Properties of calcium phosphates ceramic composites derived from natural materials

In this study, ceramics containing mixed phases of hydroxyapatite/beta-tricalcium phosphate (HA/β-TCP) were fabricated by a solid-state reaction technique. The HA powder was synthesized from cockle shells while the β-TCP powder was synthesized from egg shells. Pure HA and β-TCP fine powders were successfully obtained. The HA and β-TCP were mixed and subjected to a thermal treatment up to 1100 °C. To form the mixed phase ceramics, the resulting powders were sintered at 1350 °C. Effects of HA concentration on the properties of the studied ceramic were investigated. X-ray diffraction analysis revealed that all samples presented multiphase of calcium phosphate compounds. Average grain size of the ceramics decreased with the HA additive content. The 75 wt% HA ceramic showed the maximum hardness value (5.5 GPa) which is high when compared with many calcium phosphate ceramics. In vitro bioactivity test indicated that apatite forming increased with the HA additive content. To increase antibacterial activity, selected ceramics were coated with AgNO₃. Antibacterial test suggested that an Ag compound coating on the ceramics could improve the antibacterial ability of the studied ceramics. In addition, the antibacterial ability for the Ag coated ceramics depended on the porosity of the ceramics.     

Effects of strontium substitution on the phase transformation and crystal structure of calcium phosphate derived by chemical precipitation

This study focused on the effects of strontium substitution on the phase transformation and crystal structure of calcium phosphate. Chemical precipitation was used to prepare Sr-doped hydroxyapatite (HA) precursor powders. The phase transformation of the as-prepared samples during sintering was analyzed. The powders were characterized by X-ray diffraction, X-ray fluorescence spectroscopy, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Quantitative analysis of the phase content and fine structure was performed by Rietveld refinement. Sr doping was found to facilitate the phase transformation from HA to beta-tricalcium phosphate (β-TCP) at 1000 °C. The β-TCP content increased with increasing Sr content, causing a decline in the ratio of HA to β-TCP. With Sr contents of ≤5 mol%, HA remained the major phase in the biphasic mixtures; in contrast, with Sr contents of ≥15 mol%, the mass fraction of β-TCP exceeded 50%. The incorporation of Sr²⁺ into HA and β-TCP caused the lattice parameters of both phases to increase. Additionally, Sr incorporation slightly enhanced the binding energy of Ca. The study confirmed that Sr doping could be used to modulate the phase fractions of HA and β-TCP. The effective partial substitution of Sr in both HA and β-TCP makes these materials promising for bone repair.     

Preparation and characterization of single phase,biphasic and triphasic calcium phosphate whisker-like fibers by homogenous precipitation using urea

Whisker-like calcium phosphate fibers were prepared by a homogenous precipitation method under refluxing conditions in solutions with a Ca/P molar ratio of 1.67 at 90 °C and pH 3.0 using urea as additive. The precipitates were characterized by XRD, FTIR, SEM, and FE-SEM. The ICP spectroscopy and elemental analyzer were used to determine Ca/P ratio and carbonate content of products, respectively. The results indicate that the morphology and structural characteristics of the precipitates depend on the urea concentration and reaction time. Whisker-like biphasic monetite/hydroxyapatite (HA) fibers with a mean length of 60 µm and a mean width of 1.0 µm were obtained with the use of a low concentration of urea. Calcination of the biphasic calcium phosphate at 800 °C led to the formation of a triphasic mixture of HA/whitlockite (β-TCP)/calcium pyrophosphate (CPP) with a whisker-like morphology. The use of a higher concentration of urea was caused to form a mixed morphology of spherulites/whisker-like fibers consisting of octacalcium phosphate (OCP) and HA phases. When the reaction time was increased to 10 days, both the above biphasic calcium phosphates transformed to a single phase HA which its morphology and growth pattern were similar to those of a whisker, according to FE-SEM images. The HA whiskers produced by a lower amount of urea had a lower carbonate content compared to those obtained using a higher urea concentration.     

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.     

