Nucleation of Hydroxyapatite Crystal through Chemical Interaction with Collagen

The nucleation of hydroxyapatite (HAp) crystal through chemical interaction with collagen was investigated. A collagen membrane was soaked in a supersaturated simulated body fluid (1.5 SBF) solution with ion concentrations at 1.5 times that of normal simulated body fluid (1.0 SBF). A few carbonate-containing HAp crystals were formed mostly on the edge-side of the collagen membrane after 4 weeks. In the Fourier-transform infrared spectometry (FTIR) results, the carboxylate band of the collagen membrane showed red chemical shifts after the formation of HAp crystals, which coincided well with the decrease of the calculated bond orders of the carboxylate group when chelated with a calcium ion, which emulated the first-step nucleation of HAp crystal on the carboxylate group of collagen. The result implies that the binding of a calcium ion to the carboxylate group of collagen is one of the key factors for the nucleation of HAp crystals in a 1.5 SBF solution.     

Interconnected Hydroxyapatite Nanofibers in the Biomimetic Coating of a Bioinert Ceramic Substrate

A bioinert ceramic substrate, α-Al₂O₃, has been coated with hydroxyapatite (HAp) by the biomimetic route using simulated body fluid (SBF) solution at room temperature. The substrate was incubated at 37°C in SBF for 6 days with a periodic replacement with freshly prepared SBF at 48-h intervals. After 6 days, continuous nanofiber-like structures of HAp (5–35 μm in length, 0.05 μm diameter) were obtained, connecting the intra- and interglobular clusters, within the coated mineral layer on the substrate surface. This is a unique and new observation, and this phenomenon has been demonstrated by a simple fractal growth model.     

Effect of Albumin on the Growth Characteristics of Hydroxyapatite Coatings on Alumina Substrates

A potent biomolecule, egg albumin (EA), has been used in the biomimetic coating of hydroxyapatite (HAp) on α-Al₂O₃ substrate. The same experiment was repeated without using EA and the results have been compared. In both cases, the substrates were incubated up to 6 days in SBF (simulated body fluid) at 37°C with a periodic replacement by freshly prepared SBF at 48-h intervals. The presence of EA influenced the growth and morphology of HAp crystals. Without EA, the HAp coating comprises irregular-shaped, serrated-edged plates whereas with EA, thick and regular-edged HAp crystal plates were observed on the α-Al₂O₃ substrate.     

Apatite-Forming Ability of Sodium-Containing Titania Gels in a Simulated Body Fluid

An essential condition for an artificial material to bond to living bone is the formation of bonelike apatite on its surface in the living body. The bonelike apatite can be reproduced on the bone-bonding material even in an acellular simulated body fluid (SBF) with ion concentrations almost equal to those of human blood plasma. In the present study, the dependence of the apatite-forming abilities of sodium-containing titania gels in a SBF on composition and structure is examined. The sodium-containing titania gels are model substances produced on the surface layer of bioactive titanium metal prepared by sodium hydroxide solution and heat treatments. When sodium-containing titania gels are immersed in the SBF, Na⁺ ions incorporated in the gels are exchanged with the H₃O⁺ ions in the SBF. This ion exchange causes an accompanying increase in the pH of the SBF and increases its ionic activity product, thus providing favorable conditions for apatite nucleation on the surfaces of the gels. Nevertheless, sodium-containing titania gels that do not contain anatase do not form apatite on their surfaces. Independent of the composition, the gels form apatite on their surfaces in the SBF, specifically when they contain anatase. These results imply that the Ti-OH groups on titania, which have been proposed to be responsible for the apatite formation, are effective for apatite nucleation when they are arranged in a specific structural unit based on the anatase structure.     

