This study compares the penetration characteristics of impact-induced jets with those of laser-induced jets, focusing on the underlying penetration mechanism rather than device performance for needle-free injection. Using an impact-induced jet system capable of ejecting a highly focused liquid jet at high speed without the use of lasers, we examine jet penetration into skin-simulating materials. Unlike conventional needle-free injectors that produce diffused liquid jets, the impact-induced method generates a highly focused jet that limits the injected area, thereby reducing invasiveness. Comparative experiments with laser-induced jets show that, even at similar jet tip velocities, impact-induced jets achieve greater penetration depth. The penetration depth remains constant regardless of the offset distance D from the target, owing to the high and nearly uniform velocity of the cylindrical jet root region, indicating that penetration is governed by the cylindrical jet structure. Furthermore, we systematically vary the liquid viscosity, jet inertia, and elastic modulus of the skin-simulating material. To account for cylindrical liquid jet penetration, a shear deformation model is proposed, in which the jet kinetic energy is dissipated through deformation of the gelatin. The model shows good agreement with experimental results and provides a unified physical basis for liquid jet penetration.
Needle-free injection has attracted attention as a method of avoiding needlestick injuries, disposal problems, and pain or fear associated with needles [1,2,3,4,5,6].
A comparison of conventional needle-free injectors and laser-induced jets is illustrated in Fig. 1 and highlights the trade-offs between cost, safety, invasiveness, and injection volume. Various needle-free injection (NFI) technologies have been developed using spring-driven [4,7], compressed gasdriven [5,8], and lorentz force-driven systems [6,9]. As shown in Fig. 1, these conventional devices mainly target deeper injections, such as those into the muscle layer, and commonly produce diffused jets that expand wider than the nozzle orifice, causing excessive injection volume and strong stimulation of pain receptors [10,11,12,13]. To address these issues, laser-induced jets have been proposed as a new generation of focused jet injection systems [14,15,16,17,18]. Their high-speed and highly focused jets are well suited for shallow skin delivery, where they can target langerhans cells located near the epidermal surface, enabling strong immune responses with a small dose [10,19,20,21]. Because such jets are thinner than the nozzle diameters, they enable precise small-volume drug delivery [10,22], achieving accurate dose control with minimal pain and reduced dispersion compared to diffused jets [10,13,22,12]. However, the use of lasers introduces major challenges, including cavitation damage [23,24], drug thermal denaturation [10,25], and high device costs [26].
Here, we focus on a new focused jet generation technique, impact-induced jet generation, as a cost-effective, laser-free alternative. This mechanism produces a focused jet by accelerating liquid through impact and focusing it at a concave gas-liquid interface when the container is abruptly stopped [27,28,29,30]. Although previous designs could eject high-viscosity liquids [31,32], they did not reach the 75 m/s velocity required for human skin penetration [22]. By adopting a tapered nozzle shape, we have developed an impactinduced method for generating focused jets that exceed 100 m/s, which is sufficient to penetrate human skin [22,33,34], even for liquids with viscosities as high as 200 mm 2 /s. As shown in Fig. 2, this compact and low-cost system generates focused jets without lasers and can target the “Target” region in Fig. 1, resulting in low-cost, minimally invasive, and small-volume injection applications.
The penetration of liquid jets into skin-simulating materials has been widely studied [11,12,22,35,36,37,38,39,40,41,42]. Previous studies have revealed that the penetration depth is not only determined by the representative velocity of the jet, but also by the morphology of the jet and its internal velocity distribution [11,12,40,41]. In previous studies, the penetration behavior of liquid jets into skin-simulating materials has been investigated mainly by varying the jet inertia and the material elasticity, and the results have often been organized using the elastic Froude number F r e (F r e = inertial force / material elasticity) [37,35,38,43]. It has also been suggested that the viscous force of the liquid plays an important role in the penetration process and that the penetration behavior can be described in terms of the Reynolds number Re (Re = inertial force / viscous force) [22,42]. However, due to the technical difficulty of ejecting high-viscosity liquids at sufficiently high speeds, no systematic investigation covering a wide range of liquid viscosities has been conducted yet [44,45,46].
In this study, the penetration behavior of impact-induced jets capable of ejecting high-viscosity liquids into skin-simulating materials is investigated. Analysis of the internal velocity distribution of the liquid jets reveals a distinct penetration mechanism in the impact-induced jet. In this case, the penetration depth is not governed by the jet tip, as commonly assumed, but by the flow in the root region of the jet. Furthermore, systematic penetration experiments are performed by varying the inertial, viscous, and elastic forces of the liquid-solid system. On the basis of these experiments, a new penetration model, termed the shear deformation model, is proposed, in which the kinetic energy of the liquid jet is dissipated through the shear deformation of the skin-simulating materials, and the validity of this model is experimentally verified. We systematically vary the jet inertia, the liquid viscosity (by two orders of magnitude, 1 -200 mm 2 /s), and the elasticity of skin-simulating materials to establish a broadly applicable penetration model for needle-free injection applications.
We first conducted experiments to investigate differences in the penetration behavior of focused liquid jets penetrating skin-simulating materials when generated by impact-induced jets and laser-induced jets. Subsequently, we systematically varied the jet velocity of the impact-induced jets, the v
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