Existence of Posner's Cluster in Vacuum

Surface functionalization of hydroxyapatite nanoparticles for biomedical applications

This review completely covers the various aspects of hydroxyapatite (HAp) nanoparticles and their role in different biological situations, and provides the surface and interface contents on (i) hydroxyapatite nanoparticles and their hybridization with organic molecules, (ii) surface designing of hydroxyapatite nanoparticles to provide their biocompatibility and photofunction, and (iii) coating technology of hydroxyapatite nanoparticles. In particular, we summarized how the HAp nanoparticles interact with the different ions and molecules and highlighted the potential for hybridization between HAp nanoparticles and organic molecules, which is driven by the interactions of the HAp nanoparticle surface ions with several functional groups of biological molecules. In addition, we highlighted the studies focusing on the interfacial interactions between the HAp nanoparticles and proteins for exploring the enhanced biocompatibility. Such studies focus on how these interactions affect the hydration layers and protein adsorption. However, the hydration layer state involves diverse molecular interactions that can alter the shape of the adsorbed proteins, thereby affecting cell adhesion and spreading on the surfaces. We also summarized the relationship between the surface properties of the HAp nanoparticles and the hydration layer. Furthermore, we spotlighted the cytocompatible photoluminescent probes that can be developed by designing HAp/organic nanohybrid structures. We then emphasized the importance of photofunctionalization in theranostics, which involves the integration of diagnostics and therapy based on the surface design of the HAp nanoparticles. Furthermore, the coating techniques using HAp nanoparticles and HAp nanoparticle/polymer composites were outlined for fusing base biomaterials with biological tissues. The advantages of HAp/biocompatible polymer composite coatings include the ability to effectively cover porous or irregularly shaped surfaces while controlling the thickness of the coating layer, and the addition of HAp nanoparticles to the polymer matrix improves the mechanical properties, increases the roughness, and forms the morphologies that mimic bone nanostructures. Therefore, the fundamental design of hydroxyapatite nanoparticles and their surfaces was suggested from various aspects for biomedical applications.

Synthetic amorphous calcium phosphates (ACPs): preparation, structure, properties, and biomedical applications

Amorphous calcium phosphates (ACPs) represent a metastable amorphous state of other calcium orthophosphates (abbreviated as CaPO₄) possessing variable compositional but rather identical glass-like physical properties, in which there are neither translational nor orientational long-range orders of the atomic positions. In nature, ACPs of a biological origin are found in the calcified tissues of mammals, some parts of primitive organisms, as well as in the mammalian milk. Manmade ACPs can be synthesized in a laboratory by various methods including wet-chemical precipitation, in which they are the first solid phases, precipitated after a rapid mixing of aqueous solutions containing dissolved ions of Ca²⁺ and PO₄^3- in sufficient amounts. Due to the amorphous nature, all types of synthetic ACPs appear to be thermodynamically unstable and, unless stored in dry conditions or doped by stabilizers, they tend to transform spontaneously to crystalline CaPO₄, mainly to ones with an apatitic structure. This intrinsic metastability of the ACPs is of a great biological relevance. In particular, the initiating role that metastable ACPs play in matrix vesicle biomineralization raises their importance from a mere laboratory curiosity to that of a reasonable key intermediate in skeletal calcifications. In addition, synthetic ACPs appear to be very promising biomaterials both for manufacturing artificial bone grafts and for dental applications. In this review, the current knowledge on the occurrence, structural design, chemical composition, preparation, properties, and biomedical applications of the synthetic ACPs have been summarized.

