Hydroxyapatite substituted by transition metals: experiment and theory

Evaluation of incipient enamel-carious-like lesion treated with hydroxyapatite-chitosan nanocomposite hydrogel

BACKGROUND

Non-invasive restoration of tooth enamel is a topic of high relevance in dental material science. Multiple approaches have been proposed to reach optimum reconstruction results. The current study was performed to evaluate the cross-sectional microhardness besides mineral quality and content in artificially induced carious enamel after treatment with hydroxyapatite-chitosan (HAp-CS) nanocomposite gel.

METHODS

Artificially carious lesions were induced by immersion of teeth in acidic carboxymethyl cellulose gel (pH 4.95-5) for 24- and 72-hours periods. Two different compositions of HAp-CS nanocomposite hydrogel were prepared with two different ratios 50/50 (%) and 70/30 (%), respectively. Additionally, sodium fluoride gel (1000 ppm concentration) was prepared and used as reference. Gels were applied to carious lesions twice/day for 3 min/each. After 45 days of application, surface morphology, energy dispersive x-ray spectroscopy, micro-Raman analysis in addition to cross-sectional microhardness were evaluated. Statistical analysis was performed using two-way ANOVA and Tukey's post hoc statistical tests.

RESULTS

Surface morphological evaluation of treated surfaces showed obliteration of surface irregularities. Groups demineralized for 24 hours and treated with 70/30 (HAp-CS) showed highest significant cross-sectional-microhardness (P ≤ 0.05). Evaluated subsurface cross-sectional microhardness showed better mineral quality for groups demineralized for 24 hours and treated with HAp-CS nanocomposite gels.

CONCLUSIONS

Nanocomposite gel with 70/30 (HAp-CS) could efficiently improve cross-sectional microhardness and both minerals composition and quality for lesions demineralized for 24 hours. More severely induced lesions, as demineralized for 72 hours, need more powerful agent compositions and/or prolonged application protocols for improvement.

Dynamic Nuclear Polarization Enhanced Multiple-Quantum Spin Counting of Molecular Assemblies in Vitrified Solutions

Crystallization pathways are essential to various industrial, geological, and biological processes. In nonclassical nucleation theory, prenucleation clusters (PNCs) form, aggregate, and crystallize to produce higher order assemblies. Microscopy and X-ray techniques have limited utility for PNC analysis due to the small size (0.5-3 nm) and time stability constraints. We present a new approach for analyzing PNC formation based on 31P nuclear magnetic resonance (NMR) spin counting of vitrified molecular assemblies. The use of glassing agents ensures that vitrification generates amorphous aqueous samples and offers conditions for performing dynamic nuclear polarization (DNP)-amplified NMR spectroscopy. We demonstrate that molecular adenosine triphosphate along with crystalline, amorphous, and clustered calcium phosphate materials formed via a nonclassical growth pathway can be differentiated from one another by the number of dipolar coupled 31P spins. We also present an innovative approach for examining spin counting data, demonstrating that a knowledge-based fitting of integer multiples of cosine wave functions, instead of the traditional Fourier transform, provides a more physically meaningful retrieval of the existing frequencies. This is the first report of multiquantum spin counting of assemblies formed in solution as captured under vitrified DNP conditions, which can be useful for future analysis of PNCs and other aqueous molecular clusters.

Chemical-physical behavior of Hydroxyapatite: A modeling approach

Hydroxyapatite (HAp) is a ceramic composed of calcium phosphate, frequently employed as a bone substitute material due to its biocompatibility and bioactivity. Over the past century, there has been substantial attention in fields such as orthopedics and plastic surgery. Remarkably, synthetic HAp exhibits properties akin to those found in natural bone and teeth. Computational theoretical chemistry focuses on numerically computing molecular electronic structures and interactions. As chemistry education evolves, it's imperative to acknowledge the increasing significance of computational tools in research. Density Functional Theory (DFT) stands out as the most widely adopted method in contemporary computational chemistry. In this study, we synthesized Hydroxyapatite (HAp) via the double decomposition method using synthetic sources. The synthesized materials underwent thorough characterization, including X-ray Diffraction (XRD), UV-visible spectroscopy, and Fourier Transform Infrared (FTIR) spectroscopy under various conditions. Additionally, we performed quantum mechanical computations on the HAp molecule using density functional theory. Our results were then compared with experimental data. Our experimental findings highlight the successful synthesis of HAp, particularly under specific temperature conditions. Moreover, the quantum chemistry calculations exhibited excellent agreement with the experimental results, especially in terms of spectroscopic characterizations.

