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.