Synthesis and sintering of nanocrystalline hydroxyapatite powders by citric acid sol-gel combustion method

The citric acid sol-gel combustion method has been used for the synthesis of nanocrystalline hydroxyapatite (HAP) powder from calcium nitrate, diammonium hydrogen phosphate and citric acid. The phase composition of HAP powder was characterized by X-ray powder diffraction analysis (XRD). The morphology of HAP powder was observed by transmission electron microscope (TEM). The HAP powder has been sintered into microporous ceramic in air at 1200 °C with 3 h soaking time. The microstructure and phase composition of the resulting HAP ceramic were characterized by scanning electron microscope (SEM) and XRD, respectively. The physical characterization of open porosity and flexural strength have also been carried out.     

Microwave sintering of calcium phosphate ceramics

Microwave sintering of hydroxyapatite (HAP) and tri-calcium phosphate (TCP) ceramics were studied using a commercial 2.45 GHz, 3 KW fully automated microwave system. Four different powder compositions, apart from pure HAP and TCP, were prepared with small amount of different oxide additives e.g., magnesium oxide, zinc oxide and titanium oxide. All samples were densified at 1250 °C for 30 min. Sintered samples had high density and homogeneous microstructure for all compositions. Phase analysis using XRD showed no major variations from starting materials. Presence of additives increased compressive strength between 100 and 200%. Fractured surfaces showed clear signs of intergranular fracture in undoped HAP and TCP while in doped samples, fracture modes were either transgranular or mixed mode. In comparison to conventional sintering, microwave sintering of calcium phosphate ceramics was found to be economically exciting due to substantial reduction in processing time and energy expenditure due to volumetric heating of samples.     

微波法快速制备高稳定性纳米羟基磷灰石

以Ca(NO₃)₂·4H₂O和(NH₄)₂HPO₄为前驱物,采用微波法快速制备出由30nm左右的小颗粒自组装成短捧状的纳米羟基磷灰石粉末,并分析了其自组装的原因。采用XRD,IR,TG-DTA,TEM对其进行了测试,并探讨了不同的微波辐射时间、微波功率、低温煅烧对羟基磷灰石粉体的热稳定性的影响。研究结果证实:随微波辐射反应溶液时间的延长,HAP的热稳定性提高,微波辐射反应溶液的最佳时间是1h;随微波功率的升高,HAP的热稳定性提高;微波辐射反应溶液时间为1h、微波功率为700W、500℃保温HAP3h,HAP在1200℃稳定存在。     

Carbon-centered radicals in γ-irradiated bone substituting biomaterials based on hydroxyapatite

Gamma irradiated synthetic hydroxyapatite, bone substituting materials NanoBone(?) and HA Biocer were examined using EPR spectroscopy and compared with powdered human compact bone. In every case, radiation-induced carbon centered radicals were recorded, but their molecular structures and concentrations differed. In compact bone and synthetic hydroxyapatite the main signal assigned to the CO(2) (-) anion radical was stable, whereas the signal due to the CO(3) (3-) radical dominated in NanoBone(?) and HA Biocer just after irradiation. However, after a few days of storage of these samples, also a CO(2) (-) signal was recorded. The EPR study of irradiated compact bone and the synthetic graft materials suggest that their microscopic structures are different. In FT-IR spectra of NanoBone(?), HA Biocer and synthetic hydroxyapatite the HPO(4) (2-) and CO(3) (2-) in B-site groups are detected, whereas in compact bone signals due to collagen dominate.     

Production and characterization of pure and Cr<Superscript>3+</Superscript>-doped hydroxyapatite for biomedical applications as fluorescent probes

Pure and Cr³⁺-doped hydroxyapatite (HAP) were prepared via chemical precipitation route. The XRD measurements revealed that the typical HAP powder pattern was obtained. SEM analysis indicated that aggregates of nanoparticles were formed. EDX analysis indicates that the [Ca]/[P] concentration ratio was higher than the expected values but can be explained by the presence of carbonate groups as dopants. The optical absorption spectra of the doped samples presented absorption bands typical of Cr³⁺ occupying to different crystalline sites. From the position of the bands, it was possible to estimate the crystal field parameters for both sites of Cr³⁺ in the HAP matrix. The emission spectra of the Cr-doped samples were also investigated and typical transitions of the dopant ion, in trivalent state, were identified. The potential use of the Cr³⁺-HAP as fluorescent probes for medical applications was discussed.     