Anaerobic syntrophic system composed of phosphate solubilizing bacteria and dissimilatory iron reducing bacteria induces cadmium immobilization via secondary mineralization

Secondary mineralization is a promising method for remediating cadmium (Cd) pollution in sediments, but the poor stability of Cd-containing secondary minerals is a bottleneck that limits the development of this approach. The existence of phosphate can enhance the formation of stable secondary minerals and points a new direction for Cd immobilization. In this research, a novel syntrophic system composed of phosphate solubilizing bacteria (PSB) and dissimilatory iron reducing bacteria (DIRB) was established and the effect and mechanism of Cd immobilization in the system were also explored. The results showed that under the conditions of DIRB:PSB (V:V)= 3:1, syntrophic bacteria dosage of 5% and glucose dosage of 5 g/L, Cd incorporated in the secondary minerals could account for about 60% of the total Cd. In the pH range of 5-9, alkaline environment was conducive to the immobilization of Cd and the percentage of combined Cd was up to 58%, while the combined Cd in secondary minerals decreased from 62% to 56% with the increase of initial Cd concentration from 0.1 to 0.3 mmol/L. In addition, XRD, XPS, Mössbauer and other characterization results showed that secondary minerals, such as Cd exchange hydroxyapatite (Cd-HAP) and kryzhanovskite (Fe₃(PO₄)₂(OH)₃) were formed in this new system. The established syntrophic system of PSB and DIRB is thus a prospective bioremediation technology for Cd immobilization in sediments and can avoid the potential risk might be caused by the addition of phosphorus-containing materials.

Compositional Analysis of the Dental Biomimetic Hybrid Nanomaterials Based on Bioinspired Nonstoichiometric Hydroxyapatite with Small Deviations in the Carbonate Incorporation

In our paper, we discuss the results of a comprehensive structural-spectroscopic and microscopic analysis of non-stoichiometric nanocrystalline hydroxyapatite (CHAp) with low carbonate anion content and biomimetic hybrid nanomaterials produced on its basis. It was shown that hydroxyapatite nanocrystals synthesized by chemical precipitation and biogenic calcium source mimic the properties of biogenic apatite and also have a morphological organization of "core-shell" type. The "core" of the CHAp nanocrystal is characterized by an overabundance of calcium Ca/P~1.9. Thus "a shell" with thickness of ~3-5 nm is formed from intermediate apatite-like phases where the most probable are octocalcium phosphate, dicalcium phosphate dihydrate and tricalcium phosphate. The multimode model of the Raman profile of samples CHAp and biomimetic composites for spectral region 900-1100 cm-1 proposed in our work has allowed to allocate precise contribution of B-type carbonate substitution, taking into account the presence on a surface of "core" HAp nanocrystal of various third-party intermediate apatite-like phases. The calibration function constructed on the basis of the described model makes it possible to reliably determine small concentrations of carbonate in the structure of hydroxyapatite with the application of Raman express method of diagnostics. The results of our work can inspire researchers to study the processes of induced biomineralization in mineralized tissues of the human body, using non-destructive methods of control with simultaneous analysis of chemical bonding, as well as determining the role of impurity atoms in the functions exhibited by biotissue.

Essence of hydroxyapatite in defluoridation of drinking water: A review

Hydroxyapatite (HAP) is an easily synthesizable, low-cost mineral that has been recognized as a potential material for fluoride removal. Some of the synthesis methods of HAP are quite straightforward and cost-effective, while some require sophisticated synthesis techniques under advanced laboratory conditions. This review assesses the physicochemical characteristics of HAP and HAP-based composites produced via various techniques, their recent development in defluoridation and most importantly, the fluoride removal performances. For the first time, fluoride removal performances of HAP and HAP composites are compared based on partition coefficient (K_D) instead of maximum adsorption capacity (Q_max), which is significantly influenced by initial loading concentrations. Novel HAP tailored composites exhibit comparatively high K_D values indicating the excellent capability of fluoride removal along with specific surface areas above 120 m²/g. HAP doped with aluminium complexes, HAP doped ceramic beads, HAP-pectin nanocomposite and HAP-stilbite nanocomposite, HAP decorated nanotubes, nanowires and nanosheets demonstrated high Q_max and K_D. The secret of HAP is not the excellent fluoride removal performances but best removal at neutral and near-neutral pH, which most of the defluoridation materials are incapable of, making them ideal adsorbents for drinking water treatment. Multiple mechanisms including physical surface adsorption, ion-exchange, and electrostatic interactions are the main mechanisms involved in defluoridation. Further research work must be focused on upscaling HAP-based composites for defluoridation on a commercial scale.

