Search

The mineralized collagen fibril is the basic building block of bone, and is commonly pictured as a parallel array of ultrathin carbonated hydroxyapatite (HAp) platelets distributed throughout the collagen. This orientation is often attributed to an epitaxial relationship between the HAp and collagen molecules inside 2D voids within the fibril. Although recent studies have questioned this model, the structural relationship between the collagen matrix and HAp, and the mechanisms by which collagen directs mineralization remain unclear. Here, we use XRD to reveal that the voids in the collagen are in fact cylindrical pores with diameters of ~2 nm, while electron microscopy shows that the HAp crystals in bone are only uniaxially oriented with respect to the collagen. From in vitro mineralization studies with HAp, CaCO3 and γ-FeOOH we conclude that confinement within these pores, together with the anisotropic growth of HAp, dictates the orientation of HAp crystals within the collagen fibril., Mineralized collagen is the building block of bone but how the collagen directs hydroxyapatite formation remains unclear. Here, the authors demonstrate cylindrical pores in collagen and how the anisotropic growth of hydroxyapatite directs the orientation of crystal growth in mineralized collagen.

The mineralized collagen fibril is the basic building block of bone, commonly pictured as a parallel array of ultrathin carbonated hydroxyapatite (HAp) platelets distributed throughout the collagen. This orientation is often attributed to an epitaxial relationship between the HAp and collagen molecules inside 2D voids within the fibril. Although recent studies have questioned this model, the structural relationship between the collagen matrix and HAp, and the mechanisms by which collagen directs mineralization remain unclear. Here, we use XRD to reveal that the voids in the collagen are in fact cylindrical pores with diameters of ∼2 nm, while electron microscopy shows that the HAp crystals in bone are only uniaxially oriented with respect to the collagen. Fromin vitromineralization studies with HAp, CaCO3and γ-FeOOH we conclude that confinement within these pores, together with the anisotropic growth of HAp, dictates the orientation of HAp crystals within the collagen fibril.

Inspired by the observation that crystalline calcium carbonate and calcium phosphate biominerals frequently form via amorphous precursors, a wide range of studies have been performed which demonstrate that many inorganic crystals can precipitate from solution via amorphous phases. This article considers the crystallization mechanism of calcium oxalate, which is a significant biomineral in many plants and the primary constituent of kidney stones in vertebrates, and shows that this can also precipitate via an amorphous precursor phase from aqueous solution. A range of approaches were employed to study calcium oxalate formation, including precipitation in bulk solution in the presence and absence of additives and in the spatially confined volumes offered by track etched membranes and a crossed cylinders apparatus. A freeze concentration method was also used to generate sufficient quantities of amorphous calcium oxalate (ACO) for analysis. The results show that amorphous calcium oxalate crystallizes rapidly in bulk solution but can be significantly stabilized through the concerted activity of additives and confinement. We also demonstrate that ACO has a composition of ˜CaC2O4:H2O. These data suggest that calcium oxalate biominerals, in common with their carbonate and phosphate counterparts, may also precipitate via amorphous phases.

While negatively charged organic additives are widely used as an effective means to control CaCO3 precipitation, positively charged additives are generally considered to be much less active. Nevertheless, the cationic polyelectrolyte poly(allylamine hydrochloride) has recently been shown to exert significant control over CaCO3 precipitation, driving the formation of thin films and fibers, and other examples suggest that many positively charged additives promote vaterite formation. This article aims to bring together these sometimes conflicting views of the activity of positively charged additives. The effect of a series of polyamines on CaCO3 precipitation was studied, where the polyamines were selected such that the amine group type, the pKa value (of the corresponding conjugated acid), the molecular weight, and the side chain length of the polymers could be evaluated. The results unambiguously demonstrate that polyamines carrying primary amine groups are capable of exerting a significant effect and that...

That the cationic polyelectrolyte poly(allylamine hydrochloride) (PAH) exerts a significant influence on CaCO3 precipitation challenges the idea that only anionic additives have this effect. Here, we show that in common with anionic polyelectrolytes such as poly(aspartic acid), PAH supports the growth of calcite thin films and abundant nanofibers. While investigating the formation of these structures, we also perform the first detailed structural analysis of the nanofibers by transmission electron microscopy (TEM) and selected area electron diffraction. The nanofibers are shown to be principally single crystal, with isolated domains of polycrystallinity, and the single crystal structure is even preserved in regions where the nanofibers dramatically change direction. The formation mechanism of the fibers, which are often hundreds of micrometers long, has been the subject of intense speculation. Our results suggest that they form by aggregation of amorphous particles, which are incorporated into the fibers uniquely at their tips, before crystallizing. Extrusion of polymer during crystallization may inhibit particle addition at the fiber walls and result in local variations in the fiber nanostructure. Finally, we investigate the influence of Mg2+ on CaCO3 precipitation in the presence of PAH, which gives thinner and smoother films, together with fibers with more polycrystalline, granular structures.