Properties of sintered zinc hydroxyapatite bioceramic prepared using waste chicken eggshells as calcium precursor

In the recent years, research on the development of hydroxyapatite (HA) using calcium from natural resources such as limestone, mammalian bones, marine shells and avian eggshells have been extensively studied. However, many studies focused on the properties of prestine HA without incorporation of dopants for strengthening effect. In this work, HA bioceramic was prepared using waste chicken eggshells calcium with addition of various concentrations of zinc dopant (1, 3 and 5 mol% Zn). In this work, the zinc-doped HA (ZnHA) was synthesized using a wet-chemical precipitation technique followed by oven drying and unixial pressing to formed green compacts. Pressureless sintering was carried out at 1200, 1250 and 1300 °C. The results showed that 5 mol% ZnHA (5ZnHA) exhibited the overall best properties after sintering at 1250 °C. The improvement in the fracture toughness was attributed to the formation of β-TCP phase when zinc ion was incorporated into HA, combined with enhanced densification. It was observed that the HA grains were coarser and more densely when sintered at 1250 °C, indicating that there was strong interaction between pores and grain boundaries. However, fracture toughness slightly declined after sintering at 1300 °C due to rapid and abnormal HA grain growth.     

Calcium phosphate bioceramics synthesized from eggshell powders through a solid state reaction

Everyday millions of tons of eggshells are produced as biowaste around the world. Most of this waste is disposed of in landfills without any pretreatment. Eggshells in landfills produce odors and promote microbial growth as they biodegrade. The present invention provides an environmentally beneficial and cost-effective method of producing calcium phosphate bioceramics (hydroxyapatite or tricalcium phosphate) from eggshell waste. In this investigation, heat treatment produced solid state reactions between eggshell powders and dicalcium phosphate dihydrate (CaHPO₄·2H₂O, DCPD) or calcium pyrophosphate (Ca₂P₂O₇). When eggshell powders (CaO) and DCPD were heat treated at 1150 °C for 3 h, only a single hydroxyapatite (HA) phase was found; no diffraction peaks of starting materials and no β-TCP were observed. The XRD patterns of the product fabricated from raw eggshell powders (CaCO₃) and Ca₂P₂O₇ heat treated at 1100 °C for 3 h showed that almost only pure β-TCP remained with a trace amount of HA. The calcium phosphate ceramic synthesized from eggshell powders contains several important trace elements such as Na, Mg and Sr.     

Influence of sodium on the properties of sol-gel derived hydroxyapatite powder and porous scaffolds

This study investigates the properties of sol-gel derived sodium (Na)-doped hydroxyapatite (HA) powder. Different amounts of Na (1, 5, 10 and 15 mol%) were prepared and the sintered bodies were characterized to determine the current phases, microstructural evolution and mechanical properties. X-ray diffraction analysis reveals that a phase pure HA of crystallite sizes, which varied from 35 nm to 65 nm, was obtained in the synthesized powder after calcining from 500 °C to 1000 °C. Scanning electron microscopy examination shows evidence of larger particle sizes, particularly in samples that contain higher amounts of Na concentration. The resultant powders were subsequently used to prepare porous NA-doped HA bodies through a polymeric sponge method. The addition of 5% Na resulted in a porous body with 27% porosity and was beneficial in enhancing the compressive strength of HA 17-fold compared with undoped HA. The prepared scaffold also shows suitable pore interconnectivity with pore sizes that vary between 100 and 300 µm which is suitable for use as porous bone substitutes.     

The Effects of Calcium‐to‐Phosphorus Ratio on the Densification and Mechanical Properties of Hydroxyapatite Ceramic

In this work, hydroxyapatite (HA) powders were synthesized using calcium hydroxide Ca(OH)₂ and orthophosphoric acid H₃PO₄ via wet chemical precipitation method in aqueous medium. Calcium‐to‐phosphorus (Ca/P) ratio was set to 1.57, 1.67, 1.87 that yield calcium‐deficient HA, stoichiometric HA, and calcium‐rich HA, respectively. These synthesized HA powders (having different Ca/P ratio) were characterized in terms of particle size and microstructural examination. Then, the densification and mechanical properties of the calcium‐deficient HA, stoichiometric HA, and calcium‐rich HA were evaluated from 1000 to 1350°C. Experimental results have shown that no decomposition of hydroxyapatite phase was observed for stoichiometric HA (Ca/P = 1.67) and calcium‐deficient HA (Ca/P = 1.57) despite sintered at high temperature of 1300°C. However, calcium oxide (CaO) was detected for calcium‐rich HA (Ca/P = 1.87) when samples sintered at the same temperature. The study revealed that the highest mechanical properties were found in stoichiometric HA samples sintered at 1100–1150°C, having relative density of ~99.8%, Young's modulus of ~120 GPa, Vickers hardness of ~7.23 GPa, and fracture toughness of ~1.22 MPam^1/2.     