氨基酸/羟基磷灰石复合材料应用于酸蚀牛牙釉质体外再矿化

早期牙釉质龋病的再矿化在龋齿的预防和修复中起着至关重要的作用。基于牙釉质基质中所含的主要氨基酸, 在10 mmol/L甘氨酸、10 mmol/L L-丝氨酸和5 mmol/L L-天冬氨酸的存在下制备了氨基酸/羟基磷灰石(AA/HAP)复合材料, 表征了其物理、化学和生物学性质, 并评估了其对酸蚀牛牙釉质的再矿化作用。与不含氨基酸的羟基磷灰石(HAP)相比, 在氨基酸的抑制作用下, AA/HAP复合材料具有更低的结晶度和更高的生物相容性。在人工唾液中用AA/HAP对酸蚀牛牙釉质进行体外再矿化。再矿化后, 分别表征了牛牙釉质样品的表面和横截面形态、成分和力学性能。结果表明, AA/HAP可以诱导表面和深层牙釉质病变的修复。复合材料中释放的氨基酸可以吸附在有机基质残基上并诱导平行排列的HAP晶体的形成, 从而使牙釉质表面显微硬度(SMH)得到显著恢复。最后, 讨论了AA/HAP复合材料对酸蚀牛牙釉质的再矿化机制。     

Processing of nanocrystalline hydroxyapatite particles via reverse microemulsions

Nanocrystalline hydroxyapatite (HAP) particles were synthesized at room temperature using reverse microemulsions, in which cyclohexane was used as the organic phase, mixed surfactant with TX-100 and 1- pentanol, and CaCl₂ solution as aqueous phase. The reactor systems with aqueous/organic volumetric ratios 1:10, 1:5, 2:5, and 1:2 were carefully selected for the microemulsion processing by the pseudo-ternary phase diagram and the electric conductivity measurement of the emulsion. The as-obtained HAP nanoparticles with carbonate substitution and broadening X-ray diffraction (XRD) traces were similar to the fine powder of human bone, despite of the aqueous/organic volumetric ratio in the emulsion. No obvious other’s phase occurred after as-obtained particles calcined under different temperature till 700 °C. In the emulsion-derived precursors, the HAP particles based on spherical morphology were prepared into the size between 15 ∼ 30 nm as a low volumetric ratio of 1:10 or 1:5 was applied. As the volumetric ratio increased to 2:5, the HAP particles with rod-like shape of (140∼280) × (10∼80) nm were formed. Practical implication of the results is that the nanocrystalline bone-like hydroxyapatite can be obtained via the emulsion processing at room temperature without further calcinations.     

Formation and Characterization of Bone-like Nanoscale Hydroxyapatite in Glass Bone Cement

Glass based bone cement (GBC) was synthesized by mixing CaO-SiO2-P2O5 based glass powder with ammonium phosphate liquid medium. Bone-like hydroxyapatite (HAP, Ca10(PO4)6(OH)2) was found to form after GBC was immersed in simulated body fluid (SBF). HAP crystal grew with an increasing time along c axle and reached about 200 nm in length after 30 days, however, the end plane granularity remained 30-50 nm. The chemical composition, crystal structure and morphology of HAP formed from GBC were proved to have great resemblance with living HAP. It is believed that GBC was a desirabie biomedicai material with high bioactivity. Furthermore, the high compressive strength guaranteed the possibility of GBC in clinical application.     