Synthesis of ZrO2 and Y2O3-Doped ZrO2 Thin Films Using Self-Assembled Monolayers

Undoped or Y₂O₃-doped ZrO₂ thin films were deposited on self-assembled monolayers (SAMs) with either sulfonate or methyl terminal functionalities on single-crystal silicon substrates. The undoped films were formed by enhanced hydrolysis of zirconium sulfate (Zr(SO₄)·4H₄O) solutions in the presence of HCl at 70°C. Typically, these films were a mixture of two phases: nanocrystalline tetragonal- ( t -) ZrO₂ and an amorphous basic zirconium sulfate. However, films with little or no amorphous material could be produced. The mechanism of film formation and the growth kinetics have been explained through a coagulation model involving homogeneous nucleation, particle adhesion, and aggregation onto the substrate. Annealing of these films at 500°C led to complete crystallization to t -ZrO₂. Amorphous Y₂O₃-containing ZrO₂ films were prepared from a precursor solution containing zirconium sulfate, yttrium sulfate (Y₂(SO₄)₃8·H₂O), and urea (NH₂CONH₂) at pH 2.2–3.0 at 80°C. These films also were fully crystalline after annealing at 500°C.     

Apatite Coating on Organic Polymers by a Biomimetic Process

Dense and uniform layers of a biologically active carbonate-containing hydroxyapatite can be formed on various kinds of organic polymers by the following biomimetic method. First, a substrate is set in contact with particles of CaO–SiO₂-based glass soaked in a simulated body fluid (SBF) with ion concentrations nearly equal to those of human blood plasma for forming the apatite nuclei on the substrate. Second, the substrate is soaked in another solution highly supersaturated with respect to the apatite, e.g, with ion concentrations 1.5 times those of SBF (1.5SBF) for making the apatite nuclei grow on the substrate in situ. The induction period for the apatite nucleation, which is defined as the time of the first treatment required for forming enough of the apatite nuclei to make the continuous layer after the second treatment, was almost 24 h for most of the examined polymers. The adhesive strength of the formed apatite layer to the polymers was as high as 3 to 4 M Pa for poly(ethylene terephthalate), poly-ether sulfone, and poly (vinyl alcohol) hydrogel. This type of apatite–organic polymer composite is expected to be useful for repairing not only living hard tissues but also soft ones.     

Particle Size Control and Dependence on Precursor pH: Synthesis Uniform Submicrometer Zinc Aluminate Particles

Zinc aluminate (ZnAl₂O₄) particles have been synthesized by the hydrothermal method using NH₃·H₂O as a pH adjustment mineralizer. Experimental results showed that ZnAl₂O₄ particle size was dependent on the precursor pH, and could be controlled through pH adjustment. It was 5.5, 11.5, and 27 nm when the precursor pH was 8.2, 9.3, and 10.5, respectively. On the other hand, the particle size distribution changed broader with increase in pH. These differences were attributable to the different NH₃·H₂O function. NH₃·H₂O was mainly used as a base at lower pH (<9.0), while its complex function predominated at higher one (>9.5). From thermodynamic viewpoint, the rate-limiting steps were dissolution of Al(OH)₃ and γ-AlO(OH) to Al(OH)₄⁻ at lower and higher pH, respectively. The newly formed γ-AlO(OH) with high reactivity was the critical factor in the synthesis of bimodal particles. Higher temperature treatment of γ-AlO(OH) could decrease the reactivity, and could be used as an aluminum source for synthesis uniform ZnAl₂O₄ particles.     

Soft-Chemistry Synthesis and Characterization of Bismuth Oxyfluorides and Ammonium Bismuth Fluorides

Two bismuth oxyfluorides, BiOF and BiO_0.5F₂, have been prepared by soft chemistry. These two compounds are of interest for application as new type of positive electrode active materials in secondary lithium battery. BiOF was obtained directly from the fluorination of Bi₂O₃ by a saturated aqueous solution of NH₄F. BiO_0.5F₂ was obtained by the hydrolysis of β-NH₄BiF₄ at 70°C. The ammonium bismuth fluoride β-NH₄BiF₄, which is a new orthorhombic form of NH₄BiF₄, was obtained by reacting BiF₃ in a saturated aqueous solution of NH₄F. Two other ammonium bismuth fluorides, (NH₄)_0.5BiF_3.5 and NH₄Bi₃F₁₀, were also prepared by annealing β-NH₄BiF₄ at a relatively low temperature under inert atmosphere. The existence of the fluorite (NH₄)_0.5BiF_3.5 is reported for the first time in this paper.     

