Hydroxyapatite Lattice Research Articles

Synthesis and characterization of nano-hydroxyapatite added with magnesium obtained by wet chemical precipitation

Biogenic hydroxyapatites from mammalian bones naturally contain traces of ions, like Mg, which play a vital role in the bone remodeling process. In this way, synthetic hydroxyapatites should include this kind of mineral. In this work, hydroxyapatite added with Mg was synthesized by wet precipitation using (NH4)H2PO4, Ca(NO3)2.4H2O, and Mg(NO3)2.6H2O (0, 0.032, 0.061, and 0.123 ​M). Inductively coupled plasma and X-ray diffraction evidenced the Mg inclusion in the hydroxyapatite lattice in the same levels reported for natural hydroxyapatites. The calcination at 600 ​°C was performed to remove the reaction by-products, but it also gave rise to physicochemical changes as the coalescence and crystals recrystallization. It produced, in turn, an improvement in the crystalline quality, according to Raman analysis. Despite those physicochemical changes, all the samples remained nanometric according to scanning electron microscopy imaging.

Effects of Zoledronic Acid on Bone Structure and Organization of Nanocomposites in Rats with Glucocorticoid-Induced Osteoporosis

Continuous use of methylprednisolone (Mps) is one of the most common causes of secondary osteoporosis resulting in alterations of the skeleton microstructure and increased fracture risk. The aim of the research was to assess the influence of zoledronic acid on mineral mass, micro- and nanostructure of the bones in rats, which received glucocorticoids. Wistar rats were randomly divided into three groups: vehicle control, the introduction of methylprednisolone, 3 mg/kg/day (Mps), and Mps combined with zoledronic acid, 0.025 mg/kg (Mps+Zol.). The relative amount of a crystalline component and collagen in the bones was detected by means of X-ray diffraction, microstructure, and calcium level through atomic absorption spectroscopy. Methylprednisolone caused a decrease in the amount of a mineral component in the femoral neck (to -39.8%) in group Mps than vehicle control. It should be emphasized that the introduction of Zol. did not allow a significant decrease in a mineral component of the bone during the experiment (-11.7% on the 24th day) compared with Mps group, and indices were almost at the same level as in the control group. Zoledronic acid can partially inhibit harmful consequences of glucocorticoids for bone structure in rats and improve mineralization without impairment of crystal parameters of hydroxyapatite lattice.

Formation of vacancy point-defects in hydroxyapatite nanobelts by selective incorporation of Fe3+ ions in Ca(II) sites. A CL and XPS study

We report a cathodoluminescence (CL) and x-ray photoelectron spectroscopy (XPS) study of the formation of calcium vacancies (VCa) and hydroxyl ion vacancies (VOH) in hydroxyapatite (HAp) nanobelts selectively doped with Fe3+ ions by substituting Ca(II) ions of the HAp lattice. Additionally, we report a paramagnetic behavior of the nanobelts doped with Fe concentrations lower than 1 atomic percent (at.%), besides the absence of superparamagnetism even at concentrations around 12 at.%. XPS spectroscopy measurements demonstrated that the Fe3+ ions incorporated selectively into the HAp lattice by specific substitution of Ca2+ (II) ions, forming Fe-PO4 bonds. CL spectra obtained on the HAp:Fe powders revealed that Fe doping promotes VCa and VOH formation. We propose that the generation of paramagnetism in the HAp:Fe samples is due to the contribution of both Fe3+ ions and the unpaired electrons of O2− anions adjacent to VCa.

Synthesis, characterization and biological performance study of Sr-doped hydroxyapatite/chitosan composite coatings

