Abstract
The fabrication of responsive soft materials that enable the controlled release of microbial induced calcium carbonate (CaCO3) 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 CaCO3 polymorphic regulation, the degree of control over the CaCO3 remains insufficient to meet these requirements. We demonstrate that the amount and location of CaCO3 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 CaCO3-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.
Highlights
Calcium carbonate (CaCO3) is one of the most abundant materials in the world, used, for example, in the cement industry,[1] for papermaking,[2] and drug delivery.[3,4] It plays an important role in nature, for example, for paleoclimate reconstructions,[5] ocean acidi cation,[6] and biomineralisation.[7,8,9] CaCO3 has three mineral polymorphs that are in order of decreasing solubility and increasing thermodynamic stability: vaterite, aragonite, and calcite.[10]
The process parameters used in this study yield a production rate of 100 beads per minute, with a mean bead diameter of $1 mm and $0.75 mm for alginate and carboxymethyl cellulose (CMC), respectively, as shown in Fig. 2A(i) and summarised in Table S1.† To ensure that enough urea is hydrolysed at a given location, and that enough CaCO3 is formed, we adjust the number of bacteria per bead to 4.4 Â 107 cfu per bead
Paper imposed on the bacteria contained in this solution during the mixing and extrusion process is much higher,[54] despite of its higher yield strain, as shown in Fig. S3.† The higher shear force required to mix the CMC solution is likely responsible for bacteria immobilised in CMC displaying a lag phase of 1 day and a slower growth rate during the rst 24 h of the exponential phase than those contained in alginate, as shown in Fig. S2A.† Within day 2, the encapsulation yield (EY) was 50.4 Æ 3.91, in agreement with the $55% reported in a previous study.[55]
Summary
Calcium carbonate (CaCO3) is one of the most abundant materials in the world, used, for example, in the cement industry,[1] for papermaking,[2] and drug delivery.[3,4] It plays an important role in nature, for example, for paleoclimate reconstructions,[5] ocean acidi cation,[6] and biomineralisation.[7,8,9] CaCO3 has three mineral polymorphs that are in order of decreasing solubility and increasing thermodynamic stability: vaterite, aragonite, and calcite.[10]. We focus on showcasing this approach to produce site-speci c biomineralised composites that bind soil particles together by embedding beads in sand specimens due to contemporary interest in the use of MICP for soil reinforcement applications (Fig. 1D).[25,27,49] as a novel biomimetic regulating technique of MICP, this platform technology opens new possibilities for the design of CaCO3 crystals with tailorable microstructures, and mechanical properties It enables the design of living materials that endow geosystems with the ability to sense, heal, and develop immunity to harmful environmental, climatic, and human actions, vital for the development of sustainable practices
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