A tattoo that is warning you for too many hours of sunlight exposure, or is alerting you for taking your medication? Next to their cosmetic role, tattoos could get new functionality using intelligent ink. That would require more precise and less invasive injection technique. Researchers of the University of Twente now develop a micro-jet injection technology that doesn’t use needles at all. Instead, an ultrafast liquid jet with the thickness of a human hair penetrates the skin. It isn’t painful and there is less waste. In their new publication in the American Journal of Applied Physics, the scientists compare both the needle and the fluid jet approach.