Hydroxyapatite precipitation on a carboxylated vinyl chloride-vinyl acetate copolymer

The kinetics of crystal growth of hydroxyapatite [Ca₅(PO₄)₃OH, HAP], the thermodynamically most stable calcium phosphate phase, on a commercially available carboxylated (1%) vinylchloride (83%) vinyl acetate (13%) copolymer was investigated at relatively low ionic strength, at 37 °C and pH 7.4. Stable supersaturated solutions were seeded with HAP seed crystals as reference and with the polymer in powder form. Well defined induction times preceded the growth of HAP on the polymeric substrate. The induction times measured were inversely proportional to the solution supersaturation. The rates of crystallization both on HAP and on the polymer were measured at conditions of constant supersaturation. HAP growth took place on both substrates and the parabolic dependence of the rates of crystal growth on the relative solution supersaturation suggested a surface diffusion controlled mechanism. The rates of HAP crystal growth on the polymeric substrate were found to depend on the amount of the seed material suggesting selective overgrowth of HAP. A possible explanation for this selectivity is the development of active growth sites through the formation of surface complexes of the calcium ions with the dissociated carboxyl groups.     

Influence of powder characteristics on the rheological behaviour of hydroxyapatite slurries

A study of the influence of several dispersants (nature and concentration), of the calcination temperature and of the concentration of hydroxyapatite (HAP), on the rheological behaviour of HAP slurries, made it possible to obtain low viscosity slurries containing 73 wt% HAP and 3.5 wt% dispersant. The calcination of the HAP powder affected both the morphology and the chemical nature of the surface of particles. This last effect was clearly shown by the decrease of the zeta potential with increasing calcination temperature. The optimal calcination temperature was chosen beyond 800 °C. Parts sintered at 1200 °C for 30 min were obtained with a density higher than 96% of theoretical by casting optimized suspensions.     

Stoichiometry of hydroxyapatite: influence on the flexural strength

This paper reports results on the sintering behaviour of hydroxyapatite (HAP) for various compositions and its resultant flexural strength. The HAP decomposition occurs at higher temperatures for sintered compacts than for powder and their OH stoichiometry depends on the porosity-closing temperature. The mechanical behaviour of HAP depends on its composition: the best results are obtained for HAP containing tricalcium phosphate (TCP), whereas for nearly pure HAP the flexural strength decreases to the lowest results corresponding to HAP containing CaO. It is suggested that the strengthening of HAP by TCP involves surface compression due to the TCP transformation.     

Synthesis and thermal stability of selenium-doped hydroxyapatite with different substitutions

Selenium (Se) plays a specific role in human health, especially for its anti-tumor effect. Incorporation of selenium into biocompatible hydroxyapatite (HAP) may endow the materials with novel characteristics. In the current work, a series of selenium-doped hydroxyapatite (Se-HAP) nanoparticles with different Se/P ratios were synthesized by a modified chemical precipitation. It was revealed that the powders with/without heat-treatment were nano-sized needle-like HAP while the heat-treated samples have high crystallinity. The addition of selenium decreases the crystallinity of the synthesized apatite, and also takes a negative effect on the thermal stability of the as-prepared powders. The Se-HAP nanoparticles with Se/P molar ratio not more than 5% sintered at 900°C can achieve good crystallinity and thermal stability.     

Creation of dense hydroxyapatite (synthetic bone) by hydrothermal conversion of seashells

Strombus gigas (conch) shells and Tridacna gigas (Giant clam) shells have dense, tailored structures that impart excellent mechanical properties to these shells. In this study, conch and clam seashells were converted to hydroxyapatite (HAP) by a hydrothermal method at different temperatures and for different conversion durations. Dense HAP structures were created from these shells throughout the majority of the samples. High temperatures were found to accelerate the conversion process, however, cracks were found on the surface of the samples converted at high temperature or for very long conversion times. The conversion at 180 °C, refreshing the diammonium hydrogen phosphate [(NH₄)₂HPO₄] solution every 2 days, produced samples of good quality. Different morphologies of HAP were found in different regions of the converted shells, which may be caused by different structural morphologies and in different amounts of porosity in the original shells. Partially converted shell samples with dense HAP layers on the surface growing inward and original shell structures inside have an average fracture stress about 137–218 MPa, which is close to the mechanical strength of compact human bone.     

