Gas-driven needle-free dental injection enables tunable and predictable soft-tissue penetration: high-speed kinematic and micro-computed tomography validation

Needle-free jet injectors are a promising alternative to conventional dental needles, particularly for patients with injection-related fear. However, the actuation mechanism can substantially affect jet formation, delivery consistency, and tissue penetration. This study compared portable gas-driven jet injectors with a conventional spring-driven device and evaluated whether jet velocity could be used to modulate penetration behavior in soft tissue. Four gas-driven injectors with different inlet-side restriction settings were compared with a spring-driven comparator. High-speed imaging was used to quantify piston motion, flow rate, and jet velocity. Soft-tissue penetration was assessed in ex vivo chicken breast using a radiopaque silver nitrate tracer and micro-computed tomography (micro-CT). Penetration volume, maximum depth, maximum cross-sectional area, depth of maximum area, and average cross-sectional area were analyzed. The gas-driven injectors showed smooth piston advancement, quasi-steady jet profiles, and complete discharge, whereas the spring-driven comparator showed an impulsive early peak, recoil, fan-spray tendency, and incomplete discharge. Mean jet velocity exceeded 13 m/s in all gas-driven groups but was 2.40 m/s in the spring-driven group despite a much higher peak velocity. Micro-CT showed no significant between-group difference in detectable tracer volume. Increasing jet velocity in the gas-driven groups was associated with deeper and narrower penetration tracts, whereas lower jet velocities produced shallower and broader dispersion. Gas-driven needle-free injection provided more stable kinematics and more controllable penetration geometry than the spring-driven comparator under the present ex vivo conditions. Velocity-based regulation may enable procedure- and site-specific optimization of needle-free dental local anesthesia, although clinical validation in intraoral tissues is still required.