Effects of Ammonium Chloride on the Rheological Properties and Sedimentation Behavior of Aqueous Silica Suspensions

The influence of ammonium chloride (NH₄Cl) on the rheological properties and sedimentation behavior of aqueous silica (SiO₂) suspensions of varying solids volume fraction (φ_s) was studied. SiO₂ suspensions with low NH₄Cl concentration (≤0.05 M , pH 5.2) exhibited Newtonian behavior and a constant settling velocity ( U ). The volume fraction dependence was well described by the Richardson–Zaki form, U = U ₀(1 −φ_s) ⁿ , where n = 4.63 and U ₀= 1.0419 × 10⁻⁵ cm/s. At higher NH₄Cl concentrations (0.07–2.0 M , pH 5.2), suspensions exhibited shear thinning and more complicated sedimentation behavior due to their aggregated nature. For all suspensions studied, however, the apparent suspension viscosity, characteristic cluster size, and initial settling velocity were greatest at ∼0.5 M NH₄Cl and exhibited a similar dependence on salt concentration. Above 0.5 M NH₄Cl, considerable restabilization was observed. This behavior cannot be explained by traditional DLVO theory.     

Colloidal Processing and Ionic Conductivity of Fine-Grained Cupric-Oxide-Doped Tetragonal Zirconia

CuO-doped tetragonal ZrO₂ (3-mol%-Y₂O₃-doped tetragonal zirconia, 3Y-TZ) green bodies were consolidated from zirconia slurries with Cu²⁺ by a pressure filtration method. The slurries were prepared by dispersing 3Y-TZ powder in a solution of [NH₄OH + NH₃NO₃] = 0.1 M at pH 11 and adding an appropriate amount of Cu(NO₃)·3H₂O solution. Green bodies with narrow pore-size distribution were obtained after cold isostatically pressing the pressure-filtrated bodies. Small amounts of CuO-doped samples were densified fully at 1200°C. The size of a grain of 0.16-mol%-CuO-doped 3Y-TZ sintered at 1200°C was 84 nm. Bulk and grain-boundary conductivities are measured by a complex impedance method. The bulk conductivity of the CuO-doped 3Y-TZ was almost equal to the undoped one, but the grain-boundary conductivity decreased with CuO addition.     

Improvement of Hydroxyapatite Sol–Gel Coating on Titanium with Ammonium Hydroxide Addition

Small amounts (1–5 vol%) of ammonium hydroxide (NH₄OH) were added to the ethanol–water-based hydroxyapatite (HA) sol–gel solution. After aging the sol, a Ti substrate was dip-coated and heat-treated at 500°C for 1 h in air. The sol properties were monitored in terms of pH, viscosity, and structure changes with aging time; also, the coating phase and structure on Ti were investigated. During aging, the viscosity of the sol increased while the pH decreased, confirming the polymerization and gelation of the sol. The addition of NH₄OH altered the sol properties significantly. Increase in the NH₄OH concentration increased the pH and viscosity of the sol considerably, especially at short aging period, followed by a saturation with further aging. The Fourier transformed-infrared analysis also confirmed a gradual structure change of the sol with NH₄OH addition. Such changes in pH, viscosity, and structure bands of the sol driven by the NH₄OH addition were attributed to the improved polymerization and gelation of the sol. The improved gelation shortened the aging time needed for crystallization of the apatite coating, i.e., increase in the NH₄OH addition improved the crystallization of the coatings. Within 24 h aging time, the coatings on Ti containing over 3.5 vol% NH₄OH showed characteristic HA phase and structure bands, at which period no apatite peaks were observed on the HA coating obtained without NH₄OH addition.     

Biomimetic Preparation and Characterization of Bioactive Coatings on Alumina and Zirconia Ceramics

For the preparation of bioactive coatings on alumina and zirconia ceramic surfaces a fast biomimetic method using a supersaturated solution containing Na⁺, Ca²⁺, Cl⁻, HCO₃⁻, and PO₄³⁻ ions was used. The coatings were analysed with the use of an X-ray diffraction spectrometer and a transmission electron microscope equipped with an energy-dispersive spectroscopy detector. After the precipitation both coatings were composed of poorly crystallized, nanosized, plate-like particles with the octacalcium phosphate (OCP) crystal structure. The adhesion of the coatings was improved by a heat treatment at 1050°C for 1 h. During this heat treatment the calcium phosphate layer, deposited from a supersaturated solution onto the surface of the substrates, was sintered to form a dense coating. At the same time the OCP crystal structure was transformed into that of hydroxyl apatite, the coating's crystallinity was increased, and the particles grew isotropically up to 300 nm in size. The bioactivity of the coated ceramic was confirmed before and after the heat treatment using a simple simulated body fluid test.     