The Dynamical Ensemble of the Posner Molecule Is Not Symmetric

The Posner molecule, Ca₉(PO₄)₆, has long been recognized to have biochemical relevance in various physiological processes. It has found recent attention for its possible role as a biological quantum information processor, whereby the molecule purportedly maintains long-lived nuclear spin coherences among its ³¹P nuclei (presumed to be symmetrically arranged), allowing it to function as a room temperature qubit. The structure of the molecule has been of much dispute in the literature, although the S₆ point group symmetry has often been assumed and exploited in calculations. Using a variety of simulation techniques (including ab initio molecular dynamics and structural relaxation), rigorous data analysis tools, and by exploring thousands of individual configurations, we establish that the molecule predominantly assumes low-symmetry structures (C_s and C_i) at room temperature, as opposed to the higher-symmetry configurations explored previously. Our findings have important implications for the viability of this molecule as a qubit.

A soft-chemistry approach to the synthesis of amorphous calcium ortho/pyrophosphate biomaterials of tunable composition

The development of amorphous phosphate-based materials is of major interest in the field of biomaterials science, and especially for bone substitution applications. In this context, we herein report the synthesis of gel-derived hydrated amorphous calcium/sodium ortho/pyrophosphate materials at ambient temperature and in water. For the first time, such materials have been obtained in a large range of tunable orthophosphate/pyrophosphate molar ratios. Multi-scale characterization was carried out thanks to various techniques, including advanced multinuclear solid state NMR. It allowed the quantification of each ionic/molecular species leading to a general formula for these materials: [(Ca²⁺_y Na⁺_z H⁺_3+x-2y-z)(PO₄^3-)_1-x(P₂O₇^4-)_x](H₂O)_u. Beyond this formula, the analyses suggest that these amorphous solids are formed by the aggregation of colloids and that surface water and sodium could play a role in the cohesion of the whole material. Although the full comprehension of mechanisms of formation and structure is still to be investigated in detail, the straightforward synthesis of these new amorphous materials opens up many perspectives in the field of materials for bone substitution and regeneration. STATEMENT OF SIGNIFICANCE: The metastability of amorphous phosphate-based materials with various chain length often improves their (bio)chemical reactivity. However, the control of the ratio of the different phosphate entities has not been yet described especially for small ions (pyrophosphate/orthophosphate) and using soft chemistry, whereas it opens the way for the tuning of enzyme- and/or pH-driven degradation and biological properties. Our study focuses on elaboration of amorphous gel-derived hydrated calcium/sodium ortho/pyrophosphate solids at 70 °C with a large range of orthophosphate/pyrophosphate ratios. Multi-scale characterization was carried out using various techniques such as advanced multinuclear SSNMR (³¹P, ²³Na, ¹H, ⁴³Ca). Analyses suggest that these solids are formed by colloids aggregation and that the location of mobile water and sodium could play a role in the material cohesion.

Simulation of Calcium Phosphate Prenucleation Clusters in Aqueous Solution: Association beyond Ion Pairing

Classical molecular dynamics simulations and free energy methods have been used to obtain a better understanding of the molecular processes occurring prior to the first nucleation event for calcium phosphate biominerals. The association constants for the formation of negatively charged complexes containing calcium and phosphate ions in aqueous solution have been computed, and these results suggest that the previously proposed calcium phosphate building unit, [Ca(HPO4)3]4-, should only be present in small amounts under normal experimental conditions. However, the presence of an activation barrier for the removal of an HPO4 2- ion from this complex indicates that this species could be kinetically trapped. Aggregation pathways involving CaHPO4, [Ca(HPO4)2]2-, and [Ca(HPO4)3]4- complexes have been explored with the finding that dimerization is favorable up to a Ca/HPO4 ratio of 1:2.

Freeman C, Harding JH. Interaction of stable aggregates drives the precipitation of calcium phosphate in supersaturated solutions

Using molecular dynamics simulations, we show for the first time that calcium phosphate nanoparticles of eight formula units are thermodynamically stable and could be key in the nucleation of amorphous calcium phosphate.

