High speed imaging of solid needle and liquid micro-jet injections

We have used high speed imaging to capture the fast dynamics of two injection methods. The first one and perhaps the oldest known is based on solid needles and used for dermal pigmentation, popularly known as tattooing. The second is a novel needle-free microjet injector based on thermocavitation. Injections in agarose gel skin surrogates were made with both methods and ink formulations having different fluidic properties. Water, a glycerin–water mixture, and commercial inks were used with both injectors to understand better end-point injection. The agarose deformation process due to the solid needle injection helped establish an assessment of penetration potential by using the dimensionless penetration strength quantity. We found that microjet injections are superior than solid injections in terms of energy and volumetric delivery efficiencies per injection for three different liquids. The microjet injector could reduce the environmental impact of used needles and benefit millions of people using needles for medical and cosmetic use.