> The ink hardened as it dried, but its final physical properties depended on how much iron and phosphate were added to the mix. The more that was added, the more nanoparticles formed, and the stiffer and harder the final material became. By tweaking the recipe in this way, the researchers could create objects that were as flexible and rubbery as a squid or as stiff and hard as bone.

> “It should be possible to mix the ink at a ratio that you can change immediately prior to printing,” Dr. Joester said. “And that would allow you to to change the composition, the amount of nanoparticles, and therefore the strength of the material on the fly. Meaning that you can print materials where the strength changes very dramatically over relatively short distances.”

> The technique might be useful in the burgeoning field of soft robotics, allowing engineers to create machines that are hard and stiff in some places and soft and pliable in others, Dr. Joester said: “I think it would be amazing if you could print all of these gradients into the structure.”