Growth kinetics of spherulitic apatite in some MgO-CaO-SiO2-P2O5 glasses

Crystallization of glasses with compositions (wt%) of 11.2 MgO, 33.3 SiO₂, (55.5–x) CaO, and xP₂O₅ (x=18.3, 16.65, 15.825 and 15.0) resulted in a spherulitic apatite phase with different crystal morphologies. An ellipsoidal morphology was observed for x=18.3, 16.65 and 15.825, and an anomalous morphology was observed for x=15.0. A metastable phase, which was similar in some characteristics to apatite, was also found for x=15.0. The growth kinetics of the spherulitic apatite crystals were investigated to explain the above observations. Both the dendrite arms along the [0001] and [1 1¯20] directions of the apatite crystals showed constant growth rates in each glass. Growth-rate anisotropy was found between these two directions. The ellipsoidal shape of the apatite crystals is explained by this growth-rate anisotropy. The growth rates, and the growth-rate anisotropy, varied with the P₂O₅ content in such a manner that the changes in phase formation behaviour can be explained on the basis of the kinetic results.     

Fluorescence detection and imaging of amino-functionalized organic monolayer

Amino-terminated organic monolayer formed on silicon covered with native oxide (SiO₂/Si) was directly visualized under observation with fluorescent microscopy. This successful fluorescence visualization was achieved by a combination of fluorescamine method and photopatterning of the amino-terminated surface. As a typical example, an amino-terminated self-assembled monolayer (SAM) was formed on SiO₂/Si substrate in a vapor of 12.5 vol.% solution of N-(6-aminohexyl)-3-aminopropyltrimethoxysilane [H₂N(CH₂)₆NH(CH₂)₃Si(OCH₃)₃, AHAPS] diluted with absolute toluene. A micropattern of AHAPS-SAM was photolithographycally prepared using 172 nm vacuum ultraviolet (VUV) light under a reduced pressure of 10 Pa for 30 min through a photomask. The resultant micropattern composed of AHAPS- and SiOH-covered regions was provided to fluorescamine method. Due to a nonluminescence of fluorescamine itself under UV/visible irradiation, a fluorescent emission could not be observed on SiOH regions of the micropattern. In contrast, fluorescamine reacted with the outermost amino group of the AHAPS-SAM to give a fluorescent emission. A comprehensible fluorescence method for verifying formation of an amino-terminated organic monolayer has been developed.     

Compositional dependence of formation of an apatite layer on glass-ceramics in simulated physiological solution

In simulated physiological solution, an apatite layer is formed on the surface of apatite-containing glass-ceramics having the ability to bond to living bone. In this study, the influence of composition in the system CaO-P₂O₅-SiO₂-MgO, Al₂O₃ on apatite layer formation is investigated. On CaO-P₂O₅-SiO₂ glass-ceramics, an apatite layer was formed rapidly in simulated physiological solution. However, a solution containing an excess of Mg²⁺ prevented apatite layer formation. On glass-ceramics containing MgO, the amount of apatite formed on the surface decreased. An apatite layer was not formed on glass-ceramics containing Al₂O₃. The prevention of apatite layer formation on glass-ceramics containing MgO is attributed to an increase of Mg²⁺ concentration in the solution. It is thought that glass-ceramics containing Al₂O₃ form are Al₂O₃-rich layer, and that this layer prevents the formation of an apatite layer.     

Apatite formation from simulated body fluid on various phases of TiO2 thin films prepared by filtered cathodic vacuum arc deposition

Hydroxyl (OH⁻)-free TiO₂ thin films with amorphous and crystalline phases were deposited onto (100) silicon substrates using filtered cathodic vacuum arc deposition in order to investigate the in vitro apatite formation in simulated body fluid (SBF). The surface morphology, composition and structure of the TiO₂ thin films were characterized. The X-ray photoelectron spectroscopy results confirmed the presence of calcium and phosphorus on all TiO₂ thin film surfaces after immersion in SBF at 37 °C. Fourier transform infra red results showed the presence of carbonated apatite on the surface of these films. Amorphous structured TiO₂ thin film showed poor ability to form apatite on its surface in SBF. Apatite formation was more pronounced on the surfaces of the anatase films in comparison to those of rutile. The carbonated apatite deposition rate increased significantly when the TiO₂ film was illuminated with UV light prior to immersing in the SBF. In particular, the UV-treated anatase and rutile films showed increased rates of carbonated apatite formation on their surfaces in comparison to samples not treated with radiation. The increase in hydrophilicity due to UV treatment appears beneficial for the apatite growth on these surfaces.     

