Amorphous Calcium Carbonate in Biomineralisation and Synthetic/Biomimetic Chemistry


Student thesis: Doctoral Thesis

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Awarding Institution
Award date17 Aug 2021


Since calcium carbonate is one of the most abundant and common biogenic minerals in nature, it is not surprising that many studies have focused on its formation and application. Amorphous calcium carbonate is the most unstable polymorph in the calcium carbonate family. Organisms can use this adjustable intermediate state (amorphous state of calcium carbonate) to produce their desired structure. For example, the exoskeleton of some organisms (such as crustaceans, sea-shell nacre, and brittle star eyes) is based on calcium carbonate. However, the mechanism of biomineralization has always been covered with a veil of mystery. In addition, with the controllability of amorphous calcium carbonate, many researchers have been inspired to design and synthesize new materials by using ionic additives or organic polymers. Therefore, the research of this thesis is based on the amorphous calcium carbonate system, deeply excavating the mechanism of the biomineralization process, and ingeniously designing two novel materials.

The first work in this thesis (Chapter 2) discloses that biomineralization likely operates through a supervariate mechanism based on multi-ionic solutions, a mechanism that enables convenient phase and kinetic regulation through stress control. Specifically, from solutions of multiple ionic components, bioceramics with highly variable (supervariate) compositions are first produced in a gelatinous state of exceptional stability, which offers convenience in material storage, transportation, molding, and processing. Counter-intuitively, the supervariate wet gels can be solidified by simply compacting them under a mild force, while the formulas (e.g., carbonates or phosphates), hydration levels, and phases (amorphous or crystalline) of the resultant bioceramics can be tailored. Furthermore, we propose that the biogenic amorphous minerals (e.g., amorphous calcium carbonate, ACC) are very likely stabilized by constricting their volume at the microscale, so that they are prohibited from undergoing the prerequisite dehydration step (which requires extra volume) preceding crystallization. The new biomineralization mechanism described here answers a pivotal question on bioceramics of life.

In our second work (Chapter 3), we have designed and fabricated a series of amorphous hybrid salts derived from amorphous calcium carbonate. The as-prepared amorphous hybrid salts (CaMgNiCoCu-AHS) exhibited favourable light absorption capacity within UV-Vis-NIR region and demonstrated great potential as a photothermal material for applications on solar-driven water purification. The typical sample achieved a high evaporation rate of 0.74 kg·m−2·h−1 under one sun illumination and a good stability. Low cost and simple preparation process can provide convenience for large-scale production of this material. This novel material holds great potential for solar energy utilization, seawater desalination and sewage treatment systems in the future. In addition, the cation used in the synthesis process can also be extended to many other ions not mentioned in this study for further exploring the characteristics of various amorphous hybrid salts.

In our last work (Chapter 4), we successfully fabricated a pressure sensor with high sensitivity and good biocompatibility by introducing a biosimilar ACC-based multi-ion system (Ca2+, Mg2+, CO32-, PO42-) into the organic matrix. The obtained mineral hydrogel exhibits good mechanical properties. The pressure sensitivity of the mineral hydrogel-based capacitive sensor shows almost linear response up to 1.0 kPa, which allows it to sense gentle finger tapping and human motion. The stability of the device has also been verified by being subjected to 50 cycles of compression. Multiple advantages make this mineral hydrogel-based ionic skin bioavailable in many potential applications, such as human motion detection, personal healthcare, and wearable devices, etc. Moreover, excellent anti-swelling and anti-icing properties make it optional for applications in complex environments.