Synthetic bone printed on a 3D printer stimulates bone regeneration when implanted into animals, according to a new study. Ramille Shah of Northwestern University and colleagues printed “hyperelastic bone” out of hydroxyapatite (a mineral similar to that in natural bone) mixed with one of two polymers already used in medicine. Implanted into mice, rats and one macaque, the samples were integrated into tissue and caused new bone to grow without negative side effects. A printed “bone” shaped like a human femur was able to withstand natural loads, and the ease with which various shapes can be printed makes the technique promising for humans.