Hydroxyapatite (HA) is widely used in bone repair and regeneration because of its composition and structure similar to human bone. However, the artificially synthesized HA has poor mechanical properties, so it is often necessary to add a second phase to the powder as a supporting phase to enhance its mechanical properties. Strontium (Sr) has the properties of promoting the growth of bone cells, which can increase bone formation in the body and reduce bone self-absorption. Nevertheless, Sr-doped HA composites have been rarely reported in the literature in recent years. In this paper, Sr-doped HA was employed to be complexed with chitosan (CS) for the first time to prepare a composite coating that is dense and uniform, with good adhesion and ductility, in order to expand its clinical applications. Therefore, a series of HA samples doped with different Sr mole fraction (0 mol% (pure HA), 0.5 mol%, 1 mol%, 2 mol%, 4 mol%) were prepared by chemical precipitation method. Then the HA/CS composite coating doped with Sr was prepared on the stainless steel substrate by spin-coating technology. The results showed that Sr2+ ions had replaced part of Ca2+ ions and have been successfully doped into the HA lattice. The synthesized HA particles showed a shortrod-like morphology, and the substitution of Sr ions promoted the growth of Sr-doped HA crystals and increased its grain size. In addition, the synthesized Sr-doped HA/CS composite coating is dense and uniform. In vitro cell culture experiments showed that all composite coatings have good biocompatibility. The HA/CS coating with 4 mol% Sr ion doping is more conducive to cell proliferation, adhesion and osteogenic differentiation.

Ionic substituted hydroxyapatite for bone regeneration applications: A review

Biological apatites are characterised by various ionic substitutions within the HAp lattice that are crucial for bone metabolism. The introduction of key role elements within synthetic calcium phosphates (CaP), mainly hydroxyapatite (HAp), can increase osteogenesis and enhance bone regeneration process. The lattice structure of HAp enables cationic and anionic substitutions leading to the enhanced biological performance of synthetic bone graft materials. This review summarises recent and relevant studies on cationic and anionic substitutions in the HAp lattice that are commonly found in the human body. Furthermore, co-substituted HAp obtained from synthetic and biological precursors, along with their influence on the bone regeneration process, has been discussed. Finally, future perspectives for the use of substituted HAp have been presented.

Preparation and antibacterial properties of cerium-zinc co-doped hydroxyapatite-graphene composite

Cerium (Ce) and zinc (Zn) are common trace elements in human bodies. Ce-Zn co-doped hydroxyapatite (HA) composites has good application value in biomedical field. In this study, Ce-Zn co-doped hydroxyapatite-graphene (HA-GP) composite powders are successfully synthesized by a one-pot hydrothermal method. Then, the Ce-Zn co-doped hydroxyapatite-graphene composites are characterized by a set of techniques including X-ray diffraction (XRD), Fourier transform infrared (FTIR), Raman infrared (Raman), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDX) and transmission electron microscopy (TEM). The results show that Ce and Zn ions have been successfully doped into HA lattice by replacing partial Ca ions. However, due to the difference between the radii of Ce, Zn and Ca ions, the amorphous phases in the obtained composites increase with the increase of doping ratios, and the agglomeration phenomenon begins to appear as well. The antibacterial tests show that doping of Ce and Zn ions greatly improves the antibacterial properties of the composites. The sterilization rate reaches up to 93.2% as the doping content is increased to 10% (molar ratio), which implies that Ce-Zn co-doped HA-GP composite has a good antibacterial application prospect.

Regulating the Superficial Vacancies and OH− Orientations on Polarized Hydroxyapatite Electrocatalysts

AbstractSmart designs of hydroxyapatite (HAp) materials with customized electrical properties are drawing increasing attention for their wide range of potential applications. Such enhanced electrical properties directly arise from the number and orientation of OH− groups in the HAp lattice. Although different polarization treatments have been proposed to enhance the final conductivity by generating vacancies at high temperatures and imposing specific OH− orientations through electric voltages, no direct measurement showing the evolution that OH− groups undergo has been described yet. In this article, the first direct empirical observation that allows the characterization of both the generation of vacancies and the polarization of OH− groups is reported. The mechanisms behind the electrical enhancement are elucidated allowing to distinguish between charge accumulation at the crystal grains, which is due to the formed vacancies, and charge accumulation in the boundaries of particles. In addition, a linear dependence between the number of vacancies and the superficial charge is observed. Therefore, it is demonstrated that the charge accumulation at the micrometric grain boundaries has a great impact on the catalytic properties of the thermally stimulated polarized HAp. These results will be used for further optimization of the catalyst properties.

Effect of zoledronic acid on bone nanocomposites organization and prevention of bone mineral density loss in ovariectomized rats.