A key feature of biomineralization processes is that they take place within confined volumes, in which the local environment can have significant effects on mineral formation. Herein, we investigate the influence of confinement on the formation mechanism and structure of calcium phosphate (CaP). This is of particular relevance to the formation of dentine and bone, structures of which are based on highly mineralized collagen fibrils. CaP was precipitated within 25–300 nm diameter, cylindrical pores of track etched and anodised alumina membranes under physiological conditions, in which this system enables systematic study of the effects of the pore size in the absence of a structural match between the matrix and the growing crystals. Our results show that the main products were polycrystalline hydroxapatite (HAP) rods, together with some single crystal octacalcium phosphate (OCP) rods. Notably, we demonstrate that these were generated though an intermediate amorphous calcium phosphate (ACP) phase, and that ACP is significantly stabilised in confinement. This effect may have significance to the mineralization of bone, which can occur through a transient ACP phase. We also show that orientation of the HAP comparable, or even superior to that seen in bone can be achieved through confinement effects alone. Although this simple experimental system cannot be considered, a direct mimic of the in vivo formation of ultrathin HAP platelets within collagen fibrils, our results show that the effects of physical confinement should not be neglected when considering the mechanisms of formation of structures, such as bones and teeth.

Since calcium carbonate is one of the most abundant biogenic minerals found in nature, it is no surprise that there has been a huge focus on its formation and use. In this review, we intend to cover the use of amorphous calcium carbonate, which is the most unstable polymorph of calcium carbonate, for the design of new materials. Amorphous calcium carbonate has been used to manipulate the morphology of new materials, and to create strong inorganic/organic hybrid materials based on biological examples. The exoskeletons of crustaceans, sea shell nacre, and brittle star eyes are a few of the examples discussed here, and researchers have looked at these biominerals for the design of new materials. By using polymer additives and organic synthetic layers to substitute for the natural proteins used in biological systems, interesting hybrid materials have been developed. By taking inspiration from this research, new ideas for the design of the fusion materials can be achieved.

Soluble macromolecules are essential to Nature's control over biomineral formation. Following early studies where macromolecules rich in aspartic and glutamic acid were extracted from nacre, research has focused on the use of negatively charged additives to control calcium carbonate precipitation. It is demonstrated that the positively charged additive poly(allylamine hydrochloride) (PAH) can also cause dramatic changes in calcite morphologies, yielding thin films and fibers of CaCO3 analogous to those produced with poly(aspartic acid) via a so-called PILP (polymer-induced liquid precursor) phase. The mechanism by which PAH induces these effects is investigated using a range of techniques including cryo transmission electron microscopy (TEM), Raman microscopy, and thermogravimetric analysis, and the data show that hydrated Ca2+/PAH/CO32- droplets initially form in solution, before coalescing and ultimately crystallizing to give calcite, together with small quantities of vaterite. It is suggested that it is the initial formation of hydrated Ca2+/PAH/CO32- droplets that is key to this process, rather than a specific polymer/mineral interaction. These results are discussed in terms of their relevance to biomineralization processes and highlight the opportunity for using counter-ion-induced phase separation of polyelectrolytes as a method for generating minerals with non-crystallographic morphologies.

We have developed pH- and magnetic-responsive hydrogels that are stabilized by both covalent bonding and catechol/Fe3+ ligands. The viscoelastic properties of the gels are regulated by the complexation valence and can be used to tune drug release profiles. The stable incorporation of magnetic nanoparticles further expands control over the mechanical response and drug release, in addition to providing magnetic stimuli-responsivity to the gels. Published version

Many questions remain regarding the formation of ultrathin hydroxapatite (HAP) crystals within the confines of collagen fibrils of bones. These structures form through the interplay of the collagen matrix and non-collagenous proteins, and in vitro mineralization studies employing poly(aspartic acid) (PAsp) as a mimic of the non-collagenous proteins have generated mineralized fibrils with structures comparable to their biogenic counterparts. In this article, we employ the nanoscale cylindrical pores perforating track-etch filtration membranes to investigate the role of PAsp in controlling the infiltration and crystallization of calcium phosphate (CaP) within confined volumes. Oriented polycrystalline HAP and non-oriented octacalcium phosphate (OCP) rods precipitated within the membrane pores via an amorphous calcium phosphate (ACP) precursor, where PAsp increased the proportion of OCP rods. Further, ACP crystallized faster within the membranes than in bulk solution when PAsp was present, suggesting that PAsp inhibits crystallization in solution, but promotes it when bound to a substrate. Finally, in contrast to the collagen system, PAsp reduced the yield of intra-membrane mineral and failed to enhance infiltration. This suggests that a specific interaction between the collagen matrix and ACP/PAsp precursor particles drives effective infiltration. Thus, while orientation of HAP crystals can be achieved by confinement alone, the chemistry of the collagen matrix is necessary for efficient mineralisation with CaP.