The technology was just coming into its own in the early 21 st century. A century later, printers could build virtually anything solid, one atom at a time, as long as the raw materials were available. The catch was energy . It took a lot of energy to reduce source materials to their monatomic form, and it took as much energy to drop them into the proper place in the creation matrix. Such 3D printing had had to wait for cheap fusion energy before becoming practical technology.