The effect of metal rate on the gamma shielding parameters of hydroxyapatite at medical treatment energies

The hydroxyapatite (HAp) is a kind of biomaterial which is used for bone treatment applications. We have scrutinized the gamma attenuation parameters such as such as the effective atomic number (Zeff), electron density (Nel), mass attenuation coefficient (MAC), linear attenuation coefficient (LAC), half value layer (HVL), tenth value layer (TVL) and mean free path (MFP) for only single liquid gamma source and a narrow beam geometry for the energy (medical treatment energy) ranging from 778 keV to 1408 keV (Eu-152) for the animal bone, iron, cobalt, copper, and zinc decorated Nano hydroxyapatite (nFeHAp, nCoHAp, nCuHAp, and nZnHAp) artificial bone powders. The gamma-rays were counted with using Ultra Low Energy Germanium detection system with a resolution 150 eV at 5,95 keV and a high purity germanium detector with a resolution of 1.85 keV at 1.33 MeV experimentally. The gamma ray attenuation parameters are calculated for the metal doped hydroxyapatite and compare with the animal bone. The results were compared with the output XCOM NIST data. While the mass absorption coefficient values for animal bone range from 0.08 to 0.05 at current energy levels, the values for metal-added artificial bone powders range from 0.07 to 0.05. While the linear absorption coefficient values for existing energy values for animal bone range from 0.04 to 0.02, they range from 0.03 to 0.02 for metal-added artificial bone powders. Mean free path values for real bone range from 24 to 36 at current energies, while half value layer values range from 16 to 25 and tenth value layer values range from 56 to 83. For metal-doped artificial bone powders, these parameters range from 26 to 35, 18 to 24, and 61 to 80, respectively. The results points that, the data of the gamma ray attenuation parameters are very close to the value of the animal bone due to the removal of calcium atoms from the structure when metal is added.

Functional role of inorganic trace elements in dentin apatite tissue-Part 1: Mg, Sr, Zn, and Fe

Many essential elements exist in nature with significant influence on dentin and bone apatite tissue. Hydroxyapatite (HAp) is the major inorganic crystalline structure of dentin that provides a site for various physiological functions such as surface layer ion exchange. Decades of apatite research have shown that enamel is a high-substituted crystalline apatite, but recent findings suggest that dentin apatite may play a more important role in regulating ion exchange as well as mineral crystallinity. This article is the first part of a review series on the functional role of inorganic trace elements including magnesium, strontium, zinc, and iron in dentin hydroxyapatite. The morphology, physiology, crystallinity, and solubility of these elements as they get substituted into the HAp lattice are extensively discussed. An electronic search was performed on the role of these elements in dentin apatite from January 2007 to September 2021. The relationship between different elements and their role in the mineral upkeep of dentin apatite was evaluated. Several studies recognized the role of these elements in dentinal apatite composition and its subsequent effects on morphology, crystallinity, and solubility. These elements are of great importance in physiological processes and an essential part of living organisms. Magnesium and strontium stimulate osteoblast activity, while zinc can improve overall bone quality with its antibacterial properties. Iron nanoparticles are also vital in promoting bone tissue growth as they donate or accept electrons in redox reactions. Thus, understanding how these elements impact dentin apatite structure is of great clinical significance.

Iron in Hydroxyapatite: Interstitial or Substitution Sites?

Iron-doped hydroxyapatite (Fe-HAp) is regarded as a promising magnetic material with innate biocompatibility. Despite the many studies reported in the literature, a detailed theoretical description of Fe inclusions is still missing. There is even no consensual view on what kind of Fe defects take place in Fe-HAp-iron interstitial or calcium substitutions? In order to address these questions, we employ modern first-principles methodologies, including hybrid density functional theory, to find the geometry, electronic, magnetic and thermodynamic properties of iron impurities in Fe-HAp. We consider a total of 26 defect configurations, including substitutional (phosphorus and calcium sites) and interstitial defects. Formation energies are estimated considering the boundaries of chemical potentials in stable hydroxyapatite. We show that the most probable defect configurations are: Fe3+ and Fe2+ substitutions of Ca(I) and Ca(II) sites under Ca-poor conditions. Conversely, Fe interstitials near the edge of the hydroxyl channel are favored in Ca-rich material. Substitutional Fe on the P site is also a probable defect, and unlike the other forms of Fe, it adopts a low-spin state. The analysis of Fe K-XANES spectra available in the literature shows that Fe-HAp usually contains iron in different configurations.

