Chemical preparation of carbonated calcium hydroxyapatite powders at 37°C in urea-containing synthetic body fluids

An important inorganic phase of synthetic bone applications, calcium hydroxyapatite (HA, Ca₁₀(PO₄)₆(OH)₂), was prepared as a single-phase ceramic powder. Carbonated HA powders were formed from calcium nitrate tetrahydrate and di-ammonium hydrogen phosphate salts dissolved in aqueous ‘synthetic body fluid’ (SBF) solutions, containing urea (H₂NCONH₂), at 37 °C and pH of 7·4, by using a novel chemical precipitation technique. These powders were also found to contain trace amounts of Na and Mg impurities in them, originated from the use of SBF solutions, instead of pure water, during their synthesis. The characterization and chemical analysis of the synthesized powders were performed by powder X-ray diffraction (XRD), Fourier-transformed infra-red spectroscopy (FT–IR), and inductively-coupled plasma atomic emission spectroscopy (ICP–AES).     

Bone substitute of poly(amino acid)/β‐Ca2SiO4 biocomposite: Degradability and bioactivity

A poly(amino acid)/β‐Ca₂SiO₄(PAA/β‐Ca₂SiO₄) bioactive composite was prepared by in situ melting polymerization. The composition, structure, and morphology were characterized by infrared spectrometry, X‐ray diffraction, X‐ray photoelectron spectroscopy, scanning electron microscopy, and differential scanning calorimeter. The results indicated that the β‐Ca₂SiO₄ particles were uniformly distributed in the PAA matrix and some interaction was found at the interface between PAA and β‐Ca₂SiO₄. The crystallinity of PAA in the composite was found decreasing with the increase of β‐Ca₂SiO₄ content. The bioactivity of the composite was evaluated by soaking the composite in simulated body fluid (SBF) and results showed that the PAA/β‐Ca₂SiO₄ composite (PSC) could induce a dense and continuous layer of apatite after soaking for 1 week. In addition, the PSC was soaked SBF for 2 months, and the weight loss reached 8.77%, showing the composite could be degradable. Collectively, these results suggested that the incorporation of β‐Ca₂SiO₄ produced a biocomposite with enhanced bioactivity and might have potential applications as a bone tissue substitute. POLYM. COMPOS., 37:1335–1341, 2016. © 2014 Society of Plastics Engineers     

Hydrothermal Synthesis of Si‐doped Hydroxyapatite Nanopowders: Mechanical and Bioactivity Evaluation

Si‐doped hydroxyapatite nanoparticles (n‐Si_xHA) were prepared by hydrothermal synthesis from calcium nitrate tetrahydrate and diammonium hydrogen orthophosphate. A rod‐like morphology was obtained for all the powders irrespective of the incorporated Si‐doping level. But the crystallinity of the n‐Si_xHA powders, the density achieved upon sintering powder compacts and their mechanical properties (three‐point‐bending strength), as well as their biomineralization activity evaluated by immersing them into simulated body fluid (SBF) were found to be dependent on the Si‐doping amount.     

Review on calcium silicate-based bioceramics in bone tissue engineering

Calcium (Ca) and silica (Si) ions have attracted intense interest in biomedical applications. The two ions are directly involved in many biological processes; for instance, Ca plays a key role in regulating cellular responses to bioceramics, promoting cell growth, and differentiation into osteoblasts. Si plays a significant role in bone calcification and is helpful for bone density improvement and inhibiting osteoporosis. Calcium silicate ceramics including a large group of trace metal containing calcium silicate-based compounds are involved in biomedical applications such as repairing hard tissue texture, bone scaffolds, bone cements, or implant coatings. The aim of the study is to provide a comprehensive overview of developments in research on calcium silicate-based ceramics, such as wollastonite (CaSiO₃), diopside (CaMgSi₂O₆), akermanite (Ca₂MgSi₂O₇), bredigite (Ca₇Mg(SiO₄)₄), merwinite (Ca₃MgSi₂O₈), monticellite (CaMgSiO₄), hardystonite (Ca₂Zn(Si₂O₇), and baghdadite (Ca₃ZrSi₂O₉), including degradation, apatite mineralization, and mechanical properties. Finally, the biological in vitro and in vivo presentation for bone tissue repair are summarized, which show promise with regard to application of calcium silicate-based ceramics as bone repair and replacement materials.     