Reticulated ceramic foams from alumina-chromia solid solutions: A feasibility study

Open cellular ceramic foams were manufactured from plain and chromia-doped alumina, with a chromium concentration ranging between 1.25 mol% and 5.0 mol%. The (AlCr)₂O₃ starting powders were prepared by precipitation of a chromia precursor onto the surface of an alumina powder and subsequent calcination. Characterization of the starting powders as well as the foam samples made therefrom were carried out with respect to the chromium concentration in the alumina phase and the influence of the dopant on the cellular structure and sintering behavior of the doped material. While no positive effect on the compressive strength of the ceramic foams was found, the dopant influences the sintering behavior resulting in an increased shrinkage and in a reduction of total porosity.     

Non-isothermal decomposition kinetics of nano-scale CaCO3 as a function of particle size variation

We report the synthesis of nanocrystalline calcium carbonate with varying particle sizes by precipitation techniques from an aqueous solution of calcium nitrate and sodium carbonate at controlled pH. The particle size of the carbonate powder was precisely controlled by changing the precursor concentration. The synthesized carbonate powders were characterized by using scanning electron microscopy, X-ray diffraction technique, and transmission electron microscopy. The particle size, along with the crystallite size of as-synthesized carbonate powder, decreases with increasing precursor concentration. The non-isothermal decomposition kinetics of the carbonate powder was also evaluated by using near to the modified Arrhenius equation's exact solution. The experimental results were best fitted at n = 0.5, and the one-dimensional diffusion-controlled transport process mechanism (D₁) and one-dimensional phase boundary movement mechanism (R₁) was found to be very close fit of the corresponding evaluated g(α) value. The apparent activation energy of the nano calcium carbonate decomposition was found in the range of 120–175 kJ/mol, which is also inherently functioning with the average particle size. The apparent activation energy of decomposition of CaCO₃ found to be decreased with decreasing average particle size of nanocrystalline calcium carbonate.     

Effects of powder stoichiometry on the sintering of β-tricalcium phosphate

The impact of weak stoichiometry variations on β-TCP sintering behaviour was studied. β-Tricalcium phosphate (β-TCP) powders were synthesised by chemical precipitation through aqueous solution of diammonium phosphate and calcium nitrate. Excess or deficiency of nitrate salt leads to compositions with Ca/P ratios below or over 1.5. These powders, calcined at various temperatures (800–950 °C), were shaped by slip casting process and sintered at 1100 °C. The microstructure, phase composition, specific surface area and density of powders and sintered compacts were analysed by SEM, XRD, FTIR, BET, Archimedes methods and dilatometry.This study shows that the presence of calcium pyrophosphate or the hydroxyapatite phases affects considerably the physical characteristics of the β-TCP powders and in particular specific surface area and consequently their sinterability.A precise determination of the β-TCP chemical composition after synthesis allows to adapt the calcination temperature of the raw powder in order to obtain a maximum densification of the compact. The beneficial role of small quantity of HA phase inside β-TCP powder on their sinterability was also shown in this work.     

Characterization of a high strength hydroxyapatite cement with dual chelate-setting using phytic acid and citric acid

With the development of biomaterials, a hydroxyapatite (HA) bone cement based on chelation has gradually attracted attention. This paper presents an investigation on the micromorphology and mechanical property of HA bone cement prepared by HA powders modified by inositol hexaphosphate (IP6, phytic acid). With the citric acid monohydrate (CA) solution used as setting liquid, scanning electron microscopy (SEM) and universal testing machine were employed to investigate the influence of parameters including concentrations of CA as well as powder–liquid ratio on the properties of HA bone cement. In addition, the setting mechanism of chelating cement was analyzed. The results showed that when CA concentration was more than 20 wt.%, the curing products of IP6/CA dual chelating HA cement contained calcium citrate tetrahydrate and tricalcium phosphate (TCP) besides HA. The compressive strength of dual chelated cement increased with the CA concentration. With 10 000 ppm-IP6-HA used as the starting powder, the maximum compressive strength of bone cement prepared with 40 wt.% CA as the setting liquid was up to 57 MPa. Furthermore, the temperature, pH, antibacterial activity measurement, and cell studies in vitro were carried on, suggesting that chelate-setting HA cement has potential development in orthopedic materials.     

