Preparation of hollow hydroxyapatite microspheres

The preparation of hollow hydroxyapatite (HA) microspheres as potential drug-delivery vehicles was investigated. A lithium-calcium-borate (10Li₂O-15CaO-75B₂O₃) (mol%) glass, made by fusing the components at 1100°C for 1 h, was ground to a powder and passed through a flame at ∼1400°C to spheroidize the particles. The resulting glass microspheres (106–125 μm in diameter) were reacted in 0.25 M K₂HPO₄ solution for 5 days at 37°C and pH 10–12, resulting in the formation of porous, hollow microspheres of a calcium phosphate (Ca-P) material with external diameters similar to those of the original glass particles. Heat treatment at 600°C for 4 h partially converted the Ca-P material to HA, as confirmed by X-ray diffraction, and also increased the strength of the hollow microspheres.     

Evaluation of hydroxyapatite microspheres made from a borate glass to separate protein mixtures

A hydroxyapatite (HA, Ca₁₀(PO₄)₆(OH)₂), transformed from a calcium-containing borate glass, has been investigated for its protein adsorption and chromatographic characteristics. Microspheres of the borate glass were transformed into HA by reacting them with a 0.25 M phosphate (K₂HPO₄) solution for 24 h at 37 °C (pH 9.0). The HA microspheres with a diameter of 45–90 μm were hand packed into a steel column (4.6 mm × 80 mm) and used to separate a binary protein mixture of bovine serum albumin (BSA) and lysozyme. HA microspheres, with a diameter <45 μm, were used for separating a protein mixture of BSA, myoglobin, and lysozyme. These microspheres had a diameter that was 20–30 times larger than commercial HA column packing spherical particles, 2–3 μm, but these microspheres had a six times larger surface area and a more uniform spherical shape. These advantages compensated for their larger size and the separation results were comparable to those commercially available HA columns in the separation of the proteins studied. These unique HA microspheres, made from microspheres of a borate glass, are considered to be useful as packing materials for protein separation in chromatography.     

Formation of water-soluble and biocompatible TOPO-capped CdSe quantum dots with efficient photoluminescence

Hollow hydroxyapatite (HA) microspheres (diameter = 100–800 μm) were prepared by reacting solid Li₂O–CaO–B₂O₃ glass spheres in 0.25 M K₂HPO₄ solution at 37°C. The influence of subsequent heating on the microstructure, surface area, and compressive strength of the HA microspheres was evaluated using scanning electron microscopy, the BET method, and nano-mechanical testing. The surface area and rupture strength of the as-prepared microspheres were 135 m²/g and 1.6 ± 0.6 MPa, respectively. On heating for 8 h at 600°C, the surface area decreased to 27 m²/g, but there was no increase in the compressive strength (1.7 ± 0.4 MPa). Heating to 800°C (8 h) resulted in a marked decrease in the surface area (to 2.6 m²/g) and a sharp increase in the compressive strength (to >35 ± 8 MPa). These hollow HA microspheres may be useful as devices for drug or protein growth factor delivery or as scaffolds for engineered tissues.     

Evaluation of BSA protein release from hollow hydroxyapatite microspheres into PEG hydrogel

Implants that simultaneously function as an osteoconductive matrix and as a device for local drug or growth factor delivery could provide an attractive system for bone regeneration. In our previous work, we prepared hollow hydroxyapatite (abbreviated HA) microspheres with a high surface area and mesoporous shell wall and studied the release of a model protein, bovine serum albumin (BSA), from the microspheres into phosphate-buffered saline (PBS). The present work is an extension of our previous work to study the release of BSA from similar HA microspheres into a biocompatible hydrogel, poly(ethylene glycol) (PEG). BSA-loaded HA microspheres were placed in a PEG solution which was rapidly gelled using ultraviolet radiation. The BSA release rate into the PEG hydrogel, measured using a spectrophotometric method, was slower than into PBS, and it was dependent on the initial BSA loading and on the microstructure of the microsphere shell wall. A total of 35–40% of the BSA initially loaded into the microspheres was released into PEG over ~ 14 days. The results indicate that these hollow HA microspheres have promising potential as an osteoconductive device for local drug or growth factor delivery in bone regeneration and in the treatment of bone diseases.     

