There is provided a 3D printing system, methods, and materials for the 3D printing of objects that include a cured hydrogel material, an uncured hydrogel material, and a support material. The cured hydrogel material may define a scaffold for organs or other biological structures. The 3D printing system selectively deposits the hydrogel material and support material, dries the hydrogel material, and selectively applies a catalyst to the hydrogel material to selectively cure the hydrogel material. Once the 3D printing has completed, the uncured hydrogel material may be drained and the support material may be melted or dissolved leaving a scaffold of cured hydrogel material that may be infused with living cells of the desired organ or biological structure.