Osteoporosis often occurs in individuals of different age groups, frequently during menopause and after ovariectomy. It increases the risk of pathological fractures almost twice. The aim of our research was to assess bone metabolism, nanocomposite structure of the tibia under conditions of ovariectomy and zoledronic acid treatment. X-ray diffraction has been performed for nanostructure analysis of mineral crystallites and crystal lattice of hydroxyapatite in the tibia samples of ovariectomized rats with additional application of bisphosphonate zoledronic acid (0.025mg/kg). Markers of remodeling- osteocalcin, alkaline phosphatase, tartrate resistant acid phosphatase 5b- were determined. Quantitative amount of calcium in the bones was detected by atomic absorption method. Zoledronic acid prevented loss of mineral mass after ovariectomy. Rats after ovariectomy, treated with zoledronic acid, showed statistically higher (р<0.05) values of crystalline phase and calcium content compared with the SHAM-surgery and ovariectomy groups (р<0.05). Zoledronic acid inhibited bone remodeling, which is proved by tartrate resistant acid phosphatase 5b reduction and inhibition of osteoclasts during the experiment. These results enable to suggest that zoledronic acid can improve mineral mass of the bone during menopause in individuals of different age groups.

Mesoporous Iron(III)-Doped Hydroxyapatite Nanopowders Obtained via Iron Oxalate

Mesoporous hydroxyapatite (HA) and iron(III)-doped HA (Fe-HA) are attractive materials for biomedical, catalytic, and environmental applications. In the present study, the nanopowders of HA and Fe-HA with a specific surface area up to 194.5 m2/g were synthesized by a simple precipitation route using iron oxalate as a source of Fe3+ cations. The influence of Fe3+ amount on the phase composition, powders morphology, Brunauer–Emmett–Teller (BET) specific surface area (S), and pore size distribution were investigated, as well as electron paramagnetic resonance and Mössbauer spectroscopy analysis were performed. According to obtained data, the Fe3+ ions were incorporated in the HA lattice, and also amorphous Fe oxides were formed contributed to the gradual increase in the S and pore volume of the powders. The Density Functional Theory calculations supported these findings and revealed Fe3+ inclusion in the crystalline region with the hybridization among Fe-3d and O-2p orbitals and a partly covalent bond formation, whilst the inclusion of Fe oxides assumed crystallinity damage and rather occurred in amorphous regions of HA nanomaterial. In vitro tests based on the MG-63 cell line demonstrated that the introduction of Fe3+ does not cause cytotoxicity and led to the enhanced cytocompatibility of HA.

Investigation of Mammoth Tusk Mammuthus Primigenius by Thermogravimetry and X-ray Diffraction Analysis

The article represents the results of the research of the mammoth tusk Mammuthus Primigenius using thermogravimetric (TGA) and X-ray diffraction analysis (XRD). The stages of thermal destruction of mammoth tusk components and their temperature limits are revealed. XRD of mammoth tusk showed that heat treatment leads to a change in the structure of the mineral component of the material under study from hydroxyapatite (HAP) to whitlockite. It was found that this process is caused by the presence of magnesium ions in the crystal lattice of HAP.

Influence of magnesium and strontium substitutions in the structure of hydroxyapatite lattice on the deposition rate and properties of the CaP coatings formed via RF-sputtering of the powder targets

This work is dedicated to studying of the properties of the calcium phosphate (CaP) coatings deposited on Ti substrates by radio-frequency magnetron sputtering (RFMS) of three hydroxyapatite-based powder targets: pure hydroxyapatite (HA), Mg-substituted HA (Mg-HA, Mg = 0.93 ± 0.13 at.%) and Sr-substituted HA (Sr-HA, Sr ∼ 0.47 at.%). The influence of ionic substitutions in the structure of the sputtered targets on the surface morphology, physicochemical properties of the coatings and their wettability were studied. It is revealed that Mg and Sr ionic substitutions in the crystal lattice of HA at these concentrations don’t affect deposition rate, however, it influences morphology, wettability and elemental and phase composition of deposited coatings.