Long-term conversion of 45S5 bioactive glass–ceramic microspheres in aqueous phosphate solution

The conversion of 45S5 glass and glass–ceramics to a hydroxyapatite (HA)-like material in vitro has been studied extensively, but only for short reaction times (typically <3 months). In this paper, we report for the first time on the long-term conversion of 45S5 glass–ceramic microspheres (designated 45S5c) in an aqueous phosphate solution. Microspheres of 45S5c (75–150 μm) were immersed for 10 years at room temperature (~25 °C) in K₂HPO₄ solution with a concentration of 0.01 M or 1.0 M, and with a starting pH of 7.0 or 9.5. The reacted 45S5c microspheres and solutions were analyzed using structural and analytical techniques. Only 25–45 vol% of the 45S5c microspheres were converted to an HA-like material after the 10 year reaction. In solutions with a starting pH of 9.5, an increase in the K₂HPO₄ concentration from 0.01 to 1.0 M resulted in a doubling of the volume of the microspheres converted to an HA-like material but had little effect on the composition of the HA-like product. In comparison, reaction of the 45S5c microspheres in the solution with a starting pH of 7.0 resulted in an HA-like product in the 0.01 M K₂HPO₄ solution but a calcium pyrophosphate product, Ca₁₀K₄(P₂O₇)₆.9H₂O, in the 1.0 M solution. The consequences of these results for the long-term use of 45S5 glass–ceramics in biomedical applications are discussed.     

Kinetics and mechanisms of converting bioactive borate glasses to hydroxyapatite in aqueous phosphate solution

Borate bioactive glasses are receiving increasing attention as scaffold materials for bone repair and regeneration. In this study, the kinetics and mechanisms of converting three groups of sodium–calcium–borate glasses with varying CaO:B₂O₃ ratio to hydroxyapatite (HA) in 0.25 M K₂HPO₄ solution were investigated at 10–70 °C. Glass disks with the composition 2Na₂O·(2 − x)CaO·(6 + x)B₂O₃ (x = 0, 0.5, and 1.0) were immersed for up to 8 days in the potassium phosphate solution. The conversion kinetics to HA were monitored by measuring the weight loss of the glass, while X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy were used to study structural and compositional changes. All three groups of glasses formed HA on their surfaces, showing that the glasses were bioactive. At 10–37 °C, the conversion kinetics was well fitted by the contracting sphere model. Also, the contracting sphere model has a good fit for the early stage of conversion at 70 °C, whereas a three-dimensional (3D) diffusion model provided a good fit to the data of the later stage. The results of this study provide kinetic and structural data for the design of borate bioactive glasses for potential applications in bone tissue engineering.     

Reaction of sodium calcium borate glasses to form hydroxyapatite

This study investigated the transformation of two sodium calcium borate glasses to hydroxyapatite (HA). The chemical reaction was between either 1CaO · 2Na₂O · 6B₂O₃ or 2CaO · 2Na₂O · 6B₂O₃ glass and a 0.25 M phosphate (K₂HPO₄) solution at 37, 75 and 200 °C. Glass samples in the form of irregular particles (125–180 μm) and microspheres (45–90 and 125–180 μm) were used in order to understand the reaction mechanism. The effect of glass composition (calcium content) on the weight loss rate and reaction temperature on crystal size, crystallinity and grain shape of the reaction products were studied. Carbonated HA was made by dissolving an appropriate amount of carbonate (K₂CO₃) in the 0.25 M phosphate solution. X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy were used to characterize the reaction products. The results show that sodium calcium borate glasses can be transformed to HA by reacting with a phosphate solution. It is essentially a process of dissolution of glass and precipitation of HA. The transformation begins from an amorphous state to calcium-deficient HA without changing the size and shape of the original glass sample. Glass with a lower calcium content (1CaO · 2Na₂O · 6B₂O₃), or reacted at an elevated temperature (75 °C), has a higher reaction rate. The HA crystal size increases and grain shape changes from spheroidal to cylindrical as temperature increases from 37 to 200 °C. Increase in carbonate concentration can also decrease the crystal size and yield a more needle-like grain shape.     

Effect of pyrophosphate ions on the conversion of calcium–lithium–borate glass to hydroxyapatite in aqueous phosphate solution

The conversion of glass to a hydroxyapatite (HA) material in an aqueous phosphate solution is used as an indication of the bioactive potential of the glass, as well as a low temperature route for preparing biologically useful materials. In this work, the effect of varying concentrations of pyrophosphate ions in the phosphate solution on the conversion of a calcium–lithium–borate glass to HA was investigated. Particles of the glass (150–355 μm) were immersed for up to 28 days in 0.25 M K₂HPO₄ solution containing 0–0.1 M K₄P₂O₇. The kinetics of degradation of the glass particles and their conversion to HA were monitored by measuring the weight loss of the particles and the ionic concentration of the solution. The structure and composition of the conversion products were analyzed using X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy. For K₄P₂O₇ concentrations of up to 0.01 M, the glass particles converted to HA, but the time for complete conversion increased from 2 days (no K₄P₂O₇) to 10 days (0.01 M K₄P₂O₇). When the K₄P₂O₇ concentration was increased to 0.1 M, the product consisted of an amorphous calcium phosphate material, which eventually crystallized to a pyrophosphate product (predominantly K₂CaP₂O₇ and Ca₂P₂O₇). The consequences of the results for the formation of HA materials and devices by the glass conversion route are discussed.     