Posner qubits: spin dynamics of entangled Ca9(PO4)6 molecules and their role in neural processing

It has been suggested that 31P nuclear spins in Ca9(PO4)6 molecules could form the basis of a quantum mechanism for neural processing in the brain. A fundamental requirement of this proposal is that spins in different Ca9(PO4)6 molecules can become entangled and remain so for periods (estimated at many hours) that hugely exceed typical 31P spin relaxation times. Here, we consider the coherent and incoherent spin dynamics of Ca9(PO4)6 arising from dipolar and scalar spin-spin interactions and derive an upper bound of 37 min on the entanglement lifetime under idealized physiological conditions. We argue that the spin relaxation in Ca9(PO4)6 is likely to be much faster than this estimate.

Structures and infrared spectra of calcium phosphate clusters by ab initio methods with implicit solvation models

Since the first detection of pre-nucleation clusters during the formation of calcium phosphate minerals, determining such clusters' compositions and structures has become crucial for understanding the early-stage nucleation of these minerals in solutions. In previous experimental studies, the composition and sizes of pre-nucleation clusters have been calculated, but their structural information has been difficult to determine because they are very small (<1 nm). In this study, we examined the structures and infrared spectra of small- and medium-sized calcium phosphate clusters using ab initio calculations combined with implicit solvation models. Adding solvent effects increased the possibility of the existence of alternative configurations of calcium phosphate clusters other than their compact configurations. The calcium atoms had a tendency to be located outside of the clusters to coordinate with water molecules in the aqueous environment. The computed infrared spectra of extended small calcium phosphate clusters captured some of the features measured in the in situ infrared spectra, which supports the network structures proposed by large-scale molecular dynamics studies and X-ray adsorption near-edge spectra. The relative stabilities of medium-sized Ca₉(PO₄)₆ clusters with respect to the stability of Posner's cluster in water were also reviewed. We found that in water, alternative structures with low symmetry or large dipole moments had lower energies than Posner's cluster.

Posner molecules: from atomic structure to nuclear spins

The Posner molecule, Ca₉(PO₄)₆, a possible intermediate step in bone growth, may also protect the constituent ³¹P spins from decoherence.

Improving the biocompatibility of tobermorite by incorporating calcium phosphate clusters

BACKGROUND

In our earlier work, tobermorite containing calcium phosphate (CP) clusters (CP-Tob) was hydrothermally prepared in the CaO-SiO2-P2O5-H2O system for biomedical applications.

OBJECTIVE

CP-Tob was used to investigate the influence of CP cluster incorporation on its biocompatibility.

METHODS

Tobermorite samples with and without CP clusters were hydrothermally prepared at 180°C for 40 h. The biocompatibility, structure, and density of states of the tobermorite samples were investigated by experimental and first principles methods.

RESULTS

The amounts of lysozyme and bovine serum albumin adsorbed on CP-Tob were higher than those on tobermorite without CP clusters. Cluster incorporation caused a decrease in the solubility, resulting in the enhancement of the cell compatibility. The calculated results indicated that incorporating clusters, which interact with the silicate units of tobermorite, led to a change of the density of states of tobermorite.

CONCLUSIONS

Incorporation of CP clusters in tobermorite led to improvement of the biocompatibility evaluated by biological and computational analyses.

Regulatory inhibition of biological tissue mineralization by calcium phosphate through post-nucleation shielding by fetuin-A

In vertebrates, insufficient availability of calcium and inorganic phosphate ions in extracellular fluids leads to loss of bone density and neuronal hyper-excitability. To counteract this problem, calcium ions are usually present at high concentrations throughout bodily fluids-at concentrations exceeding the saturation point. This condition leads to the opposite situation where unwanted mineral sedimentation may occur. Remarkably, ectopic or out-of-place sedimentation into soft tissues is rare, in spite of the thermodynamic driving factors. This fortunate fact is due to the presence of auto-regulatory proteins that are found in abundance in bodily fluids. Yet, many important inflammatory disorders such as atherosclerosis and osteoarthritis are associated with this undesired calcification. Hence, it is important to gain an understanding of the regulatory process and the conditions under which it can go awry. In this manuscript, we extend mean-field continuum classical nucleationtheory of the growth of clusters to encompass surface shielding. We use this formulation to study the regulation of sedimentation of calcium phosphate salts in biological tissues through the mechanism of post-nuclear shielding of nascent mineral particles by binding proteins. We develop a mathematical description of this phenomenon using a countable system of hyperbolic partial differential equations. A critical concentration of regulatory protein is identified as a function of the physical parameters that describe the system.

