Stoichiometric Hydroxyapatite Research Articles

Substituted hydroxyapatite and β-tricalcium phosphate as osteogenesis enhancers

Hydroxyapatite and β-tricalcium phosphate are the most popular calcium phosphates used for bone tissue regeneration and in the manufacture of prostheses for bone replacement. Incorporation of different ions in low concentrations into the crystal lattice of calcium phosphates allows enhancing or modifying their properties. In this work, stoichiometric hydroxyapatite, calcium deficient hydroxyapatite and β-tricalcium phosphate co-substituted with zinc, magnesium, and silicate were synthesized using a mechanochemical approach. The compositions of the materials were investigated by powder X-ray diffraction, FTIR spectroscopy and scanning electron microscopy. The materials prepared were assessed for dissolution in water and implanted into rat skull bones. For the first time, it was demonstrated that the substituted calcium-deficient hydroxyapatite has an increased solubility in water compared with β-tricalcium phosphate and hydroxyapatite. In vivo studies revealed that the calcium-deficient hydroxyapatite promotes better osteogenesis than other calcium phosphates investigated here, showing almost complete remodeling of the damaged bone tissue already after 5 months. In this regard, calcium-deficient hydroxyapatite with zinc, magnesium, and silicate substitution is the most promising material for bone tissue regeneration.

Synthesis and characterisations of clinoptilolite enriched hydroxyapatite nanoceramics by sol-gel route for bone regeneration

Hydroxyapatite (HA) has great bioactive potential in bone tissue engineering (BTE). However, the high crystalline stability of synthetic HA restricts its in-vivo bioresorbability which extends the healing time of bone defects. This study investigates doping HA with biocompatible Clinoptilolite (CLP) mineral rich in silicate and other biomimetic ions (Na+, K+, Ca+2, Mg+2 etc.). Sol-gel-based CLP doped HA nanoceramics are aimed to be exploited as bone filler substitutes with biomimetic morphology, composition, and improved biocompatibility and bioactivity. The pure and CLP (5%, 10% and 20%) doped HA ceramics were synthesised by sol-gel method to provide molecular level interactions of ions and sintered at different temperatures (800 °C, 950 °C, and 1100 °C). Morphological analyses by SEM and S-TEM showed biomimetic nano rod-like HA particles are attached to dispersed CLP sheets. Sintering of composite ceramics led to a cell-friendly interconnected porous microstructure even at 1100 °C, where the pure HA had a compact non-porous structure. The Ca/P ratio of CLP-doped HA samples was lower than the stoichiometric HA which indicates a partial reduction of stability that is supposed to induce bioresorption capacity. Cytotoxicity tests by WST-8 with Saos-2 cell lines showed that CLP addition into HA has significantly improved cell viability. The HA-CLP nanoceramics are expected to promote bone regeneration due to cell-friendly microstructure and biomimetic composition which can be supported by further in-vivo clinical studies.

Surface immobilization mechanisms of cobalt ions on hydroxyapatite catalyst supports

Cobalt deposition in an excess of solution was used to design Co-modified hydroxyapatite materials with various Co loadings and controlled dispersion. It is rationalized how the properties of hydroxyapatite supports (more or less stoichiometric compositions, nanorod or platelet morphologies and crystalline (100) zig-zag termination or non-apatitic hydrated external layer), influence the immobilization processes of Co by operating either with slightly acidic (natural) or basic pH of the suspension media. Four cobalt immobilization mechanisms impacting the final dispersion of Co on hydroxyapatites were identified by combining structural (XRD, 1H and 31P solid-state NMR, UV-Vis, Raman and X-ray fluorescence spectroscopies), surface (XPS) characterizations of Co-modified hydroxyapatites after drying and thermal treatment at 500 °C, and monitoring of the pH and the composition of the supernatant solutions during the Co deposition step. On dried Co-modified crystalline stoichiometric hydroxyapatite nanorods, cobalt is highly dispersed through cationic exchange or strong electrostatic adsorption (SEA) at slightly acidic or basic pH, respectively. After thermal treatment at 500 °C, only cation exchange preserved atomic dispersion of Co(II) ions since Co3O4 nanoparticles were observed on samples for which Co deposition occurred via SEA. On defective hydroxyapatite platelets, cobalt deposited at acidic natural pH could diffuse in an external non-apatitic layer, whereas under basic pH media, this surface layer was hydrolysed, resulting in the formation of a cobalt-substituted hydroxyapatite layer in which only a limited fraction of the surface cobalt species could be probed by XPS.

