Confinement Increases the Lifetimes of Hydroxyapatite Precursors

Abstract

The mineral component of bone is a carbonated, non-stoichiometric hydroxyapatite (calcium phosphate) that forms in nanometer confinement within collagen fibrils, the principal organic constituent of bone. We here employ a model system to study the effect of confinement on hydroxyapatite precipitation from solution under physiological conditions. In common with earlier studies of calcium carbonate and calcium sulfate precipitation, we find that confinement significantly prolongs the lifetime of metastable phases, here amorphous calcium phosphate (ACP) and octacalcium phosphate (OCP). The effect occurs at surprisingly large separations of up to 1 m, and at 0.2 m the lifetime of ACP is extended by at least an order of magnitude. The soluble additive poly(aspartic acid), which in bulk stabilizes ACP, appears to act synergistically with confinement to give a greatly enhanced stability of ACP. The reason for the extended lifetime appears to be different from that found with CaCO 3 and CaSO 4 , and underscores both the variety of mechanisms whereby 2 confinement affects the growth and transformation of solid phases, and the necessity to study a wide range of crystalline systems to build a full understanding of confinement effects. We suggest that in the case of ACP and OCP the extended lifetime of these metastable phases is chiefly due to a slower transport of ions between a dissolving metastable phase, and the more stable, growing phase. These results highlight the potential importance of confinement on biomineralisation processes.3

Molecular Dynamics Simulations of Hydroxyapatite Nanopores in Contact with Electrolyte Solutions: The Effect of Nanoconfinement and Solvated Ions on the Surface Reactivity and the Structural, Dynamical, and Vibrational Properties of Water

“…Specifically, it is well-recognized that many features of crystal growth are affected by confinement [1][2][3][4][5][6][7] and solution composition [2,[8][9][10][11][12]. Confinement can enable control over size, polymorphism and orientation, single crystal/polycrystalline structure, and morphology [13].…”

Effect of Nanoscale Confinement on the Crystallization of Potassium Ferrocyanide

“…19,25,28,61 Focusing in particular on inorganic solids, the crossed cylinder apparatus has been particularly valuable in studying crystals formed between surfaces separated by distances as small as 200 nm.…”

“…As an alternative to additive-directed crystallization, it is also becoming increasingly evident that the micro-environments in which crystals form can themselves define crystallization pathways and products. [15][16][17][18][19] Not only is this of particular importance to processes that naturally occur in confinement such as weathering, biomineralization and the manufacture of nanomaterials, but confinement can potentially provide a stand-alone means of controlling crystallization.…”

“…18,20,21 Looking then at biologically-relevant solids such as calcium carbonate and calcium sulfate, confined volumes can provide environments that can control the formation of single crystals with complex morphologies. 7,[22][23][24] Using systems including a crossedcylinders apparatus, 19,25,26 arrays of picolitre droplets, 27 vesicles that offer confinement in the general range 50 nm -50 m, [28][29][30] and the pores of track-etched membranes, [31][32][33] it has also been demonstrated that the lifetimes of amorphous precursor phases and metastable crystalline polymorphs of calcium carbonate, calcium phosphate, calcium sulfate and calcium oxalate can be significantly extended, even in micron-scale environments. While this length scale is relevant to many biomineralization processes, crystallization in the environment (eg.…”

Preparation of graphene oxide reinforced hydroxyapatite Poly (vinyl alcohol) nanocomposite materials

“…Due to the developments of nanotechnologies, considerable research efforts have been attracted to develop HA nanomaterials for suitable bone and teeth substitutes because of their similarities with inorganic mineral components in human bodies [1][2][3][4][5][6]. A number of applications have been reported for the nanoscale HA in various fields in column chromatography in life science, ceramic reinforcement in medical science, bone defect filler, ion exchange of heavy metal, and amino acids purifications by virtue of their own physical properties depending on morphologies [5][6][7][8][9][10]. HA based polymer nanocomposites are one of these promising nanomaterials which take the advantages of nanotechnologies in the materials researches.…”