It does not exist. There are "e-tattoos" that are the same exact thing as adhesive label RFID / NFC tags, but use glue that is designed for skin. There are also now "tech tats" which are basically conductive ink that is laid down on skin instead of paper to create rudimentary circuits.
There is no case in which the traditional tattooing process could create conductive pathways in the skin. There are many reasons for this, but the top two are:
1) Ink that is injected into the skin via tattoo needle becomes separated into small droplets as the skin heals and integrates the ink. Each droplet is separated by skin cells and collagen, and they are not congruous.
2) Skin itself is conductive. Your body is full of water and salt, and without a proper insulator in place, any conductive circuit pathway running through the skin or body will see huge losses and leakage current issues. Even electric eels shock themselves in the process of hunting their prey.. nature hasn't figured out a good insulator yet.