Phosphoserine – a convenient compound for modification of calcium phosphate bone cement collagen composites

Temporary bone replacement materials on the basis of calcium phosphates and hydroxyapatite (HAP) are used in surgery for filling bone defects. Components which are able to control the nucleation and crystal growth of HAP through their functional groups and which can additionally activate bone cells may be helpful in the development of materials with enhanced remodelling in vivo. In this study, the influence of O-phospho-L-serine (PS) on the materials properties of calcium phosphate bone cement composites was investigated. For up to an addition of 25 mg/g PS a strong increase in the stability of the cements under load was determined. The material was studied by scanning electron microscopy and transmission electron microscopy. A more dense microstructure and a plate-like morphology of the HAP-crystals were detected in the modified composites compared with the non-modified samples. By X-ray powder diffraction an inhibition of the dissolution of alpha-tricalcium phosphate (alpha-TCP) and dicalciumphosphate anhydrous (DCPA) particles was found. alpha-TCP and DCPA are the main constituents of the cement precursor. The results of cell culture studies using rat calvaria osteoblasts demonstrate a good viability of the cells on the PS-modified material. Furthermore, the proliferation and differentiation were found to be enhanced on the PS-modified material.     

Hydroxyapatite, a biomaterial: Its chemical synthesis, characterization and study of biocompatibility prepared from shell of garden snail, Helix aspersa

The shell of garden snail (Helix aspersa) is basically made of calcium carbonate. An attempt is made to convert calcium carbonate of garden snail shell to hydroxyapatite. The snail shell was found to decompose within 850 $^{\circ}$ C to all the carbonate phases. The calcined snail shells were then treated with acids followed by different chemicals in ammoniacal media maintaining proper stoichiometry to produce fine hydroxyapatite (HAP) as filter cake with a Ca $/$ P molar ratio of 1 ${\cdot }$ 67. The dried HAP powder was extremely pure with a specific surface area of 15 m $^{\bf 2}/$ g. The different characterization techniques were adopted both for calcined snail shell and HAP synthesized by X-ray diffraction (XRD), thermal analysis (DTA $/$ TGA), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The surface area and particle size of HAP powder prepared by chemical precipitation route, were also determined by BET and Malvern particle size analyser, respectively. The synthesized powder was soaked in stimulated body fluid (SBF) medium for various periods of time in order to evaluate its bioactivity. The changes of pH of SBF medium were measured. High bioactivity of prepared HAP powder due to the formation of apatite on its surface was observed.     

Effect of metal-ion-to-fuel ratio on the phase formation of bioceramic phosphates synthesized by self-propagating combustion

Synthetic calcium hydroxyapatite (HAP, Ca₁₀ (PO₄)₆ (OH)₂) is a well-known bioceramic material used in orthopedic and dental applications because of its excellent biocompatibility and bone-bonding ability due to its structural and compositional similarity to human bone. Here we report, for the first time, the synthesis of HAP by combustion employing tartaric acid as a fuel. Calcium nitrate is used as the source of calcium and diammonium hydrogen phosphate serves as the source of phosphate ions. Reaction processing parameters such as the pH, fuel-oxidant ratio and autoignition temperature are controlled and monitored. The products were characterized by powder x-ray diffraction, which revealed the formation of a hexagonal hydroxyapatite phase. Fourier transform infrared spectroscopy (FT-IR) spectra showed that the substitution of a carbonate ion occurs at the phosphate site. The morphology of the particles was imaged by scanning electron microscopy, which also revealed that the particles are of submicron size. Thermal analysis showed that the phase formation takes place at the time of combustion. Surface area and porosity analysis showed that the surface area is high and that the pores are of nanometer size. The mean grain size of the HAP powder, determined by the Debye–Scherrer formula, is in the range 20–30 nm. Chemical analyses to determine the Ca : P atomic ratio in synthesized ceramics were performed, and it was found to be 1 : 1.66.     

