Hydroxyapatite pattern formation in PVA gels

Sodium hydroxide solution was allowed to diffuse from the edge of an acidic poly(vinyl alcohol) gel sheet containing dissolved calcium and phosphate ions, and calcium phosphate was observed to precipitate as the pH rose. The precipitation pattern changed depending on the solute concentration near the reaction front; precipitate “walls” were formed in areas in which the calcium phosphate concentration was higher or the sodium hydroxide concentration was lower than the conditions for homogeneous precipitation, and within a very limited concentration combination of calcium phosphate and sodium hydroxide, a regular stripe pattern with a pitch of about 100 μm was formed. The calcium phosphate precipitate obtained was a single phase of hydroxyapatite. It was also found that apatite ceramic sheets with periodic porous structures or with undulate patterns could be manufactured by sintering gels with stripe patterns.     

Hydroxyapatite particles are capable of inducing osteoclast formation

Hydroxyapatite (HA) coatings have been used to improve implant fixation by promoting bone formation around the prosthesis. A macrophage response to HA particulates has been noted around loosened HA-coated prostheses. As biomaterial wear particle-associated macrophages are known to be capable of differentiating into osteoclasts that are capable of bone resorption, we examined whether particulate HA could similarly induce macrophage-osteoclast differentiation. HA-associated macrophages were isolated from granulomas, formed by subcutaneous implantation of HA, and co-cultured with UMR 106 osteoblast-like cells in the presence of 1,25-dihydroxyvitamin D₃ for up to 14 days on glass coverslips and bone slices. HA-associated macrophage-osteoclast differentiation was evidenced by the formation of numerous multinucleated tartrate resistant acid phosphatase (TRAP)-positive cells which formed lacunar resorption pits on bone slices. Polymethylmethacrylate (PMMA) particle-associated macrophages, isolated from subcutaneous PMMA-containing granulomas, caused significantly more osteoclast formation and bone resorption than HA-associated macrophages. These results indicate that macrophages responding to HA particles are capable of osteoclast differntiation. They also suggest that particles derived from uncemented (HA-coated) implants are likely to induce less osteoclast formation and osteolysis than cemented implants. © 2001 Kluwer Academic Publishers     

Hydroxyapatite formation on metallurgical grade nanoporous silicon particles

Studies into bone-like apatite or hydroxyapatite (HA) growth on potential biomaterials when in contact with simulated body fluid (SBF) not only establish a general method for determining bioactivity but coincidently lead to the design of new bioactive materials in biomedical and tissue engineering fields. Previous studies of HA growth on porous silicon (PS) have examined electrochemically etched silicon substrates after immersion in a SBF. This study differs from previous work in that it focuses on characterising HA growth on chemically etched metallurgical grade nanoporous silicon particles. The PS used in this study is comprised of nanosponge particles with disordered pore structures with pore sizes ranging up to 10 nm on micron-sized particles. The silicon particles are analysed before and after immersion into SBF using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) analysis and X-ray photoelectron spectroscopy (XPS). Results indicate that a HA layer forms on the surface of the nanosponge particles. Experimental analysis indicates that the morphology and calcium-to-phosphorus ratio (Ca/P) verify the formation of crystalline HA on the nanoporous silicon particles.     

Hydroxyapatite formation on cellulose cloth induced by citric acid

Hydroxyapatite (HAp) formation on cellulose cloth with the aid of citric acid was investigated. The cellulose cloths were soaked in simulated body fluid (SBF) solutions (1.5 SBF) with ion concentrations 1.5 times that of SBF (1.0 SBF) with and without citric acid and carbonate containing HAp crystals were found to form only in the 1.5 SBF solution that contained citric acid. The results were explained in terms of hydrogen bonding of citric acid to the cellulose cloth and its chelating ability of calcium ions. Practical application may involve the inclusion of citric acid in the 1.5 SBF solution to promote formation of HAp on previously bioinert cellulose cloth.     