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).     

Mechanochemical Synthesis of Hydroxyapatite from Calcium Oxide and Brushite

Fine hydroxyapatite (HA) powders were prepared by mechanically activating a mixture of calcium oxide and brushite powders in a high-energy shaker mill. A defective HA phase was formed when the starting powder mixture was activated for ≤20 h. When the mixture was calcined at 800°C, it was converted to β-tricalcium phosphate. In contrast, a nanocrystalline HA phase was formed when the mechanical activation was extended to 30 h. The material was transformed to a HA compound (Ca₁₀(PO₄)₆(OH)₂) of high crystallinity when it was calcined at 800°C.     

Effect of Mn doping on hydrolysis of low-temperature synthesized metastable alpha-tricalcium phosphate

In the present work, effect of Mn doping on hydrolysis rate of low-temperature synthesized metastable α-tricalcium phosphate (α-TCP) was investigated. α-TCP powders containing different amount of Mn²⁺ ions (0, 0.5 and 1 mol%) were synthesized by wet co-precipitation process, followed by annealing and crystallization of as-precipitated amorphous calcium phosphate at 700 °C. It was demonstrated that the presence of Mn²⁺ ions significantly retards hydrolysis rate of α-TCP. While pristine α-TCP fully hydrolyzed with a conversion to calcium-deficient hydroxyapatite in 10 h, complete hydrolysis of α-TCP doped with 0.5 and 1 mol% of Mn occurred only after 20 and 35 h, respectively. Initial and final products were characterized by X-ray diffraction (XRD) analysis, infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). Chemical composition of starting and fully hydrolyzed α-TCP powders was determined by inductively coupled plasma optical emission spectrometry (ICP-OES).     

Influences of gallium substitution on the phase stability,mechanical strength and cellular response of β-tricalcium phosphate bioceramics

β-tricalcium phosphate (β-TCP), a well-accepted synthetic bone grafting biomaterial, is confronted with limitations of poor phase stability and lacking the capacity to mediate the biological functions. In the current study, gallium (Ga) was substituted for calcium in the β-TCP, and the influences of Ga substitution on the phase stability, compressive strength and cellular response of β-TCP bioceramics were investigated. The results indicated that substitution of at least 2.5 mol% Ga for calcium prevented the β-TCP from transforming into α-TCP at 1250 °C. The β-TCP bioceramics substituted with 2.5 mol% Ga attained the highest compressive strength. The β-TCP bioceramics substituted with 2.5 and 5 mol% Ga showed good cytocompatibility, and suppressed in vitro osteoclastic activity as well as osteoblastic differentiation. Considering the favorable mechanical strength and the inhibitory effect on the osteoclastic activity, the β-TCP bioceramics substituted with 2.5 mol% Ga are promising for treating the bone defect in the pathological state of excessively rapid bone resorption.     

