That tattoo needle doesn’t do what you think it’s doing

CHICAGO—When you get a tattoo, a cluster of needles jabs your skin as many as 200 times per second, hopefully under the guidance of a highly skilled artist who crafts that dragon or lotus flower you’ve always wanted. Strictly speaking, however, the needles do not push ink into your flesh. Rather, each opens a void that sucks in the ink as the needles lift out of the skin.

The observation isn't new, but it could be exploited to deliver certain vaccines in a new way, Idera Lawal, a chemical engineer at Texas Tech University, reported here this week at the March meeting of the American Physical Society.

The hollow hypodermic needles typically used to deliver vaccines depend on positive pressure. The needle sinks below your skin or into your muscle; the person administering the vaccine then pushes on the needle's plunger, driving the liquid through the needle's motionless tip.

However, that approach may not work for every type of vaccine, Lawal says. For example, vaccines under development that consist of DNA are generally too thick to flow through a standard hypodermic needle.

It may be possible to deliver such vaccines via tattooing instead, Lawal says. That's because the physics of a tattoo needle differ greatly from those of a hypodermic needle.

Unlike a hypodermic needle, a tattoo needle does not inject liquid when it is sunk into the skin. Rather, as the solid, ink-coated stylus descends, it opens a small hole up to 2 millimeters deep. Only as the needle pulls out of the skin does the vacuum in the hole draw ink into the skin—as Lawal showed in this video in which a clear gel serves as a surrogate for flesh. He says he could find no reference to the effect in the scientific literature—although it's clearly familiar to tattoo artists.

"It's a very pretty video," says Thomas Sykes, a physicist at the University of Oxford who was not involved with the work. Lawal's data show half the ink is delivered in 10 of 50 jabs, Skyes notes, suggesting there may be a smaller optimal number of jabs for delivering a load such as a vaccine. But whether your friends would consider you cool for getting a vaccine via tattoo remains to be seen.