Microstructure and mechanical properties of composites of bovine derived hydroxyapatite (BHA) reinforced with MgF2

Composites of calcinated bovine bone derived hydroxyapatite (BHA) doped 1 and 2 wt% MgF2 were prepared by a sintering process. Microstructure and crystallographic analyses along with measurements of density, compression strength, and microhardness were carried out in the produced samples. The experimental results indicated a beneficial effect of MgF2 in the matrix of BHA reflected in the significant increase of compression strength and microhardness up to 143 MPa and 313 HV, respectively, achieved after sintering at 1300 0C for 2% MgF2 addition. The presence of MgF2 reduced the onset of sintering towards lower temperatures (i.e. 1100 0C) and increased the stability of hydroxyapatite towards transformation to TCP at 1300 0C. The influence of Mg2+ and F- ions in the lattice of hydroxyapatite is discussed.

Biocompatible magnesium-doped biphasic calcium phosphate for bone regeneration.

Autologous bone grafting remains the gold standard for almost all bone void-filling orthopedic surgery. However, autologous bone grafting has several limitations, thus scientists are trying to identify an ideal synthetic material as an alternative bone graft substitute. Magnesium-doped biphasic calcium phosphate (Mg-BCP) has recently been in the spotlight and is considered to be a potential bone substitute. The Mg-BCP is a mixture of two bioceramics, that is, hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP), doped with Mg2+ , and can be synthesized through chemical wet-precipitation, sol-gel, single diffusion gel, and solid state reactions. Regardless of the synthesis routes, it is found that the Mg2+ preferentially accommodates in β-TCP lattice instead of the HA lattice. The addition of Mg2+ to BCP leads to desirable physicochemical properties and is found to enhance the apatite-forming ability as compared to pristine BCP. In vitro results suggest that the Mg-BCP is bioactive and not toxic to cells. Implantation of Mg-BCP in in vivo models further affirmed its biocompatibility and efficacy as a bone substitute. However, like the other bioceramics, the optimum physicochemical properties of the Mg-BCP scaffold have yet to be determined. Further investigations are required regarding Mg-BCP applications in bone tissue engineering.

Pourbaix-Guided Mineralization and Site-Selective Photoluminescence Properties of Rare Earth Substituted B-Type Carbonated Hydroxyapatite Nanocrystals

Rare-earth labeling in biological apatite could provide critical information for the pathologic transition (osteoclastic) and physiologic regeneration (osteogenesis) of bone and teeth because of their characteristic site-sensitive fluorescence in different coordinative conditions of various tissues in many biological processes. However, the rare-earth labeling method for biological apatites, i.e., carbonated-hydroxyapatite, has been rarely found in the literature. In this paper, we report a Pourbaix-diagram guided mineralizing strategy to controllable carbonation and doping of rare-earth ions in the hydroxyapatite (HA) lattice. The carbonation process of hydroxyapatite was achieved by controllable mineralization in hydrothermal condition with K2CO3 as the carbonate source, which results into the pure B-type carbonated hydroxyapatite (CHA) with tunable carbonate substitution degree. All of the as-synthesized materials crystalized into P63/m (No. 176) space group with the lattice parameter of a decreases and c increases with the increasing of carbonate content in the reactants. Structural refinement results revealed that the substitution of planar CO32− is superimposed on one of the faces of PO43− tetrahedral sub-units with a rotation angle of 30° in reference to c-axis. All of the hydrothermally synthesized CHA nanocrystals show hexagonal rod-like morphology with the length of 70–110 nm and diameter of 21–35 nm, and the decreasing length/diameter ratio from 3.61 to 2.96 from low to high carbonated level of the samples. Five rare-earth cations, of Pr3+, Sm3+, Eu3+, Tb3+, and Ho3+, were used as possible probe ions that can be doped into either HA or CHA lattice. The site-preference of Tb3+ doping is the same in the crystallographic site of HA and CHA according to characteristic emission peaks of 5D4–7Fj (j = 3–6) transitions in their photoluminescent spectroscopy. Our work provides a controllable carbonation method for rare-earth labeling hydroxyapatite nanomaterials with potential biologically active implant powders for bone repair and tissue regeneration.