Preparation of highly porous hydroxyapatite from cuttlefish bone

Hydroxyapatite structures for tissue engineering applications have been produced by hydrothermal (HT) treatment of aragonite in the form of cuttlefish bone at 200°C. Aragonite (CaCO₃) monoliths were completely transformed into hydroxyapatite after 48 h of HT treatment. The substitution of CO₃ ²⁻ groups predominantly into the PO₄ ³⁻ sites of the Ca₁₀(PO₄)₆(OH)₂ structure was suggested by FT-IR spectroscopy and Rietveld structure refinement. The intensity of the ν₃PO₄ ³⁻ bands increase, while the intensity of the ν₂CO₃ ²⁻ bands decrease with the duration of HT treatment resulting in the formation of carbonate incorporating hydroxyapatite. The SEM micrographs have shown that the interconnected hollow structure with pillars connecting parallel lamellae in cuttlefish bone is maintained after conversion. Specific surface area (S _BET) and total pore volume increased and mean pore size decreased by HT treatment.     

Hydroxyapatite nanoparticles as a controlled-release carrier of BMP-2: absorption and release kinetics in vitro

Recently, nanoparticles have been extensively developed as controlled-release carriers; however, there has been little research on hydroxyapatite nanoparticles (HANPs) and their potential applications. In this study, HANPs were investigated as a controlled-release carrier of bone morphogenetic protein-2 (BMP-2), the absorption and release kinetics of which were analyzed in vitro. Different concentrations of BMP-2 solution were used to evaluate the adsorptive properties of HANPs. It was observed that the amount of BMP-2 adsorbed onto HANPs could be as high as 70 μg/mg and that adsorption rate was highly correlated with the concentration of BMP-2 solution used. After absorption, the suspension of HANPs absorbed BMP-2 (HANPs/BMP-2) was incubated at 37°C for 15 days and the release kinetics of BMP-2 from HANPs/BMP-2 was determined daily. The release profile showed sustained release of BMP-2 over the period of the investigation. Collectively, these results suggest that HANPs has the potential to function as a carrier for drug delivery systems and as a scaffold material in bone tissue engineering.     

天然多孔材料水热合成羟基磷灰石

研究发现牡蛎白垩层是一种天然方解石质多孔微结构材料,并以其为原料,采用水热法在90℃水浴、常压条件下与磷酸氢二铵溶液反应生成羟基磷灰石.产物通过傅立叶红外光谱(FTIR)、X射线粉末衍射(XRD)、扫描电子显微镜(SEM)进行分析.结果表明,常压下反应15min就出现了羟基磷灰石.48h后白垩层几乎完全转化为羟基磷灰石,并且生成的羟基磷灰石仍完好的保留原多孔微结构.这种转化是通过两种历程完成的,即方解石表面的溶解-重结晶及其内部的固态局域规整离子交换反应.     

Alkaline hydrothermal kinetics in titanate nanostructure formation

In this study, the mechanism of precursor dissolution and the influence of kinetics of dissolution on titanate nanotube formation were investigated. This comparative study explored the dissolution kinetics for the case of commercial titania powders, one composed of predominantly anatase (>95%) and the other rutile phase (>93%). These nanoparticle precursors were hydrothermally reacted in 9 mol L⁻¹ NaOH at 160 °C over a range of reaction times of between 2 and 32 h. The high surface area nanotube-form product was confirmed using X-ray diffraction, FT-Raman spectroscopy, and transmission electron microscopy. The concentration of nanotubes produced from the different precursors was established using Rietveld analysis with internal and external corundum standardization to calibrate the absolute concentrations of the samples. Interpretation of the dissolution process of the precursor materials indicated that the dissolution of anatase proceeds via a zero-order kinetic process, whereas rutile dissolution is through a second-order process. The TiO₂ nanostructure formation process and mechanism of TiO₂ precursor dissolution was confirmed by non-invasive dynamic light scattering measurements. Significant observations are that nanotube formation occurred over a broad range of hydrothermal treatment conditions and was strongly influenced by the order of precursor dissolution.     

Biological hydroxyapatite obtained from fish bones

In this study biological HA has been obtained from fish bones, which are available as waste from fishing activities. Fish bone can be used as a cheap source of biological HA contributing at the same time to give added value to fishing by-products as well as reducing the undesirable environmental impact. For this purpose, fish bones of sword fish and tuna have been cleaned and subjected to heat treatment. Material obtained at 600 °C is a B type hydroxyapatite. At 950 °C a biphasic material was found: biological hydroxyapatite/beta-TCP in a 87/13 ratio. The in vitro cytotoxicity test assessed that all materials are non-cytotoxic.These materials present a promising future because the raw material are wastes, while using a biological substituted apatite containing Mg and Sr as bone substitutes, instead of synthetic apatite without them, would be much beneficial for bone defect healing.     