Facile fabrication of nano-hydroxyapatite/silk fibroin composite via a simplified coprecipitation route

Nano-hydroxyapatite/silk fibroin (n-HA/SF) composite was successfully fabricated based on a simplified coprecipitation route. In detail, the degummed SF was first dissolved in CaCl₂ aqueous/ethanol solution without desalting procedure, and then (NH₄)₂HPO₄ solution and NH₄OH were dropped into the above solution to form n-HA/SF composite. The structure and morphology of n-HA/SF composite were investigated by Transmission electron microscopy, Fourier transform infrared spectrometry, energy dispersive X-Ray spectrum, X-ray diffraction, and thermogravimetric analyses. Results indicate that the inorganic phase is carbonate-substituted HA with low crystallinity and similar to the crystals of human bone. The HA crystals have diameter of around 20–30 nm and length of about 200–500 nm. The content of SF in the composite is about 30%, and the two phases bonded each other strongly. In addition, a formation mechanism of n-HA/SF was proposed.     

Sodium silicate bonded borate glass scaffolds for tissue engineering

Borate and silicate glass particles and microspheres with size distributions in the range of approximately 100–400 micron were loosely compacted and bonded by sodium silicate solution to prepare resorbable, porous glass constructs with porosity 30–50%. Conversion of the binding borate glass to hydroxyapatite was investigated by measuring the weight loss of the constructs in a solution of 0.25 M K₂HPO₄ with a pH value of 9.0 at 37 °C, as a function of time. Almost full conversion of the borate glass to hydroxyapatite was achieved in less than 6 days. X-ray diffraction revealed an initially amorphous product that subsequently crystallized to hydroxyapatite.     

Preparation of hollow hydroxyapatite microspheres by the conversion of borate glass at near room temperature

Hollow hydroxyapatite microspheres, consisting of a hollow core and a porous shell, were prepared by converting Li₂O-CaO-B₂O₃ glass microspheres in dilute phosphate solution at 37 °C. The results confirmed that Li₂O-CaO-B₂O₃ glass was transformed to hydroxyapatite without changing the external shape and dimension of the original glass object. Scanning electron microscopy images showed the shell wall of the microsphere was built from hydroxyapatite particles, and these particles spontaneously align with one another to form a porous sphere with an interior cavity. Increase in phosphate concentration resulted in an increase in the reaction rate, which in turn had an effect on shell wall structure of the hollow hydroxyapatite microsphere. For the Li₂O-CaO-B₂O₃ glass microspheres reacted in low-concentration K₂HPO₄ solution, lower reaction rate and a multilayered microstructure were observed. On the other hand, the glass microspheres reacted in higher phosphate solution converted more rapidly and produced a single hydroxyapatite layer. Furthermore, the mechanism of forming hydroxyapatite hollow microsphere was described.     

Characterization of self-organized TiO<Subscript>2</Subscript> nanotubes on Ti–4Zr–22Nb–2Sn alloys and the application in drug delivery system

In this study, the self-organized TiO₂ nanotubes grown by anodization of Ti–4Zr–22Nb–2Sn at different potentials, concentration of NH₄F and anodization time was investigated. The morphology of nanotubes was observed by FE-SEM. The drug-loaded nanotubes were also fabricated in aqueous media containing minocycline hydrochloride. They were characterized by SEM, XPS and FT-IR. The results showed that the drug of minocycline hydrochloride (MH) was loaded in the nanotubes. The release effects were studied in phosphate buffer solution (PBS). The release rate of MH from TiO₂ nanotubes with shorter tube length in PBS was lower than the one of MH from longer nanotubes. The sustaining release time could last at least 150 h. Hence, it is a promising method to eliminate the harmful reactions by carrying drug in the tubes when the titanium alloys were used as biomedical implants.     

