Dynamics of Sorption—Desorption of 223Ra Therapeutic α-Emitter on Granulated Hydroxyapatite

The dynamics of the ²²³Ra distribution in the volume of spherical hydroxyapatite (HAP) granules in the course of the ²²³Ra sorption from aqueous solutions onto sorbent particles and desorption was studied by α-track radiography. The optimum time of contact of the sample with a detector (exposure) was found, and a procedure for preparing experimental samples was suggested. Taking into account the density of the porous sorbent, the ranges of the α-particles emitted by ²²³Ra and its daughter products and of the recoil nuclei were estimated. The averaged effective range of the α-particles in HAP is ～35 μm. A mathematical model of the Ra diffusion into the depth of the porous sorbent, taking into account the sorption, was developed on the basis of the parameters obtained. The effective diffusion coefficient was estimated at ～3 × 10^–5 cm² s^–1. Correlation be¬tween the sorbent particle size, radionuclide sorption time, and the absorbed dose produced by the particle in a biological tissue was demonstrated.

     

The crystallization of Hydroxyapatite in the presence of sodium alginate

The effect of sodium alginate on the crystal growth of hydroxyapatite (HAP) was investigated at sustained supersaturation by the constant composition technique. Sodium alginate was found to inhibit HAP crystal growth at low concentrations and reduced the crystal growth rates by 42–86% for inhibitor concentrations of 2.1 × 10^{− 7}–12.6 × 10^{− 7} mol/l. The inhibition effect on the crystal growth rate may be explained possibly through adsorption onto the active growth sites. A detailed kinetics analysis suggested a Langmuir-type adsorption of the alginate on HAP surface and a value of 1.63 × 10⁷ l/mol was obtained for the affinity constant of sodium alginate for the surface of HAP. The apparent order for the crystallization reaction was determined to be approximately 2, thus suggesting a surface diffusion controlled spiral growth mechanism.     

Radionuclide sorption diagnostics of nanoagglomerates and textures based on them

Diagnostics of nanoagglomerates of hydroxyapatite Ca₁₀(PO₄)₆(OH)₂ and of hierarchic structures based on them by the method of adsorption of tritium-labeled sodium succinate is made. The adsorption kinetics is one-step in the case of hydroxyapatite nanocrystals and two-step in the case of textured hydroxyapatite. The parameters of the S-shaped sorption isotherms are calculated; they are described by the Guggenheim-Fowler-Frumkin equation. The specific surface area of hydroxyapatite nanoagglomerates is 650–700 m² g^?1, which is close to the theoretical density of individual nanocrystals (900 m² g^?1), and the specific surface area of textured hydroxyapatite (macrospheroids) is 250–300 m² g^?1. Adsorption of succinate ions on the surface of hydroxyapatite nanocrystals leads to the formation of a tightly bound monolayer, which may lead to structural rearrangement of the sorbent.     

Factors influencing the removal of divalent cations by hydroxyapatite

The effect of pH, contact time, initial metal concentration and presence of common competing cations, on hydroxyapatite (HAP) sorption properties towards Pb(2+), Cd(2+), Zn(2+), and Sr(2+) ions was studied and compared using a batch technique. The results strongly indicated the difference between the sorption mechanism of Pb(2+) and other investigated cations: the removal of Pb(2+) was pH-independent and almost complete in the entire pH range (3-12), while the sorption of Cd(2+), Zn(2+) and Sr(2+) generally increased with an increase of pH; the contact time required for attaining equilibrium was 30 min for Pb(2+) versus 24h needed for other cations; maximum sorption capacity of HAP sample was found to be an order of magnitude higher for Pb(2+) (3.263 mmol/g), than for Cd(2+) (0.601 mmol/g), Zn(2+) (0.574 mmol/g) and Sr(2+) (0.257 mmol/g); the selectivity of HAP was found to decrease in the order Pb(2+)>Cd(2+)>Zn(2+)>Sr(2+) while a decrease of pH(PZC), in respect to the value obtained in inert electrolyte, followed the order Cd(2+)>Zn(2+)>Pb(2+)>Sr(2+); neither of investigated competing cations (Ca(2+), Mg(2+), Na(+) and K(+)) influenced Pb(2+) immobilization whereas the sorption of other cations was reduced in the presence of Ca(2+), in the order Sr(2+)>Cd(2+)>or=Zn(2+). The pseudo-second order kinetic model and Langmuir isotherm have been proposed for modeling kinetic and equilibrium data, respectively. The sorption of all examined metals was followed by Ca(2+) release from the HAP crystal lattice and pH decrease. The ion exchange and specific cation sorption mechanisms were anticipated for Cd(2+), Zn(2+) and Sr(2+), while dissolution of HAP followed by precipitation of hydroxypyromorphite (Pb(10)(PO(4))(6)(OH)(2)) was found to be the main operating mechanism for Pb(2+) immobilization by HAP, with the contribution of specific cation sorption.     

