Characterization of skin blebs from intradermal jet injection: Ex-vivo studies

This paper presents results from an ex-vivo study of intradermal jet injections, which is an attractive method to achieve both needle-free and fractional dose delivery of vaccines. Due to the fact that fluid properties of many novel therapeutics and vaccines can vary significantly, a key parameter for our study is the fluid viscosity, whilst the main focus is on determining the best correlation between the delivered volume and geometrical dimensions of the fluid deposit. For this we use a combination of top-view (skin wheal), underside (below the dermis), and cross-section (true skin bleb) perspectives and find that the top-view alone, as done in clinical practice, is insufficient to estimate the volume deposited in the dermis. Overall, the best correlation is found between the injection volume and cross-sectional diameter, however there is significant variation amongst the different fluids. For mean injection volumes of 60 μL the mean bleb diameter is ≈8 mm, with mean aspect ratio h¯/d=0.38, indicating the blebs are mostly oblate. However, the shape varies with viscosity and the higher viscosity does not spread laterally to the same degree as lower viscosity fluids. In addition, our high-speed video observations of the injection process, reveal some interesting dynamics of the jet injection method, and we modeled the bleb growth with an exponential saturation.