Unwind by Neal Shusterman
In a society where unwanted teens are dissected so that their body parts may be harvested for transplant, three runaways fight the system that makes it lawful to "unwind" them.
Disturbing! The suspense of this tale will have you biting your nails until the very end, upon which you will cringe as the process of being "unwound" is finally described! This title is not for the faint of heart.