Improvement in crystallinity of apatite coating on titanium with the insertion of CaF2 buffer layer

In the apatite coatings on Ti the heat treatment process is necessary to crystallize the apatite structure for improved chemical stability and biological properties. However, the heat treatment normally degrades the mechanical strength of the coating layer associated with thermally induced stress. In this study, we aimed to improve the crystallization of apatite coating by using calcium fluoride (CaF₂) as a buffer layer. The insertion of a thin layer of CaF₂ (0.2–1 μm) between apatite and Ti significantly improved the crystallization behavior of apatite. Moreover, this crystallization was more enhanced as the thickness of CaF₂ was increased. When a 1 μm-thick CaF₂ was inserted, the crystallization of apatite initiated at a temperature as low as 320 °C, being a dramatic improvement in the crystallization when considering the crystallization initiation temperature of a bare apatite coating on Ti was ∼450 °C. As a result of this crystallization enhancement, the dissolution behavior of CaF₂-inserted apatite coatings was more stable than that of the bare apatite coating, showing much reduced initial-burst effect. Preliminary cellular assay showed the CaF₂-inserted apatite coating provided a substrate for cells to spread and grow favorably, as being similar to the bare apatite coating. This novel way of apatite coating on Ti using CaF₂ buffer layer may be useful in the coating systems particularly requiring low temperature processing and increased crystallinity with high chemical stability.     

Preparation of a bone-like apatite foam cement

The preparation of a porous bone-like calcium deficient apatite implant material was investigated. A novel cement system composed of an equimolar mixture of Ca₄(PO₄)₂O, Ca(H₂PO₄)₂{H₂O, and CaCO₃ was used. At a liquid/powder ratio of 0.83 ml/g low density open framework foam cements were formed due to the rapid evolution of CO₂. The initial product of the reactants was CaHPO₄{2H₂O which then reacted with Ca₄(PO₄)₂O, forming a calcium deficient carbonated apatite, upon soaking of the cement blocks in SBF. Foam-like cements were composed of a plate-like apatite due to epitaxial overgrowth and conversion of the brushite plate precursor. Cylinders of the foam cement were reinforced with an outer layer of a solid apatite cement to form a material suitable for application as a bone-section implant. © 2001 Kluwer Academic Publishers     

One-step synthesis of petal-like apatite/titania composite coating on a titanium by micro-arc oxidation

Petal-like apatite/titania (TiO₂) coating was prepared on commercially pure titanium (Ti) by micro-arc oxidation in electrolyte containing calcium and phosphate for the first time. The surface morphology, crystalline structure, chemical composition and binding state of the apatite/TiO₂ composite coating were characterized. The coating consists of a double-layer (apatite layer and TiO₂ layer) structure. The average thickness of the inner TiO₂ layer and the outer apatite layer is about 6 μm and 16 μm respectively. The outer apatite layer is porous and exhibits petal-like pattern. The apatite layer consists of hydroxyapatite (HA) and carbonate-apatite and the inner TiO₂ layer consists of anatase and rutile.     

