Biomechanical style tattoo designs can make extremely unique and effective tattoos, particularly when the natural form of the human body is used for the "bio" and the tattoo implies some sort of mechanical understructure, or cyborg body. These designs are very simple to make as long you keep in mind the two major aspects of them: machines or machinery and biological structures which can be anything that grows, from plants to the more common animals and humans.
Decide what you want to draw: a machine animal, a cyborg foetus, a human eating biomechanical tree, or maybe to simply a few cracks in the human exterior to show the machine underneath.
Draw the bio part of your image first, whether it's the outline of a crack with a few muscles beneath, a foetus, a plant or an animal.
Detail the drawing as you want the tattoo to look, but for now only focus on the bio-anatomical characteristics.
Add in the mechanical. This part is easy and fun, essentially just add in machine parts wherever you want. Erase the bio parts that are underneath and replace them with things like springs, screws, watch parts.
Your design will be biomechanical so long as you have at least one mechanical aspect, but you can have as many as you want so long as you don't forget the bio-anatomical aspects. You can even take inspiration from real life instances of biomechanics like artificial hearts or hearing apparatus.
To avoid confusion in your design, try to stick to a single time period; avoid mixing hardware pieces from computers (chips, batteries, etc.) with antique radio machinery for example.