Bio-mechanical tattoos are basically tattoos depicting human robotics. They are a style of tattoo popularized by illustrator H. R. Giger, the designer of the creature from the Alien movies. Bio-mechanical tattoos are most commonly recognized as showing flesh intertwined with machines or mechanics.
They were popular in the late 80's and early 90's but seem to be witnessing a resurgence in interest. The majority of people requesting bio-mechanical tattoos are guys who get them done mostly on their upper arms, legs, back and wrists. They can be done as small pieces say like a gash looking into the flesh or as an entire piece on the back for example.
The most common colors for bio-mechanical tattoos are usually black and gray but colors are used to great effect in the 'android' style.