Sorption of Radioactive Iodine and Fluoride Ion on Hydroxyapatite from the Aqueous Phase

Sorption of the ¹³¹I^-, ¹³¹IO₃ ^-, and F^- ions on various samples of hydroxyapatite (HAP) from aqueous solutions was studied. The HAP samples were prepared by the following reactions: Ca(OH)₂ + H₃PO₄ HAP, CACl₂ + Na₃PO₄ + NaOH HAP, and Ca(NO₃)₂ + HAP seed + (NH₄)₂HPO₄ + NH₃ HAP. None of the HAP samples sorb ionic species of radioactive iodine from aqueous solutions. As for F^-, the best sorption properties are exhibited by the HAP sample prepared by the second reaction. The degree of recovery of fluoride ion with the HAP precipitate in 5 min, 4 h, and 5 days of the contact of the solid and liquid phases is 54, 66, and >95% of the initial F^- amount, respectively. The distribution factor K _d of the fluoride ion between the HAP solid phase and solution decreases with increasing V/m ratio and pH of the initial solution.     

Sorption of cesium and strontium radionuclides onto crystalline alkali metal titanosilicates

Sorption of tracer amounts of ¹³⁷Cs and ⁹⁰Sr radionuclides from model solutions of various compositions onto synthetic titanosilicates, framework ivanyukite and layered SL3, both synthesized at the Center for Nanomaterials Science, Kola Scientific Center, Russian Academy of Sciences, was studied. Synthetic ivanyukite and titanosilicate SL3 well compete with Termoxid-25 ferrocyanide sorbent in the ability to take up cesium from neutral NaNO₃ solutions and from a simulated solution of bottom residue from a nuclear power plant with RBMK reactors. The maximal sorption of ¹³⁷Cs onto ivanyukite is observed at pH 6–7. The dependence of the ¹³⁷Cs distribution coefficient (K _d) on ivanyukite on the concentration of sodium and potassium ions in the solution was studied. Potassium ions affect the cesium sorption more strongly than sodium ions do. In the ability to take up ⁹⁰Sr, synthetic ivanyukite well competes with synthetic zeolite of type A and with the sorbent based on modified manganese dioxide. The dependences of K _d of ⁹⁰Sr on the concentrations of the Na⁺ and Ca²⁺ ions in the solution were determined. Calcium ions affect the strontium sorption more strongly than sodium ions do. Ivanyukite and SL3 show promise as sorbents for removing cesium and strontium radionuclides from multicomponent salt solutions.     

Microwave sintering improves the mechanical properties of biphasic calcium phosphates from hydroxyapatite microspheres produced from hydrothermal processing

Starting from two microspherical agglomerated HAP powders, porous biphasic HAP/TCP bioceramics were obtained by microwave sintering. During the sintering the HAP powders turned into biphasic mixtures, whereby HAP was the dominant crystalline phase in the case of the sample with the higher Ca/P ratio (HAP1) while α-TCP was the dominant crystalline phase in the sample with lower Ca/P ratio (HAP2). The porous microstructures of the obtained bioceramics were characterized by spherical intra-agglomerate pores and shapeless inter-agglomerate pores. The fracture toughness of the HAP1 and HAP2 samples microwave sintered at 1200 °C for 15 min were 1.25 MPa m^1/2. The phase composition of the obtained bioceramics only had a minor effect on the indentation fracture toughness compared to a unique microstructure consisting of spherical intra-agglomerate pores with strong bonds between the spherical agglomerates. Cold isostatic pressing at 400 MPa before microwave sintering led to an increase in the fracture toughness of the biphasic HAP/TCP bioceramics to 1.35 MPa m^1/2.     