Hydroxyapatite formation on porous ceramics of alpha-tricalcium phosphate in a simulated body fluid

Alpha-tricalcium phosphate (α-TCP) ceramic is a bioresorbable material that degrades in bone tissue after implantation, since it exhibits higher solubility than beta-tricalcium phosphate (β-TCP) ceramics. The high solubility of α-TCP in an aqueous solution causes its transformation into hydroxyapatite (HAp) through hydrolysis. While one expects the formation of hydroxyapatite after exposure to an aqueous solution mimicking a body environment, we occasionally find variation in HAp formation in the simulated body fluid (SBF). In the present study, HAp formation resulting from exposure to SBF was investigated for some types of α-TCP ceramics with different porosities and specific surface area. Reduced porosity and large surface area of porous specimens may increase the local density of Ca²⁺ in the surrounding SBF to increase the degree of supersaturation with respect to HAp. Thus, the porosity and specific surface area are significant parameters for determining not only bioabsorbability but also the ability to form HAp.     

Wet Chemical Synthesis of Hydroxyapatite Particles from Nonstoichiometric Solutions

Hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂; or HAp] particles with controlled stoichiometry were precipitated from solution by forced hydrolysis of mixtures of calcium and phosphate salts at a high pH and temperature. The mixing procedure determines the final stoichiometry of the precipitated HAp. The resulting panicles were characterized by FTIR, XRD, TEM, and light scattering and their chemical composition was determined by ICP, indicating that pure crystalline HAp with a narrow size distribution was indeed formed, despite some aggregation being detected by light scattering. The Ca/P ratio was found to depend on the specific preparation route.     

Mechanochemical effects on the kinetics of zinc titanate formation

In this work, mixtures Zn-TiO₂ (anatase) in molar ratio 1:1 were mechanochemically activated in air atmosphere, and submitted to thermal treatments in order to study its thermal transformations. The behavior of the system during the milling was followed by X-ray diffraction (XRD), differential thermal analyses (DTA) and thermogravimetric analyses (TGA). Mechanochemical activation produces a progressive loss in crystallinity of the starting powders, with simultaneous oxidation of metallic Zn. However, the formation of neither ZnTiO₃ nor Zn₂TiO₄ could be detected. At temperatures above 600°C, the thermal treatments resulted in the formation of ZnTiO₃ and Zn₂TiO₄, at lower temperatures and shorter holding times for samples activated during longer times. The non-activated mixture exhibited a very different behavior, yielding Zn₂Ti₃O₈ and Zn₂TiO₄ without evidence of ZnTiO₃ formation. The obtained results are explained on the basis of reaction mechanisms taking place in the activated and non-activated samples.     

Anomalous kinetics of lath martensite formation in stainless steel

The kinetics of lath martensite formation in Fe–17·3 wt-%Cr–7·1 wt-%Ni–1·1 wt-%Al–0·08 wt-%C stainless steel was investigated with magnetometry and microscopy. Lath martensite forms during cooling, heating and isothermally. For the first time, it is shown by magnetometry during extremely slow isochronal cooling that transformation rate maxima occur, which are interrupted by virtually transformation free temperature regions. Microscopy confirms martensite formation after athermal nucleation of clusters followed by their time dependent growth. The observations are interpreted in terms of time dependent autocatalytic lath martensite formation followed by mechanical stabilisation of austenite during the transformation process.     

聚乙二醇对相转化成膜过程凝胶动力学的影响研究

以酚酞基聚芳醚砜/聚乙二醇/N,N-二甲基乙酰胺为铸膜液体系,考察了聚乙二醇对非对称膜成膜过程凝胶动力学的影响.结果表明,成膜过程中凝胶动力学的研究结果得到与Strathman等人不同的结果,凝胶前锋位移的平方与时间不是简单的线性关系,凝胶动力学过程不能简单的用Fick扩散定律来描述.提出将凝胶过程与非对称膜的结构相对应分为4个连续的凝胶过程,凝胶速度曲线由4段具有不同速度常数的线段组成,每一段的速度常数大小和膜横截面上的不同结构相对应.在凝胶过程中,凝胶速度常数最大的是皮层.聚乙二醇浓度增加,在热力学上加速了铸膜液的相分离,铸膜液黏度的自然对数值线性增加,铸膜液黏度对凝胶速度的影响比热力学因素的影响要大.     

