RESUMEN
Precipitates formed at an early stage (during the first 6 h) of the hydroxyapatite crystallization of a solution were studied. A nitrous synthesis was used (0.583M (NH(4))(2)HPO(4) and 0.35 M Ca(NO(3))(2).4H(2)O solutions at pH 11-12, 21 degrees C, fast mixing, lyophilization of aliquots). Although XRD patterns indicated an amorphous calcium phosphate (ACP), IR spectra revealed apatite nanocrystals in the precipitates. Some amount of free calcium was found in the mother solution by mass spectrometrical analysis of the aliquots. This amount considerably decreased as the synthesis proceeded, however, the decrease had a slight effect on the crystallinity of the precipitates. A new suggestion on the nature of delayed crystallization (under conditions as those in the present study) was proposed. The free calcium adsorbed by the nanoparticles from the solution formed a shell around a particle because the calcium diffusion into the bulk was poor at the low synthesis temperature. As such, the encapsulation delayed the crystallization of the nanoparticles. Evidence for this suggestion was given. New possibilities were proposed for preparation of bioactive materials of desired composition based on the structural and compositional peculiarities of the X-ray diffraction-amorphous calcium phosphates.
Asunto(s)
Fosfatos de Calcio/química , Precipitación Química , Materiales Biocompatibles/química , Fosfatos de Calcio/análisis , Cristalización , Durapatita/química , Nanopartículas/química , Espectrofotometría Infrarroja , Temperatura , Difracción de Rayos XRESUMEN
Lattice and surface impurity reactions and structural changes induced by them in slightly carbonated hydroxyapatite (SCHA) treated at 25-1100 degrees C were comprehensively studied. The SCHA was processed by a conventional wet synthesis at a high possible temperature(96 degrees C) using ammonium containing parent reagents. IR-spectroscopy, XRD, TG-DTA technique and mass spectrometric thermal analysis (MSTA) were employed for characterization of the samples. NH4+ with H3O+ in cationic-and CO3(2-) (A- and B-positions) with HPO4(2-) in anionic sites, and H2O, CO3(2-)(HCO3(-)) NO3(-), NxHy on the surface of particles were found and considered as impurity groups. Complicated changes in lattice constants of theSCHA stepwise annealed in air (for 2 h) were revealed; the changes were associated with reactions of the impurity groups. Filling the hexed sites with hydroxyl ions above 500 degrees C was shown to happen partly due to lattice reactions but was mainly owing to hydrolysis of the SCHA by water molecules in air. Decomposition of CO3(2-) groups proceeded through both thermal destruction and reactions with some of the impurity ions. The decarbonation in A-sites occurred at much lower temperatures (450-600 degrees C) than in B-sites (700-950 degrees C) and was first revealed to happen in two stages: due to an impurity reaction around 500 degrees C, and then through thermal destruction at 570 degrees C. A redistribution of CO3(2-) ions, decreasing in amount on the whole, was observed upon annealing above 500 degrees C. To avoid possible erroneous conclusions from TG-data, a sensitive method was shown to be required for monitoring gaseous decomposition products (such as the MSTA in this study), in case several impurity groups were present in a SCHA.