Non-covalent interactions and adsorption affinities of doxycycline and tetracycline with the (010) and (001) surfaces of hydroxyapatite. A DFT and experimental study

Understanding the antibiotic interaction processes with hydroxyapatite (HA) surface is crucial for designing effective antibiotic release systems for treating inflammatory processes in calcified tissues. In this study, we investigated the non-covalent interactions and adsorption sites of the antibiotics doxycycline (DOX) and tetracycline (TC) on the (001) and (010) HA surfaces using transmission electron microscopy. The needle-shaped HA nanoparticles exhibited PO4-rich (010-PO4) and PO4-deficient (010-OH) surfaces. Adsorption isotherms of nearly stoichiometric HA (Ca10(PO4)6(OH)2) demonstrated Langmuir–Freundlich kinetics, with DOX showing a greater affinity than TC for the HA surface. Density functional theory atomistic simulations with three configurations of DOX and TC interacting with the 001, 010-PO4, and 010-OH surfaces revealed van der Waals interactions and hydrogen bonds at the interface. However, DOX exhibited higher adsorption energy than TC. Both molecules preferred to interact with the 010-PO4 surface in their stable configurations. However, the π-cation interactions between the aromatic ring of DOX and the Ca atoms on the HA surface were not observed in TC. Our findings further our understanding of the non-covalent interactions and adsorption sites of cyclines on the HA surface, thus promoting the design of more effective antibiotic release systems.

CALCIUM PHOSPHATE-BASED NANOCOATING IN ORTHOPAEDICS: INFLUENCE OF COMPOSITION AND SUBSTRATE HEATING ON FILM PROPERTIES AND BM-MSC BEHAVIOUR

Calcium phosphates-based (CaPs) nanocoatings on metallic prosthesis are widely studied in orthopedics and dentistry because they mimic the mineral component of native human bone and favor the osseointegration process. Despite the fact that different calcium phosphates have different properties (composition, crystallinity, and ion release), only stoichiometric hydroxyapatite (HA) films have been analyzed in deep. Here, we have realized films of different CaPs (HA, beta-tricalcium phosphate (β-TCP) and brushite (DCPD)) onto Ti6Al4V microrough substrates by Ionized Jet Deposition (IJD). We have implemented the heating of substrates at 400°C during deposition to see the effect on coating properties.Different film features are evaluated: morphology and topography (FEG-SEM, AFM), physical-chemical composition (FT-IR and EDS), dissolution profile and adhesion to substrate (scratch test), with a focus on how the different CaPs and temperature changed the coating features. After coating optimization, we have studied the in vitro BM-MSC behavior, in term of viability and early adhesion.We have obtained good transfer of fidelity in composition from target to coating for all CaPs, with nanostructured films formed by globular aggregates (~300 nm diameter), with homogeneous and uniform coverage of the substrate surface, without cracks. The heating during deposition has increased the adhesion of the films to the substrate, with higher stability in medium immersion and wettability, features that can improve the biological behavior of cells. All CaP coatings have showed excellent biocompatibility, with DCPD coating that promote higher cells viability at 14 days respect to HA and β- TCP films. About the early cell adhesion, the BM-MSC have showed switch from a globular to an elongated morphology at 6 hours in all coatings respect to the uncoated titanium, sign of better adhesion.From these results, the fabrication of different CaP nanocoatings with IJD can be a promising for applications in orthopedics and dentistry.