Synthesis of calcium orthophosphates by chemical precipitation: re-evaluation of the solubility products of hydroxyapatites

For the first time, a linear correlation was found between the specific solubility products of calcium orthophosphates and their molar ratio Ca/P (the correlation coefficient being R2=0.9742). Nevertheless, the values of solubility products of tricalcium phosphate (pKS=26–29), hydroxyapatite (pKS=116.8) and its Ca-deficient forms (pKS~85) cannot be correlated. We proposed to adjust these values of solubility products in accordance with the obtained correlation dependence as follows: pKS is 40, 155 and 114–155 for tricalcium phosphate, hydroxyapatite and Ca-deficient hydroxyapatite, respectively. The calculated solubility diagrams (isotherms) with adjusted solubility products agree reasonably well with the known experimental data, which could not be explained and were not accepted by the scientific community up to the present day. Based on well-known ideas of chemical thermodynamics, we suggested an explanation of the correlation between the specific solubility products of calcium orthophosphates and their molar ratios Ca/P. The developed model of dissolution process is based on a comparison between crystal lattice energy and hydration energy of calcium ions. The experiments on chemical precipitation of orthophosphates were performed at pH 4–10 and at a constant molar ratio Ca/P=1.5; their results showed that only one single metastable form, calcium hydrogen phosphate, is precipitated within the entire pH range at the temperature of 200С. The obtained sediments were isothermally exposed at the temperature of 2500С in a mother solution for 6 hours; such a treatment resulted in a full dehydration of calcium hydrogen phosphate and its partial transformation into a more stable form (hydroxyapatite). The mass fraction of hydroxyapatite increases from 14% to 70% and the degree of crystallinity decreases from 20% to 5% with increasing the value of solution pH. It was concluded that the rate of phase transition from CaHPO4 to Ca10(PO4)6(OH)2 in influenced by the concentration of hydroxide ions that are incorporated into a crystal lattice of hydroxyapatite.

Physical Fundamentals of Biomaterials Surface Electrical Functionalization.

This article is focusing on electrical functionalization of biomaterial’s surface to enhance its biocompatibility. It is an overview of previously unpublished results from a series of experiments concerning the effects surface electrical functionalization can have on biological systems. Saccharomyces cerevisiae cells were used for biological experiments. The hydroxyapatite (HAp) specimens were used to investigate influence of structural point defects on the surface electrical charge. Threshold photoelectron emission spectroscopy was used to measure the electron work function of HAp and biologic samples. The density functional theory and its different approximations were used for the calculation of HAp structures with defects. It was shown that the electrical charge deposition on the semiconductor or dielectric substrate can be delivered because of production of the point defects in HAp structure. The spatial arrangements of various atoms of the HAp lattice, i.e., PO4 and OH groups, oxygen vacancies, interstitial H atoms, etc., give the instruments to deposit the electrical charge on the substrate. Immobilization of the microorganisms can be achieved on the even surface of the substrate, characterized with a couple of nanometer roughness. This cells attachment can be controlled because of the surface electrical functionalization (deposition of the electrical charge). A protein layer as a shield for the accumulated surface charge was considered, and it was shown that the protein layer having a thickness below 1 µm is not crucial to shield the electrical charge deposited on the substrate surface. Moreover, the influence of surface charge on the attachment of microorganisms, when the surface roughness is excluded, and the influence of controlled surface roughness on the attachment of microorganisms, when surface charge is constant, were also considered.

Thermo-physical insights into a series of strontium substituted hydroxyapatite

In this study, we investigated the effect of Sr substitution (2, 4, 8, 16 mol. %) on the thermo-physical properties of hydroxyapatite (HA). FTIR and Raman analysis revealed that Sr substitution incorporated CO32− and HPO42− into the HA lattice and consequently SrHA lost more mass according to the TG studies. The substitution of Sr reduced the dehydroxylation peak temperatures from 1266 °C to 1226 °C and decomposition peak temperatures from 1472 °C to 1432 °C. Heat treated HA, 2SrHA, 4SrHA and 8SrHA preserved dominant HA phase (>99%). However, 16SrHA decomposed to TCP (>23 wt %). Moreover, Sr substitution increased the “a” and “c” lattice constants and thus, enlarging the crystal volume of HA. Raman studies confirmed that Sr substituted HAs up to 8 mol % decomposed slightly to oxyhydroxyapatite (OHA). We employed a deconvolution process to the Raman peaks at ~960 cm−1 to calculate the degree of the OHA decomposition by Sr substitution.