Study of Electron-Nuclear Interactions in Doped Calcium Phosphates by Various Pulsed EPR Spectroscopy Techniques

Substituted calcium phosphates (CaPs) are vital materials for the treatment of bone diseases and repairing and replacement of defects in human hard tissues. In this paper, we present some applications of the rarely used pulsed electron paramagnetic resonance (EPR) and hyperfine interaction spectroscopy approaches [namely, electron spin-echo envelope modulation (ESEEM) and electron-electron double-resonance detected nuclear magnetic resonance (EDNMR)] to investigate synthetic CaPs (hydroxyapatite, tricalcium, and octacalcium phosphate) doped with various cations (Li+, Na+, Mn2+, Cu2+, Fe3+, and Ba2+). These resonance techniques provide reliable tools to obtain unique information about the presence and localization of impurity centers and values of hyperfine and quadrupole tensors. We show that revealed in CaPs by EPR techniques, radiation-induced stable nitrogen-containing species and carbonate radicals can serve as sensitive paramagnetic probes to follow CaPs' structural changes caused by cation doping. The most pulsed EPR, ESEEM, and EDNMR spectra can be detected at room temperature, reducing the costs of the measurements and facilitating the usage of pulsed EPR techniques for CaP characterization.

Advanced Materials Based on Nanosized Hydroxyapatite

The development of new materials based on hydroxyapatite has undergone a great evolution in recent decades due to technological advances and development of computational techniques. The focus of this review is the various attempts to improve new hydroxyapatite-based materials. First, we comment on the most used processing routes, highlighting their advantages and disadvantages. We will now focus on other routes, less common due to their specificity and/or recent development. We also include a block dedicated to the impact of computational techniques in the development of these new systems, including: QSAR, DFT, Finite Elements of Machine Learning. In the following part we focus on the most innovative applications of these materials, ranging from medicine to new disciplines such as catalysis, environment, filtration, or energy. The review concludes with an outlook for possible new research directions.

X-Ray Diffraction and Multifrequency EPR Study of Radiation-Induced Room Temperature Stable Radicals in Octacalcium Phosphate

Octacalcium phosphate (OCP) {Ca8H2(PO4)6×5H2O] has attracted increasing attention over the last decade as a transient intermediate to the biogenic apatite for bone engineering and in studies involving the processes of pathological calcification. In this work, OCP powders obtained by hydrolysis of dicalcium phosphate dehydrate were subjected to X- and γ-ray irradiation and studied by means of stationary and pulsed electron paramagnetic resonance at 9, 36 and 94 GHz microwave frequencies. Several types of paramagnetic centers were observed in the investigated samples. Their spectroscopic parameters (components of the g and hyperfine tensors) were determined. Based on the extracted parameters, the induced centers were ascribed to H0, CO33-, CO2- and nitrogen-centered (presumably NO32-) radicals. The spectroscopic parameters of the nitrogen-centered stable radical in OCP powders were found to be markedly different from those in hydroxyapatite. According to X-ray diffraction data, γ-ray irradiation allowed the phase composition of calcium phosphates to change; all minor phases with the exception of OCP and hydroxyapatite disappeared, while the OCP crystal lattice parameters changed after irradiation. The obtained results could be used for the tracing of mineralization processes from their initiation to completion of the final product, identification of the OCP phase, and to follow the influence of radiation processes on phase composition of calcium phosphates.

Fibrous scaffolds of Ag/Fe co-doped hydroxyapatite encapsulated into polycaprolactone: Morphology, mechanical and in vitro cell adhesion

The development of a scaffold matrix to promote wound healing is a critical requirement to improve the health care system. For this purpose, electrospun scaffolds of polycaprolactone (PCL) have been encapsulated with hydroxyapatite (HAP) doped with different contributions Ag ions. The obtained scaffolds have been investigated by XRD, FTIR and FESEM. It was shown that scaffolds were configured as cross-linked network with diameters around 0.6, 0.9, 2.1, and 2.5 μm for 0.0Ag/Fe-HAP@PCL, 0.4Ag/Fe-HAP@PCL, 0.6Ag/Fe-HAP@PCL, and 0.8Ag/Fe-HAP@PCL, respectively. Additionally, the composition of 0.8Ag/Fe-HAP@PCL exhibited the highest roughness average of 34 nm, while the inorganic root of co-dopant HAP recorded 44.8 nm. The mechanical properties have been investigated and showed that the maximum strain at break was about 129.31 ± 5.4% at no additional Ag ions, and reached its lowest value of 103.02 ± 3.5% at 0.2Ag/Fe-HAP@PCL. On the other hand, cell viability increased from 94.74 ± 4 to 98.9 ± 4% for 0.0Ag/Fe-HAP@PCL and 0.6Ag/Fe-HAP@PCL, respectively. Further, the antibacterial activity was investigated and exhibited that the inhibition zones of E. coli increased from 0.0 at 0.0Ag/Fe-HAP@PCL to 7.5 ± 1.3 mm for 0.8Ag/Fe-HAP@PCL. Moreover, the in vitro cell attachment showed that fibroblast cells proliferated and spread on the fibers' surface and through scaffolds' porosity.

