Caner Durucan

alpha-Tricalcium phosphate hydrolysis to hydroxyapatite at and near physiological temperature.

The kinetics of hydroxyapatite (HAp) formation by direct hydrolysis of α-tricalcium phosphate (α-TCP) [α-Ca3(PO4)2] have been investigated. Transformation kinetics were examined for reactions at 37 °C, 45 °C and 56 °C by isothermal calorimetric analysis. Setting times and morphologies of the resultant HAp were found to be strongly dependent on reaction temperature. XRD analysis accompanied by FTIR confirmed that phase pure calcium-deficient hydroxyapatite (CDHAp) [Ca10-x(HPO4)x(PO4)6-x(OH)2-x] was formed. Complete reaction occurs within 18, 11, 6.5 h at 37, 45 and 56 °C, respectively. The extent of HAp formation differs for particulate slurries and pre-shaped forms of reactant α-TCP. Formation of hydroxyapatite in pre-formed pellets was hindered due to limited water penetration, but enhanced with the presence of NaCl as a pore generator. Regardless of the precursor characteristics and temperature, HAp formation is characterized by an initial period of wetting of the α-TCP precursor, an induction period and a growth period during which the bulk transformation to HAp occurs. The microstructures of the resultant HAp at all temperatures were generally similar and are characterized by the formation porous flake-like morphology. Microstructural coarsening was observed for the CDHAp formed above the physiological temperature. The hardening generated by the hydrolysis reaction was demonstrated using diametrical compression tests. The original tensile strength of 56% dense α-TCP increased from 0.70±0.1 MPa to 9.36±0.4 MPa after hydrolysis to CDHAp at 37 °C, corresponding to a density of 70%. ©2000 Kluwer Academic Publishers

Low temperature formation of calcium-deficient hydroxyapatite-PLA/PLGA composites.

Hydroxyapatite-biodegradable polymer composites have been formed by a low temperature chemical route. Precomposite structures were prepared by combining alpha-Ca(3)(PO(4))(2) (alpha-tricalcium phosphate or alpha-TCP) with poly(L-lactic) acid and poly(DL-lactide-co-glycolide) copolymers. The final composite structure was achieved by in situ hydrolysis of alpha-TCP to Ca(9)(HPO(4))(PO(4))(5)OH (calcium deficient hydroxyapatite or CDHAp) either in solvent cast or pressed precomposites. Hydrolysis was performed at 56 degrees C-a temperature slightly above the glass transition of the polymers. The effects of polymer chemistry, composite formation technique, and porosity on hydrolysis kinetics and degree of transformation were examined with isothermal calorimetry, X-ray diffraction (XRD), Fourier transform infrared spectroscopy, and scanning electron microscopy. Calorimetric data and XRD analyses revealed that hydrolysis reactions were inhibited in the presence of the polymers. Isothermal calorimetry indicated the extent of the alpha-TCP to CDHAp transformation in 24 h to be 85% in the solvent cast composites containing PLGA (85:15) copolymer; however, XRD analyses suggested almost complete reaction. The CDHAp formation extent was 26% for the pressed composites containing the same polymer. In the presence of NaCl as a pore generator, 81% transformation was observed for the pressed composites. This transformation occurred without any chemical reaction between the polymer-inorganic components, as determined by Fourier transform infrared spectroscopy. Minimal transformation to CDHAp occurred in composites containing poly(L-lactic) acid.

Calcium‐deficient hydroxyapatite‐PLGA composites: Mechanical and microstructural investigation

The microstructural and mechanical properties of composites composed of calcium deficient hydroxyapatite (CDHAp) and poly(lactide-co-glycolide) (PLGA) have been investigated. The composites were formed by hydrolysis of alpha-tricalcium phosphate (alpha-TCP) to CDHAp in pressed precomposite compacts of alpha-TCP-PLGA-NaCl. The differences in hydrolysis of alpha-TCP-PLGA-NaCl for two compositions of 80:10:10 wt % and 60:20:20 wt %. were monitored by isothermal calorimetry and X-ray diffraction. The microstructural evolution and variance in final composite microstructure after hydrolysis at 37 degrees C, 45 degrees C, and 56 degrees C were examined by scanning electron microscopy. HAp-PLGA composite formed from the alpha-TCP-PLGA-NaCl (80:10:10) precomposites at 37 degrees C developed a tensile strength of 13.3 +/- 0.9 MPa, a flexural strength of 24.8 +/- 1.7 MPa, and Youngs modulus of 2.8 +/- 0.3 GPa. These values were 12.00 +/- 0.2 MPa, 36.1 +/- 2.1 MPa, and 5.5 +/- 0.8 GPa for the precomposite composition 60:20:20. All these mechanical properties showed a variation with hydrolysis temperature and composition. The differences in mechanical properties were related to the final microstructures of the composites, which are governed by the morphological changes in the polymer structure at its glass transition temperature and the extent of cement-type formation of CDHAp by hydrolysis of alpha-TCP.

