These injectors contain a highly pressurized cartridge of inert gas (normally not higher than 8 bars), usually helium or carbon dioxide, that propels a piston. The piston is locked in place by a trigger mechanism, and when the piston is released, it compresses a column of fluid that is then forced through an orifice to create a liquid jet. Gas-powered injectors exhibit greater stability throughout injection delivery due to a more constant application of force on the driving piston. The pressure decay in the compressed gas cartridge is negligible during a typical injection, which is on the order of 100 ms [25]. Although there is still a pressure peak during the first few milliseconds of injection followed by an average injection pressure, the average injection pressure can be maintained at a specified level and does not decay as rapidly as spring-powered injectors. This makes it possible to target specific areas more easily, as well as giving gas-powered injectors the ability to target deep subcutaneous tissues. Recently, a similar, alternative injector design was also proposed using a hydro-pneumatic mechanism [28]. Although both gas-powered and spring-powered injectors form the majority of power sources available in commercial injectors, these types of power sources lack controllability. This implies that, once they begin to drive the injection, there is no means of modifying the application of power such that variable factors, such as friction and fluid damping, can be considered. Thus, the stagnation pressure profile produced by the injectors is fixed from its design stages and cannot be modified. Furthermore, because the power application is limited with these types of injectors, it is necessary to couple the orifice diameter with the total volume of liquid to be delivered. This usually results in the use of larger jet diameters to limit the time duration of the injection. The use of larger-diameter jets causes pain, bruising and hematomas, which are among the primary reasons that have limited the widespread use of liquid jet injectors. In order to overcome the aforementioned limitations and improve the controllability of liquid jet injection, it is necessary to precisely control the pressure profile exhibited by the liquid jet. Among more recently proposed concepts to drive needle-free liquid injectors, such as: the use of a single piezoelectric crystal [29] or a piezoelectric crystal stack [30] to actuate a piston, expansion of vapor bubbles using laser radiation [31; 32; 33; 34; 35; 36; 37], electrical current discharge [38], and pyrotechnics [39], it seems that electronic linear actuators provide one of the most practical and efficient methods for complete closed-loop control of liquid jet production. They offer real-time control and do not require high voltages or subject the fluid to electric charges or laser radiation as in other experimental techniques. This technology has been applied to an experimental device proposed by [40] and was subsequently used in many studies, e.g., [41; 42; 43; 44; 45], and in commercialization [46; 47], whereby a Lorentz force actuator based upon a voice coil was used to compress a column of fluid, using a plunger traveling at a specified velocity profile. The results of this study illustrated that a semi-closed-loop system consisting of a simple coil/one magnet actuator coupled to a linear variable differential transformer (LVDT) could successfully be used to dynamically shape a specific pressure profile. Although the system produced controllable traces, there is a lack of information on its precision.