Characteristics of sintered calcium deficient hydroxyapatite scaffolds produced by 3D printing

This study investigates the parametric quality and reliability of 3D-printed scaffolds using a composite filament comprising a thermoplastic polymer and hydroxyapatite in 1:1 wt ratio. Employing a fused filament fabrication printer, we verified the 3D printing strategy, sintering temperature range, and preparation of hydroxyapatite scaffolds. To complete the study, in vitro cytotoxicity tests using animal and human cell models were conducted. The calcium-deficient hydroxyapatite (Ca/P ∼ 1.54) used in preparation of filament exhibited after sintering differences in crystalochemical phases in different rations, comparing to stoichiometric hydroxyapatite (Ca/P ≅ 1.67). The FFF process successfully produced scaffold macropores suitable for tissue vascularization (∼380 µm or smaller). Optimal sintering temperature for cell proliferation was identified at 1300 °C, especially effective for used investigated calcium-deficient hydroxyapatite. Cytotoxicity assessment with murine and human fibroblastic cells demonstrated differing behaviour, emphasizing the need for careful material property modifications for practical scaffold utilization.

Reactions on the surface of stoichiometric hydroxyapatite of different crystallinities in a Zn<sup>2+</sup> aqueous solution

Reactions on the surface of stoichiometric hydroxyapatite of different crystallinities in a Zn<sup>2+</sup> aqueous solution

In vitro bioactivity and biocompatibility behaviour of atmospheric plasma sprayed Indian clam seashell derived hydroxyapatite coating on Ti-alloy

Initially, various hydroxyapatite (HA) powders were derived from the Indian clam seashell by the hydrothermal reaction method with varying heating temperatures (700–1000 ℃). Further, the synthesized and agglomerated pure HA powders were sprayed on Ti-6Al-4 V (ELI) alloy to deposit different HA coatings through the atmospheric plasma spraying (APS) technique. In the current work, the in vitro bioactivity and biocompatibility properties of the fabricated HA coatings are examined to find the usefulness of such coated samples for biomedical applications. In the first test, the coatings are exposed to the simulated body fluid (SBF) for 1, 3, and 5 days of duration. The XRD (X-ray diffractometry) and FESEM (Field Emission Scanning Electron Microscopy) analyses of the treated HA coatings reveal the formation of apatite structure, which has grown with immersion time. The biocompatibility of the HA coatings is examined through MTT (3-(4,5-dimethylthiazol-2-yl)− 2,5-diphenyltetrazolium bromide) assay analysis by using human osteosarcoma cell line (MG-63 cell). All the HA coatings are found to show improvement in cell density with an increase in incubation time from 1 to 5 days. From these tests, the seashell-derived HA coatings are found to be both bioactive and biocompatible in nature. The variation in biological properties observed among the HA coatings is due to their different microstructural features resulting from the melting and solidification behavior of the HA feedstocks. As the deposition parameters are kept constant, the synthesis condition of HA powders, such as the reaction temperature, has played a key role in varying the phases and stoichiometry of the resulting coating. With the highest amount of metastable and near stoichiometric HA phase content, the HA-1000 coating has shown the maximum amount of cell proliferation and apatite growth among all. However, from the alkaline phosphatase (ALP) activity test, the HA-800 coating is found to show the maximum ALP expression level. This is attributed to the highest thickness of such coating resulting from the high deposition rate of HA-800 powder. Possibly, the high porosity of HA-800 coating has allowed a good amount of HA material to participate in the reaction process.

Cell viability and bioactive coating on additive manufactured polycaprolactone substrate for osteointegration applications

Polycaprolactone (PCL) finds its application in the medical field in the areas of suture materials, scaffolds for tissue engineering and medical devices fabrication. To improve the anchoring or osteointegration of PCL, bioactive material will be reinforced or deposited as a layer over the PCL substrate. This experimental work aims to deposit Hydroxyapatite (HA),a bioactive material over PCL substrate material using Electron Beam Deposition (EBD). The results of the characterization studies reveal that the HA was successfully deposited over PCL substrate with suitable crystalline and surface roughness. It was also evident that Ca/P ratio of deposited HA was reasonably closer to that of standard stoichiometric HA. The HA layer deposited by Pulsed Laser Deposition (PLD) process could be an interesting option for enhancing the biocompatibility of additive manufactured polyamide. The results revealed that the water contact of HA coated PCL, the substrate has an angle of 113° with a hydrophobic surface and surface energy of 14.32 mJ/m2. However, the cell growth on coated sample was increased by 22 % and 45 % for 7th and 15th day respectively. This increment in coated sample was due to release of Ca2+ ions Fluorescent microscopic image of cells seeded on coated sample clearly revealed that the viable cells (green in colour) growth was improved in coated samples.

