There's a hint at the limitations in the article: While developing the process, the team produced 3-D printed structures made from copper, nickel, silver, and

There's a hint at the limitations in the article: While developing the process, the team produced 3-D printed structures made from copper, nickel, silver, and various metal alloys. Copper, nickel, and silver are all relatively "noble" metals, easily reduced to the metallic state from their salts or oxides. I'm guessing that this technique does not work with more reactive metals like aluminum, titanium, or magnesium. It may not even work with iron. EDIT: the full article in Nature is open access and has more details: https://www.nature.com/articles/s41586-022-05433-2 https://www.nature.com/articles/s41586-022-05433-2 Calcination in air converts the metal-salt-swollen hydrogels to metal oxides, and subsequent reduction in forming gas (95% N2, 5% H2) yields metal or alloy replicas of the designed architecture. So this process is limited to elements that can be reduced from the oxide to the metal by gaseous hydrogen. Iron could work but aluminum, titanium, magnesium, and other reactive metals would not.