Template-free hydrothermal synthesis of hollow hematite microspheres

Hollow hematite (α-Fe₂O₃) microspheres with an average diameter of 3-4 μm and a shell thickness of approximate 150 nm was synthesized by a simple hydrothermal route using FeCl₃·6H₂O solution and acetic acid without using any templates. The hollow microspheres were composed of α-Fe₂O₃ nanoparticles with the diameter range from 20 to 40 nm. The effects of reaction parameters such as reaction time, temperature, concentration of FeCl₃·6H₂O solution, and initial pH on the morphology of the final products were investigated. A possible formation mechanism of hollow α-Fe₂O₃ microspheres was also proposed, where the acetic acid played a role of etching in the formation of hollow structure.     

Photocatalytic activities of hollow and hierarchical Bi<Subscript>3.15</Subscript>Nd<Subscript>0.85</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> microspheres synthesized through a hydrothermal process

Hollow and hierarchical Bi_3.15Nd_0.85Ti₃O₁₂ (BNdT) microspheres of 0.5–1.2 μm in diameter were synthesized through a hydrothermal process. Each hollow microsphere is constructed across by many single-crystalline BNdT nanoplates with in-plane dimension of ~400 nm. The BNdT nanoplates are of layered perovskites. The UV–visible absorption characteristics demonstrated that the band gap of the BNdT microspheres is 3.33 eV. The hierarchical microspheres exhibit significant photocatalytic activity. Up to 75% methyl orange was decolorized after UV irradiation for 210 min, whereas lower than 10% methyl orange decolorized using BNdT powders prepared from single crystals as catalyst. The photocatalytic decolorization of methyl orange solution is a pseudo-first-order reaction and its kinetics can be expressed as ln(C/C ₀) = kt.     

Preparation and characterization of hollow hydroxyapatite microspheres by spray drying method

Hollow hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) microspheres were prepared using a simple spray drying method. The incorporation of ammonium bicarbonate could produce carbon dioxide and ammonia gas bubbles during the spraying, and thus created a hollow inner structure in the resultant microspheres. The hollow microspheres prepared using different amounts of ammonium bicarbonate were also characterized. These microspheres were composed of nanoparticles with an average crystallite size of 15 nm. A high surface area (80 m²/g) and porosity of the microspheres could be achieved when the concentration of ammonium bicarbonate was about 5 wt.%. Fourier transform infrared results showed that CO₃^2? was incorporated into the HA microspheres. These hollow microspheres have many potential uses such as injectable drug-delivery carriers.     

Bioactive borate glass scaffolds: in vitro and in vivo evaluation for use as a drug delivery system in the treatment of bone infection

The objective of this work was to evaluate borate bioactive glass scaffolds (with a composition in the system Na₂O–K₂O–MgO–CaO–B₂O₃–P₂O₅) as devices for the release of the drug Vancomycin in the treatment of bone infection. A solution of ammonium phosphate, with or without dissolved Vancomycin, was used to bond borate glass particles into the shape of pellets. The in vitro degradation of the pellets and their conversion to a hydroxyapatite-type material in a simulated body fluid (SBF) were investigated using weight loss measurements, chemical analysis, X-ray diffraction, and scanning electron microscopy. The results showed that greater than 90% of the glass in the scaffolds degraded within 1 week, to form poorly crystallized hydroxyapatite (HA). Pellets loaded with Vancomycin provided controlled release of the drug over 4 days. Vancomycin-loaded scaffolds were implanted into the right tibiae of rabbits infected with osteomyelitis. The efficacy of the treatment was assessed using microbiological examination and histology. The HA formed in the scaffolds in vivo, resulting from the conversion of the glass, served as structure to support the growth of new bone and blood vessels. The results in this work indicate that bioactive borate glass could provide a promising biodegradable and bioactive material for use as both a drug delivery system and a scaffold for bone repair.     

Hydroxyapatite formation from cuttlefish bones: kinetics

Highly porous hydroxyapatite (Ca₁₀(PO₄)₆·(OH)₂, HA) was prepared through hydrothermal transformation of aragonitic cuttlefish bones (Sepia officinalis L. Adriatic Sea) in the temperature range from 140 to 220°C for 20 min to 48 h. The phase composition of converted hydroxyapatite was examined by quantitative X-ray diffraction (XRD) using Rietveld structure refinement and Fourier transform infrared spectroscopy (FTIR). Johnson–Mehl–Avrami (JMA) approach was used to follow the kinetics and mechanism of transformation. Diffusion controlled one dimensional growth of HA, predominantly along the a-axis, could be defined. FTIR spectroscopy determined B-type substitutions of CO₃ ²⁻ groups. The morphology and microstructure of converted HA was examined by scanning electron microscopy. The general architecture of cuttlefish bones was preserved after hydrothermal treatment and the cuttlefish bones retained its form with the same channel size (~80 × 300 μm). The formation of dandelion-like HA spheres with diameter from 3 to 8 μm were observed on the surface of lamellae, which further transformed into various radially oriented nanoplates and nanorods with an average diameter of about 200–300 nm and an average length of about 8–10 μm.     