Carbonated Hydroxyapatite Extracted from Indonesian’s Eggshell Biogenic Wastes as Bioceramic Materials

Carbonated hydroxyapatite (CHA), with a chemical composition close to the mineral found in human bone, represented higher solubility than stoichiometric hydroxyapatite (HA). Therefore, the B-type CHA is commonly used for bone tissue engineering. This study fabricated B-type CHA using Indonesian eggshells from chicken, organic chicken, and duck because of the high calcium carbonate (CaCO3) content (94%). A co-precipitation method was used for synthesizing CHA. The physicochemical properties of the CHA were characterized using Scanning Electron Microscopy-Energy Dispersive X-Ray Spectroscopy (SEM-EDS), X-Ray Diffractometer (XRD), and Fourier Transform Infrared Spectroscopy (FTIR). Based on FTIR results for CHA, the stretching functional groups of B-type CO3 were detected at 1452-1453 cm-1, 1417-1418 cm-1, and 873-874 cm-1, which indicated the formation of B-type CHA. Meanwhile, CHA from organic chicken eggshells had low crystalline properties and the best morphology due to a more homogeneous and uniform agglomeration. More specifically, CHA based on organic chicken eggshells has a Ca/P molar ratio following natural human bone, which is 1.71. Therefore, all B-type CHA samples are candidates in bioceramic materials for bone tissue engineering applications.

Evaluation of the acute toxicity by Artemia salina of hydroxyapatite nanoparticles obtained via sol-gel in an aqueous medium without using additives

Hydroxyapatite (Hap) is one of the most important calcium phosphate bioceramics applied to bone tissue regeneration. Synthesizing Hap nanoparticles from easily accessible and low-cost alternative sources of calcium precursors remains a challenge, as well as defining an ideal and reproducible synthesis route without using additives to control the pH of the reaction and entirely performed at room temperature. This study proposes a route for the synthesis of hydroxyapatite by the sol-gel method without the addition of additives for pH control, carried out at room temperature. The Hap samples were characterized by FTIR, XRD, SEM, and BET. The synthesized Hap presented a spherical morphology without the formation of unwanted phases or residues. Samples calcined at 600 and 750 °C resulted in stoichiometric hydroxyapatite with 100% purity and average particle sizes of 24 and 53 nm, respectively. On the other hand, the samples calcined at 900 and 1050 °C presented a specific content of β-calcium phosphate and average particle sizes of 118 and 732 nm, respectively. Acute toxicity was evaluated by Artemia salina nauplii instar I and II for 24 and 48 h of exposure. The tests were conducted on 10, 100, and 1000 ppm of Hap dissolutions. The highest death rate and more significant morphological alterations were observed in A. salina nauplii instar II, exposed to Hap at a concentration of 1000 ppm for 48 h.

Incorporation/Enrichment of 3D Bioprinted Constructs by Biomimetic Nanoparticles: Tuning Printability and Cell Behavior in Bone Models.

Reproducing in vitro a model of the bone microenvironment is a current need. Preclinical in vitro screening, drug discovery, as well as pathophysiology studies may benefit from in vitro three-dimensional (3D) bone models, which permit high-throughput screening, low costs, and high reproducibility, overcoming the limitations of the conventional two-dimensional cell cultures. In order to obtain these models, 3D bioprinting offers new perspectives by allowing a combination of advanced techniques and inks. In this context, we propose the use of hydroxyapatite nanoparticles, assimilated to the mineral component of bone, as a route to tune the printability and the characteristics of the scaffold and to guide cell behavior. To this aim, both stoichiometric and Sr-substituted hydroxyapatite nanocrystals are used, so as to obtain different particle shapes and solubility. Our findings show that the nanoparticles have the desired shape and composition and that they can be embedded in the inks without loss of cell viability. Both Sr-containing and stoichiometric hydroxyapatite crystals permit enhancing the printing fidelity of the scaffolds in a particle-dependent fashion and control the swelling behavior and ion release of the scaffolds. Once Saos-2 cells are encapsulated in the scaffolds, high cell viability is detected until late time points, with a good cellular distribution throughout the material. We also show that even minor modifications in the hydroxyapatite particle characteristics result in a significantly different behavior of the scaffolds. This indicates that the use of calcium phosphate nanocrystals and structural ion-substitution is a promising approach to tune the behavior of 3D bioprinted constructs.

