Influence of oxide-based sintering additives on densification and mechanical behavior of tricalcium phosphate (TCP)

In this research, we studied and analyzed the effects of four different oxide-based sintering additives on densification, mechanical behavior, biodegradation and biocompatibility of tricalcium phosphate (TCP) bioceramics. Selective sintering additives were introduced into pure TCP ceramics, in small quantities, through homogeneous mixing, using a mortar and pestle. The consequent powders of different compositions were pressed into cylindrical compacts, uniaxially and sintered at elevated temperatures, 1150°C and 1250°C, separately in a muffle furnace. X-ray powder diffraction technique was used to analyze the phase-purity of TCP after sintering. Hardness of these sintered specimens was evaluated using a Vickers hardness tester. Sintered cylindrical samples were tested under uniaxial compressive loading, as a function of composition to determine their failure strength. Biodegradation studies conducted using simulated body fluid under dynamic environment, revealed that these additives could control the rate of resorption and hardness degradation of TCP ceramics.     

Hydrothermal synthesis of porous triphasic hydroxyapatite/(alpha and beta) tricalcium phosphate

A novel, porous triphasic calcium phosphate composed of nonresorbable hydroxyapatite (HAp) and resorbable tricalcium phosphate (alpha- and beta-TCP) has been synthesized hydrothermally at a relatively low temperature. The calcium phosphate precursor for hydrothermal treatment was prepared by gel method in the presence of ascorbic acid. XRD, FT-IR, Raman analyses confirmed the presence of HAp/TCP. The surface area and average pore size of the samples were found to be 28 m2/g and 20 nm, respectively. The samples were found to be bioactive in simulated body fluid (SBF).     

Processing of bovine hydroxyapatite (HA) powders and synthesis of calcium phosphate silicate glass ceramics using DC thermal plasma torch

In the present work, hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) was prepared from bovine bones with calcination method (up to 850 °C).The calcinated hydroxyapatite was powdered (30-40 μm) using a mechanical grinder; the particles were highly irregular in shape with sharp edges, angular, rounded, circular, dentric, porous and fragmented morphologies. The irregular shaped calcinated hydroxyapatite was plasma processed to produce spherical powders for thermal spray coating applications. More over; calcium phosphate silicate glass ceramics was produced by plasma melting of ball milled hydroxyapatite-borosilicate glass (50 wt.%) mixture. The samples were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffractometer (XRD) and energy dispersive X-ray analysis (EDX). The morphology was determined using scanning electron (SEM) and optical microscopy (OM). The microhardness, density and porosity measurements for the synthesized samples were made.     

Immersion behavior of hydroxyapatite (HA) powders before and after sintering

The immersion behavior of two different hydroxyapatite (HA) powders before and after sintering was investigated by soaking them in simulated body fluid (SBF) for various periods. The results showed that the mechanism of formation of bone-like apatite on the two HA powders was different due to their different phase composition. Moreover, after being sintered at a proper elevated temperature, the bioactivity of HA powders was increased.     

Phase development and sintering behaviour of biphasic HA-TCP calcium phosphate materials prepared from hydroxyapatite and bioactive glass

The composites of hydroxyapatite (HA) with 2.5 and 5 wt% of a double oxide (50 mol% CaO and 50 mol% P₂O₅) glass were prepared using the conventional powder mixing and sintering method. The addition of the glass significantly enhanced the decomposition process of HA into alpha tricalcium phosphate (α-TCP) for bodies sintered at 1,300 and 1,350 °C and β-TCP phases for the ones sintered at 1,200, 1,250 and 1,300 °C. Microstructural characteristics, phase development and thermal behaviour were studied by SEM, XRD and STA. The effects of TCP phase content and phase transformation from β-TCP to α-TCP on the sintering are discussed. The characterizations revealed considerable content of TCP in the form of large semi-islands due to important reactions between the fine HA and the glass mixed powders.     

Dynamic study of calcium phosphate formation on porous HA/TCP ceramics

Bone-like apatite formation on porous calcium phosphate ceramics was investigated in static simulated body fluid (SBF) and dynamic SBF at different flowing rates. The results of a 14-day immersion in static SBF showed that the formation of bone-like apatite occurred both on the surface and in the pores of the samples. When SBF flowed at the physiological flow rate in muscle (2 ml/100 ml⋅min), bone-like apatite could be detected only in internal surface of the pores of samples. The result that bone-like apatite formation could only be found in the pores when SBF flowed at physiological flow rate was consistent with that of porous calcium phosphate ceramics implanted in vivo: osteoinduction was only detected inside the pores of the porous calcium phosphate ceramics. This result implicates that the bone-like apatite may play an important role in the osteoinduction of Ca-P materials. The dynamic model used in this study may be better than usually used static immersion model in imitating the physiological condition of bone-like apatite formation. Dynamic SBF method is very useful to understand bone-like apatite formation in vivo and the mechanism of ectopic bone formation in calcium phosphate ceramics.     

