Visualization of drug delivery via tattooing: effect of needle reciprocating frequency and fluid properties

Tattooing is a commonplace practice among the general populace in which ink is deposited within dermal tissue. Typically, an array of needles punctures the skin which facilitates the delivery of a fluid into the dermis. Although, a few studies in the past have investigated the potential of tattooing as an intradermal (ID) drug injection technique, an understanding of the fluid dynamics involved in the delivery of fluid into skin is still lacking. Herein, we sought to provide insight into the process via an in vitro study. We utilize a five needle flat array (5F) with a tattoo machine to inject fluids into gelatin gels with modulus \(\sim\) 15 \(\,\mathrm {kPa}\). High-speed imaging was used to visualize the injection process and estimate the amount of fluid delivered after each injection up to the \(50 \mathrm {th}\) injection. We investigate the role of reciprocating frequency (\(f = O(10 - 100)\, \mathrm {Hz}\)) and physical properties of the fluids on the volume infused (\(V_o \sim O(100) \,\mathrm {nL}\)) after injection. We find that \(V_\mathrm {o}\) is only slightly dependent on f, however volume delivered increases with decreasing viscosity. In addition, we illustrate the physical mechanism of infusion during tattooing, which has not been reported. An understanding of the injection process via tattooing can be useful in the development of ID tattoo injectors as drug delivery devices.