The Binding of Isaac

To unlock this item you need to use the Blood Donation machine 20 times over the course of your playthroughs. Blood Donation machines can be found randomly in any room on a floor and can be fairly common on the Basement/Cellar floors or they can generally be found in an Arcade room. To find an Arcade you need to enter the second floor of any chapter (Basement II, Caves II etc) with at least 5 cents then you must pay one cent to enter and a Blood Donation machine should be there. Just run into it 20 times to unlock the item.

Items that give temporary invincibility (Chariot card, Book of Shadows, Celtic Cross etc) help make this easier as you can use the machine without losing health while the invincibility is active.