Needle-like apatite-leucite glass-ceramic as a base material for the veneering of metal restorations in dentistry

A needle-like apatite-leucite glass-ceramic was prepared in the SiO₂-Al₂O₃-Na₂O-K₂O-P₂O₅-F system. Nucleation and crystallization processes were studied in bulk and powdered samples. The crystallization of leucite follows the mechanism of surface crystallization. After the precipitation of NaCaPO₄ crystals and another unknown crystal phase, the formation of needle-like apatite is based on a volume nucleation and crystallization process. The mechanism of the formation of needle-like apatite differs to those of apatite precipitation in glass-ceramics. The morphology of needle-like apatite is comparable to that of apatite in natural teeth.The properties of the glass-ceramic, especially the good chemical durability, the optical properties, as well as mechanical and thermal properties allow glass-ceramic to be used as a main component in a bio-material for the veneering of metal restorations in dentistry.     

A nitrogen doped TiO<Subscript>2</Subscript> layer on Ti metal for the enhanced formation of apatite

Biomedical titanium metals subjected to gas under precisely regulated oxygen partial pressures (P_O2) from 10⁻¹⁸ to 10⁵ Pa at 973 K for 1 h were soaked in a simulated body fluid (SBF), whose ion concentrations were nearly equal to those of human blood plasma, at 36.5°C for up to 7 days. The effect of oxygen partial pressures on apatite formation was assessed using X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) measurements. After heating, the weight of the oxide layer (mainly TiO₂) formed on the titanium metal was found to increase with increased oxygen partial pressure. Nitrogen (N)-doped TiO₂ (Interstitial N) was formed under a P_O2 of 10⁻¹⁴ Pa. At lower P_O2 (10⁻¹⁸ Pa), only a titanium nitride layer (TiN and Ti₂N) was formed. After soaking in SBF, apatite was detected on heat-treated titanium metal samples. The most apatite was formed, based on the growth rate calculated from the apatite coverage ratio, on the titanium metal heated under a P_O2 of 10⁻¹⁴ Pa, followed by the sample heated under a P_O2 of 10 and 10⁴ Pa (in N₂). The titanium metal heated under a P_O2 of 10⁵ Pa (in O₂) experienced far less apatite formation than the former three titanium samples. Similarly, very little weight change was observed for the titanium metal heated under a P_O2 of 10⁻¹⁸ Pa (in N₂). During the experimental observation period (5 days, 36.5°C, SBF), the following relationship held: The growth rate of apatite decreased in the order P_O2 of 10⁻¹⁴ Pa > P_O2 of 10 Pa ≥ P_O2 of 10⁴ Pa > P_O2 of 10⁵ Pa > > P_O2 of 10⁻¹⁸ Pa. These results suggest that N-doped TiO₂ (Interstitial N) strongly induces apatite formation but samples coated only with titanium nitride do not. Thus, controlling the formation of N-doped TiO₂ is expected to improve the bioactivity of biomedical titanium metal.     

Composition and structure of apatite formed on organic polymer in simulated body fluid with a high content of carbonate ion

Apatite layer was formed on polyethyleneterephthalate (PET) substrate by the following biomimetic process. The PET substrate was placed on granular particles of a CaO, SiO₂-based glass in simulated body fluid (SBF) with ion concentrations nearly equal to those of human blood plasma to form apatite nuclei on their surfaces. The apatite nuclei was then grown into a continuous layer by subsequently soaking the substrate in SBF under air or CO₂ atmosphere in which CO₂ partial pressure in the ambient was adjusted to 14.8 kPa to increase the content of carbonate ion to a level nearly equal to that of blood plasma. The increase in the content of carbonate ions in SBF changed the Ca/P atomic ratio of the apatite from 1.51 to 1.63, content of CO₃ ^2- ions from 2.64 to 4.56 wt %, and lattice constants a from 94.32 to 94.23 nm and c from 68.70 to 68.83 nm, respectively. The Ca/P ratio and lattice constants of the apatite formed in SBF under CO₂ atmosphere were approximately identical to those of bone apatite, i.e. Ca/P atomic ratio 1.65, content of CO₃ ^2- ion 5.80 wt % and lattice constants a 94.20 and c 68.80 nm. This indicates that an apatite with composition and structure nearly identical to those of bone apatite can be produced in SBF by adjusting its ion concentrations including the content of carbonate ions to be equal to those of blood plasma.     