Development of chitosan/gelatin/keratin composite containing hydrocortisone sodium succinate as a buccal mucoadhesive patch to treat desquamative gingivitis

The aim of this research was to develop chitosan/gelatin/keratin composite containing hydrocortisone sodium succinate as a buccal mucoadhesive patch to treat desquamative gingivitis, which was fabricated through an environmental friendly process. Mucoadhesive films increase the advantage of higher efficiency and drug localization in the affected region. In this research, mucoadhesive films, for the release of hydrocortisone sodium succinate, were prepared using different ratios of chitosan, gelatin and keratin. In the first step, chitosan and gelatin proportions were optimized after evaluating the mechanical properties, swelling capacity, water uptake, stability, and biodegradation of the films. Then, keratin was added at different percentages to the optimum composite of chitosan and gelatin together with the drug. The results of surface pH showed that none of the samples were harmful to the buccal cavity. FTIR analysis confirmed the influence of keratin on the structure of the composite. The presence of a higher amount of keratin in the composite films resulted in high mechanical, mucoadhesive properties and stability, low water uptake and biodegradation in phosphate buffer saline (pH?=?7.4) containing 10⁴?U/ml lysozyme. The release profile of the films ascertained that keratin is a rate controller in the release of the hydrocortisone sodium succinate. Finally, chitosan/gelatin/keratin composite containing hydrocortisone sodium succinate can be employed in dental applications.     

Development of separation technology for the removal of radium-223 from targeted thorium conjugate formulations. Part II: purification of targeted thorium conjugates on cation exchange columns

Tumor targeting pharmaceuticals will play a crucial role in future pharma pipelines. The targeted thorium conjugate (TTC) therapeutic platform could provide real benefit to patients, whereby targeting moieties like monoclonal antibodies are radiolabelled with the alpha-emitting radionuclide thorium-227 (²²⁷Th, t_1/2?=?18.7?days). A potential problem could be the accumulation of the long-lived daughter nuclide radium-223 (²²³Ra, t_1/2?=?11.4?days) in the drug product during manufacturing and distribution. Therefore, the level of ²²³Ra must be standardized before administration to the patient. The focus in this study has been the removal of ²²³Ra, as the other progenies will have a very limited stay in the formulation. In this study, the purification of TTCs labeled with decayed ²²⁷Th has been explored. Columns packed with a strong cation exchange resin have been used to sequester ²²³Ra. The separation of TTC from ²²³Ra has been evaluated as influenced by both formulation and process parameters with a design of experiments (DOE) study; including citrate or acetate buffer, pH, buffer concentration, presence or absence of pABA?+?EDTA, resin amount and sodium chloride concentration. The aim was to achieve a separation with high sorption of ²²³Ra and accompanying low TTC sorption. The results were analyzed by multivariate analysis. Four regression models of TTC and ²²³Ra sorption from citrate and acetate buffered formulations were developed. The predictive accuracy of sorption in the four statistical models was given by standard deviations and confidence intervals. The TTC sorption in citrate and acetate buffered formulations was affected by the identical variables and the variation in TTC sorption was comparable for the two models. However, the DOE variables had a significantly stronger impact on the ²²³Ra sorption in citrate buffered formulations than the ²²³Ra sorption in acetate buffer. An optimal separation with a TTC sorption below 25% and ²²³Ra sorption above 90% can be achieved in both citrate and acetate buffered formulations. Stability studies of radiochemical purity (RCP) indicated that the measured ²²⁷Th values may be partly due to free ²²⁷Th and not TTC, but the results indicate that TTC stability may be controlled by optimizing formulation parameters. Hence, the sorption data and the regression models presented must be reviewed and further explored with regard to what is known about the stability of the TTC in the different buffered formulations.     

Development of cerium and silicon co-doped hydroxyapatite nanopowder and its in vitro biological studies for bone regeneration applications

Multi-ion, co-substituted bioactive glass ceramics play a significant role in the stimulation of physical and biological properties for outstanding effects in biomedical application. The following work attempts to develop HAP as a parent material doped with a combination of cerium (Ce⁴⁺ @1.25?wt%) and silicon (Si⁴⁺ @1, 3 and 5?wt%) by refluxing based sol-gel technique. The anti-bacterial tests exhibit E. coli showing higher inhibition efficiency, in vitro hemolytic test exhibit good compatible nature of dual doped HAP with erythrocytes (<5% of hemolytic). In vitro bioactivity assay confirms that the developed dual doped HAP possesses excellent bone-like apatite layer formation on their surfaces. In vitro cellular study was performed for Ce/Si-HAP@5% powder against MG-63 cells, which demonstrated the good cell viability at higher concentrations (up to 800?µg/ml). Further, dual doped HAP powders were characterized by various analytical techniques such as ATR-FTIR, Powder-XRD, TGA-DTA, SEM-EDS, TEM and XPS analysis. The studies confirm that the synthesized dual doped HAP will act as better bioactive glass ceramics for potential orthopedic and dentistry applications.     

