Amorphous Calcium Phosphate Mediated Hydroxyapatite Nucleation Kinetics

Vertebrate bones, teeth and mollusk shells are biological mineralization materials, which have excellent mechanical properties. So, biological minerals have attracted much attention and fascination in bio-inspired or biomimetic materials chemistry. However, the challenges for biomimetic mineralization are that the composition and structure of synthesized bone, tooth or shell is very different from biological minerals. Considering biological minerals has specific size, shape, orientation, composition and hierarchical organization. So increased understanding of biomineralization mechanisms has greatly enhanced the possibilities of biomimetic mineralization approaches.Moreover, increasing evidences believed that biological minerals are formed from amorphous phase which is considered as precursor phase in organism. So it is very important and urgent to know how these amorphous phases transformed into crystallites. In this thesis, we studied the processes of amorphous calcium phosphate (ACP) mediated hydroxyapatite nucleation kinetics. In simulated body fluid (SBF), we build an ACP-mediated HAP nucleation kinetics model. And then based on this model, we studied the effect of ACP surface modification, the aggregation state of ACP and pH on HAP nucleation kinetics. This thesis contained six chapters:In chapter 1, we briefly introduced the basic concepts in biomineralization, including the variety of biological minerals and the process of mineralization in living organism. After that the theory of classical crystallization and non-classical crystallization pathway was described in detail, especially the amorphous mediated nucleation pathway. Based on above understanding, the challenges of classical nucleation theory were mentioned and inspired us to build an amorphous mediated nucleation kinetics modelIn chapter 2, we conducted a series of experiments to obtain nucleation kinetics data in simulated body fluids (SBFs) under different supersaturations and temperatures. It turned out that these kinetics data are hard to explain by classical nucleation theory (CNT). By further characterization, there is amorphous calcium phosphate (ACP) existed in solution before hydroxyapatite (HAP) formation. Due to amorphous mediated nucleation pathway is different from classical nucleation pathway, so we build an ACP-mediated HAP nucleation kinetics model. This model indicates that the amounts of ACP and the activities of free calcium are the keys to control HAP nucleation. In addition, we considered that HAP nucleation occurred on ACP-solution interface. These results help us to understand the mechanism of mineralization and how to control the nucleation rate.In chapter 3, we investigated the ACP surface modification on HAP nucleation kinetics. By different introducing protocols, we found that polymer additives can adsorbed on ACP surface or embedded inside ACP. Compare to control, we can see that when polymer additives adsorbed on ACP surface the induction time (ti) was prolonged significantly. According to our ACP-mediated hydro xyapatite nucleation model, the ACP-solution interface is the key for HAP nucleation. So when polymer additives adsorbed on ACP surface, thus block HAP nucleation site. This results consistent with our nucleation model.In chapter 4, we demonstrated the effect of ACP aggregation state on ACP-mediated HAP nucleation. According to our ACP-mediated HAP nucleation kinetics model, due to HAP nucleation occurred on ACP-solution interface. So changing the aggregation state of ACP can increase the effective surface area of ACP, thus promote HAP nucleation. Since collagen-I molecular can self-assembled into collagen-I fibrils, and in previous understanding, during mineralization collagen-I fibrils are always considered as a passive template. In our study when preventing ACP self-aggregation, HAP nucleation was promoted. However, when ACP in aggregation state, no matter how much collagen-I fibrils were introduced the nucleation rate of HAP almost has no change. These results mean that the aggregation control of ACP is important in HAP nucleation control and prove that collagen-I fibrils were just as a passive template in our system.In chapter 5, the effect of pH on HAP nucleation was studied based on our above ACP-mediated HAP nucleation model. We found that HAP nucleation faster at low pH (low supersaturation), and this phenomenon contradicts CNT understanding. In CNT, the crystal nucleus is directly formed from ions in supersaturated solutions. But using ACP-mediated HAP nucleation kinetics model, we successfully explain this abnormal phenomenon and found that the activity of free calcium can be controlled by pH. So in the view of nucleation kinetics, we confirmed that ACP-mediated nucleation pathway is different from classical nucleation and the model built on chapter 2 is still valid in different pH.In chapter 6, we summarized the ACP-mediated HAP nucleation kinetics and how to control this process (For example, ACP surface modification, ACP aggregation state and pH). Moreover, according to these conclusions, we also analysis the shortages of this work and unsolved issues for further study.