Dynamic study of calcium phosphate formation on porous HA/TCP ceramics

Bone-like apatite formation on porous calcium phosphate ceramics was investigated in static simulated body fluid (SBF) and dynamic SBF at different flowing rates. The results of a 14-day immersion in static SBF showed that the formation of bone-like apatite occurred both on the surface and in the pores of the samples. When SBF flow at the physiological flow rate in muscle (2 ml/100 ml min1), bone-like apatite could be detected only in internal surface of the pores of samples. The result that bone-like apatite formation could only be found in the pores when SBF flown at physiological flow rate was consistent with that of porous calcium phosphate ceramics implanted in vivo: osteoinduction was only detected inside the pores of the porous calcium phosphate ceramics. This result implicates that the bone-like apatite may play an important role in the osteoinduction of Ca-P materials. The dynamic model used in this study may be better than usually used static immersion model in imitating the physiological condition of bone-like apatite formation. Dynamic SBF method is very useful to understand bone-like apatite formation in vivo and the mechanism of ectopic bone formation in calcium phosphate ceramics.     

Effect of titanium and zirconium substitutions for calcium on the formation and structure of tricalcium phosphate and hydroxyapatite

The effect of Ti and Zr substitutions for Ca cations on the formation of tricalcium phosphate and hydroxyapatite has been studied in a wide concentration range: from 0.1 to 20 mol %. Upon the incorporation of Ti and Zr cations into tricalcium phosphate, the major forming phase is β-tricalcium phosphate. On the addition of low substituent concentrations to hydroxyapatite, we observe the formation of a single-phase material with the apatite structure. Increasing the substituent concentration to 10–20 mol % Ti or 20 mol % Zr leads to the formation of tricalcium phosphate. The unit-cell volume of the cation-substituted tricalcium phosphates has been shown to decrease with increasing substituent concentration. In the zirconium-containing hydroxyapatites, the unit-cell volume decreases with increasing zirconium concentration, whereas the titanium-containing hydroxyapatites exhibit an opposite tendency.     

A simple sol-gel technique for synthesis of nanostructured hydroxyapatite, tricalcium phosphate and biphasic powders

Hydroxyapatite (HAP), β-tricalcium phosphate (β-TCP) and biphasic calcium phosphate (BCP) nanocrystalline powders were prepared by a simple sol-gel approach. Because of the unique characteristic of the phosphorous source ((CH₃O)₃P) and the proper uses of calcic and phosphorous sources with Ca/P molar ratio between 1.4 and 1.67, three different kinds of nanostructured calcium phosphate powders were achieved by changing the ratio of calcic and phosphorous sources. For HAP and β-TCP, pure phases were prepared. For BCP, the proportion of HAP and β-TCP could be changed by thermal treatment.     

Synthesis of calcium phosphate powder from calcium lactate and ammonium hydrogen phosphate for the fabrication of bioceramics

A calcium phosphate powder has been synthesized from aqueous 0.25, 0.5, and 1.0 M calcium lactate and ammonium hydrogen phosphate solutions atat a Ca/P = 1, without pH adjusting. According to X-ray diffraction data, the as-synthesized powder consisted of brushite (CaHPO₄ · 2H₂O) and octacalcium phosphate (Ca₈(HPO₄)₂(PO₄)₄ · 5H₂O). After heat treatment in the range 500–700°C, the powders were gray in color because of the destruction of the reaction by-product. The powders heat-treated in the range 500–700°C consisted largely of γ-Ca₂P₂O₇. The ceramics prepared from the synthesized powders by firing at 1100°C consisted of β-Ca₂P₂O₇ and β-Ca₃(PO₄)₂.     