Synthesis of Tetraethoxysilane-Reacted Hydroxyapatite Nanoparticles and Their Stabilization in Phosphate-Buffered Saline.

Hydroxyapatite (HA) particle, which is an inorganic component of biological hard tissues, is being applied as a bioceramic for biotechnology and medicine fields. However, early bone formation is difficult in the implantation of well-known stoichiometric HA into our body. To solve this problem, it is important to control the shapes and chemical compositions of the physicochemical properties of HA to be functionalized as the state similar to the biogenic bone. In this study, the physicochemical properties of the HA particles synthesized in the presence of tetraethoxysilane (TEOS) (SiHA particles) were evaluated and investigated. In particular, the surface layers of the SiHA particles were successfully controlled by adding silicate and carbonate ions in the synthetic, which would be involved in the bone formation process, and their elusive reaction behavior with phosphate-buffered saline (PBS) was also evaluated. The results showed that the ions in the SiHA particles increased with the increase in the added TEOS concentration, and the silica oligomer was also formed on the surfaces. The ions were present not only in the HA structures but also on the surface layers, indicating the formation of the non-apatitic layer containing the hydrated phosphate and calcium ions. The change in state of the particles with the immersion in PBS was evaluated, the carbonate ions eluted from the surface layer into PBS, and the free water component in the hydration layer increased with the immersion time in PBS. Therefore, we successfully synthesized the HA particles containing silicate and carbonate ions, suggesting the important state of the surface layer consisting of the characteristic non-apatitic layers. It was found that the ions in the surface layers can react with PBS and leach out, weakening the interaction of hydrated water molecules on the particle surfaces to increase the free water component in the surface layer.

Synthesis and Characterization of Nano-Hydroxyapatite Obtained from Eggshell via the Hydrothermal Process and the Precipitation Method

Hydroxyapatite (HA) is a major component of the inorganic minerals in the hard tissues of humans and has been widely used as a biomedical ceramic material in orthopedic and dentistry applications. Because human bone contains several impurities, including carbonates, chlorides, fluorides, magnesium, and strontium, human bone minerals differ from stoichiometric HA. Additionally, natural bone is composed of nano-sized HA, and the nanoscale particles exhibit a high level of biological activity. In this paper, HA is prepared via the hydrothermal process because its reaction conditions are easy to control and it has been shown to be quite feasible for large-scale production. Therefore, the hydrothermal process is an effective and convenient method for the preparation of HA. Furthermore, eggshell is adopted as a source of calcium, and mulberry leaf extract is selectively added to synthesize HA. The eggshell accounts for 11% of the total weight of a whole egg, and it consists of calcium carbonate, calcium phosphate, magnesium carbonate, and organic matter. Eggshell contains a variety of trace elements, such as magnesium and strontium, making the composition of the synthesized HA similar to that of the human skeleton. These trace elements exert considerable benefits for bone growth. Moreover, the use of eggshell as a raw material can permit the recycling of biowaste and a reduction in process costs. The purpose of this study is to prepare HA powder via the hydrothermal method and to explore the effects of hydrothermal conditions on the structure and properties of the synthesized HA. The room-temperature precipitation method is used for the control group. Furthermore, the results of an immersion test in simulated body fluid confirm that the as-prepared HA exhibits good apatite-forming bioactivity, which is an essential requirement for artificial materials to bond to living bones in the living body and promote bone regeneration. In particular, it is confirmed that the HA synthesized with the addition of the mulberry leaf extract exhibits good in vitro biocompatibility. The morphology, crystallite size, and composition of the carbonated nano-HA obtained herein are similar to those of natural bones. The carbonated nano-HA appears to be an excellent material for bioresorbable bone substitutes or drug delivery. Therefore, the nano-HA powder prepared in this study has great potential in biomedical applications.

