Experimental investigation on the injection mechanism for a pneumatic needle-free injector under different skin tissues

Needle-free injection technology possess several advantages over traditional needle injections. This type of device is designed to achieve drug delivery by interacting with the skin through a high-velocity microjet stream, rather than delivering the liquid drug through a hollow needle. This work focuses on the influence of the jet velocity and interlayer combinations of the penetrated medium on the injection process of the pneumatic needle-free injection. Skin tissues with different Young's modulus and tissue layer thicknesses were modelled by varying the combination of gels with different mass fractions (from 0.5 % to 6.0 %) or tissue layer thicknesses (from 5 mm to 20 mm) in single-layered, two-layered and multi-layered gels, respectively. The high-speed imaging data showed that the injection process of jets generated by pneumatic needle-free injection into gels can be divided into three phases: erosion, stagnation and diffusion. Meanwhile, the thickness of the subcutaneous layer of the multi-layered gel was 5 mm–10 mm corresponding to intramuscular injection and 20 mm or more corresponding to subcutaneous injection at a jet velocity of 127.44 m/s. It was also found that the jet velocity can be adjusted according to the mechanical properties of the skin tissue, allowing subcutaneous or intramuscular injection. Our findings contribute to a better understanding of the injection mechanism of the pneumatic needle-free injection in skin with different mechanical properties.