Controlling the calcium carbonate microstructure of engineered living building materials.

The fabrication of responsive soft materials that enable the controlled release of microbial induced calcium carbonate (CaCO 3 ) precipitation (MICP) would be highly desirable for the creation of living materials that can be used, for example, as self-healing construction materials. To obtain a tight control over the mechanical properties of these materials, needed for civil engineering applications, the amount, location, and structure of the forming minerals must be precisely tuned; this requires good control over the dynamic functionality of bacteria. Despite recent advances in the self-healing of concrete cracks and the understanding of the role of synthesis conditions on the CaCO 3 polymorphic regulation, the degree of control over the CaCO 3 remains insufficient to meet these requirements. We demonstrate that the amount and location of CaCO 3 produced within a matrix, can be controlled through the concentration and location of bacteria; these parameters can be precisely tuned if bacteria are encapsulated, as we demonstrate with the soil-dwelling bacterium Sporosarcina pasteurii that is deposited within biocompatible alginate and carboxymethyl cellulose (CMC) hydrogels. Using a competitive ligand exchange mechanism that relies on the presence of yeast extract, we control the timing of the release of calcium ions that crosslink the alginate or CMC without compromising bacterial viability. With this novel use of hydrogel encapsulation of bacteria for on-demand release of MICP, we achieve control over the amount and structure of CaCO 3 -based composites and demonstrate that S. pasteurii can be stored for up to 3 months at an accessible storage temperature of 4 °C, which are two important factors that currently limit the applicability of MICP for the reinforcement of construction materials. These composites thus have the potential to sense, respond, and heal without the need for external intervention.