Phase formation kinetics of nanoparticle-seeded strontium bismuth tantalate powder

Strontium bismuth tantalate (SBT) having composition of Sr_0.7Bi_2.3Ta₂O₉ has been prepared through sol-gel method using their corresponding metal alkoxides as precursors. Seeded SBT powder was prepared by the addition of 5 wt% of nanometer-sized SBT particles to the sol followed by pyrolysis. Differential thermal analyses (DTA) were performed on the unseeded and seeded powder and Aurivillius phase formation temperatures were found to be reduced by 60°C in the seeded ones. Non-isothermal kinetic analyses were applied to the DTA results to obtain activation energy and Avrami exponent values for the Aurivillius phase formation in unseeded and seeded samples. The activation energy for the Aurivillius phase formation was found to be 268 kJ/mol for the seeded ones, while 375 kJ/mol for the unseeded ones, which plays a major role for the enhanced kinetics in the seeded ones. The Avrami exponent values for the Aurivillius phase formation in unseeded and seeded ones were determined as 2.80 and 1.15, respectively, revealing different nucleation and crystal growth mechanisms.     

羟基磷灰石热稳定性研究进展

概述了化学计量比HA及缺钙型HA在高温下的热分解机制,综述了国内外对HA热稳定性研究的成果,总结了通过提高钙磷比、向晶格中引入F-或K+、改变测试气氛等提高HA热稳定性的方法及其机理,展望了制备高温稳定的HA粉末和晶须的研究方向和可能的途径.     

Hydroxyapatite starting from calcium carbonate and orthophosphoric acid: synthesis,characterization, and applications

Hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂], Ca-HA, is the emblematic mineral phase of bones, and is known for its complexity and difficult to reproduce chemical synthesis. Among the routes developed for obtaining this calcium phosphate, the so-called double-decomposition method is well described and often utilized. However, the Ca-HA synthesized by this way forms a larger mass of ammonium nitrate by-product than the desired product itself. Pure Ca-HA for orthopedic or dental applications usually uses thermal treatment to eliminate residual nitrogen compounds by releasing them in the atmosphere. Contemporary sol–gel methods currently in fashion produce even more degradation products including solvents and precursor organics. We now report on a green synthesis procedure which makes pure Ca-HA with minimum by-product. The synthesis calls for reacting phosphoric acid with calcium carbonate in water suspension to form a Ca-HA gel of fine particles. This gel can be filtered and the solids recovered, dried, and sintered, but can also be used as-is for environmental applications such as heavy metal ions or textile dye removal from polluted waste streams. This green Ca-HA has been used to trap heavy metals in flue gases and in municipal waste water treatment plants. This low-cost and low-environmental impact material can be developed for medical use because of its absence of impurities, and in catalytic productions for remediation of many environmental problems. Recent results show Ca-HA can also serve in reforming biogas compositions into useful products, after deposition of selected metal elements. Some of these results will be communicated in this paper.     

Effect of surface condition on the kinetics of scandium-deuteride formation

We studied the effects of surface oxidation on the kinetics of deuterium (D₂) absorption by Sc. A Sieverts apparatus was used to measure the D₂ absorption kinetics and X-ray photoelectron spectroscopy (XPS) was used to determine the surface oxidation state during heat treatments in ultra-high vacuum. In the as-received condition, the bulk Sc sample was covered by an Sc₂O₃ film. However, the Sc₂O₃ phase decomposed dramatically at 573 K due to the fast diffusion of oxygen from the surface to the bulk. The initial surface of Sc was cleaned of most oxygen by heating up to 873 K whereas the obtained fresh surface at temperatures over 873 K was contaminated by oxygen again when cooled down to room temperature. Due to fresh surfaces available for D₂ absorption, the sample at temperatures over 873 K started to absorb D₂ as soon as it was introduced. The absorption rate at these temperatures appears to be governed by D diffusion in the Sc bulk. However, the sample deuterated at room temperature presented an incubation time before the absorption was observed. And this stage was followed by a surface controlled process that changed gradually to a stage related to deuterium diffusion into the bulk.