On enhancement and control of green emission of rare earth co-doped hydroxyapatite nanoparticles: synthesis and upconversion luminescence properties

In this work, low-temperature hydrothermal synthesis of a series of xMo–1%Er–10%Yb (x:mol%) doped hydroxyapatite (HA) phosphors was studied.

Empirical and theoretical insights into the structural effects of selenite doping in hydroxyapatite and the ensuing inhibition of osteoclasts

The use of ions as therapeutic agents has the potential to minimize the use of small-molecule drugs and biologics for the same purpose, thus providing a potentially more economic and less adverse means of treating, ameliorating or preventing a number of diseases. Hydroxyapatite (HAp) is a solid compound capable of accommodating foreign ions with a broad range of sizes and charges and its properties can dramatically change with the incorporation of these ionic additives. While most ionic substitutes in HAp have been monatomic cations, their lesser atomic weight, higher diffusivity, chaotropy and a lesser residence time on surfaces theoretically makes them prone to exert a lesser influence on the material/cell interaction than the more kosmotropic oxyanions. Selenite ion as an anionic substitution in HAp was explored in this study for its ability to affect the short-range and the long-range crystalline symmetry and solubility as well as for its ability to affect the osteoclast activity. We combined microstructural, crystallographic and spectroscopic analyses with quantum mechanical calculations to understand the structural effects of doping HAp with selenite. Integration of selenite ions into the crystal structure of HAp elongated the crystals along the c-axis, but isotropically lowered the crystallinity. It also increased the roughness of the material in direct proportion with the content of the selenite dopant, thus having a potentially positive effect on cell adhesion and integration with the host tissue. Selenite in total acted as a crystal structure breaker, but was also able to bring about symmetry at the local and global scales within specific concentration windows, indicating a variety of often mutually antagonistic crystallographic effects that it can induce in a concentration-dependent manner. Experimental determination of the lattice strain coupled with ab initio calculations on three different forms of carbonated HAp (A-type, B-type, AB-type) demonstrated that selenite ions initially substitute carbonates in the crystal structure of carbonated HAp, before substituting phosphates at higher concentrations. The most energetically favored selenite-doped HAp is of AB-type, followed by the B-type and only then by the A-type. This order of stability was entailed by the variation in the geometry and orientation of both the selenite ion and its neighboring phosphates and/or carbonates. The incorporation of selenite in different types of carbonated HAp also caused variations of different thermodynamic parameters, including entropy, enthalpy, heat capacity, and the Gibbs free energy. Solubility of HAp accommodating 1.2 wt% of selenite was 2.5 times higher than that of undoped HAp and the ensuing release of the selenite ion was directly responsible for inhibiting RAW264.7 osteoclasts. Dose-response curves demonstrated that the inhibition of osteoclasts was directly proportional to the concentration of selenite-doped HAp and to the selenite content in it. Meanwhile, selenite-doped HAp had a significantly less adverse effect on osteoblastic K7M2 and MC3T3-E1 cells than on RAW264.7 osteoclasts. The therapeutically promising osteoblast vs. osteoclast selectivity of inhibition was absent when the cells were challenged with undoped HAp, indicating that it is caused by selenite ions in HAp rather than by HAp alone. It is concluded that like three oxygens building the selenite pyramid, the coupling of (1) experimental materials science, (2) quantum mechanical modeling and (3) biological assaying is a triad from which a deeper understanding of ion-doped HAp and other biomaterials can emanate.

Biomimetic trace metals improve bone regenerative properties of calcium phosphate bioceramics

The bone regenerative capacity of synthetic calcium phosphates (CaPs) can be enhanced through the enrichment with selected metal trace ions. However, defining the optimal elemental composition required for bone formation is challenging due to many possible concentrations and combinations of these elements. We hypothesized that the ideal elemental composition exists in the inorganic phase of the bone extracellular matrix (ECM). To study our hypothesis, we first obtained natural hydroxyapatite through the calcination of bovine bone, which was then investigated its reactivity with acidic phosphates to produce CaP cements. Bioceramic scaffolds fabricated using these cements were assessed for their composition, properties, and in vivo regenerative performance and compared with controls. We found that natural hydroxyapatite could react with phosphoric acid to produce CaP cements with biomimetic trace metals. These cements present significantly superior in vivo bone regenerative performance compared with cements prepared using synthetic apatite. In summary, this study opens new avenues for further advancements in the field of CaP bone biomaterials by introducing a simple approach to develop biomimetic CaPs. This work also sheds light on the role of the inorganic phase of bone and its composition in defining the regenerative properties of natural bone xenografts.