Asunto(s)
Materiales Biocompatibles/química , Carbono/química , Durapatita/química , Ensayo de Materiales , Modelos Químicos , Modelos Moleculares , Materiales Biocompatibles/análisis , Carbono/análisis , Durapatita/análisis , Calor , Conformación MolecularRESUMEN
A modified processing route for fabricating dense and porous biphasic calcium phosphate (BCP) ceramics of desired and reproducible phase composition (hydroxyapatite (HA)/beta-tricalcium phosphate (beta-TCP) ratio) has been developed. The principal idea of the route was combining a precipitation and a solid phase methods. First, a nonstoichiometric (slightly carbonated calcium-deficient) HA (CdHA) precipitate was synthesized by mixing a calcium carbonate (CaCO(3)) water suspension with an orthophosphoric acid (H(3)PO(4)) solution in abundance (related to the amount resulting in a stoichiometric HA) under definite conditions, and a powder of the precipitate was prepared and calcinated in air (860 degrees C, 1.5 h). In the second stage, a BCP ceramics of the composition determined by the calcium-deficiency in a calcinated powder (the acid abundance in a mixture) was processed by sintering powder compacts with or without a porosizer under appropriate conditions (1,200 degrees C, 2h). A calibrating dependence of the HA/beta-TCP ratio in the ceramics on the acid abundance has been plotted which enabled a controlled preparation of BCP ceramics. A correlation based on unresolved bands in nu(4)-PO (4) (3-) domain in IR-spectra of nanostructured BCP materials was found. Using the correlation, the process of CdHA --> beta-TCP transformation could be easily monitored. The density and microhardness of the BCP ceramics neglectly depended on the composition, however, the compressive strength did: the lower the HA/beta-TCP ratio, the higher the strength in the dense materials.
Asunto(s)
Fosfatos de Calcio/síntesis química , Cerámica , Fosfatos de Calcio/química , Microscopía Electrónica de Rastreo , Espectrofotometría Infrarroja , Difracción de Rayos XRESUMEN
Two kinds of calcium phosphate ceramic (CPC) granules of high porosity (50 +/- 5%) and improved (for such materials) compressive strength (10-25 MPa) consisted of hydroxyapatite (PHA) and a mixture of hydroxyapatite (HA) and beta-tricalcium phosphate (beta-TCP) in 60 HA/40 beta-TCP composition (PCPC) were developed. A comparative study of in vivo behavior of the materials implanted into an almost unloaded (greater trochanter of femur) and loaded (distal methaphysis of femur) zones in the skeleton of rabbits was performed. Significant activating influence of loading on the processes of new bone formation and reconstruction in macropores of both materials during all periods of implantation (up to 6 months) was observed. The role of relevant cells in the processes in the unsoluble PHA and the degradable PCPC (in which the processes was observed to intensify due to dissolution of the material) was studied and is discussed. Great disturbance in pore structure of the BCPC was revealed in more late periods of implantation. After 6 months, presence of large composite fragments located in intertrabecula spaces of greater trochanter was a characteristic feature of the PCPC crushing. The developed CPC materials seems to have good perspective for using in bone defect plasty in some loaded areas of the skeleton.
Asunto(s)
Sustitutos de Huesos/química , Huesos/metabolismo , Fosfatos de Calcio/química , Cerámica/química , Animales , Materiales Biocompatibles , Fuerza Compresiva , Durapatita/química , Ensayo de Materiales , Porosidad , Presión , Conejos , Propiedades de Superficie , Factores de TiempoRESUMEN
A new method for the preparation of nonstoichiometric hydroxyapatite (HA) "dense" and porous granules, round in form and up to 8 mm in sizes designed for application in orthopaedic surgery has been developed. The "dense" granules' porosity was up to 32% and they only contained micropores. They differed from that kind of granules by increased values of compression strength (up to 48 MPa). The porous granules contained a system of interconnected micro- and macropores. The porosity value (up to 70%) and the porosity structure were similar to those in the mineral framework of a spongy bone. The compression strength of the porous granules (up to 25 MPa) was high enough for various kinds of application. Granules of both sorts were used in performing 42 operations on the locomotor system. Depending upon localization and supposed level of the injured area loading, "dense" or porous granules were used. The postoperative observations (up to four years in length) have attested to the high quality of the granules.