Aqueous co-precipitation of Pd-doped cerium oxide nanoparticles: chemistry, structure, and particle growth

Nanoparticles of palladium-doped cerium oxide (Pd–CeO₂) have been prepared by aqueous co-precipitation resulting in a single phase cubic structure after calcination according to X-ray diffraction (XRD). Inhomogeneous strain, calculated using the Williamson–Hall method, was found to increase with palladium content, and the lattice contracts slightly, relative to nano-cerium oxide, as palladium content is increased. Moreover, high resolution transmission electron microscopy reveals some instances of defective microstructure. These factors combined imply that palladium is in solid solution with CeO₂ in these nanoparticles, but palladium (II) oxide (PdO) peaks in the Raman spectra indicate that solid solution formation is partial and that highly dispersed PdO is present as well as the solid solution. Nevertheless, the addition of palladium to the CeO₂ lattice inhibits the growth of the 6% Pd–CeO₂ particles compared to pure CeO₂ between 600 and 850 °C. Activation energies for grain growth of 54 ± 7 and 79 ± 8 kJ/mol were determined for 6% Pd–CeO₂ and pure CeO₂, respectively, along with pre-exponential Arrhenius factors of 10 for the doped sample and 600 for pure cerium oxide.     

Sustained release optimized formulation of anastrozole-loaded chitosan microspheres: in vitro and in vivo evaluation

The purpose of this study was to develop sustained release formulation of anastrozole-loaded chitosan microspheres for treatment of breast cancer. Chitosan microspheres cross-linked with two different cross-linking agents viz, tripolyphosphate (TPP) and glutaraldehyde (GA) were prepared using single emulsion (w/o) method. A reverse phase HPLC method was developed and used for quantification of drug in microspheres and rat plasma. Influence of cross-linking agents on the properties of chitosan microspheres was extensively investigated. Formulations were characterized for encapsulation efficiency (EE), compatibility of drug with excipients, particle size, surface morphology, swelling capacity, erosion and drug release profile in phosphate buffer pH 7.4. EE varied from 30.4 ± 1.2 to 69.2 ± 3.2% and mean particle size distribution ranged from 72.5 ± 0.5 to 157.9 ± 1.5 μm. SEM analysis revealed smooth and spherical nature of microspheres. TPP microspheres exhibited higher swelling capacity, percentage erosion and drug release compared to GA microspheres. Release of anastrozole (ANS) was rapid up to 4 h followed by slow release status. FTIR analysis revealed no chemical interaction between drug and polymer. DSC analysis indicated ANS trapped in the microspheres existed in amorphous form in polymer matrix. The highest correlation coefficients (R ²) were obtained for Higuchi model, suggesting a diffusion controlled mechanism. There was significant difference in the pharmacokinetic parameters (AUC_0−∞, Kel and t_1/2) when ANS was formulated in the form of microspheres compared to pure drug. This may be attributed to slow release rate of ANS from chitosan microspheres and was detectable in rat plasma up to 48 h which correlates well with the in vitro release data.     

Hydrogen peroxide versus water synthesis of bioglass–nanocrystalline hydroxyapatite composites

This article reports a comparison of the structural and textural properties of bioglass–hydroxyapatite (HA) composites obtained in the SiO₂–CaO–P₂O₅ system by sol–gel method, with different amounts of hydrogen peroxide (3% H₂O₂) or water (H₂O). X-ray diffraction, Raman, and FT-IR spectroscopy reveal the presence of nanocrystalline HA. Scanning electron microscopy images illustrate that the HA phase is mainly distributed on the glass surface. The results point out that the sintering at 550 °C of a sol–gel derived SiO₂–CaO–P₂O₅ bioglass leads to a single crystalline phase of HA, and validate a new processing method for obtaining bioglass–HA composites. Structural analyses of the investigated composites indicate the existence of a silicate network built up from Q₃ and Q₂ units. The replacement of water with hydrogen peroxide has as consequence the increase of depolymerization degree of silica network. Textural properties were investigated with N₂-adsorption technique. The composites prepared with hydrogen peroxide exhibit a more uniform and narrow mesoporous distribution that recommends them for drug uptake and release applications. It was found that the specific surface area and pore volume are clearly influenced by the H₂O₂(H₂O):TEOS molar ratio.