Reinforcing hydrogels with in situ formed amorphous CaCO3

Mineralizing hydrogels with CaCO3 is an attractive strategy to reinforce them, because of the excellent mechanical and biological properties of CaCO3. Unfortunately, the degree of hydrogel reinforcement achieved with CaCO3 remains limited. An important contributing factor is the poor control over the CaCO3 formation that leads to an incomplete understanding of its reinforcing mechanism. To address this challenge, we systematically investigate the influence of the formation of CaCO3 on the mechanical properties of a model hydrogel, poly(acrylamide) (PAM), that is reinforced by these minerals. We demonstrate that the amount, size, structure and morphology of CaCO3 significantly influences the mechanical properties of mineralized hydrogels. For example, while the fracture energy of PAM hydrogels is increased 3-fold if reinforced with individual micro-sized CaCO3 crystals, it increases by a factor of 13 if reinforced with a percolating ACC nano-structure that forms in the presence of a sufficiently high quantity of Mg2+ additives. These insights will likely enable a more targeted reinforcement of hydrogels with CaCO3 by tuning the mechanical properties of the resulting composites over a much wider range than is currently possible