Adsorption of Mo on γ-Al2O3 Samples of Various Structures

Adsorption of Mo on -Al₂O₃ samples of various structures was studied with the aim of using these sorbents in production of chromatographic 99mTc generators. The sorption capacities of the oxides for Mo were determined. The optimal concentrations of HCl and HNO₃ for converting the sorbent to the H⁺ form were found. In all the cases, treatment with HCl ensures higher sorption capacity than treatment with HNO₃. The sorption capacity of aluminum oxides prepared by ac electrochemical synthesis was studied in relation to pH of sodium polymolybdate solution and sorption time. The sorption capacity of these samples for Mo is several times higher than that of the chromatographic oxide traditionally used in the generator technology.     

The influence of transition metal ions on collagen mineralization

The ions in body fluid play an important role in bone formation besides being a synthesizing material. Transition metal ions Co^2 +, Ni^2 +, Zn^2 +, Fe^3 +, Mn^2 +, Cu^2 +, Cd^2 + and Hg^2 + doped hydroxyapatite (HAP)/collagen composites were synthesized successfully in the presence of collagen traces at mild acidic pH for the first time. However, the amount of doped Hg^2 + and Cd^2 + was relatively low. Meanwhile, through soaking the collagen sponge as a template in simulated body fluid (SBF) which contains different transition metal ions (Mn^2 +, Cu^2 +, Ni^2 +, Co^2 +, Cd^2 +, Hg^2 +), bone-like HAP/collagen composites were synthesized. Hg^2 + had a certain inhibitory effect on the formation of HAP crystals on the surface of the collagen sponge while Co^2 + can promote the formation of HAP on the collagen sponge. For both HAP/collagen composites and HAP/collagen sponge, it was found that transition metal ions Mn^2 + had a significant effect on the morphology of HAP particles and could induce to form floc-like HAP particle aggregates.     

Structure reinforcement of porous hydroxyapatite pellets using sodium carbonate as sintering aid: Microstructure,secondary phases and mechanical properties

Porous HAP pellets suitable for loading therapeutic agents were prepared using microcrystalline cellulose (MCC) as pore former and sodium carbonate as sintering aid (SAID). The effect of sintering temperature on the microstructure, mechanical properties and disintegration of pellets prepared at different SAID content was studied. Pellets were characterized by SEM, image analysis, porosimetry and surface area. Secondary phases were identified by PXRD, ATR-FTIR and Raman spectroscopy. Increasing the sintering temperature decreased the diameter, porosity, surface area and friability of the pellets but increased the pore size, tensile strength and disintegration time. The effect of SAID was dependent on sintering temperature. With 5% SAID, a secondary β-tricalcium phosphate (β-TCP) phase was formed, indicated by FTIR peak at 980 cm^?1 and characteristic PXRD reflections, whereas with 10%, a secondary B-type carbonated hydroxyapatite phase (CHA) formed, indicated by FTIR peaks at 878 and 1450 cm^?1, a broad Raman peak in the region 1020 to 1050 cm^?1 and PXRD reflections. Pellets prepared with SAID showed high strength and also porosity. The biphasic HAP/β-TCP pellets exhibited remarkably great strength (4.39 MPa) at the high sintering temperature, while still retaining 43.9% porosity. Relationships were established between the mechanical properties or disintegration time of the porous pellets and the microstructural parameters.     