Detection of Posner's clusters during calcium phosphate nucleation: a molecular dynamics study

Molecular dynamics simulations of calcium and phosphate ions in water show that Posner-like clusters originate during the aggregation process.

Research Perspectives on Functional Micro and Nano Scale Coatings

Research topics related to the production of nanocomposites are the most important directions of development of new semiconductor engineering, ensuring high nanocomposites obtaining useful properties in the scope of biophysical characteristics, biomedical and piezoelectric applications. We present two case studies as Hydroxyapatite are in medical applications and aluminum nitride as acoustic wave sensor. Hydroxyapatite, is the main inorganic structure of the tooth enamel and bone and is a biomaterial that is commonly used in biomedical applications that involve bone substitution, drug delivery and bone regeneration because of its excellent biocompatibility, high bioactivity and good osseoconductivity. Since the past decade. Aluminum nitride (AlN), an electrical insulating ceramic with a wide band gap of 6.3 eV, is a potentially useful dielectric material very important in fields such as optoelectronic and micro electronics.

Growth of crystalline hydroxyapatite thin films at room temperature by tuning the energy of the RF-magnetron sputtering plasma

Right angle radio frequency magnetron sputtering technique (RAMS) was redesigned to favor the production of high-quality hydroxyapatite (HA) thin coatings for biomedical applications. Stoichiometric HA films with controlled crystallinity, thickness varying from 254 to 540 nm, crystallite mean size of 73 nm, and RMS roughness of 1.7 ± 0.9 nm, were obtained at room temperature by tuning the thermodynamic properties of the plasma sheath energy. The plasma energies were adjusted by using a suitable high magnetic field confinement of 143 mT (1430 G) and a substrate floating potential of 2 V at the substrate-to-magnetron distance of Z = 10 mm and by varying the sputtering geometry, substrate-to-magnetron distance from Z = 5 mm to Z = 18 mm, forwarded RF power and reactive gas pressure. Measurements that were taken with a Langmuir probe showed that the adjusted RAMS geometry generated a plasma with an adequate effective temperature of Teff ≈ 11.8 eV and electron density of 2.0 × 10(15) m(-3) to nucleate nanoclusters and to further crystallize the nanodomains of stoichiometric HA. The deposition mechanism in the RAMS geometry was described by the formation of building units of amorphous calcium phosphate clusters (ACP), the conversion into HA nanodomains and the crystallization of the grain domains with a preferential orientation along the HA [002] direction.

Calcium orthophosphates: occurrence, properties, biomineralization, pathological calcification and biomimetic applications

The present overview is intended to point the readers' attention to the important subject of calcium orthophosphates. This type of materials is of special significance for human beings, because they represent the inorganic part of major normal (bones, teeth and antlers) and pathological (i.e., those appearing due to various diseases) calcified tissues of mammals. For example, atherosclerosis results in blood vessel blockage caused by a solid composite of cholesterol with calcium orthophosphates, while dental caries and osteoporosis mean a partial decalcification of teeth and bones, respectively, that results in replacement of a less soluble and harder biological apatite by more soluble and softer calcium hydrogenphosphates. Therefore, the processes of both normal and pathological calcifications are just an in vivo crystallization of calcium orthophosphates. Similarly, dental caries and osteoporosis might be considered an in vivo dissolution of calcium orthophosphates. Thus, calcium orthophosphates hold a great significance for humankind, and in this paper, an overview on the current knowledge on this subject is provided.

