Barcode

Barcode tattoos are a very common tattoo. The tattooed codes can be machine read, assuming the tattoo is of sufficient quality (although the vast majority are not of sufficient quality, and the odds of them being readable as they age are slim to none).

The easiest way to make a barcode that "says" what you want it to is to download a barcode font. It will probably come with a readme file that explains how to use it and what its limitations are—there are a wide variety of different symbolic elements which allow for different types of encoding:

UPC, EAN, Bookland, and ISSN

These are numeric only, fixed length, and contain at least one check digit (for error checking).

Code 39

This is the most common code set for inventory tracking and ID. It can be any length, and supports alphanumeric — there is even a full ASCII version.

POSTNET

These are used for ZIP codes, and rather than varying in width, they vary in height. This method of encoding would not be particularly relevant to tattooing.

Code 128

To simplify, this is a "dense" version of Code 39. There's even a subset of Code 128 that is numeric only and employs compression — in the "real world", this method is used when space is at a premium.

Note that there are many more encoding schemes than the ones mentioned above.

Barcode

Barcode

Barcode

Barcode

YouTube video of voice-synth barcode reading "tattoo" style stickers