Chemical reaction of bioactive glass and glass-ceramics with a simulated body fluid

Glass-ceramic A-W containing crystalline apatite and wollastonite in an MgO-CaO-SiO₂ glassy matrix bonds to living bone through an apatite layer which is formed on its surface in the body. The parent glass G of glass-ceramic A-W and glass-ceramic A, which has the same composition as glass-ceramic A-W but contains only the apatite, also bond to living bone through the surface apatite layer, whereas glass-ceramic A-W(Al), which contains the apatite and wollastonite in an MgO-CaO-SiO₂-Al₂O₃ glassy matrix, neither forms the surface apatite layer nor bonds to living bone. In the present study, in order to reveal the mechanism of formation of the surface apatite layer, changes in ion concentrations of a simulated body fluid with immersion of these four kinds of glass and glass-ceramics were investigated. Bioactive glass G and glass-ceramics A and A-W all showed appreciable increases in Ca and Si concentrations, accompanied by an appreciable decrease in P concentration, whereas non-bioactive glass-ceramic A-W(Al) hardly showed any element concentration change. It was speculated from these results that dissolution of the Ca(II) and Si(IV) ions from bioactive glass and glass-ceramics plays an important role in forming the apatite layer on their surfaces in the body.     

Effects of ions dissolved from bioactive glass-ceramic on surface apatite formation

Non-bioactive glass-ceramic A-W(Al) containing apatite and wollastonite in a MgO–CaO–SiO₂–Al₂O₃ glassy matrix did not form an apatite layer on its surface in a simulated body fluid with ion concentrations nearly equal to those of human blood plasma and also in the fluids with small amounts of the calcium and silicate ions added individually, but formed the apatite layer in the fluid with the calcium and silicate ions added simultaneously. This indicates that the calcium and silicate ions dissolved from bioactive glass-ceramic A-W containing the apatite and wollastonite in a MgO–CaO–SiO₂ glassy matrix play a cooperative and important role in forming an apatite layer on its surface in the body, to give the glass-ceramic bioactivity. The calcium ion might increase the degree of the supersaturation of the surrounding body fluid, and the silicate ion might provide favourable sites for nucleation of the apatite on the surfaces of glass-ceramic.     

Compositional dependence of bioactivity of glasses in the system CaO-SiO2-Al2O3: itsin vitro evaluation

In order to investigate fundamentally the effect of Al₂O₃ on the bioactivity of glasses and glass-ceramics, the compositional dependence of bioactivity of glasses in the system CaO-SiO₂-Al₂O₃ was studiedin vitro. It is already known that the essential condition for glasses and glass-ceramics to bond to living bone is the formation of an apatite layer on their surfaces in the body, and that the surface apatite layer can be reproduced even in an acellular simulated body fluid which has almost equal ion concentrations to those of the human blood plasma. In the present study, bioactivity of the glasses was evaluated by examining apatite formation on their surfaces in the simulated body fluid with thin-film X-ray diffraction, Fourier transform infrared reflection spectroscopy and scanning electron microscopic observation. Only CaO-SiO₂-Al₂O₃ glasses containing Al₂O₃ less than 1.5 mol % formed the surface apatite as well as Al₂O₃-free CaO-SiO₂ glasses, but CaO-SiO₂-Al₂O₃ containing Al₂O₃ more than 1.7 mol % did not form it as well as an SiO₂-free CaO-Al₂O₃ glass. This indicates that only a small amount of addition of Al₂O₃ to glass compositions suppresses the bioactivity of glasses and glass-ceramics by suppressing apatite formation on their surfaces in the body.     

