We have used the pH-induced self-assembly of a peptide-amphiphile to make a nanostructured fibrous scaffold reminiscent of extracellular matrix. The design of this peptide-amphiphile allows the nanofibers to be reversibly cross-linked to enhance or decrease their structural integrity. After cross-linking, the fibers are able to direct mineralization of hydroxyapatite to form a composite material in which the crystallographic c axes of hydroxyapatite are aligned with the long axes of the fibers. This alignment is the same as that observed between collagen fibrils and hydroxyapatite crystals in bone.

http://science.sciencemag.org/content/sci/294/5547/1684.full.pdf

Self-assembly and mineralization of peptide-amphiphile nanofibers

EVOLUTION: Of Mice . . .

Nanoscale hydroxyapatite particles for bone tissue engineering

Synthesis methods for nanosized hydroxyapatite with diverse structures.

1.1. Biomineralization
The study of biomineralization is not only important to gain an understanding of how mineral-rich tissues are created in vivo but also because it is a great source of inspiration for the design of advanced materials.1-7 Mineralized tissues have remarkable hierarchical structures that have evolved over time to achieve great functions in a large variety of organisms. Organic phases play a key role in templating the structure of mineralized tissues; therefore, their matrices are often hybrid in composition, varying widely in the relative content of organic and inorganic substances. Understanding the complex integration of hard and soft phases that biology achieves in mineralized matrices across scales and its link to properties is knowledge of great value to materials chemistry. At the same time, the synthetic mechanisms used by biology to create mineralized matrices could also offer some useful strategies to create synthetic hybrid materials. Often, the amount of organic material utilized by Nature to modify mechanical properties of mineralized structures is vanishingly small. One example is the role of occluded proteins in the toughness of biogenic calcite.8 The study of mammalian bone and teeth in the biomineralization and biomimetic context is particularly interesting since the information derived could contribute a significant biomedical impact on therapies and strategies to repair or regenerate human mineralized tissues. This is an important area given the continually rising average life span of humans. The materials of interest could be highly sophisticated bioactive scaffolds to regenerate bone and possibly dental tissues as well. This review focuses on the formation of hydroxyapatite (HA) in synthetic systems designed primarily in the biomimetic context of bone or enamel mineralization for therapeutic approaches in repair of human tissues. Bone and enamel share the same mineral composition, HA, but have different morphologies and organic content. Enamel is almost entirely inorganic in composition, and bone has a relatively high organic composition. Knowledge acquired in this field may inspire the chemical synthesis of novel hybrid materials, including apatite-based structures for the regeneration of human bone and dental tissues.

Biomimetic Systems for Hydroxyapatite Mineralization Inspired By Bone and Enamel

/pdf/dental-caries-the-disease-and-its-clinical-management-4y07j170wq.pdf

Dental caries; the disease and its clinical management

Self-assembly of synthetic hydroxyapatite nanorods into an enamel prism-like structure.

Dental enamel is the outermost layer of teeth and the hardest mineralized tissue in the human body. It consists of nanorod-like hydroxyapatite (HA) crystals arranged into a highly organized micro-architectural unit called an enamel prism. These special units play an important role in determining the unique physicochemical properties of dental enamel. Cells (ameloblasts) and enamel proteins are thought to be intimately involved in vivo in producing this unique structure. The aim of this research was to create a similar structure directly by crystal growth without using cells and/or enamel proteins. Using a hydrothermal method we have been able to synthesize these prism-like structures, consisting of fluorapatite (FA) crystals, which are similar to the dimensions of those seen in human enamel. Dental pulp stem cells (DPSCs) were cultured on these crystals and showed the excellent biocompatibility of the FA crystals. The growth mechanism of this structure is outlined and its use in tissue repair is discussed. Ninety-five percent (by volume) of human enamel is comprised of nanorod-like calcium hydroxyapatite crystals, which have an approximate cross section of 25–100 nm and an undetermined length of 100 nm to 100 lm or longer along the c-axis. The typical human enamel prism structure is approximately 5 lm in cross section, and can span the entire enamel thickness, that is, approximately 1–2 mm in length. The prevailing theory is that the ameloblasts secrete amelogenin, a major enamel protein constituting approximately 90 % of all organic matrix material in developing enamel, and this protein plays a vital role in enabling crystallites to form a well-organized prism pattern. These prisms project from the dentino–enamel junction to the enamel surface. The prisms are aligned parallel to each other and separated by an interprismatic structure to form a densely compacted enamel layer. Unlike other calcified tissues, such as dentin and bone, there are no living cells in the mature enamel. The ameloblast cells die after the enamel is formed. Thus, when the enamel is damaged, the body has no ability to regenerate it. The approach to create artificial bone and tooth structures has attracted the interest of many researchers. Kniep and co-workers have described a fluorapatite–gelatine system that resembles the biosystem hydroxyapatite–collagen in both bone and dentine. Recently, Yamagishi et al. have reported a paste of fluoridated hydroxyapatite that could be used to repair a small carious lesion. Although the interface between the precipitated layer from the paste and the tooth surface contains elongated crystals some of which are orientated towards the tooth surface, the unique enamel prism structure is not visible from the transmission electron microscopy (TEM) images. In our previous paper, we reported a way to mimic the natural biomineralization process to create these special structures by modifying synthetic hydroxyapatite nanorods with surfactants that allow the nanorods to self-assemble into an enamel prismlike structure at a water/air interface. However, those self-assembled enamel prism-like structures are small, only about 400 nm in length and 100 nm in cross section, and dispersed randomly. Very recently, Fowler et al. reported that they were able to synthesize an HA bundle structure directly from a solution containing the surfactant bis(2-ethylhexyl)sulfosuccinate sodium salt (AOT), water, and oil. These bundles were only 750 nm to 1 lm in length, 250–350 nm wide and they were dispersed randomly. These unique structures, therefore, have limited applications, and the introduction of other chemicals, for example AOT may cause unnecessary biological effects. The development of nanotechnology has created many ways to grow 1D nanostructures. Among them, the hydrothermal method is a widely adopted technique to create nanorods, nanowires, and whiskers and has already been shown to be an effective way to create long hydroxyapatite nanorods and whiskers. In this study, we demonstrate a direct growth method to produce fluorapatite dental enamel prismlike structures using a hydrothermal technique. A film of compacted well-aligned FA crystals, which was grown on metal plates, had a structure C O M M U N IC A TI O N S