Bismuth-Doped Nanohydroxyapatite Coatings on Titanium Implants for Improved Radiopacity and Antimicrobial Activity

This study aims to present the possibility to obtain bismuth-doped nanohydroxyapatite coatings on the surface of the titanium implants by using a solution-derived process according to an established biomimetic methodology. The bioactivity of the titanium surface was increased by an alkali-thermal treatment. Then, under biomimetic conditions, the titanium surface was coated with a Bi-doped nanohydroxyapatite layer by using a modified supersaturated calcification solution (SCS) containing a bismuth salt. The apatite deposits were analyzed by scanning electron microscopy coupled with X-ray analysis (SEM-EDX), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and digital X-rays radiography method. The results indicate that the Bi-doped nanohydroxyapatite coatings on titanium surface were produced. These coatings exhibit a good radiopacity, thus enhancing their applications in dental and orthopedic fields. Additionally, the Bi-doped nanohydroxyapatite coatings show significant antimicrobial activity against Escherichia coli and Staphylococcus aureus bacteria.

Rare-earth (Gd3+,Yb3+/Tm3+, Eu3+) co-doped hydroxyapatite as magnetic, up-conversion and down-conversion materials for multimodal imaging

Taking advantage of the flexibility of the apatite structure, nano- and micro-particles of hydroxyapatite (HAp) were doped with different combinations of rare earth ions (RE³⁺ = Gd, Eu, Yb, Tm) to achieve a synergy among their magnetic and optical properties and to enable their application in preventive medicine, particularly diagnostics based on multimodal imaging. All powders were synthesized through hydrothermal processing at T ≤ 200 °C. An X-ray powder diffraction analysis showed that all powders crystallized in P6₃/m space group of the hexagonal crystal structure. The refined unit-cell parameters reflected a decrease in the unit cell volume as a result of the partial substitution of Ca²⁺ with smaller RE³⁺ ions at both cation positions. The FTIR analysis additionally suggested that a synergy may exist solely in the triply doped system, where the lattice symmetry and vibration modes become more coherent than in the singly or doubly doped systems. HAp:RE³⁺ optical characterization revealed a change in the energy band gap and the appearance of a weak blue luminescence (λ_ex = 370 nm) due to an increased concentration of defects. The "up"- and the "down"-conversion spectra of HAp:Gd/Yb/Tm and HAp:Gd/Eu powders showed characteristic transitions of Tm³⁺ and Eu³⁺, respectively. Furthermore, in contrast to diamagnetic HAp, all HAp:RE³⁺ powders exhibited paramagnetic behavior. Cell viability tests of HAp:Gd/Yb/Tm and HAp:Gd/Eu powders in human dental pulp stem cell cultures indicated their good biocompatibility.

Synergistic Effects of Novel Superparamagnetic/Upconversion HA Material and Ti/Magnet Implant on Biological Performance and Long-Term In Vivo Tracking

To solve the clinical challenges presented by the long-term tracking of implanted hydroxyapatite (HA) bone repair material and to investigate the synergistic effects of superparamagnetic HA and a static magnetic field (SMF) on the promotion of osteogenesis, herein a new type of superparamagnetic/upconversion-generating HA material (HYH-Fe) is developed via a two-step doping method, as well as a specially-designed titanium implant with a built-in magnet to provide a local static magnetic field in vivo. The results show that the prepared HYH-Fe material maintains the crystal structure of HA and exhibits good cytocompatibility. The combined use of the superparamagnetic HYH-Fe material and SMF can effectively and synergistically promote osteogenesis/osteointegration surrounding the Ti implants. In addition, the HYH-Fe material exhibits distinct advantages in terms of both long-term fluorescence tracking and microcomputed tomography (micro-CT) tracking. The new material and tracking strategy in this study provide scientific feasibility and will have important clinical value for long-term tracking and evaluation of implanted materials and the bone repair effect.

Magnetic Silicium Hydroxyapatite Nanorods for Enhancing Osteoblast Response in Vitro and Biointegration in Vivo

Osteoblast behavior playing an important role in the biointegration of the Ti implant with host bone in vivo can be regulated by surface properties and magnetic field. In order to endow the Ti surface with good osteogenesis activity, Si monosubstituted and Fe and Si cosubstituted hydroxyapatite (HAp) nanorods were fabricated on microporous TiO₂ by microarc oxidation (MAO) followed with hydrothermal treatment (HT). The surface properties including microstructure, microroughness, hydrophilicity, ion release, magnetic property, cytocompatibility, and biointegration of substituted HAp nanorods were observed and evaluated, together with pure HAp nanorods and microarc oxidated (MAOed) TiO₂ as controls. After being doped with Fe, MAOed TiO₂ has no changes in phase composition and microroughness, whereas it displays weakly ferromagnetic behavior and can enhance osteoblast differentiation in vitro and formation of new bone in vivo, compared with the undoped one. The substituted HAp nanorods adhere firmly to TiO₂ and have almost the same wettability and microroughness but additional Si, Fe, and/or Ca released into the medium, compared with pure HAp nanorods. Moreover, the cosubstituted HAp has a small ferromagnetic signal, while its saturation magnetization value is less than that of the MAOed doped with Fe. Compared to pure HA nanorods, the substituted HAp nanorods not only improve cell proliferation and differentiation in vitro, but also enhance the ability of bone integration in vivo, especially for the cosubstituted one, which should be ascribed to the combined effect of microstructure, magnetic property, and released ions.

