Rheological Characterization of Bioinspired Mineralization in Hydrogels

With increasing amounts of CO₂ in the atmosphere linked to potentially catastrophic climate change, it is critical that we find methods to permanently sequester and store CO₂. Inspired by the natural biomineralization of calcium carbonate (CaCO₃), one future goal of this project is to understand the mechanisms of CaCO₃ mineralization in order to ultimately optimize a bioinspired hydrogel system, which produces high value industrial powders that consume CO₂ as a feedstock. Along the way, we are developing a rheological technique to study mineral nucleation and growth events by measuring the modulations in mechanical properties of a hydrogel system during mineralization. Although the structure and formation of natural CaCO₃ materials have been intensely studied, questions regarding the mineralization pathways and early thermodynamics and kinetics of nucleation and growth still remain. Given its suitability for use with hydrated samples, rheology is a useful tool to study these hard-soft viscoelastic systems to try to capture early mineralization kinetics, which are difficult to study using traditional microscopy techniques. Our system consists of a gelatin hydrogel matrix, which is preloaded with calcium ions, and an aqueous solution of carbonate ions, which are allowed to diffuse through the gel to initiate the mineralization process. In order to monitor how the growth of minerals affects the mechanical properties of the gel network, we measure the storage (G’) and loss (G”) moduli and relaxation modulus of the system using a parallel plate rheometer. We have found that gels with minerals exhibit higher storage and loss moduli than those without minerals and relax on longer timescales with additional modes of relaxation. We believe these differences are results of interactions between the CaCO₃ and gelatin interfaces. By learning how these mechanical signatures are linked to the physical and chemical interactions of the system, we will be able to use nondestructive rheological characterization to understand how modulations in the gel result in changes in the grown mineral. Exploiting this new knowledge, we strive to tailor hydrogel systems to produce a wide variety of CaCO₃ powders with different polymorphs and morphology, which can be used for industries such as plastics, paper, cosmetics, paints, and pharmaceuticals.