Effect of metal-ion-to-fuel ratio on the phase formation of bioceramic phosphates synthesized by self-propagating combustion

Abstract

Synthetic calcium hydroxyapatite (HAP, Ca₁₀ (PO₄)₆ (OH)₂) is a well-known bioceramic material used in orthopedic and dental applications because of its excellent biocompatibility and bone-bonding ability due to its structural and compositional similarity to human bone. Here we report, for the first time, the synthesis of HAP by combustion employing tartaric acid as a fuel. Calcium nitrate is used as the source of calcium and diammonium hydrogen phosphate serves as the source of phosphate ions. Reaction processing parameters such as the pH, fuel-oxidant ratio and autoignition temperature are controlled and monitored. The products were characterized by powder x-ray diffraction, which revealed the formation of a hexagonal hydroxyapatite phase. Fourier transform infrared spectroscopy (FT-IR) spectra showed that the substitution of a carbonate ion occurs at the phosphate site. The morphology of the particles was imaged by scanning electron microscopy, which also revealed that the particles are of submicron size. Thermal analysis showed that the phase formation takes place at the time of combustion. Surface area and porosity analysis showed that the surface area is high and that the pores are of nanometer size. The mean grain size of the HAP powder, determined by the Debye–Scherrer formula, is in the range 20–30 nm. Chemical analyses to determine the Ca : P atomic ratio in synthesized ceramics were performed, and it was found to be 1 : 1.66.     

Development of calcium phosphate based apatite from hen’s eggshell

Stoichiometric hydroxyapatite with Ca/P molar ratio, 1.67, was synthesized using hen’s eggshell as calcium source and phosphoric acid by precipitation method. Conventional EDTA titration and gravimetric methods were adopted to estimate the amount of calcium and phosphorous, respectively. Fourier-transform infrared (FT-IR) and X-ray diffraction (XRD) techniques were employed to investigate the formation of the HAP phase. Thermal analysis (TG-DTA) was carried out to investigate the thermal stability of HAP powder. FT-IR spectra show the characteristic peaks for phosphate and hydroxyl groups. XRD results reveal that the major characteristic peaks of HAP appear in the region of approximately 26°, 28°, 29°, 30–35°, 39°, 46°, 49° and 50° (2θ) and also indicate that there are no occurrences of secondary phases during HAP formation. TG-DTA result depicts that the synthesized HAP was stable up to 1300°C.     

A novel, low temperature method for the preparation of ß-TCP/HAP biphasic nanostructured ceramic scaffold from natural cancellous bone

In this study, a novel method was used to synthesize ß-TCP/HAP biphasic ceramic scaffold from natural cancellous bone. Bovine bone was calcined to remove the organic content. The remaining material was then soaked in P₂O₅ solution for different periods. P ions were doped into cancellous bone and reacted with hydroxyapatite (HAP) to produce ß-tricalcium phosphate (ß-TCP) at room temperature without any subsequent high-temperature heat treatment. By increasing the soaking time, the crystalline phase composition of the calcined bone gradually changed from HAP into a HAP/ß-TCP biphasic structure with different HAP/TCP ratios. The crystallite size of HAP and ß-TCP was smaller than 100 nm.     

Preparation and thermal stability analysis of hydroxyapatite derived from the precipitation process and microwave irradiation method

Thermally stable hydroxyapatite (HAP) polycrystals were prepared using H₃PO₄, glucose and Ca(NO₃)₂·4H₂O, as starting materials with microwave irradiation method and coprecipitation method, respectively. The crystal structure was monitored by XRD. The calcium/phosphorous molar ratio of the different as-precipitated hydroxyapatite was determined by ICP. The results show that various parameters such as aging time, microwave irradiation power and time have significant effects on thermal stability of hydroxyapatite. The thermal stability of HAP increases with protraction of aging time, microwave irradiation time and power, respectively. Aging time reached to 24 h; HAP made by precipitation can exist stably at calcination of 1200 °C. Microwave time and power was 1 h or 700 W, respectively; the sample quenched from 1200 °C is still HAP.