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...

The effective permeability (μeff) was measured and calculated for composites consisting of micron- or submicron-sized nanocrystalline iron particles embedded in a nonmagnetic matrix. The intrinsic permeability of iron particles was obtained from the calculation for a random spatial distribution of magnetic domains and its analytical model is derived from the Landau-Lifshitz-Gilbert equation. In the calculation, each grain is assumed to be a single magnetic domain because of its nano size. The effective permeability was calculated using three methods—Bruggeman’s effective medium theory, extended Bruggeman’s effective medium theory with the consideration of the skin effect, and a simulation method which was developed in the present work. The skin effect was considered in our simulation work. Our simulation agrees well with the experimental data. Our work has shown clearly that the magnetic domain structure with a random spatial distribution of magnetic easy axes and the skin effect need to be considered to calculate the complex permeability of polycrystalline magnetic materials.

Biomineralization is an important process, which is often assisted by biomolecules. In this paper, the effect of chondroitin sulfate on the crystallization of hydroxyapatite was examined quantitatively based on a generic heterogeneous nucleation model. It is found that chondroitin sulfate can suppress the supersaturation-driven interfacial structure mismatch between the hydroxyapatite crystal and the substrate and promote the formation of ordered hydroxyapatite nanocrystallite assemblies. The nucleation mechanism of selfaligned hydroxyapatite nanocrystallites was examined from the viewpoints of kinetics and interfacial structure and properties, which contributes to an understanding of the fundamentals of biomineralization of self-assembled structures. The results obtained from this study will provide a basic principle to design and fabricate highly orderly organic-inorganic hybrid materials.Natural materials such as bones and teeth consist of biocomposites with well organized and assembled hydroxyapatite (HAP 2 ; Ca 5 (PO 4 ) 3 OH) nanobiominerals, to perform important biological functions. Although such composites in hard tissues are formed under mild conditions, they exhibit unusual mechanical properties that outperform synthetic materials (1, 2). How can biomineral nanocrystallites be assembled to form a synergetic structure? Why are some molecules, in particular biomacromolecules, capable of fabricating different patterns of biominerals? Recently, these questions concerning the biological mechanisms that control mineralized tissue construction have attracted a great deal of attention in fields ranging from biology and chemistry to materials science and bioengineering (3-7).It is well documented that the ordered structures in mineralized tissues appeared to originate from organized assemblies of biomacromolecules, such as proteins, polysaccharides, or proteoglycans, and inorganic compound salts (8 -10). Previous studies indicate that some biomacromolecules are involved in controlling the nucleation, growth, size, and shape of the mineral phases (11), since they can act as templates through self-assembly to facilitate interaction with an insoluble matrix and to induce the desired stereochemistry for the construction of organized structures (12, 13). Normally, these molecules are functionalized with acidic groups such as carboxylic acid, sulfonate, and phosphate moieties, which enable them to become an effective metal ion chelator to combine with the inorganic matrix (14 -16). Chondroitin 4-sulfate (ChS) is just one of these biomolecules. It belongs to the family of glycosaminoglycans, which can be found on cell surfaces and in the extracellular matrix of cartilage and bone. Large amounts of it in the cartilage permit diffusion of substances between blood and vessels. ChS consists of repeated disaccharide units; one of the two monosaccharides is N-acetylgalactosamine sulfate (GalNAc-OSO 3 Ϫ ), and the other is glucuronic acid that contains a carboxylate group (cf. Fig. 1). Previous investigation indicated that ...

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Coherent growth and mechanical properties of AlN/VN multilayers J. Appl. Phys. 95, 92 (2004); Biominerals in the hard tissues of many organisms exhibit superior mechanical properties due to their unique hierarchical nanostructures. In this article, we show the microstructure of human tooth enamel examined by position-resolved small-angle x-ray scattering and electron microscopy. It is found that the degree of ordering of the biominerals varies strikingly within the dental sample. Combined with nanoindentation, our results show that both the hardness and the elastic modulus increase predominantly with the ordering of the biomineral crystallites. This can be attributed to the fact that the ordered structure helps sustain a more complex mechanical stress.