Current state of the art of biphasic calcium phosphate bioceramics

We have developed 15 years ago, with the collaboration of Lynch, Nery, and LeGeros in the USA, a bioactive concept based on biphasic calcium phosphate (BCP) ceramics. The concept is determined by an optimum balance of the more stable phase of HA and more soluble TCP. The material is soluble and gradually dissolves in the body, seeding new bone formation as it releases calcium and phosphate ions into the biological medium. The bioactive concept based on the dissolution/transformation processes of HA and TCP has been applied to both Bulk, Coating and Injectable Biomaterials. The events at the calcium phosphate (CaP) biomaterial/bone interface represent a dynamic process, including physico-chemical processes, crystal/proteins interactions, cells and tissue colonization, bone remodeling, finally contributing to the unique strength of such interfaces. An important literature and numerous techniques have been used for the evaluation of the fundamental physico chemical and biological performance of BCP concept. This type of artificial bone used from a long time in preclinical and in clinical trial, revealed the efficiency for bone filling, performance for bone reconstruction and efficacy for bone ingrowth at the expense of the micro macroporous BCP bioceramics.     

Sintering and characterization of HA and TCP bioceramics with control of their strength and phase purity

HA and -TCP-based ceramics were prepared using commercial powders. Powder characteristics were defined and the processing parameters studied, aimed at the production of samples with improved microstructural and mechanical properties. The behaviour of HA powder subjected to various thermal treatments was investigated in order to control the formation of secondary phases (- and -TCP) during sintering. The optimal thermal treatment required to prepare pure -TCP powder from the precursors (HA and DCP) was determined and the sintering method required to prepare fully dense -TCP completely free from -form, was identified. Translucent hot-pressed -TCP ceramics with potential applications in aesthetic restorative prostheses were prepared and characterized. The interval of existence of -TCP and -TCP as secondary products was also defined. Crystallographic analysis was carried out on the imperfectly known low-temperature -TCP phase, and a proper monoclinic unit cell determined.     

Rational synthesis of a nanocrystalline calcium phosphate cement exhibiting rapid conversion to hydroxyapatite

The rational synthesis, comprehensive characterization, and mechanical and micromechanical properties of a calcium phosphate cement are presented. Hydroxyapatite cement biomaterial was synthesized from reactive sub-micrometer-sized dicalcium phosphate dihydrate and tetracalcium phosphate via a dissolution-precipitation reaction using water as the liquid phase. As a result nanostructured, Ca-deficient and carbonated B-type hydroxyapatite is formed. The cement shows good processibility, sets in 22 ± 2 min and entirely transforms to the end product after 6 h of setting reaction, one of the highest conversion rates among previously reported for calcium phosphate cements based on dicalcium and tetracalcium phosphates. The combination of all elucidated physical-chemical traits leads to an essential bioactivity and biocompatibility of the cement, as revealed by in vitro acellular simulated body fluid and cell culture studies.The compressive strength of the produced cement biomaterial was established to be 25 ± 3 MPa. Furthermore, nanoindentation tests were performed directly on the cement to probe its local elasticity and plasticity at sub-micrometer/micrometer level. The measured elastic modulus and hardness were established to be E_s = 23 ± 3.5 and H = 0.7 ± 0.2 GPa, respectively. These values are in close agreement with those reported in literature for trabecular and cortical bones, reflecting good elastic and plastic coherence between synthesized cement biomaterial and human bones.     

磷酸盐生物玻璃粘接剂对HA植入体烧结的影响

通过在原料中添加磷酸盐玻璃粘接剂改善HA生物陶瓷的烧结性能，测定试样的线收缩率，研究了粘接剂对HA植入体烧结的影响。结果表明磷酸盐生物玻璃粘接剂能形成液相而促进材料烧结，降低烧结温度。     

A novel,environmentally friendly process for the fabrication of calcium phosphate bioceramics

Problems are addressed that relate to the use of standard processes (involving several consecutive steps) for the fabrication of calcium phosphate bioceramics: process duration (up to 72 h), low chemical yield (notably lower than 100%), and a large amount of byproducts (largely in the form of alkaline (pH ∼ 10) aqueous solutions). A new, environmentally friendly, one-step process is proposed for the preparation of calcium phosphate bioceramics. Original Russian Text ? S.V. Dorozhkin, 2008, published in Neorganicheskie Materialy, 2008, Vol. 44, No. 2, pp. 253–256.     

