Biomechanical tattoos on the back
Now, if you want to expand even more with a biomechanical design, what I recommend is that you choose the back. The most recurrent are the sketches that include the spinal column and turn it into a mechanical piece with electronic devices and other robotic accessories.
Inside the back, you can also play with the ribs. Choosing one or both sides, the alternatives range from bones replaced by metal tubes, to resistant armor made of steel materials.