Electrochemical Properties of Hydroxyapatite Crystal Surfaces on Anatase Photocatalysts

Hydroxyapatite (HAp) was crystallized on anatase titanium dioxide (TiO₂) photocatalytic crystals or their thin films using of a pseudo-body solution method, and electrochemical properties of the HAp-adhered anatase TiO₂ photocatalytic surfaces were discussed. Decomposition rates of methylene blue were faster for the HAp-adhered anatase TiO₂ photocatalysts, although specific surface areas were smaller than those for the commercial anatase TiO₂ ones. Surface potential dispersions on the HAp-adhered anatase TiO₂ thin films before and after an ultraviolet light irradiation were measured by an atomic force microscopy. Changes in the color of leucocrystalviolet mixed in the HAp-adhered anatase TiO₂ photocatalysts or the commercial anatase TiO₂ ones with experimental durations were compared. Movements of electrons from the anatase TiO₂ photocatalytic surfaces to the HAp, crystals and oxidizing reactions on the HAp-adhered anatase TiO₂ photocatalytic surfaces in the dark were discussed on the basis of these experimental results.     

Cation-Exchange Characteristics of Sintered Hydroxyapatite in the Strongly Acidic Region

The ion-exchange behavior of hydroxyapatite (Ca₁₀(PO₄)₆-OH)₂) (HAp), fired at high temperatures, has been investigated in the strongly acidic region (pH 2 and 3). According to the present study, HAp fired above 1000°C can exchange ions from Ca²⁺ to Pb²⁺, even at pH 2, without dissolution. The molar ratios of Pb²⁺/Ca²⁺ after ion exchange are approximately unity in all cases. Ion exchange occurs in a thin layer near the surface of HAp particles fired at 1300°C. After ion exchange, Pb-Cl-apatite crystals are created in the strongly acidic region (pH 2).     

Biomimetic apatite coating on P(EMA-co-HEA)/SiO2 hybrid nanocomposites

P(EMA-co-HEA)/SiO₂ nanocomposites with silica contents in the range of 0-30 wt% were prepared by co-polymerization of the organic monomers during the simultaneous sol-gel polymerization of the silica precursor. The ability of the hybrids to form hydroxyapatite (HAp) on their surfaces was tested in vitro by soaking the samples in a simulated body fluid (SBF) solution for different times up to 35 days. On the one hand, the composition and morphology of the HAp layer formed were characterized by SEM, EDS, FTIR and XRD; on the other, the exchange of soluble silicates and calcium and phosphate ions, and the structural changes taking place in the nanohybrids when immersed in SBF were analyzed by SEM/EDS. This is, up to our knowledge, the first time the HAp nucleation mechanism has been proposed for organic-silica nanohybrids and correlated with their respective nanostructures. The results revealed that the formation of a HAp coating was in all cases limited by the nucleation induction time, but the mechanism and rate of HAp nucleation were found to be different depending on the nanostructure of the samples, which differs, in turn, with the silica content as a consequence of the differing connectivity of the silica network. The nanohybrids with silica contents in the range of 10-20 wt% proved to be the most suitable for the development of bioactive synthetic scaffolds for bone or other mineralized tissues.     

Stabilization of Ethanol-Based Alumina Suspensions

Al₂O₃ powders have been successfully dispersed in ethanol by varying the suspension acidity. An operational pH (O.pH) was defined to measure the acidity of these ethanol-based suspensions. The isoelectric point of Al₂O₃ in ethanol was at an O.pH of 8. According to Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, suspensions between an O.pH of 3.5–10.5 possessed only attractive inter-particle potential. Suspensions below 3.5 had high ζ potential, fine particle size, and were Newtonian. However, suspensions at high pH were shear thinning and consisted of agglomerates, despite their high ζ potential. The use of citric acid as a dispersant has also been investigated. At an O.pH of 3, optimum additions of citric acid between 0.6 and 1.0 wt% decreased the particle size, resulted in repulsive inter-particle potentials and increased the solid loading capacity to 15 vol% from 2 vol% while maintaining Newtonian behavior and similar viscosity to suspensions at O.pH 2. Addition of citric acid created agglomerated suspensions that were negatively charged at O.pH 10.5 (obtained by adding NH₄OH), but positively charged suspensions at O.pH 13.6 (obtained by adding tetramethylammonium hydroxide).     