Asunto(s)
Enfermedades Óseas/cirugía , Sustitutos de Huesos/química , Cristalización/métodos , Durapatita/química , Adolescente , Adulto , Anciano , Enfermedades Óseas/diagnóstico , Fuerza Compresiva , Femenino , Humanos , Masculino , Ensayo de Materiales , Microesferas , Persona de Mediana Edad , Conformación Molecular , Ortopedia/métodos , Porosidad , Polvos , Propiedades de Superficie , Resultado del TratamientoRESUMEN
It has been shown that reduction of sintering temperature of hydroxyapatite is connected to the use of a powder, the particles of which consist of crystalline and amorphous-like constituents. Shrinkage of the pressings made of the powder starts at the middle temperatures (600-700 degrees C) and is realized by a mechanism of a mutual sliding of the particles. If the firing temperature of the pressings increases, the second stage, realized by a diffusion mechanism (over approximately 900 degrees C), begins. A shrinkage curve on the sliding stage has a stepped character. It is caused by a stopping effect on the shrinkage of the pressure of a gas, which is exuded as a result of crystallization of the amorphous-like constituent, and partly accumulates in closed cavities and pores. The contribution of the two mechanisms to the shrinkage process is determined by the degree of crystallinity of the powder (i.e. the ratio of the crystalline and the amorphous-like constituents). If a powder has a higher degree of crystallinity, higher temperatures result, and the stepped character of the shrinkage curve is smoothed. The sliding stage is practically lacking for pressings of crystalline powder, and a noticeable shrinkage starts at high temperatures (over approximately 900 degrees C) as diffusive processes develop.
Asunto(s)
Durapatita/química , Cerámica , Cristalización , Calor , Termodinámica , Difracción de Rayos XRESUMEN
Inhomogeneous ceramics of hydroxyapatite (HA) were prepared by sintering briquettes in which an inhomogeneous distribution of density was made by pressing HA powder into a die with rough walls. The resulting sample of such a ceramic is a hard thin shell with a loose core, and it is characterized by an inhomogeneous macro- and microstructure. It is a sintered conglomerate from HA grains containing grain boundary macropores in contact with the surface and micropores located inside the grains, part of which are also associated with the free surface. The highest value of microhardness is fixed on the surface of the sample. The radial distribution of microhardness in the (cylindrical) sample has an axisymmetric, nonmonotonic character and, on the whole, shows the decrease of microhardness (the increase of porosity) from the surface to the center. The highest values of microhardness, crushing strength, and fracture strength are close to those known for ceramics of moderate strength.
Asunto(s)
Materiales Biocompatibles/química , Cerámica/química , Hidroxiapatitas/química , Dureza , Porosidad , Polvos , Espectrofotometría Infrarroja , Propiedades de SuperficieRESUMEN
Some physics during heating and sintering of powder pressings of hydroxyapatite (HA) under conventional (usual) conditions have been studied. It is revealed that heating and firing of the pressings of a middle-dispersity powder are accompanied by release of gases. The gas release hinders and can stop the shrinkage (sintering). The microhardness is low and has a complicated distribution on the surface and in the volume of ceramics. A slight degassing (drying) of the pressing before sintering heightens the density and improves the microhardness characteristics of the ceramics. The shrinkage stop effect is eliminated in pressings of a fine powder. On the basis of the results, a technique and some methods for quality improvement of ceramics were proposed and approbated. They consisted of the manufacture of samples of a mixture of two powders with different dispersity, use of a press technique with two male dies, thermal treatment of pressing before sintering, and the choice of moderate sintering conditions. The resulting ceramics had a density close to the theoretical, almost homogeneous microhardness distribution in the sample and much higher values of microhardness and compressive strength.
Asunto(s)
Materiales Biocompatibles , Cerámica , Durapatita , Materiales Biocompatibles/química , Materiales Biocompatibles/aislamiento & purificación , Cerámica/química , Cerámica/aislamiento & purificación , Durapatita/química , Durapatita/aislamiento & purificación , Dureza , Tamaño de la Partícula , Polvos , Espectrofotometría InfrarrojaRESUMEN
The formation of bone tissue in pores, and around, a block made of partially dehydroxylated porous hydroxyapatite ceramic and implanted into a rat's femur using X-ray diffraction (XRD), infrared spectroscopy (IRS) and histological methods were investigated. Structural characteristics and composition of new bone tissue formed three months after implantation, in fact, do not differ from those of the normal femur. A model of osteogenesis based on dissolution-precipitation reactions is given, taking into account peculiarities of structure, deficiency and impurity composition for the material of the implant.