The effect of chemical composition and morphology on the drug delivery properties of hydroxyapatite-based biomaterials

In the case of orthopedic and dental interventions, local antibiotic therapy reduces significantly the risk associated with the intervention. The aim of this study was the preparation and characterization of pure hydroxyapatite (HA), Si- and Mg-doped HA, which ensures the sustained release of doxycycline, and the investigation of the parameters, which were crucial for the drug release. The carriers were synthesized using the precipitation method. In order to achieve different morphologies, traditional drying and spray drying methods were used: Si-doped HA was prepared using two different sources of Si, Na₂SiO₃ and Ludox AS-40, while (Mg(NO₃)₂)*6H₂O was used for substitution with Mg. The carriers were characterized by XRD, SEM, EDX and TG/DTA methods, and the ion incorporation was also confirmed by lattice parameters calculations. Doxy was bound on the carriers by physical adsorption, the adsorption capacity increased proportionally by increasing the concentration of the initial Doxy solutions (10, 15, 20, 25 g/L). The investigated systems showed different releases with the change of the dissolution medium (in the case of HA microspheres, the release in PBS was twice as high as in SBF), chemical composition and morphology of the carriers. The retard effect of the carriers was improved by the spherical morphology, and the reduced release by ion substitution in both SBF and PBS increased as follows: HA < HASi1<HAMg < HASi2. The release mechanism of Doxy was discussed through five different release kinetics models.     

Phosphate content affects structure and bioactivity of sol-gel silicate bioactive glasses

Bioactive glasses can heal bone defects and bond with bone through formation of hydroxyl carbonate apatite (HCA) surface layer. Sol-gel derived bioactive glasses are thought to have potential for improving bone regeneration rates over melt-derived compositions. The 58S sol-gel composition (60 mol% SiO₂, 36 mol% CaO, and 4 mol% P₂O₅) has appeared in commercial products. Here, hydroxyapatite (HA) was found to form within the 58S glass during sol-gel synthesis after thermal stabilization. The preformed HA may lead to rapid release of calcium orthophosphate, or nanocrystals of HA, on exposure to body fluid, rather than the release of separate the calcium and phosphate species. Increasing the P₂O₅ to CaO ratio in the glass composition reduced preformed HA formation, as observed by XRD and solid-state NMR. Instead, above 12 mol% phosphate, a phosphate glass network (polyphosphate) formed, creating co-networks of phosphate and silica. Nanopore diameter of the glass and rate of HCA layer formation in simulated body fluid (SBF) decreased when the phosphate content increased.     

Mechanical properties and bioactivity of β-Ca2SiO4 ceramics synthesized by spark plasma sintering

Bioactive beta-dicalcium silicate ceramics (β-Ca₂SiO₄) were fabricated by spark plasma sintering (SPS). The relative density of as-prepared β-Ca₂SiO₄ ceramics reached 98.1% when sintered at 1150 °C, leading to great improvement in bending strength (293 MPa), almost 10 times higher than that of the specimen prepared by conventional pressureless sintering (PLS). High fracture toughness (3.0 MPa m^1/2) and Vickers hardness (5.8 GPa) of β-Ca₂SiO₄ ceramics were also achieved by SPS at 1150 °C. The simulated body fluid (SBF) results showed that β-Ca₂SiO₄ ceramics had a good in vitro bioactivity to induce hydraxyapatite (HAp) formation on their surface, which suggests that β-Ca₂SiO₄ ceramics are promising candidates for load-bearing bone implant materials.     

Synthesis of silicon-substituted hydroxyapatite by a hydrothermal method with two different phosphorous sources

