Here's a series of sketches I did by tracing photographs of myself with my left shoulder in multiple poses. I had sketched a really rough design for the tattoo on my shoulder to give me an idea of how and where it would stretch and flex, and I was really pleased with the result.

The idea here is to make it look as much as possible like a robotic shoulder joint, made of segmented panels of skin only interrupted by the seams you see here. My desire was to make this eventually look heavily anime-robot inspired, as I'm not a fan of the more 'bio flesh cyborg' tattoos that are more common.