Reactivity of biological and synthetic hydroxyapatite towards Zn(II) ion, solid-liquid investigations

The reaction of biological and synthetic hydroxyapatite Ca₅(PO₄)₃OH (HAP) with Zn²⁺ ions is investigated as a function of Zn²⁺/Ca²⁺ molar ratio, time, temperature and electrolyte type (NaCl, NaHCO₃, Na₂HPO₄) by means of pH, pZn, pCa measurements, in aqueous solution. Biological powdered HAP invariably affords an almost quantitative reaction, while Zn²⁺ precipitated only partially by reaction with cubelets of biological HAP. Using powdered biological HAP and synthetic HAP (dried at 100 °C), the reaction with Zn²⁺ ion is fast and takes place without addition of precipitating anion; synthetic HAP (dried at 1000 °C) reacts if free phosphate ions are present. The solid phases separated after different reaction times are investigated by means of X-ray diffraction (XRD), IR, SEM techniques and elemental analysis (C,H,N). The solid phases contain Zn₃(PO₄)₂ { 4H₂O (Hopeite) at the beginning of reaction and CaZn₂(PO_{4 2} { 2H₂O (Scholzite) at the equilibrium.     

Production and characterization of pure and Cr<Superscript>3+</Superscript>-doped hydroxyapatite for biomedical applications as fluorescent probes

Pure and Cr³⁺-doped hydroxyapatite (HAP) were prepared via chemical precipitation route. The XRD measurements revealed that the typical HAP powder pattern was obtained. SEM analysis indicated that aggregates of nanoparticles were formed. EDX analysis indicates that the [Ca]/[P] concentration ratio was higher than the expected values but can be explained by the presence of carbonate groups as dopants. The optical absorption spectra of the doped samples presented absorption bands typical of Cr³⁺ occupying to different crystalline sites. From the position of the bands, it was possible to estimate the crystal field parameters for both sites of Cr³⁺ in the HAP matrix. The emission spectra of the Cr-doped samples were also investigated and typical transitions of the dopant ion, in trivalent state, were identified. The potential use of the Cr³⁺-HAP as fluorescent probes for medical applications was discussed.     

Resorbable, porous glass scaffolds by a salt sintering process

Resorbable, porous glass scaffolds for tissue engineering were prepared by sintering borate glass with salt (sodium chloride). Subsequently, the sodium chloride was dissolved in water resulting in a highly porous material. By modifying the process parameters including salt particle size, salt volume percentage, sintering temperature and sintering time, sintered matrix structures were optimized. Analysis of the structure data indicates that the 50 vol% glass—50 vol% salt with particle sizes from 250–315 μm sintered at a temperature of 520°C for 10 min resulted in an optimum structure with 76.5% porosity and 29.3 N/cm² compressive strength. The process of HAP formation on the scaffolds in 0.25 M K₂HPO₄ solutions with pH 9.0 at 37°C was evaluated. The structural changes were analyzed by X-ray diffraction and scanning electron microscopy. An amorphous phosphate was formed on the surface of the scaffolds within 1d and crystalline hydroxyapatite (HA) within 10d.     

Development of separation technology for the removal of radium-223 from targeted thorium conjugate formulations. Part I: purification of decayed thorium-227 on cation exchange columns

Targeted thorium conjugates (TTCs) are being explored as a potential future platform for specific tumor targeting pharmaceuticals. In TTCs, the alpha emitting radionuclide thorium-227 (²²⁷Th) with a half-life of 18.697 d is labeled to targeting moieties, such as monoclonal antibodies (mAbs). The amount of daughter nuclide radium-223 (²²³Ra, t_1/2?=_?11.435 d) will increase during manufacture and distribution, and so a technology for purification is required to assure an acceptable level of ²²³Ra is administrated to the patient. Since ²²³Ra is the only progeny of ²²⁷Th with a long half-life (days), the progenies of ²²³Ra will have a very limited stay in the formulation once ²²³Ra is removed. The focus in this study has, therefore, been on the removal of ²²³Ra. In this study, the sorption and separation of ²²³Ra (radium(II)) and ²²⁷Th (thorium(IV)) on cation exchange columns has been evaluated as a purification method of decayed ²²⁷Th (i.e. prior to radiolabelling of a mAb and formation of TTC). The goal is to minimize the sorption of ²²⁷Th and maximize the sorption of ²²³Ra. Statistical experimental design with formulation and process parameters, including buffered formulations comprising citrate and acetate, at various concentrations and pH, presence of free radical scavenger and chelator, and resin amount have been evaluated for impact on the purification process. The studies have been interpreted by the aid of multivariate data analysis. The correlations between design of experimental variables and sorption are summarized by regression models. The predictive accuracy of radionuclide sorption was given by standard deviation and 95% confidence intervals originating from statistical cross validation. Experimental results and statistical models for citrate-buffered formulations verified reproducible and acceptable sorption levels of ²²³Ra and ²²⁷Th under selected conditions. For acetate-buffered formulations, prediction of ²²⁷Th sorption was influenced by complex variable relationships and hence a risk of obtaining irreproducibility. Fine-tuned variable levels showed, however, variable combinations predicting high sorption of ²²³Ra (>90%) and low sorption of ²²⁷Th (<3%) also for the acetate-buffered formulations. The optimal separation conditions should be decided based on tuning the variables levels for ²²³Ra in the citrate-buffered formulations, while for acetate, the optimal separation should be based on tuning variable levels for ²²⁷Th sorption. The ionic strength of the formulation also seemed to affect the radionuclide sorption. Labeling of an antibody-chelator conjugate with purified ²²⁷Th (i.e. preparation of TTC) was successful in the selected citrate-buffered formulations tested.     