Biomimetic coating of an apatite layer on poly(<Emphasis Type="SmallCaps">l</Emphasis>-lactic acid); improvement of adhesive strength of the coating

A biodegradable polymer coated with a bonelike apatite layer on its surface would be useful as a scaffold for bone tissue regeneration. In this study, poly(l-lactic acid) (PLLA) was treated with oxygen plasma to produce oxygen-containing functional groups on its surface. The plasma-treated specimen was then alternately dipped in aqueous CaCl₂ and K₂HPO₄·3H₂O solutions three times, to deposit apatite precursors onto the surface. The surface-modified specimen then successfully formed a dense and uniform bonelike surface apatite layer after immersion for 24 h in a simulated body fluid with ion concentrations approximately equal to those of human blood plasma. The adhesive strength between the apatite layer and the specimen surface increased as the power density of the oxygen plasma used increased. The maximum adhesive strength of the apatite layer to the specimen was significantly higher than that to the commercially available artificial bone, HAPEX^TM. The resultant bonelike apatite–PLLA composite would be useful as a scaffold for bone tissue regeneration.     

In vitro apatite formation on organic–inorganic hybrids in the CaO–SiO<Subscript>2</Subscript>–PO<Subscript>5/2</Subscript>–poly(tetramethylene oxide) system

The osteoconduction potential of artificial materials is usually evaluated in vitro by apatite formation in a simulated body fluid (SBF) proposed by Kokubo and his colleagues. This paper reports the compositional dependence of apatite formation on organic–inorganic hybrids in the CaO–SiO₂–PO_5/2–poly(tetramethylene oxide) system, initiated from tetraethoxysilane (TEOS), triethyl phosphate (OP(OEt)₃), calcium chloride (CaCl₂) and poly(tetramethylene oxide)(PTMO) modified with alkoxysilane. Formation of an apatite layer was observed on the surface of the organic–inorganic hybrids with molar ratios of TEOS/OP(OEt)₃ ranging from 100/0 to 20/80. The rate of apatite formation remarkably decreased when the hybrids were synthesized with TEOS/OP(OEt)₃ ratios of 40/60 or less. Hybrids without TEOS showed no apatite formation in SBF for up to 14 days. Addition of small amounts of OP(OEt)₃ to TEOS in the hybrids led to the high dissolution of calcium and silicate, while addition of large amounts of OP(OEt)₃ decreased the dissolution of calcium and silicate ions and resulted in reduced apatite formation regardless of the dissolution of phosphate ions from the hybrids.     

Synthesis,characterization and electrical properties of a lead sodium vanadate apatite

The lacunary lead sodium vanadate apatite Pb₈Na₂(VO₄)₆ was synthesized by the solid-state reaction method. The compound was characterized by X-ray powder diffraction, infrared (IR) absorption spectroscopy and Raman scattering spectroscopy.By comparing the effect of vanadate and phosphate ions on electrical properties, it was concluded that Pb₈Na₂(VO₄)₆ apatite is better conductor than Pb₈Na₂(PO₄)₆ apatite.     

Microwave assisted-in situ synthesis of porous titanium/calcium phosphate composites and their in vitro apatite-forming capability

Microwave irradiation has been proven to be an effective heating source in synthetic chemistry, and can accelerate the reaction rate, provide more uniform heating and help in developing better synthetic routes for the fabrication of bone-grafting implant materials. In this study, a new technique, which comprises microwave heating and powder metallurgy for in situ synthesis of Ti/CaP composites by using Ti powders, calcium carbonate (CaCO₃) powders and dicalcium phosphate dihydrate (CaHPO₄·2H₂O) powders, has been developed. Three different compositions of Ti:CaCO₃:CaHPO₄·2H₂O powdered mixture were employed to investigate the effect of the starting atomic ratio of the CaCO₃ to CaHPO₄·2H₂O on the phase, microstructural formation and compressive properties of the microwave synthesized composites. When the starting atomic ratio reaches 1.67, composites containing mainly alpha-titanium (α-Ti), hydroxyapatite (HA), beta-tricalcium phosphate (β-TCP) and calcium titanate (CaTiO₃) with porosity of 26%, pore size up to 152 μm, compressive strength of 212 MPa and compressive modulus of 12 GPa were formed. The in vitro apatite-forming capability of the composite was evaluated by immersing the composite into a simulated body fluid (SBF) for up to 14 days. The results showed that biodissolution occurred, followed by apatite precipitation after immersion in the SBF, suggesting that the composites are suitable for bone implant applications as apatite is an essential intermediate layer for bone cells attachment. The quantity and size of the apatite globules increased over the immersion time. After 14 days of immersion, the composite surface was fully covered by an apatite layer with a Ca/P atomic ratio approximately of 1.68, which is similar to the bone-like apatite appearing in human hard tissue. The results suggested that the microwave assisted-in situ synthesis technique can be used as an alternative to traditional powder metallurgy for the fabrication of Ti/CaP biocomposites.     