Preparation of Titanium Nitride Films from Amide Precursors Synthesized by Electrolysis

A TiN precursor solution was synthesized by galvanostatic electrolysis of Ti metal and isopropylamine at a current density of 50 mA·cm⁻² at room temperature. TiN films were prepared by dip-coating of the precursor solution on a Si wafer, followed by two-stage heat treatment at 400°C and a fixed temperature of 800–1200°C in flowing N₂, N₂/NH₃, or NH₃ gas. The TiN films were characterized by XRD, chemical analysis, XPS, and electrical resistivity measurements. The TiN films were composed of uniform grains 20 to 200 nm in size with thicknesses ranging from 300 to 400 nm at temperatures of 800–1200°C. The effect of the heat treatment atmosphere (N₂ and NH₃) on the impurity content, crystallinity, particle size, and electrical resistivity is discussed.     

Growth of a Bonelike Apatite Layer on a Substrate by a Biomimetic Process

A dense and uniform bonelike apatite layer was formed on a substrate by the following biomimetic method. The substrate was first placed on granular particles of CaO, SiO₂-based glass soaked in a simulated body fluid (SBF) with ion concentrations nearly equal to those of human blood plasma and then soaked in another fluid highly supersaturated with respect to the apatite. The thickness of the apatite layer increased in proportion to the soaking time in the second solution. The rate of increase in the thickness of the apatite layer increased from 0.5 to 7.0 μm/day with increasing temperature of the second solution from 10° to 60°C, increased from 0.15 to 1.7 μm/day with increasing ion concentrations of the second solution from 0.2 to 1.5 times those of the SBF, and doubled by shaking the second solution. These results indicate that the growth of the apatite layer is controlled by mass transport across the interface between the crystal and the fluid.     

Stabilization and Processing of Aqueous BaTiO3 Suspension with Polyacrylic Acid

Polyacrylic acid (PAA) has been chosen to stabilize BaTiO₃ powder in aqueous solution. Zeta potential studies show that the Ba-rich particle surface is positively charged in the pH range from 2 to 11 without PAA. Adsorption of PAA decreases the zeta potential in acidic solution and changes the surface to a negative charge in basic solution. Adsorption of PAA onto BaTiO₃ surfaces is found to be related to the pH-dependent PAA conformation in solution. The amount of PAA adsorbed at pH 1.5 is one order of magnitude more than at pH 10.5, due to a more compact polymer conformation. Colloid stability of aqueous BaTiO₃ suspension is related to the dissociation of PAA as a function of pH. BaTiO₃ suspensions can be stabilized at pH values higher than 7, where more than 99% of carboxylic groups are ionized. At pH 10.5, suspension stability is related to the PAA coverage on BaTiO₃ particle surface. Stabilization can be achieved only when conditions of both PAA ionization and BaTiO₃ surface coverage are satisfied, suggesting an electrosteric stabilization mechanism. Green density of 62% can be achieved by slip casting at a pH above 10, compared to the best density of 58.6% by isostatic pressing at 242 MPa.     

Synthesis of Anatase-Type TiO2 Nanocrystallites Via a Redox Route

Anatase nanocrystallites showing high surface area (∼62 m²/g) and good photocatalytic property have been obtained by pyrolyzing at 600°C for 4 h an ammonium titanyl double sulfate precursor (α-(NH₄)₂TiO(SO₄)₂) synthesized via a redox approach, that is, by oxidizing an aqueous solution of titanium trichloride (TiCl₃) with ammonium peroxodisulfate ((NH₄)₂S₂O₈), followed by reacting with ammonium sulfate ((NH₄)₂SO₄).