Reactivity of α-tricalcium phosphate

The reactivity of α-tricalcium phosphate (α-TCP) in forming hydroxyapatite (HAp) at 37°C was investigated. The effects of synthesis route, HAp seeding and the presence of calcium salts on the mechanism and extent of HAp formation were examined by pH measurements and/or isothermal calorimetric analyses. A synthesis temperature at the lower end in the temperature range of 1100–1300°C and the reaction of α-TCP with a high specific surface area greatly improved rate and extent of HAp formation. The time for complete reaction decreased from 18 h to 14 h, when the reaction was carried out in the presence of 1 wt% of HAp seeds; the hydrolysis mechanism did not change. At HAp seeds proportion of 5 wt% and 10 wt%, transformation occurred without a nucleation period. The calcium salt additives studied were anhydrous and dihydrate form of dicalcium phosphate (CaHPO4 and CaHPO4 · 2H2O), calcium carbonate (CaCO3), and calcium sulfate hemihydrate (CaSO4 · 1/2H2O). All the additives delayed HAp formation as determined by the isothermal calorimetric analyses. Their retarding effects in decreasing order are CaCO3, CaSO4 · 1/2H2O, DCPD, DCP. CaCO3 almost completely retarded HAp formation. After 24 h, hydrolysis was complete only for pure α-TCP and for the α-TCP-DCP blend. Reaction was complete in other formulations before 48 h except for the CaCO3-containing blend. In all mixtures conversion to HAp occurred without forming any intermediates. However gypsum formed in the mixture containing CaSO4 · 1/2H2O. All the α-TCP-additive mixtures, excluding α-TCP-CaCO3, reached nominally the same strength value after 24 h of reaction as governed by the transformation of α-TCP to HAp. For phase-pure α-TCP, the average tensile strength changed from 0.36 ± 0.03 MPa to 7.26 ± 0.6 MPa. Upon hydrolysis only the CaSO4 · 1/2H2O-containing mixture exhibited slightly higher strength averaging 8.36 ± 0.9 MPa.

Effect of calcination on microstructure and antibacterial activity of silver‐containing silica coatings

Silver nanoparticle containing silica coatings on soda-lime glass were prepared by the sol-gel process. The effect of thermal densification treatment at different temperatures in the range of 100-700 degrees C on microstructure and antibacterial properties of the coatings were examined. The structural characterization of the coatings was performed by using scanning electron microscope, X-ray diffraction, UV-visible and X-ray photoelectron spectroscopy (XPS), and atomic force microscope (AFM). The antibacterial activity of the coatings was determined against Staphylococcus aureus by disk diffusion method. The mechanisms for formation and distribution of silver nanoparticles in the silica matrix with respect to the calcination temperature are discussed, and the correlation between the microstructural properties and antibacterial activity is described. The investigations revealed that silver nanoparticles were mainly in the metallic state during thermal treatments. AFM and XPS examinations proved that silver accumulated on the surface diffuse into glass substrate at higher calcination temperatures. A high level of antibacterial activity was observed for the coatings calcined at 300 degrees C or lower temperatures allowing accommodation of silver on the surface of the coating. Silver diffusion into bulk via ion-exchange with sodium and calcium ions from glass substrate during calcination at higher temperatures (500 or 700 degrees C) resulted in apparent degradation in the antibacterial activity.

Computational study of hydroxyapatite structures, properties and defects

Hydroxyapatite (HAp) was studied from a first principle approach using the local density approximation (LDA) method in AIMPRO code, in combination with various quantum mechanical (QM) and molecular mechanical (MM) methods from HypemChem 7.5/8.0. The data obtained were used for studies of HAp structures, the physical properties of HAp (density of electronic states—DOS, bulk modulus etc) and defects in HAp. Computed data confirmed that HAp can co-exist in different phases—hexagonal and monoclinic. Ordered monoclinic structures, which could reveal piezoelectric properties, are of special interest. The data obtained allow us to characterize the properties of the following defects in HAp: O, H and OH vacancies; H and OH interstitials; substitutions of Ca by Mg, Sr, Mn or Se, and P by Si. These properties reveal the appearance of additional energy levels inside the forbidden zone, shifts of the top of the valence band or the bottom of the conduction band, and subsequent changes in the width of the forbidden zone. The data computed are compared with other known data, both calculated and experimental, such as alteration of the electron work functions under different influences of various defects and treatments, obtained by photoelectron emission. The obtained data are very useful, and there is an urgent need for such analysis of modified HAp interactions with living cells and tissues, improvement of implant techniques and development of new nanomedical applications.