Asunto(s)
Remodelación Ósea , Fémur/metabolismo , Hidroxiapatitas/metabolismo , Prótesis e Implantes , Animales , Cerámica , Fémur/cirugía , Osteogénesis , Porosidad , Ratas , Ratas Wistar , Espectrofotometría Infrarroja , Difracción de Rayos XRESUMEN
Hydroxyapatite (HA) crystallites of smaller size than those formed during the spraying process are found in HA coatings on titanium as a result of the crystallization of the amorphous phase (approximately 630 degrees C) when the coatings are vacuum-heat-treated in the temperature interval 100-1000 degrees C. As the annealing temperature increases within the 630-1000 degrees C range, the size of the crystallites increases, and at 1000 degrees C reaches the size of those formed during the process of spraying. At the same time, at 800 degrees C and above, HA transforms into other calcium phosphate phases (alpha-tricalcium phosphate, beta-tricalcium phosphate, tetracalcium monoxide diphosphate). These phase transformations lead to the increase of coating roughness.
Asunto(s)
Materiales Biocompatibles/química , Durapatita/química , Cristalización , Calor , Microscopía Electrónica de Rastreo , Tamaño de la Partícula , Titanio/química , Difracción de Rayos XRESUMEN
A new effect of the amorphous component of periodic crystallization in the surface layers of coatings during hydroxyapatite (HA) plasma spraying is revealed. The effect is caused by significant temperature increases of surface layers due to plasma heating in the process of coating. Under prevalent spraying conditions, the crystallite sizes of the crystallite phase are considerably less than those in dense HA. These non-equilibrium phases crystallize consistently and, as a result, in the surface layers HA crystallites emerge and grow. This leads to oscillating type dependences of crystallite size and crystallinity on coating thickness.
Asunto(s)
Materiales Biocompatibles/química , Durapatita/química , Fosfatos de Calcio/química , Cristalización , Calor , Ensayo de Materiales , Polvos , Propiedades de Superficie , Titanio , Difracción de Rayos XRESUMEN
Plasma-sprayed hydroxyapatite coatings contain a quite large amount of amorphous phase. Infrared analysis shows that the plasma-sprayed amorphous phase is an oxyapatite and the coating predetermined as a hydroxyapatite is proved to be an oxyhydroxyapatite with a small quantity of hydroxyapatite. Heat treatment promotes the transformation of amorphous oxyapatite into a crystalline hydroxyapatite structure and reduces the dissolution rate of the coatings. Further studies focus on the properties of the amorphous phase, showing that amorphous oxyapatite is more soluble in 0.15 M lactic acid at 25 degrees C and that water molecules can accelerate the amorphous/crystalline transformation of the hydroxyapatite structure during the plasma-spraying process.
Asunto(s)
Durapatita/química , Prótesis e Implantes , Cristalización , Calefacción , Concentración de Iones de Hidrógeno , Lactatos/química , Ácido Láctico , Ensayo de Materiales , Polvos , Espectrofotometría Infrarroja , Agua/químicaRESUMEN
It is shown that under standard conditions of plasma spraying hydroxyapatite powder reaches the substrate in the form of molten drops. On striking the surface, the molten drop splatters, cools rapidly and solidifies. Consequently, a pancake particle surrounded by tiny spherical or differently shaped particles is formed on the substrate. The solidified particles are partly crystallized. Besides the crystalline hydroxyapatite, they contain some supercooled molten hydroxyapatite, i.e. amorphous hydroxyapatite. Build-up of these particles during coating gives a porous pancake-globular structure. During a brief vacuum heating at 630 degrees C, the amorphous phase is crystallized and completely crystalline hydroxyapatite coatings are obtained.