Calcium Carbonate Skeletal Material Is Synthesized via Phase Transition of the Calcium Carbonate Cartilaginous Material.

The formation mechanism of calcium carbonate (CC) skeletal tissues in biomineralization has remained poorly understood for a long time. Here, we propose an artificial CC biomineralization system equivalent to the natural one in terms of the primary physicochemical mechanism. Our system is constructed of a polymer gel and a CC solution unsaturated by a dissociated anionic polymer. The gel network consists of proton donor and proton acceptor polymers, which are analogues of polymers in the natural biomineralization system and have affinity for each other through hydrogen bonding interaction. Artificial biomineralization takes place within the polymer gel to produce a monolithic composite of the network and CC, whose powder X-ray diffraction pattern indicates calcite or calcite/vaterite. Scanning electron microscopy and energy-dispersive X-ray spectroscopy observation of the composite during the mineralization process revealed a two-phase structure (network/CC solid solution phase and CC hypercomplex gel phase). As artificial biomineralization proceeds, the solid phase grows in size at the cost of the gel phase as if the latter is substituted with the former, until the solid phase occupies the whole depth of the composite. These results suggest that the hypercomplex gel is the precursor of the resultant network/CC solid solution, and its discontinuous change is a phase transition to the solid solution. Despite minute differences in higher-order structures between our model system and the natural system, the fundamental structure of CC skeletal tissues in the latter can be interpreted as a network/CC solid solution, whereas that of CC cartilaginous tissues as a CC hypercomplex gel. Then, it can be deduced that, in biomineralization, the CC skeletal tissue is in principle formed via a phase transition of the CC cartilaginous tissue.