Fetuin-A is a mineral carrier protein: small angle neutron scattering provides new insight on Fetuin-A controlled calcification inhibition

Clinical studies and animal experiments have shown that the serum protein fetuin-A is a highly effective inhibitor of soft tissue calcification. This inhibition mechanism was elucidated on the basis of an in vitro fetuin-A-mineral model system. In a previous study, we found that in a two-stage process ∼100-nm sized calciprotein particles (CPPs) were formed whose final stage was stabilized by a compact outer fetuin-A monolayer against further growth. Quantitative small-angle neutron scattering data analysis revealed that even at a fetuin-A concentration close to the stability limit, only approximately one-half of the mineral ions and only 5% of the fetuin-A were contained in the CPPs. To uncover the interplay of the remaining supersaturated mineral ion fraction and of the 95% non-CPP fetuin-A, we explored the fetuin-A monomer fraction in solution by contrast variation small-angle neutron scattering. Our results suggest that the mineral ions coalesce to subnanometer-sized clusters, reminiscent of Posner clusters, which are stabilized by fetuin-A monomers. Hence, our experiments revealed a second mechanism of long-term mineral ion stabilization by the fetuin-A that is complementary to the formation of CPPs.

Amorphous Calcium Phosphates

Amorphous calcium phosphates (ACPs) represent a unique class of biomedically relevant calcium orthophosphate salts, in which there are neither translational nor orientational long-range orders of the atomic positions. Nevertheless, the constancy in their chemical composition over a relatively wide range of preparation conditions suggests the presence of a well-defined local structural unit, presumably, with the structure of Ca9(PO4)6 – so-called Posner’s cluster. ACPs have variable chemical but rather identical glass-like physicochemical properties. Furthermore, all ACPs are thermodynamically unstable compounds and, unless stored in dry conditions or doped by stabilizers, spontaneously they tend to transform to crystalline calcium orthophosphates. Although some order within general disorder is the most distinguishing feature of ACPs, the solution instability of ACPs and their easy transformation to crystalline phases might be of a great biological relevance. Namely, the initiating role ACPs play in matrix vesicle biomineralization raises the importance of this phase from a mere laboratory curiosity to that of a key intermediate in skeletal calcification. Furthermore, ACPs are very promising candidates to manufacture artificial bone grafts.

Amorphous calcium phosphates: synthesis, properties and uses in biomaterials

This review paper on amorphous calcium phosphates (ACPs) provides an update on several aspects of these compounds which have led to many studies and some controversy since the 1970s, particularly because of the lack of irrefutable proof of the occurrence of an ACP phase in mineralised tissues of vertebrates. The various synthesis routes of ACPs with different compositions are reported and the techniques used to characterise this phase are reviewed. We focus on the various physico-chemical properties of ACPs, especially the reactivity in aqueous media, which have been exploited to prepare bioactive bone substitutes, particularly in the form of coatings and cements for orthopaedic applications and composites for dental applications.

Calcium Orthophosphates in Nature, Biology and Medicine

The present overview is intended to point the readers’ attention to the important subject of calcium orthophosphates. These materials are of the special significance because they represent the inorganic part of major normal (bones, teeth and dear antlers) and pathological (i.e. those appearing due to various diseases) calcified tissues of mammals. Due to a great chemical similarity with the biological calcified tissues, many calcium orthophosphates possess remarkable biocompatibility and bioactivity. Materials scientists use this property extensively to construct artificial bone grafts that are either entirely made of or only surface-coated with the biologically relevant calcium ortho-phosphates. For example, self-setting hydraulic cements made of calcium orthophosphates are helpful in bone repair, while titanium substitutes covered by a surface layer of calcium orthophosphates are used for hip joint endoprostheses and as tooth substitutes. Porous scaffolds made of calcium orthophosphates are very promising tools for tissue engineering applications. In addition, technical grade calcium orthophosphates are very popular mineral fertilizers. Thus ere calcium orthophosphates are of great significance for humankind and, in this paper, an overview on the current knowledge on this subject is provided.