Antibacterial and biocompatible Zn and Cu containing CaP magnetron coatings for MgCa alloy functionalization

The application of bioresorbable magnesium-based alloys in medical implants has been limited by their rapid dissolution rate. In order to enhance their suitability for medical use, this study proposes the deposition of calcium phosphate (CaP) onto Mg (0.8) Ca alloy using an RF magnetron sputtering with stoichiometric hydroxyapatite and Zn- or Cu-substituted hydroxyapatites. The comparison study assessed structure, composition, and mechanical properties of Mg (0.8) Ca alloy, Zn, and Cu containing CaP coatings. CaP coatings showed an increased Ca/P ratio due to re-sputtering. Sputtering of Zn- and Cu-substituted hydroxyapatites resulted in CaP dopant concentrations of 0.6 ± 0.2 wt% and 0.4 ± 0.1 wt%, respectively. All deposited coatings were found to be amorphous. Addition of Cu or Zn dopant slightly changed the CaP coatings' scratch resistance, with critical loads of 2.19 ± 0.60 N, 2.55 ± 0.95 N, and 2.76 ± 1.29 N for CaP, Zn–CaP, and Cu–CaP coatings, respectively. CaP coatings reduced corrosion current and increased electrical resistance compared to MgCa samples, but Cu addition accelerated corrosion. Biological tests revealed that the hAMSCs survival was improved and the bacteriostatic effect on the pathogenic SA strain was enhanced for Zn–CaP coatings when compared to CaP and Cu–CaP coatings. These findings suggest that the deposition of CaP coatings onto Mg (0.8) Ca alloy, and particularly the use of Zn-substituted hydroxyapatites, could be a promising approach to enhance the suitability of magnesium-based alloys for medical implant applications.

Effect of solvents and pH on in situ formation of hydroxyapatite for stone conservation

In situ formation of hydroxyapatite (HAP) inside the cracks and on the surface of calcareous substrates (e.g. marble and limestone) has been proposed for consolidation and protection of cultural heritage. HAP can be formed in situ by reacting the substrate with an aqueous solution of diammonium hydrogen phosphate (DAP). Taking inspiration from literature on bioceramics and from recent computational results, in the present study different strategies were explored to improve the consolidating efficacy of the DAP solution, namely by increasing pH or by adding organic solvents. A 1 M DAP +1 mM CaCl2 solution was taken as a reference and compared to 10-fold more diluted solutions (0.1 M DAP + 0.1 mM CaCl2) at pH 8, 10 or 12 and with addition of 20 wt% ethanol, isopropanol, or acetone. The solutions were applied onto weathered marble specimens and the formation of the new phases was characterized by FEG-SEM and XRD, while the strengthening ability was assessed in terms of bulk mechanical properties and surface properties. The results of this study indicate that apatite phases (mostly likely carbonate HAP with possible stoichiometric HAP) were formed as the result of all treatments, except at pH 12 when undesired ammonium calcium phosphate hydrate was formed. In terms of strengthening efficacy, when in-depth consolidation is needed, the more concentrated DAP solutions are able to provide higher strengthening ability. When consolidation of the most superficial layer is desired, less concentrated solutions are able to provide satisfactory results, allowing for a substantial reduction in the use of DAP with economic and environmental advantages.

Nanocrystalline Apatites: Post-Immersion Acidification and How to Avoid It—Application to Antibacterial Bone Substitutes