Silicon-substituted hydroxyapatite (Si-HA) with up to 1.8 wt% Si content was prepared successfully by a hydrothermal method, using Ca(NO₃)₂, (NH₄)₃PO₄ or (NH₄)₂HPO₄ and Si(OCH₂CH₃)₄ (TEOS) as starting materials. Silicon has been incorporated in hydroxyapatite (HA) lattice by partially replacing phosphate (PO₄³⁻) groups with silicate (SiO₄⁴⁻) groups resulting in Si-HA described as Ca₁₀(PO₄)_6−x(SiO₄)_x(OH)_2−x. X-ray diffraction (XRD), Fourier transform IR spectroscopy (FTIR), inductively coupled plasma AES (ICP-AES) and scanning electron microscopy (SEM) techniques reveal that the substitution of phosphate groups by silicate groups causes some OH⁻ loss to maintain the charge balance and changes the lattice parameters of HA. The crystal shape of Si-HA has not altered compared to silicon-free reference hydroxyapatite but Si-incorporation reduces the size of Si-HA crystallites. Based on in vitro tests, soaking the specimens in simulated body fluid (SBF), and MTT assays by human osteoblast-like cells, Si-substituted hydroxyapatite is more bioactive than pure hydroxyapatite.     

In vitro behaviour of Nurse's Ass-phase: A new calcium silicophosphate ceramic

In the present study, a new single phase Si–Ca–P-based ceramic (called Nurse's A_ss) was obtained and its in vitro behaviour was explored for potential bone tissue regeneration. A porous Si–Ca–P single phase ceramic was obtained from high-temperature sintering of previously synthesised γ-dicalcium silicate and β-tricalcium phosphate. Apatite-mineralisation ability and the dissolution rate were systematically studied by immersing the material in simulated body fluid (SBF) for several time points. Massive new dense calcium deficient hydroxyapatite (CDHA) layer formation was observed at the SBF-sample interface. Adjacent to the dense CDHA layer, a porous structure developed parallel to the interface, formed by the pseudomorphic transformation of Si–Ca–P (Nurse's A_ss) into CDHA. The cell attachment test showed that the new material supported adult human bone marrow-derived mesenchymal stem cells (hMSCs) adhesion and spreading, and cells came into close contact with the ceramic surface during an extended 28-day culture. These findings indicate that the new calcium silicophosphate ceramic possesses good bioactivity and biocompatibility, and might be a promising bone graft substitute.     

Production and study of in vitro behaviour of monolithic α-tricalcium phosphate based ceramics in the system Ca3(PO4)2–Ca2SiO4

In this work a new kind of α-tricalcium phosphate (α-Ca₃(PO₄)₂) doped with dicalcium silicate (Ca₂SiO₄) ceramic materials, with compositions lying in the field of the Ca₃(PO₄)₂ solid solution in the system Ca₃(PO₄)₂–Ca₂SiO₄, were obtained. The properties of the sintered ceramics were discussed in detail as well as some in vitro relevant properties for bone repairing. Crystalline α-Ca₃(PO₄)₂ solid solution (α-TCPss) was the only phase in the ceramics containing from 1 wt% to 4 wt% of Ca₂SiO₄. The release of ionic Si in simulated body fluid increased with the content of Ca₂SiO₄ and favoured α-Ca₃(PO₄)₂ surface transformation. In addition, cell attachment test showed that the α-TCPss supported the mesenchymal stem cells adhesion and spreading, and the cells established close contact with the ceramics after 24 h of culture. According to the results, the investigated α-TCPss ceramics possesses good bioactivity, biocompatibility and mechanical properties, and might be a promising bone implant material.     

Microstructure,phase compositions and in vitro evaluation of freeze casting hydroxyapatite-silica scaffolds

Hydroxyapatite-silica (HA-SiO₂) scaffolds with different SiO₂ content (0, 2, 5 and 10 wt% SiO₂) were fabricated by freeze casting. After sintering, the scaffolds maintained the interconnected unidirectional pore channels by removing the frozen ice crystals via sublimation. X-ray diffraction (XRD) analysis indicated that SiO₂ promoted the decomposition of HA to tricalcium phosphate (TCP), comprised of α-TCP and β-TCP, which became more apparent with the increase of SiO₂ content. The microstructure observation of scanning electron microscope (SEM) showed that the scaffolds surface feature had great changes in terms of grain size and grain boundary with the addition of SiO₂. Moreover, the addition of SiO₂ could increase the porosity and pore size of the scaffolds, even allowing it to reach a maximum as the SiO₂ content increased to 5 wt%. Compression tests investigated the variation in the compressive strength of the scaffolds with the increase in the SiO₂ content, which showed first decreasing and then increasing behavior. In vitro evaluation results in simulated body fluid (1.5×SBF) revealed that the introduction of SiO₂ enhanced the growth rate of bone-like layer, especially the scaffold with 5 wt% SiO₂, which exhibited faster growth rate of bone-like layer than the other scaffolds. The XRD and fourier transformed infrared spectroscopy (FT-IR) characterization confirmed that the bone-like layer formed on the scaffold surface was a carbonate-containing hydroxyapatite bone-like layer.     

