Long-term Stabilized Amorphous Calcium Carbonate – an Ink for Bio-inspired 3D Printing

Amorphous Calcium Carbonate (ACC) is a highly unstable amorphous precursor many organisms utilize for the formation of crystals with intricate morphology and improved mechanical properties. Herein, we report for the first-time high-yield long-term stabilization of ACC, achieved via its co-precipitation in the presence of high amounts of Mg and an acetone-based storage protocol. A novel use of the formed high-Mg ACC paste as an ink for 3D printing techniques allows the formation of bio-inspired intricately shaped calcium carbonate geometries. The obtained ink can dry, though retains its amorphous nature, at a variety of temperatures ranging from 25 to 150˚C enabling various applications such as cultural heritage reconstruction and artificial reefs formation. We also show the on-demand low-temperature crystallization of the 3D printed ACC models, similar to what is achieved by organisms in nature. Using this bio-inspired crystallization route via transient amorphous precursor also enables the presence of high Mg levels within the calcite crystalline lattice, far beyond the thermodynamically stable solubility level. High levels of Mg incorporation, in turns, encompasses a great promise for the enhancement in the mechanical properties of the crystallized calcite 3D objects akin naturally found crystalline CaCO₃.