Principles of Mimicking and Engineering the Self-organized Structure of Hard Tissues

Abstract

The mechanism of the formation of a self-aligned hydroxyapatite (HAP) nanocrystallite structure was examined. It is found that the highly ordered HAP nanocrystallite assembly is attributed to the so-called self-(homo)epitaxial nucleation and growth. On the other hand, according to this mechanism, a high supersaturation will give rise to a random assembly of HAP crystallites. The effects of ions, biosubstrate, and supersaturation on the micro/nanostructure correlation between substrate and biominerals as well as their implications in hard tissue formation were examined. Surprisingly, some biomolecules are found to be able to suppress the supersaturation-driven interfacial structure mismatch and hence promote the well aligned HAP pattern formation.The biomineralization of compounds such as hydroxyapatite (HAP), 1 Ca 5 (PO 4 ) 3 (OH), and CaCO 3 plays a vital role in our life and the environment around us (1-4). HAP is the key component of teeth and bones. Many hard tissues consist of well organized and assembled aggregates of biomineral nanocrystallites. Such self-organized and assembled biomineral aggregates possess properties far superior to those of the single crystals alone. For instance, human teeth mainly consist of well aligned and compact HAP crystals, which provide high hardness and elasticity, whereas the HAP crystals of the bones are less well ordered (5, 6). Evidently, differently assembled structures among biominerals and organic substrates acquire different elastic and mechanical properties. However, the mechanism of forming these mineral structures in biological systems is not yet clear.Previous studies of biomineralization imply that nucleation plays an important part in the pattern formation of biomineral nanocrystallites through the mediating role of many organic and inorganic substrates or particles (7-9). The occurrence of these additives not only determines the crystallization process but also has a significant impact on the size, shape, aggregation pattern, and properties of the materials. Thus, a fundamental understanding of the mechanism of nucleation is essential for the methods of engineering of micro/nanostructures of materials applicable across a broad spectrum of materials-based technologies, having significant implications for life sciences. Although many authors, aware of the importance of proteins or biomolecules in the biomineralization of organisms, put forward some global nucleation models and concepts that elucidate the formation of high order organization based on aggregation-mediated processes (10), they do not give detailed insight into the pathways and rigorous concepts. To understand the precise mechanism is still a difficult task, in particular, the kinetics of nucleation. Our experiments show that biopolymeric molecules facilitate the alignment of a well ordered assembly of HAP crystallites by changing the kinetics of self-epitaxial nucleation of daughter (or secondary) HAP crystals, which nucleate and grow on the surface of existing (parent) HAP crystallites.In this work...

Mineral Trioxide Aggregate–based Endodontic Sealer Stimulates Hydroxyapatite Nucleation in Human Osteoblast-like Cell Culture

“…On the other hand, it is possible that MTA-F promoted an environment highly saturated with foreign ions, a lower number of cells, and a Ca-Si hydrogel nucleation site. It was shown that relatively high super-saturation gives rise to a compact and random assembly of HAP crystallites (23). In addition, calcium silicate hydrate is the main byproduct of the reaction of MTA with calcium silicate cements (24).…”

“…The diverse conditions of hydroxyapatite nucleation can explain this difference in nanocrystallites morphology. Nucleation is the first step in biomineralization and requires that a free energy barrier be overcome, which results in size and pattern formation of mineral crystallites through the mediating role of many organic and inorganic substrates (23). Super-saturation, foreign ions, cell membrane, and biomolecules, such as bone matrix proteins, may act as regulating agents in controlling the pattern of HAP crystallite assembly.…”

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“…m is a function of the interfacial free energy difference between the phases of crystal, fluid, and substrate, and it depends on the interaction and interfacial structural match between the crystalline phase and the substrate (27).…”

Dentine bonding agents comprising calcium-silicates to support proactive dental care: Origins, development and future

“…Thus, while the mineral constituents is being restored, the region of the exposed collagen fibrils is less susceptible to hydrolytic degradation by MMPs. Furthermore, traditional remineralization by epitaxial growth is a thermodynamically favorable process that overcomes the energy barrier of homogeneous nucleation 41) . According to Ostwald's rule 42) , normally the first occurring phase in polymorphism is the least stable and closest in free energy to the mother-phase, followed by other phases in order of increasing stability.…”