Visualization of Tattooing: What happens beneath the surface?

Since the Stone Age, the tattooing process has been used for medical and aesthetical purposes. This process involves a needle (or an array of needles) being used to deposit ink into the dermis. Over the past decade, several studies have reported the prospects of using tattooing as an intradermal (ID) drug delivery technique, however an understanding of the fluid dynamics involved in the delivery of fluid into the skin is still lacking. In this study, we probe into this process through in vitro experiments. We use both arrays, e.g., a “flat-line” 5-needle array, that is typically used by tattoo artists, as well as single-needle devices for more detailed investigation of the dynamics, to inject fluids into transparent gels with moduli ~ 15 kPa. High-speed imaging was used to visualize the injection process, along with image analysis to estimate the amount of fluid delivered after each injection up to the 50^th injection. We investigate the role of reciprocating frequency (f=O(10-100)Hz) and physical properties of the fluids on the volume infused (Vo) after injection. Based upon our observations, we hypothesize that the principal delivery mechanism is capillary imbibition.