Nurse′s A‐Phase: Synthesis and Characterization in the Binary System Ca2SiO4–Ca3(PO4)2

In the present study, a new single phase Si–Ca–P‐based ceramic was obtained by conventional sintering of compacted mixtures of calcium hydrogen phosphate anhydrous, calcium carbonate, and silicon oxide. The synthesis conditions were the followings: heated up to 1550°C for a total period of time of 72 h (3 d), with quenching in liquid nitrogen, milling, pressing, and reheating every 24 h. Second, heating at 1300°C/3 h and subsequent annealed at 1200°C/24 h. Mineralogical and microstructural characterization of the obtained Si–Ca–P‐based material was determined by Differential Thermal Analysis, X‐ray diffraction, Scanning Electron Microscopy with attached wavelength dispersive spectroscopy, Micro‐Raman and Fourier Transform Infrared Spectrometer. The results showed a single Si–Ca–P phase material with a Ca₂SiO₄/Ca₃(PO₄)₂ molar ratio equal to 2:1. The parameters of the Weibull distribution of strength, determined by diametrical compression of disks, were: modulus, m = 13, and characteristic strength σ₀ = 0.60 MPa.     

Formation and transition of calcium aluminate and calcium silicate compounds from pre-synthesized mullite in low-calcium system by solid-state reaction

The formation and transition of calcium aluminate and calcium silicate compounds from pre-synthesized mullite in low-calcium system were systematically studied by solid-state reaction at 1350–1500 °C using XRF, XRD, FTIR, SEM and PSD methods. Ca₃Al₂O₆, Ca₁₂Al₁₄O₃₃, CaAl₂O₄, Ca₂SiO₄ and Ca₂Al₂SiO₇ can form via direct reaction of mullite with CaO at the beginning of the reactions, then Ca₁₂Al₁₄O₃₃ and Ca₃Al₂O₆ react with mullite to form CaAl₂O₄ and Ca₂SiO₄, and finally Ca₂Al₂SiO₇ reacts with CaO to generate Ca₂SiO₄ and calcium aluminate compounds as the sintering process proceeds. Elevating the sintering temperature is in favor of the formation of Ca₁₂Al₁₄O₃₃, Ca₃Al₂O₆ and Ca₂Al₂SiO₇ at the initial reaction stage, but Ca₂Al₂SiO₇ cannot totally transform to calcium silicate and calcium aluminate compounds in the low-calcium system, which deteriorates the pulverization performance of the final products. Increasing the calcium dosage accelerates the transformation of Ca₂Al₂SiO₇ to Ca₁₂Al₁₄O₃₃, CaAl₂O₄ and Ca₂SiO₄, which enhances the pulverization performance. If the CaO/Al₂O₃ molar ratio exceeds 1.4, Ca₃Al₂O₆ will generate by the reactions of pre-formed CaAl₂O₄ and Ca₁₂Al₁₄O₃₃ with excessive CaO.     

Characterization and in vitro-bioactivity of natural hydroxyapatite based bio-glass–ceramics synthesized by thermal plasma processing

Natural bovine hydroxyapatite/SiO₂–CaO–MgO glass–ceramics were produced using the transferred arc plasma (TAP) processing method. Homogeneous mixtures of HA/25 wt% SiO₂–CaO–MgO and HA/50 wt% SiO₂–CaO–MgO batches obtained by dry mixing the respective compositions in a ball mill were processed in argon plasma using the TAP torch at 5 kW for 1, 2 and 3 min, respectively. The synthesized glass–ceramic samples were studied for phase composition, microstructure and bioactivity. The phase study of the synthesized glass–ceramics revealed the formation of calcium phosphate silicate with traces of calcium silicate. The structural study by SEM revealed that the prepared samples possessed smooth glassy surface morphology. The in vitro-bioactivity of the TAP synthesized glass–ceramics was examined in simulated body fluid (SBF). The SBF test results confirmed the development of crystalline carbonated apatite phase after 12 days of immersion. The cytocompatibility was evaluated through human fibroblast cell proliferation. The fibroblasts culture results showed that the sample was non-toxic and promoted cell growth.     