Biomimetic nanocrystalline apatites analogous to bone mineral can be prepared using soft chemistry. Due to their high similarity to bone apatite, as opposed to stoichiometric hydroxyapatite for example, they now represent an appealing class of compounds to produce bioactive ceramics for which drug delivery and ion exchange abilities have been described extensively. However, immersion in aqueous media of dried non-carbonated biomimetic apatite crystals may generate an acidification event, which is often disregarded and not been clarified to-date. Yet, this acidification process could limit their further development if it is not understood and overcome if necessary. This may, for example, alter biological test outcomes, during their evaluation as bone repair materials, due to potentially deleterious effects of the acidic environment on cells, especially in in vitro static conditions. In this study, we explore the origins of this acidification phenomenon based on complementary experimental data and we point out the central role of the hydrated ionic layer present on apatite nanocrystals. We then propose a practical strategy to circumvent this acidification effect using an adequate post-precipitation equilibration step that was optimized. Using this enutralization protocol, we then showed the possibility of performing (micro)biological assessments on such compounds and provide an illustration with the examples of post-equilibrated Cu2+- and Ag+-doped nanocrystalline apatites. We demonstrate their non-cytotoxicity to osteoblast cells and their antibacterial features as tested versus five major pathogens involved in bone infections, therefore pointing to their relevance in the field of antibacterial bone substitutes. The preliminary in vivo implantation of a relevant sample in a rat's calvarial defect confirmed its biocompatibility and the absence of adverse reaction. Understanding and eliminating this technical barrier should help promoting biomimetic apatites as a genuine new class of biomaterial-producing compounds for bone regeneration applications, e.g., with antibacterial features, far from being solely considered as "laboratory curiosities".

Synthesis of stoichiometric hydroxyapatite nanoparticles via aqueous solution-precipitation at 37 °C

Synthesis of stoichiometric hydroxyapatite nanoparticles via aqueous solution-precipitation at 37 °C

In vitro determination of the remineralizing potential and cytotoxicity of non-fluoride dental varnish containing bioactive glass, eggshell, and eggshell membrane.

This study compares in vitro remineralization potential and cytotoxicity of fluoride-free varnish combinations containing bioactive glass, eggshell, and membrane powder and fluoride varnish formulations on artificial caries lesions. Artificial caries lesions were formed in two windows on third molars. One of the windows was coated with one of the following varnish formulations: FV (fluoride varnish), F-BAGV (fluoride and bioactive glass containing varnish), BAGV (bioactive glass containing varnish), EPV (eggshell powder containing varnish), EMP-EPV (eggshell membrane protein and eggshell powder containing varnish), STMP-EMP-EPV (sodium trimetaphosphate-treated eggshell membrane protein and eggshell powder containing varnish). The samples were remineralized, then investigated under scanning electron microscopy, and elemental analyses were performed by X-ray dispersive analysis (EDX). In addition, the traditional colorimetric tetrazolium-based reduction assay (XTT) and the modern impedance-based real-time cell analysis system (RTCA) were used to investigate their cytotoxicity. The varnish applied area's Ca/P ratio was lower than stoichiometric hydroxyapatite except for EPV (1.66) and STMP-EMP-EPV (1.67) groups. Undiluted extracts of all varnishes, 1:2 dilutions of FV and F-BAGV groups were cytotoxic in XTT assay. In RTCA, the normalised cell index of the EMP-EPV and STMP-EMP-EPV groups was higher than the control group. Bioactive glass, eggshell, eggshell membrane proteins and STMP-treated eggshell and eggshell membrane protein containing varnish have similar remineralizing effect to fluoride-containing varnish on demineralized enamel. Integrating biological or bioactive components instead of fluoride into the dental varnishes might reduce cytotoxicity.

Reaction of stoichiometric hydroxyapatite with different crystallinity in Mg<sup>2+</sup> aqueous solution

The reaction of stoichiometric hydroxyapatite (HAp) with different crystallinity in Mg2+ aqueous solution was investigated using X-ray absorption near edge structure (XANES). Crystallinity is one of important factors for the properties of HAp. Therefore, this study aimed to clarify the effect of crystallinity on the behavior of HAp in aqueous solutions. Stoichiometric HAp samples with different crystallinities were prepared using an aqueous solution precipitation method and subsequent calcination. The Mg-K XANES spectra of the HAp pellets after immersing in Mg(NO3)2 aqueous solution were classified into two groups: those of Mg-doped HAp and Mg(OH)2. This difference is caused by the solubility of the HAp pellets in water, owing to their crystallinity. This result provides insights into the reaction mechanism of HAp in the human body and facilitates selection of suitable biomaterials.