Effects of hydrothermal treatment with CaCl<Subscript>2</Subscript> solution on surface property and cell response of titanium implants

In order to obtain early and good osteointegration after implantation of a titanium implant in the human body, the surface modified treatments using NaOH or H₂O₂ etc. were reported. In this study, titanium was hydrothermally treated with CaCl₂ solutions at 200 ^∘C for 24hr (CaCl₂-HT). Scanning electron microscope (SEM) observation clearly showed apatite deposition on the surface of CaCl₂ HT treated titanium faster than other chemical treated titanium immersion in simulated body fluid. X-ray photoelectron spectroscopy (XPS) analysis demonstrated that Ti–O–Ca bonding was formed on titanium surface by hydrothermal treatment with CaCl₂ solution. And it was revealed that thickness of TiO₂, which was known to play important roles for the formation of bone-like apatite, became approximately three times thicker than as-polished titanium. The amount of initial attached MC3T3-E1 cells on as-polished and NaOH, H₂O₂ and this CaCl₂ HT treated titanium were almost the same values. After 5 days incubation, the growth rate of MC3T3-E1 cells on CaCl₂-HT treated titanium was significantly higher than that on other chemical treated titanium. The hydrothermal treatment with 10–20 mmol/L CaCl₂ solution at 200 ^∘C was an effective method for the fabrication of titanium implant with good bioactivity and osteoconductivity.     

Studies on synthesis of calcium ferrite-based bio glass ceramics

The possibility of synthesizing a Ca-ferrite based biocompatible glass ceramic has been explored in the following two glass compositions: (i) 28Na₂O-8CaO-3P₂O₅-llFe₂O₃-50SiO₂, and (ii) 25Na₂O-8CaO-3P₂O₅-20Fe₂O₃-41SiO₂-3B₂O₃ (in weight ratio). The effect of simulated body fluid on the different glasses and glass ceramics was also investigated. While there is no direct evidence for apatite formation, the weight losses recorded and formation of a Si-rich layer at the surface appears to be an indication of onset of apatite formation. The rate of apatite formation is presumably retarded due to the presence of Al³⁺ (picked up from AL₂O₃ crucible). Ferromagnetic resonance experiments at 9.03 GHz demonstrate that these glass ceramics can possibly be used for microwave hyperthermia.     

Effect of magnesia on structure, degradability and in vitro bioactivity of CaO-MgO-P2O5-SiO2 system ceramics

CaO-MgO-P₂O₅-SiO₂ system ceramics with various magnesia contents (0, 5, 10 and 20 mol%) were successfully prepared by sintering the sol-gel-derived powder compacts. The ceramic degradation was evaluated through the weight loss in the tris-(hydroxymethyl)-aminomethane and hydrochloric acid (Tris-HCl) buffer solution, and their ability to form apatite was determined by soaking in simulated body fluid (SBF). Results indicated that the ceramics structure was greatly influenced by magnesia contents. New crystal phases of Ca₂MgSi₂O₇ and SiO₂ were formed when magnesia was added and with an increase of magnesia concentration the phase of Ca₂MgSi₂O₇ increased with a simultaneous decline of β-CaSiO₃. In addition, studies showed that magnesia played an important role in affecting the degradability and apatite forming ability of CaO-MgO-P₂O₅-SiO₂ system ceramics. It is observed that with increasing magnesia concentration, the ceramic degradability gradually decreased and the formation of apatite on samples was delayed.