Hydrogel Mineralization via Amorphous Precursors*

Marine organisms exploit varied physical and chemical interactions between calcium carbonate and organic additives to yield mineral-organic complexes that are not only visually stunning but also highly functional. Despite recent leaps in the understanding of these systems, knowledge of the relation between kinetics of mineral growth, microstructure of the composite and mechanical response is still lacking. Our aim is to mechanistically elucidate potential pathways that afford control of the mineralization pathway of hydrogels and of their microstructure, and yield composites with tunable mechanical response. Our previous studies demonstrated that the formation of amorphous calcium carbonate precursors (ACC) throughout agarose hydrogels is a diffusion-limited process, and therefore, it is strongly affected by the solution composition and by the hydrogel composition. In contrast, amorphous precursors in the hydrogel control tightly the inclusion of the polymer into calcite, and crystal morphology, as well as the rate of crystal growth, affording a uniform crystal growth throughout the hydrogels, in the absence of concentration gradients, over a wide range of solution conditions and hydrogel compositions. Based on these findings, we have extended these studies to phosphates, and show that amorphous precursors also modulate the mineralization kinetics quite tightly. Furthermore, the phosphate-mineralized hydrogels exhibit key advantages with regard to microstructure -as provided by hydroxyapatite nanosheets- and mechanical response, compared to the mineralized hydrogels in the absence of phosphates. The results of this work not only reveal an important mechanism underlying (bio)mineralization but it can also inspire new avenues to craft biomimetic materials.