http://deepblue.lib.umich.edu/bitstream/handle/2027.42/55794/1846_ftp.pdf?sequence=1

Acellular synthesis of a human enamel-like microstructure

In natural tooth enamel, fluoride is always present in the carbonated hydroxyapatite mineral and plays a key role in the prevention of tooth decay. In this study we aimed at mimicking the natural anticaries ability of the tooth by developing new, effective anticaries materials using fluorapatite nanorods or nanowires. We therefore investigated the conditions necessary to synthesize fluorapatite nanorods of different size, shape, and composition for future use either directly or indirectly, that is by incorporation into dental materials or in the treatment and prevention of caries. By controlling the chemical conditions, nanorods of desirable chemical composition and dimension were produced. The mechanism of how these structures were formed is also proposed.

Synthesis of Fluorapatite Nanorods and Nanowires by Direct Precipitation from Solution.

Exposure of both small carious lesions and artificial caries-like lesions to a synthetic calcifying fluid in vitro produced a significant degree of ‘healing’ or remineralization of the lesion. Changing the calcium concentration of the calcifying fluid had a marked effect on the degree of remineralization produced. When a low calcium concentration of 1 mM was employed, remineralization occurred throughout the entire depth of a lesion. Under these conditions, there was a mean reduction of 69% in area of the body of the lesion and a mean increase of 40% in orientated mineral. The dark zone at the advancing front of the lesion showed a dramatic increase in area of 526% and was much closer to the enamel surface relative to the control. When higher calcium ion concentrations of 3 mM were used, remineralization occurred but was limited to the surface of the lesion. Under these conditions, the mean reduction in area of the body of the lesion was 20%, brought about by a mean increase in orientated mineral of 17%. Although changes were found in relation to the dark zone, these were much smaller than those found for the 1-mM fluid, the increase in area being 38%. With respect to exposure times, results obtained using ten consecutive 24-hour exposures to the synthetic calcifying fluid were similar to those obtained after ten consecutive 1-hour exposures. Remineralization, therefore, occurred within each 1-hour exposure increment. Scanning electron microscopy showed that crystal diameters for sound enamel were in the range 35–40 nm. In the body of the lesion crystal diameters were reduced and found to be in the range 10–30 nm. In lesions remineralized with the high calcium-calcifying fluid containing 3 mM calcium, crystal diameters were larger than those found in either control lesions or in sound enamel, being in the range 50–75 nm. When the low calcium-calcifying fluid was used, remineralized lesions showed crystal diameters in the range 50–150 nm with a small number of crystals having diameters of 200 nm. Calculation of the supersaturation of the calcifying fluids revealed that the low calcium-calcifying fluid having 1 mM calcium favors crystal growth as opposed to nucleation.

Brian H. Clarkson

Papers

Dental caries; the disease and its clinical management

Self-assembly of synthetic hydroxyapatite nanorods into an enamel prism-like structure.

Acellular synthesis of a human enamel-like microstructure

Synthesis of Fluorapatite Nanorods and Nanowires by Direct Precipitation from Solution.

Remineralization of natural and artificial lesions in human dental enamel in vitro. Effect of calcium concentration of the calcifying fluid