Silicon-substituted hydroxyapatite (SiHA): A novel calcium phosphate coating for biomedical applications

Hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂], (HA) is similar in composition to bone mineral and has been found to promote new bone formation when implanted in a skeletal defect. However, its use in biomedical applications is limited by its relatively slow rate of biological interaction, and there is also a requirement to improve the success rate of HA implants in younger active patients, particularly where implants will be in place long-term. The addition of silicon (Si) into HA has been demonstrated to enhance the speed, and quality of the bone repair process. This paper describes the synthesis and detailed characterisation of nanocrystalline silicon-substituted hydroxyapatite (SiHA) thin coatings applied to a titanium substrate via a magnetron co-sputtering process. Amorphous SiHA coatings (∼1 μm thick) with varying Si content up to 4.9 wt% were produced before being transformed into crystalline films by heat-treatment. The crystalline coating was characterised by X-ray diffraction (XRD) and infrared (IR) analysis, and confirmed to be a single-phase apatite. The substitution of Si into HA resulted in an increase in both the a- and c-axes of the unit cell parameters, but a decrease in the crystallite size, with increasing Si substitution. This substitution also caused a decrease in the intensities of both the O–H and P–O bands in the IR spectra. Hence, these findings confirmed that the crystal structure of HA was altered with Si substitution. In vitro cell culture work showed that these SiHA thin coatings exhibited enhanced bioactivity and biofunctionality. An increase in the attachment and growth of human osteoblast-like (HOB) cells on these coatings was observed throughout the culture period, with the formation of extracellular matrix. In addition, confocal microscopy revealed that HOBs developed mature cytoskeletons with clear evidence of actin stress fibres, along with defined cell nuclei.     

Antimicrobial activity and biologic potential of silver-substituted calcium phosphate constructs produced with self-propagating high-temperature synthesis

There is significant demand for synthetic bone substitute materials that can decrease the incidence of implant-based bacterial infections. The intent of this research was to evaluate the antimicrobial activity and biologic potential of calcium phosphate (CaP) constructs substituted with silver (Ag) that were produced via self-propagating high-temperature synthesis (SHS). SHS is a combustion synthesis technique that has successfully generated porous CaP bioceramics intended for use in bone repair. SHS reactions are highly versatile; dopants can be added to the reactant powders to alter product chemistry and morphology. In this research, Ag powder was added to the reactants generating porous CaP constructs containing 0.5, 1, or 2 wt% Ag. Antibacterial performance of the constructs was assessed against Escherichia coli, a representative model for Gram-negative bacteria. Liquid solutions (1 μg/mL) of CaP–Ag particles to phosphate buffered saline were incubated with 10⁵ cells/mL. After 24 h, 10 μL of solution were spread on an LB agar plate and cultured for 24 h at 37 °C. Samples cultured with CaP–Ag showed complete bacterial inhibition while the controls (E. coli only and CaP without Ag) exhibited significant colony formation. The effects of Ag concentration on cytotoxicity and biocompatibility were tested in vitro. At 7 days, osteoblasts uniformly enveloped the CaP–Ag particles and displayed a healthy flattened morphology suggesting the concentrations of Ag incorporated into constructs were not cytotoxic. CaP–Ag constructs produced via SHS represent a source of synthetic bone substitute materials that could potentially inhibit, or reduce the incidence of post-operative bacterial infections.     

Effect of zirconia on the formation of calcium phosphate bioceramics under microwave irradiation

Bi-phasic calcium phosphate (BCP) bioceramics containing hydroxyapatite (HA) and tri-calcium phosphate (TCP) phases have recently attracted attention as an ideal bone graft substitute due to their controlled resorption in the body fluid upon implantation. In this study, the HA and BCP phases were prepared by in situ method, using natural goniopora under microwave irradiation. Fourier-transform infrared (FT-IR) and powder X-ray diffraction (XRD) methods were employed to investigate proof of HA and BCP formations. XRD results show that the major characteristic peaks of HA appear in the regions of approximately 26°, 28°, 29°, 30–35°, 39°, 46°, 49° and 50° (2θ). FT-IR results indicate that there are no occurrences of impurities during HA and BCP formations. Reinforcement of zirconia in the in situ formation of HA leads to a more resorbable phase of β-TCP since the influence of zirconia induces faster decomposition of HA, as indicated by differential thermal (DT) analysis. The in vitro physiological stability of prepared materials was performed in phosphate-buffered saline (PBS) of pH 7.4 at 37 °C in a thermostatic water bath, and the results indicate that the resorbable nature of BCP lies in between the resorption levels of HA and TCP. Solubility of the BCP can be controlled by the addition of zirconia corresponding to clinical applications.