The effect of Leucine on the crystal growth of calcium phosphate

Crystallization kinetics of hydroxyapatite, HAP, in the presence of Leucine, a natural aminoacid with hydrophobic side groups, were investigated at conditions of sustained supersaturation, 37 °C, pH 7.40, ionic strength 0.15 M. In the presence of Leucine, the crystal growth rates of HAP decreased markedly. Τhis action is due to adsorption and subsequent blocking of the active growth sites onto the surface of the HAP crystals. The kinetic results revealed that a Langmuir-type adsorption isotherm is followed and an affinity constants of k _aff = 20.26 × 10² L/mol for HAP crystal surface were calculated. The apparent order of the crystal growth reaction was found to be equal to two, suggesting a surface diffusion controlled mechanism.     

Antibacterial efficacy,corrosion resistance,and cytotoxicity studies of copper-substituted carbonated hydroxyapatite coating on titanium substrate

This work elucidated the antibacterial efficacy, corrosion resistance, and cytotoxicity of electroplated copper-substituted hydroxyapatite (CuHAP) coating on titanium (Ti). The fabricated CuHAP coatings were characterized by scanning electron microscopy, energy-dispersive X-ray analysis spectroscopy, Fourier transform infrared spectroscopy, and X-ray diffraction studies. The CuHAP coating had needle-like apatite crystals, the doping of Cu²⁺ into HAP reduced porosity, and the coating became denser. The CuHAP crystals were carbonated with a few of Cu²⁺ incorporation (about 0.80 wt%). The Cu²⁺ ions were homogenously deposited into HAP films. Potentiodynamic polarisation test revealed that the CuHAP coating provided good barrier characteristics and achieved superior corrosion protection for Ti substrates. The in vitro antibacterial activity of as-prepared CuHAP coating was evaluated against Escherichia coli and was found to be effectively high against bacterial colonization. Bioactivity test conducted by soaking the coatings in simulated body fluid demonstrated that CuHAP coating can quickly induce bone-like apatite nucleation and growth. In vitro biocompatibility tests, MTT, were employed to assess the cytotoxicity of CuHAP coating with osteoblast-like MC3T3-E1 cells. The obtained HAP coating doped with a low content of Cu²⁺ exhibited good cytocompatibility and had no toxicity toward MC3T3-E1.     

Growth of fibrous hydroxyapatite in the gel system

Hydroxyapatite (HAP) was prepared in the agar gel system, where Ca²⁺ ions were incorporated in the gel and PO ₄ ^3– solution was layered over the gel. When the concentration of Ca²⁺ was lower than 1.0 M and the initial solution Ca/P molar ratio was lower than about unity, fibrous HAP several centimetres to several tens of centimetres in length was grown upwards in the PO ₄ ^3– solution from the gel phase, while at the larger Ca²⁺ concentration and initial Ca/P molar ratio CaHPO₄ · 2H₂O (DCPD) in the form of gelatinous precipitate, particulate precipitates or needle-like crystals were preferred to HAP. The fibrous HAP was calcium deficient and composed of small elongated hollow ovals linked in a zigzag row. This was considered to be formed in the following manner. First, Ca²⁺ ions supplied through pores in the gel reacted with PO ₄ ^3– ions to form a small spherulite at the pore exit, then the spherulite was bloated and elongated by the osmotic pressure or capillary force until a part of the oval was broken for Ca²⁺ ions to be gushed out into the PO ₄ ^3– solution. The above two processes were repeated to form elongated hollow ovals linked to the preceding ones.