Fabrication and in vitro studies on bioactive borate glasses containing dopant chromium sulfate

This research aims to assess the bioactive properties of the modified borate glasses containing extremely low concentrations (≤5 mol.%) from chromium sulfate (Cr₂(SO₄)₃). The glasses in the system xCr₂(SO₄)₃.(60–x)B₂O₃.15CaO.15Na₂O.10P₂O₅, where x = 0, 1, 2, and 5 mol.% were prepared by the melt quenching technique. All glass samples have been treated thermally at 600 °C for 6 h. Fourier Transform Infrared (FTIR) spectroscopy, Scanning Electron Microscopy (SEM), and X-ray Diffraction (XRD) measurements were carried out to differentiate between the structural changes before and after soaking in the simulated body fluid (SBF) at about 37 °C for 1, 2, and 3 weeks. Glass-ceramic samples have showed sharper peaks that are identified using X-ray diffraction data. These crystalline phases are indexed to crystalline calcium borate (Ca₂B₂O₃) and calcium phosphate (Ca₃(PO₄)₂). In vitro tests, FTIR spectra revealed two small bands in the 560-610 cm⁻¹ range which might be assigned to the formation of a hydroxyapatite layer (HA). The formation of HA was also confirmed by XRD results, particularly after immersion in SBF for 21 days. The study suggests that the presently studied glasses containing Cr₂(SO₄)₃ can possess good bioactivity which might be considered to be suitable for some bio and medical applications.     

Synthesis and dissolution behavior of nanosized silicon and magnesium co-doped fluorapatite obtained by high energy ball milling

Nanosized hydroxyapatite (HA) powders exhibit a greater surface area than coarser crystals and are expected to show an improved bioactivity. In addition, properties of HA can be tailored over a wide range by incorporating different ions into HA lattice. The aim of this study was to prepare and characterize silicon and magnesium co-doped fluorapatite (Si–Mg–FA) with a chemical composition of Ca_9.5Mg_0.5 (PO₄)_5.5(SiO₄)_0.5F₂ by the high-energy ball milling method. Characterization techniques such as X-ray diffraction analysis (XRD), Fourier transformed infrared spectroscopy (FTIR), energy dispersive X-ray spectroscopy (EDX) and transmission electron microscopy (TEM) were utilized to investigate the structural properties of the obtained powders. Dissolution behavior was evaluated in simulated body fluid (SBF) and physiological normal saline solution at 37 °C for up to 28 days. The results of XRD and FTIR showed that nanocrystalline single-phase Si–Mg–FA powders were synthesized after 12 h of milling. In addition, incorporation of magnesium and silicon into fluorapatite lattice decreased the crystallite size from 53 nm to 40 nm and increased the lattice strain from 0.220% to 0.296%. Dissolution studies revealed that Si–Mg–FA in comparison to fluorapatite (FA), releases more Ca, P and Mg ions into SBF during immersion. 175 ppm Ca, 33.5 ppm P and 48 ppm Mg were detected in the SBF containing Si–Mg–FA after 7days of immersion, while for FA, it was 75 ppm Ca, 21.5 ppm P and 29 ppm Mg. Release of these ions could improve the bioactivity of the obtained nanopowder. It could be concluded that the prepared nanopowders have structural properties such as crystallite size (~40 nm), crystallinity degree (~40%) and chemical composition similar to biological apatite. Therefore, prepared Si–Mg–FA nanopowders are expected to be appropriate candidates for bone substitution materials and also as a phase in polymer or ceramic-based composites for bone regeneration in tissue engineering applications.     