Anisotropic demineralization and oriented assembly of hydroxyapatite crystals in enamel: smart structures of biominerals

It is interesting to note that the demineralization of natural enamel does not happen as readily as that of the synthesized hydroxyapatite (HAP), although they share a similar chemical composition. We suggest that the hierarchical structure of enamel is an important factor in the preservation of the natural material against dissolution. The anisotropic demineralization of HAP is revealed experimentally, and this phenomenon is understood by the different interfacial structures of HAP-water at the atomic level. It is found that HAP {001} facets can be more resistant against dissolution than {100} under acidic conditions. Although {100} is the largest surface of the typical HAP crystal, it is {001}, the smallest habit face, that is chosen by the living organisms to build the outer surface of enamel by an oriented assembly of the rodlike crystals. We reveal that such a biological construction can confer on enamel protections against erosion, since {001} is relatively dissolution-insensitive. Thus, the spontaneous dissolution of enamel surface can be retarded in biological milieu by such a smart construction. The current study demonstrates the importance of hierarchical structures in the functional biomaterials.

A hierarchical structure for apatite crystals

Based on the experimental results taken from the references, a reasonable hierarchical structure of apatite (both fluorapatite and hydroxyapatite) crystals has been proposed for the first time. The structure consists of four levels of the hierarchy: the smallest level is made of single unit-cells and/or Posner's clusters with the linear dimensions slightly below 1 nm, the second level comprises X-ray coherent scattering blocks of 50-80 nm in size, the third level is represented by dislocation blocks of 0.3-2.0 microm in size and, finally, there are macroblocks of 35-50 microm in size.

The effect of pH on the structural evolution of accelerated biomimetic apatite

The classic biomimetic apatite coating process can be accelerated by first immersing substrates into concentrated simulated body fluid, 5x SBF (SBF1), at 37 degrees C, to form an initial coating of precursor apatite spheres, and subsequently transferring to a second 5x SBF (SBF2) solution which is devoid of crystal growth inhibitors to promote phase transformation of SBF1-derived precursor apatite spheres into final crystalline apatite plates. Since SBF1 governs the formation kinetics and composition of the initial precursor spheres, we hypothesized that the pH of the SBF1 solution will also influence the final structure of the SBF2-derived crystalline apatite. To test this hypothesis, polystyrene substrates were immersed into SBF1 with different pH (5.8 or 6.5), and then immersed into the identical SBF2 (pH=6.0). The resultant apatites exhibited similar 2 theta XRD peaks; FTIR spectra in terms of hydroxyl, phosphate and carbonate groups; and Ca/P atomic ratio (1.42 for SBF1(5.8) apatite; 1.48 for SBF1(6.5) apatite). SEM, TEM and electron diffraction show that while SBF1(6.5) (pH 6.5) precursor spheres transform into larger, single crystals plates, SBF1(5.8) (pH 5.8) precursor spheres developed minute, polycrystalline plate-like structures over predominantly spherical precursor substrate.

Density functional study of structural, electronic and vibrational properties of mg- and zn-doped tricalcium phosphate biomaterials

Zn- and to a lesser extent Mg-releasing tricalcium phosphate (Zn- and Mg-TCP) have excellent bioactivities which do not exist in their parent TCP base. However, the mechanisms through which the dopants affect the properties are not known. In order to gain insight from geometrical and electronic structures and chemical bonding, ab initio density functional calculations have been performed for these materials using cluster models. The results show a distorted structure for Zn-TCP which may be related to its bioactivity, whereas no such distortion was found for TCP and Mg-TCP. The infrared spectra of these materials has been calculated, and the relationship to the structure investigated.

Calcium phosphate clusters

The potential energy surfaces associated with [Ca3(PO4)2n clusters are analyzed in detail using ab initio calculations for n ranging from one to four. Considering separated clusters, energy criteria favor the so-called Posner's cluster Ca9(PO4)6, which is the core of the actual structural model of amorphous calcium phosphate. This is rationalized through the existence of a distinct CaO bonding pattern in this cluster. Considering aggregated clusters as a possible model for amorphous calcium phosphate, the aggregation of Ca3(PO4)2 clusters appears as an alternative to Posner's hypothesis.