In vitro assessment of zinc apatite coatings on titanium surfaces

In this paper, coatings of hydroxyapatite partially substituted with zinc (ZnHA) were produced on titanium substrates by a two-step hydrothermal process using a precursor solution rich in calcium, phosphate and zinc. Activation of titanium surfaces was performed by oxidation with an acidic HF/HNO₃ solution. The coated substrates were then converted into HA by immersion in an alkali 0.1 M NaOH solution. The ZnHA samples were characterized by several techniques and their in vitro behavior was studied in comparison to hydroxyapatite (HA) and titanium (Ti-control) samples. A uniform and homogeneous calcium-deficient carbonate apatite coating was obtained for all samples, both doped and undoped with zinc. The percentage of zinc incorporated in the coatings is 7 at%, and the Ca/P ratio is 1.61(±0.01) for both types of samples, suggesting that Zn is incorporated substitutionally, replacing Ca atoms into the HA structure. The incorporation of Zn in the HA structure changed the crystals morphology, reduced crystals sizes and decreased the deposition rate showing that zinc is an inhibitor of the growth of HA crystal. X-ray diffraction showed that HA is the single crystalline phase present after alkali treatment. The coating adhesion strength was evaluated in terms of the critical load (Lc) obtained from scratch tests and no significant difference was found between the two tested groups, indicating the good adhesion of ZnHA to Ti substrates. The in vitro response of human osteoblasts (HOB) exposed to the surfaces of HA and ZnHA coatings was evaluated. The results of Live/Dead tests showed cell viability for all samples surfaces, but the adhesion and proliferation tests showed that ZnHA samples presented better adhered and spread cells compared with HA. ZnHA coatings presented cells with elongated or polygonal shapes and clearly more spread than HA. Quantitative analysis showed that there was a significantly higher number of cells adhered to ZnHA coatings compared to HA, indicating the zinc incorporation stimulates osteoblast proliferation.     

Preparation and characterisation of HA/TCP biphasic porous ceramic scaffolds with pore-oriented structure

Porous hydroxyapatite/tricalcium phosphate (HA/TCP) ceramic scaffolds with a uniform unidirectional pore structure were successfully fabricated by an ice-templating method by using Ca-deficient HA whiskers and phosphate bioglass. HA whiskers showed good dispersibility in the slurry and favoured the formation of interconnected pores in the scaffolds. Addition of bioglass powders enhanced the material sintering process and the phase transformation of Ca-deficient HA to β-TCP. Calcium-phosphate-based scaffolds with a composition from HA to an HA/β-TCP complex could be obtained by controlling the freezing moulding system and slurry composition. The fabricated scaffolds had a porosity of 75–85%, compressive strength of 0.5–1.0 MPa, and a pore size range of 130–200 µm.     

Fabrication of a Novel Calcium Carbonate Composite Ceramic as Bone Substitute

The coral, whose main composition is aragonite‐type calcium carbonate, has been widely used as bone substitute in clinic. However, the study on calcium carbonate bioceramic has not been largely reported due to difficulty in sintering calcium carbonate which is liable to evidently decompose at low temperature. In this study, a novel calcium carbonate composite ceramic was fabricated by sintering fast at a low temperature. A degradable, biocompatible phosphate‐based glass (PG) which grew liquid at a low temperature was added as sintering agent in the sintering process. The sintering schedule was explored by thermal analysis. The phase composition, microstructure, compressive strength, and biocompatibility of calcium carbonate composite ceramics were evaluated. The results revealed that the optimum holding time at the sintering temperature was 20 min. The obtained calcium carbonate composite ceramics did not produce calcium oxide but new compounds according to phase analysis. The compressive strength of calcium carbonate composite ceramics correspondingly increased with growing addition of PG ranging from 10 to 50 wt%. The cell proliferation on the calcium carbonate composite ceramic was not compromised but augmented compared to the neat calcium carbonate ceramic without adding PG as sintering agent. The novel calcium carbonate composite ceramic is a promising bone substitute for bone defects.     

Eggshell derived mesoporous biphasic calcium phosphate for biomedical applications using rapid thermal processing

Biphasic calcium phosphate (BCP) has received much interest for making various bone substitutes since its physicochemical properties can be easily tailored by tuning its phase composition. Due to high temperature processing, it is hard to prepare BCP with nanoscale characteristics. In the present study, we have made an attempt to optimize the heat treatment parameters for the synthesis of BCP with nanoscale characteristics from eggshell derived hydroxyapatite (HA) through rapid thermal processing (RTP). To accomplish this, eggshell derived HA was prepared by wet precipitation method and subjected to RTP at 750°C and 1150°C for 3 and 10 minutes. For comparison we have also studied conventional calcination at 750°C and 1150°C for 3 hours. XRD, FTIR, SEM, EDX, HRTEM, and BET analyses were used to understand the effect of RTP and conventional calcination on eggshell derived HA. Our results indicate that eggshell derived HA on RTP at 1150°C for 3 minutes and 10 minutes can offer nanoscale BCP with good dissolution, bioactivity, cytocompatibility, and mesoporous nature. Hence, RTP can be a potential method to prepare BCP with nanoscale features for biomedical applications.