Porous β-Ca2SiO4 ceramic scaffolds for bone tissue engineering: In vitro and in vivo characterization

The biodegradable ceramic scaffolds with desirable pore size, porosity and mechanical properties play a crucial role in bone tissue engineering and bone transplantation. A novel porous β-dicalcium silicate (β-Ca₂SiO₄) ceramic scaffold was prepared by sintering the green body consisting of CaCO₃ and SiO₂ at 1300 °C, which generated interconnected pore network with proper pore size of about 300 μm and high compressive strength (28.13±5.37–10.36±0.83 MPa) following the porosity from 53.54±5.37% to 71.44±0.83%. Porous β-Ca₂SiO₄ ceramic scaffolds displayed a good biocompatibility, since human osteoblast-like MG-63 cells and goat bone mesenchymal stem cells (BMSCs) proliferated continuously on the scaffolds after 7 d culture. The porous β-Ca₂SiO₄ ceramic scaffolds revealed well apatite-forming ability when incubated in the simulated body fluid (SBF). According to the histological test, the degradation of porous β-Ca₂SiO₄ ceramic scaffolds and the new bone tissue generation in vivo were observed following 9 weeks implantation in nude mice. These results suggested that the porous β-Ca₂SiO₄ ceramic scaffolds could be potentially applied in bone tissue engineering.     

Synthesis of chemically and structurally modified dicalcium silicate

This paper describes the synthesis of cements, chemically and structurally related to Ca₂SiO₄. Silica was obtained from rice hull after heating at 600 °C. Calcium oxide and small amounts of barium chloride were mixed in order to obtain a final (Ca/Si) or (Ca+Ba)/Si ratio equal to 1.95, 1.90, and 1.80, which is lower than in the conventional cement. The solids were mixed and ultrasonically treated for 1 h with a water/solid ratio of about 20. After drying and grinding, the mixtures were heated up to 1100 °C. It was possible, in some cases, to obtain a cementitious material. These cements are structurally related to β-Ca₂SiO₄ and the lower (Ca+Ba)/Si ratio obtained was 1.95. The initial chemical compositions of these cements are: (Ca_1.83+Ba_0.12)SiO₄ and (Ca_1.79+Ba_0.16)SiO₄. A further lowering in the (Ca+Ba)/Si ratio changes the nature of the silicates.     

Corrosion behavior of calcium–magnesium–aluminosilicate (CMAS) on sintered Gd2SiO5 for environmental barrier coatings

The Gd₂SiO₅ performed high-temperature corrosion behavior on calcium–magnesium– aluminosilicate (CMAS) for environmental barrier coatings (EBCs). The synthesized Gd₂O₃-SiO₂ powder was prepared to fabricate a sintered Gd₂SiO₅ by spark plasma sintering (SPS) at 1400°C for 20 min. CMAS was sprinkled on the sintered Gd₂SiO₅ surface and exposed for 2, 12, and 48 h at 1400°C by isothermal heat treatment. The main corrosion factor is Ca, and Ca₂Gd₈(SiO₄)₆O₂ phase is formed by reacting with Gd₂SiO₅. Extended morphology of Ca₂Gd₈(SiO₄)₆O₂ particles observed in the reaction area become thicker as the heat treatment time increases as the CMAS is dissolved. According to the results of high-temperature X-ray diffraction (HT-XRD) and differential scanning calorimetry (DSC), CMAS melted at 1243°C or a higher temperature formed the reaction area. The Ca₂Gd₈(SiO₄)₆O₂ phase was recrystallized and grown due to the reaction of Gd₂SiO₅ and Ca of the CMAS components.     

Synthesis,Characterization, and In Vitro Bioactivity of Sol‐Gel‐Derived Zn,Mg, and Zn‐Mg Co‐Doped Bioactive Glasses

Bioactive glasses in the systems CaO‐SiO₂‐P₂O₅‐ZnO, CaO‐SiO₂‐P₂O₅‐MgO, and CaO‐SiO₂‐P₂O₅‐MgO‐ZnO were prepared and characterized. Bioactive glass powders were produced by the sol‐gel method. The prepared bioactive glass powders were immersed in a simulated body fluid (SBF) for periods of up to 28 days at 310 K to investigate the bioactivity of the produced samples. Inductively coupled plasma (ICP) and ultraviolet (UV) spectroscopic techniques were used to detect changes in the SBF composition. X‐Ray diffraction (XRD) was utilized to recognize and confirm the formation of a hydroxyapatite (HA) layer on the bioactive glass powders. Microstructural characterizations of the bioactive glass samples were investigated by scanning electron microscopy (SEM) techniques. Density, porosity, and surface area values of bioactive glass powders were also determined in order to characterize the textural properties of the samples. The results revealed the growth of an HA layer on the surface of the bioactive glass samples. MgO in the glass sample increases the rate of formation of an HA layer while ZnO in the glass slows it down.