Synthesis, characterization and in vitro anti-cancer activity of vanadium-doped nanocrystalline hydroxyapatite

Nanocrystalline V(v)-doped hydroxyapatite and its reduced analogue (V(v) and V(iv) mixture) show promising in vitro cytotoxicity against cultured human bone cancer cells.

Fabrication of 6-gingerol, doxorubicin and alginate hydroxyapatite into a bio-compatible formulation: enhanced anti-proliferative effect on breast and liver cancer cells

Ample attention has been devoted to the construction of anti-cancer drug delivery systems with increased stability, and controlled and targeted delivery, minimizing toxic effects. In this study we have designed a magnetically attractive hydroxyapatite (m-HAP) based alginate polymer bound nanocarrier to perform targeted, controlled and pH sensitive drug release of 6-gingerol, doxorubicin, and their combination, preferably at low pH environments (pH 5.3). They have exhibited higher encapsulation efficiency which is in the range of 97.4-98.9% for both 6-gingerol and doxorubicin molecules whereas the co-loading has accounted for a value of 81.87 ± 0.32%. Cell proliferation assays, fluorescence imaging and flow cytometric analysis, demonstrated the remarkable time and dose responsive anti-proliferative effect of drug loaded nanoparticles on MCF-7 cells and HEpG2 cells compared with their neat counter parts. Also, these systems have exhibited significantly reduced toxic effects on non-targeted, non-cancerous cells in contrast to the excellent ability to selectively kill cancerous cells. This study has suggested that this HAP based system is a versatile carrier capable of loading various drug molecules, ultimately producing a profound anti-proliferative effect.

A Review on the Use of Hydroxyapatite-Carbonaceous Structure Composites in Bone Replacement Materials for Strengthening Purposes

Biomedical materials constitute a vast scientific research field, which is devoted to producing medical devices which aid in enhancing human life. In this field, there is an enormous demand for long-lasting implants and bone substitutes that avoid rejection issues whilst providing favourable bioactivity, osteoconductivity and robust mechanical properties. Hydroxyapatite (HAp)-based biomaterials possess a close chemical resemblance to the mineral phase of bone, which give rise to their excellent biocompatibility, so allowing for them to serve the purpose of a bone-substituting and osteoconductive scaffold. The biodegradability of HAp is low (Ksp ≈ 6.62 × 10-126) as compared to other calcium phosphates materials, however they are known for their ability to develop bone-like apatite coatings on their surface for enhanced bone bonding. Despite its favourable bone regeneration properties, restrictions on the use of pure HAp ceramics in high load-bearing applications exist due to its inherently low mechanical properties (including low strength and fracture toughness, and poor wear resistance). Recent innovations in the field of bio-composites and nanoscience have reignited the investigation of utilising different carbonaceous materials for enhancing the mechanical properties of composites, including HAp-based bio-composites. Researchers have preferred carbonaceous materials with hydroxyapatite due to their inherent biocompatibility and good structural properties. It has been demonstrated that different structures of carbonaceous material can be used to improve the fracture toughness of HAp, as they can easily serve the purpose of being a second phase reinforcement, with the resulting composite still being a biocompatible material. Nanostructured carbonaceous structures, especially those in the form of fibres and sheets, were found to be very effective in increasing the fracture toughness values of HAp. Minor addition of CNTs (3 wt.%) has resulted in a more than 200% increase in fracture toughness of hydroxyapatite-nanorods/CNTs made using spark plasma sintering. This paper presents a current review of the research field of using different carbonaceous materials composited with hydroxyapatite with the intent being to produce high performance biomedically targeted materials.

