At the moment, the inks are made of suspended ceramics, including silica and also hydroxyapatite the primary constituent of bone.

ECONOMIST: A new way to build complicated materials, micron by micron

That gives the material strength, as there are fewer places for stress to build up and fractures to form than in hydroxyapatite made haphazardly of larger, more heterogeneous grains.

ECONOMIST: Nanotechnology