Splash-back refers to contamination upon the nozzle of the jet injector. This undesirable phenomenon occurs when the jet stream impinges the skin during hole formation, in which debris—consisting of fluid, tissue cells, blood, and bodily fluid—splashes back onto the jet injector nozzle and possibly into the nozzle orifice.
Splash-back is more likely to occur when the nozzle orifice is not pressed firmly against the skin or with jet injectors that have a spacer which creates a gap in between the nozzle orifice and skin.
The following illustrations demonstrate the concept. In the first image, the jet injector is actuated and the jet stream is emitted from the nozzle orifice, as demonstrated with the blue arrow.
In this image, the jet stream penetrates the skin, initiating hole formation. This will cause debris, as mentioned above (fluid, tissue cells, blood and bodily fluids), to splatter, as demonstrated with the red arrow.
Since the nozzle of the jet injector is in the immediate splatter field it will become contaminated, as demonstrated below.
“Where is the proof?” you may question. Glad you asked.