Magnetic properties and cytocompatibility of transition-metal-incorporated hydroxyapatite

A detailed magnetization study, along with an assessment of the cellular proliferation, has been carried out on transition-metal-doped hydroxyapatite (HA), Ca_10-xM_x(PO₄)₆(OH)₂, where M = Mn, Co, and Fe. In particular, a series of MnHA powder samples with an x value of 0.04 ≤ x ≤ 1.21, one CoHA (x = 0.48) and one FeHA sample (x = 1.06) were synthesized using a wet chemical method along with an ion-exchange procedure. Characterization by transmission electron microscope (TEM), energy-dispersive X-ray spectroscopy (EDXS), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) indicated that the substitution of M elements does not change the morphology and crystalline structure of pure HA that showing a single phased HA nano-rod. In every case, the magnetization isotherms for 10 K ≤ T ≤ 300 K were linear through the origin characteristic of a paramagnetic response with no indication of superparamagnetic behavior, hysteresis, or magnetic ordering. The magnetic behavior for all samples could be fit to the Curie-Weiss law yielding values for the M ion magnetic moments. The Mn²⁺ magnetic moments were close to the spin-only value of S = 5/2 or 5.92 μ_B, while the Co²⁺ moment (4.41 μ_B) was larger than the spin-only value for S = 3/2, indicating an orbital contribution due to incomplete quenching. The magnetic behavior for the FeHA sample showed a possible spin-state transition. In addition, no statistically significant differences were observed when cells were treated with the same dose of HA or MnHA up to 50 μg/mL, suggesting that the substituted Mn introduces no cytotoxicity to the HA powders.

Cobalt immobilized on hydroxyapatite as a low-cost and highly effective heterogeneous catalyst for alkenes epoxidation under mild conditions

Transition metal Co immobilized on hydroxyapatite with a loading of 0.05 wt% (denoted 0.05 wt% Co/HAP) could catalyze partial oxidation of cyclic alkenes, aromatic alkenes and aliphatic alkenes to yield epoxide products with excellent selectivity at 30 °C with O₂ and iso-butyraldehyde as co-oxidant. The TOF value was as high as 6261 h⁻¹ for epoxidation of cyclohexene. In addition, the prepared 0.05 wt% Co/HAP catalyst can be re-used at least 6 times without significant loss of catalytic activity and selectivity.

Transition metal Co immobilized on hydroxyapatite even with low loading of 0.05 wt% is a low-cost and highly effective heterogeneous catalyst for alkenes epoxidation at 30 °C with O₂ and iso-butyraldehyde as co-oxidant.

Atomic-level design of CoOH+–hydroxyapatite@C catalysts for superfast degradation of organics via peroxymonosulfate activation

In situ formation of CoOH⁺–hydroxyapatite@C via ion exchange between Ca and Co realises the simultaneous adsorption of Co²⁺ and catalytic peroxymonosulfate oxidation for superfast oxidative degradation of organic contaminants.

Magnetic hydroxyapatite: a promising multifunctional platform for nanomedicine application

In this review, specific attention is paid to the development of nanostructured magnetic hydroxyapatite (MHAp) and its potential application in controlled drug/gene delivery, tissue engineering, magnetic hyperthermia treatment, and the development of contrast agents for magnetic resonance imaging. Both magnetite and hydroxyapatite materials have excellent prospects in nanomedicine with multifunctional therapeutic approaches. To date, many research articles have focused on biomedical applications of nanomaterials because of which it is very difficult to focus on any particular type of nanomaterial. This study is possibly the first effort to emphasize on the comprehensive assessment of MHAp nanostructures for biomedical applications supported with very recent experimental studies. From basic concepts to the real-life applications, the relevant characteristics of magnetic biomaterials are patented which are briefly discussed. The potential therapeutic and diagnostic ability of MHAp-nanostructured materials make them an ideal platform for future nanomedicine. We hope that this advanced review will provide a better understanding of MHAp and its important features to utilize it as a promising material for multifunctional biomedical applications.

Paramagnetic Manganese in the Atherosclerotic Plaque of Carotid Arteries

The search for adequate markers of atherosclerotic plaque (AP) instability in the context of assessment of the ischemic stroke risk in patients with atherosclerosis of the carotid arteries as well as for solid physical and chemical factors that are connected with the AP stability is extremely important. We investigate the inner lining of the carotid artery specimens from the male patients with atherosclerosis (27 patients, 42–64 years old) obtained during carotid endarterectomy by using different analytical tools including ultrasound angiography, X-ray analysis, immunological, histochemical analyses, and high-field (3.4 T) pulse electron paramagnetic resonance (EPR) at 94 GHz. No correlation between the stable and unstable APs in the sense of the calcification is revealed. In all of the investigated samples, the EPR spectra of manganese, namely, Mn²⁺ ions, are registered. Spectral and relaxation characteristics of Mn²⁺ ions are close to those obtained for the synthetic (nano) hydroxyapatite species but differ from each other for stable and unstable APs. This demonstrates that AP stability could be specified by the molecular organization of their hydroxyapatite components. The origin of the obtained differences and the possibility of using EPR of Mn²⁺ as an AP stability marker are discussed.