Conversion of sub-µm calcium carbonate (calcite) particles to hollow hydroxyapatite agglomerates in K2HPO4 solutions

Abstract Sub-µm CaCO3 (calcite; CC) particles were converted to calcium monohydrogenphosphate dihydrate (DCPD) and hydroxyapatite (HAp) via soaking treatments in K2HPO4 solutions with varied pH (3–12) and concentrations (0.1–1.5 M) at 37°C for up to 10 days. DCPD was derived from the solutions with pH ≤ 6; while hollow HAp was yielded when pH ≥ 7 in assemblies of petal-like crystallites. Results of magic angle spinning (MAS) and cross-polarization magic angle spinning (CP-MAS) NMR studies have shown that the HAp lattice has only PO4 2− but no HPO4 2− at B (phosphate) sites. Trace amounts of CO3 2− have occupied both A (OH) and B (PO4) sites, and H2O is adsorbed on surface crystallites. The primary crystallite size of HAp derived from Scherrer equation increases quickly in a 12 h period and becomes gradually stable afterward. Samples of particles soaked within 3 h in a temperature range of 20–80°C were analyzed by X-ray diffraction. It is shown that the rate constant of 1 M solution is about an order of magnitude greater than that of 0.1 M solution and the apparent activation energy is 33 kJ/mol. In this work, the conversion of CC to HAp can be quantitatively controlled to solve the problem of slow degradation of HAp.

Strontium substituted hydroxyapatite promotes direct primary human osteoblast maturation

Strontium-substituted hydroxyapatite (SrHAp) materials are known to actively promote bone formation. However, the optimum level of Sr inclusion needed to elicit a physiologically relevant response from bone cells is unclear and can vary dependent on the fabrication process employed. In this work hydroxyapatite (HAp), SrHAp powders (2, 5 and 10 wt% (i.e., 1, 2 and 5 at%) with respect to [Sr/(Sr + Ca) ∙100]), were synthesized with the purity and Sr-substitution was confined in the range of 1–8 wt% (1–4 at%). All SrHAp samples contained rod-like crystals (<106 nm in length), which decreased in length with increasing Sr content, and exhibited larger flatter crystals (>300 nm in length). TEM-EDX confirmed the presence of Sr and maintenance of the HAp lattice structure for both types of crystals. Qualitative in vitro evaluation using primary human ostesoblast cells (HOBC) cultured in contact with the SrHAp over 28 days showed that the presence of Sr (in particular with the highest Sr content) directly promotes the maturation of osteoblasts into osteocytes as compared to the response observed for HAp. As these materials contain no additives other than Sr, the effects observed here can only be attributed to the physiologically important levels of Sr in the samples.

Molecular Dynamics Characterization of Sr-Doped Biomimetic Hydroxyapatite Nanoparticles

Hydroxyapatite (HAP), which is the main inorganic component of human bones and teeth, has received much attention for its use as an implant material. Doping foreign ions into the HAP lattice structure is a valuable approach to endow and augment its biological characteristics. In this study, based on first-principles calculations, the force-field parameters of strontium ions substituted in HAP were first fitted by a genetic algorithm. The corresponding Sr-doped HAP bulk and surface properties were then characterized by molecular dynamics simulations. The calculated lattice structure, melting temperature, infrared spectra, and elastic parameters of 0–100 at.% Sr-doped HAP are in good agreement with the experimental observations. Several models with fully Sr-substituted HAP were constructed to understand the effects of different pH values. Our simulations indicate that Sr-substituted HAP might exhibit higher solubility along with decreasing pH values. Meanwhile, Sr ions tend to adsorb onto the HAP surface, and the amount of adsorption increases with increasing pH. Finally, the substitution of Sr ions might weaken the adsorption of Glu side-chain analogues but strengthen the adsorption of Lys side-chain analogues on the HAP surface. The force-field parameters developed for Sr in this work could be applied to subsequent studies for interactions between Sr-doped HAP and other biomolecules.