Mironov and Boland hope it will be possible to print the entire network of arteries, capillaries and veins that nourish organs. But to keep cells alive, the organs would have to be completed within a couple of hours and a growth medium circulated through the fragile new vessels. Large structures might not be strong enough to hold together if the gel is removed after such a short period. However, the team is already experimenting with adding substances such as the skin protein collagen to speed fusion and reinforce structures.