Preparation of hydroxyapatite-titania hybrid coatings on titanium alloy.

Hydroxypapatite-titania hybrid films on Ti6Al4V alloys were prepared by sol-gel technique by incorporating presynthesized hydroxypapatite (Ca(10)(PO(4))(6)(OH)(2) or HAp) powders into a titanium-alkoxide dip coating solution. Titania network was formed by the hydrolysis and condensation of Ti-isopropoxide Ti[OCH(CH(3))(2)](4)-based sols. The effect of titania sol formulation, specifically the effect of organic solvents on the microstructure of the dip coated films calcined at 500 degrees C has been investigated. The coatings exhibit higher tendency for cracking when a high vapor pressure solvent, such as ethanol (C(2)H(5)OH) is used causing development of higher macroscopic stresses during evaporation of the sol. Titania sol formulations replacing the solvent with n-proponal (CH(3)(CH(2))(2)OH) and acetly-acetone (C(5)H(8)O) combinations enhanced the microstructural integrity of the coating during evaporation and calcination treatments. Sol-gel processing parameters, such as multilayer coating application and withdrawal rate, can be employed to change the titania thickness in the range of 0.120-1.1 microm and to control the microstructure of HAp-titania hybrid coatings. A high-calcination temperature in the range of 400-600 degrees C does not cause a distinct change in crystals nature of the titania matrix or HAp, but results in more cracking due to the combined effect of densification originated stresses and thermal stresses upon cooling. Slower withdraw rates and multilayer dip coating lead to coatings more vulnerable to microcracking.

Exploring encapsulation mechanism of DNA and mononucleotides in sol-gel derived silica:

The encapsulation mechanism of DNA in sol-gel derived silica has been explored in order to elucidate the effect of DNA conformation on encapsulation and to identify the nature of chemical/physical interaction of DNA with silica during and after sol-gel transition. In this respect, double stranded DNA and dAMP (2′-deoxyadenosine 5′-monophosphate) were encapsulated in silica using an alkoxide-based sol-gel route. Biomolecule-encapsulating gels have been characterized using UV-Vis, 29Si NMR, FTIR spectroscopy and gas adsorption (BET) to investigate chemical interactions of biomolecules with the porous silica network and to examine the extent of sol-gel reactions upon encapsulation. Ethidium bromide intercalation and leach out tests showed that helix conformation of DNA was preserved after encapsulation. For both biomolecules, high water-to-alkoxide ratio promoted water-producing condensation and prevented alcoholic denaturation. NMR and FTIR analyses confirmed high hydraulic reactivity (water adsorption) for more silanol groups-containing DNA and dAMP encapsulated gels than plain silica gel. No chemical binding/interaction occurred between biomolecules and silica network. DNA and dAMP encapsulated silica gelled faster than plain silica due to basic nature of DNA or dAMP containing buffer solutions. DNA was not released from silica gels to aqueous environment up to 9 days. The chemical association between DNA/dAMP and silica host was through phosphate groups and molecular water attached to silanols, acting as a barrier around biomolecules. The helix morphology was found not to be essential for such interaction. BET analyses showed that interconnected, inkbottle-shaped mesoporous silica network was condensed around DNA and dAMP molecules.

Fabrication of D-type fiber optic sensors with a long interaction length and studying effects of critical parameters on sensorresponse

Today evanescent wave based fiber optic (F/O) platforms are in favor of the use for monitoring molecular interactions since they are practical, economic and easy to operate which make them ideal turnkey systems for clinical, pharmaceutical, environmental and security applications. The side polishing is one of techniques for reshaping the geometry of the waveguide to make the F/O sensor more sensitive to surrounding refractive index (RI) in evanescent field. In this study D-type F/O sensors with a 25 mm-long interaction lengths are fabricated. In addition to that, effects of the critical parameters such as the polishing depth, the wavelength, and the temperature on the sensor response are determined for the RI in the range of 1.33 - 1.47. The developing key of these F/O sensors is reaching high strength and penetration depth of evanescent wave in varying RI of the surrounding bio-layer. Development steps of D-type F/O sensors are; fabrication of supporting elements - silicon V channels, F/O cable preparation, adhesion, lapping and polishing, fusing the FC connectors, construction of the optical system, and RI measurements. Details of these steps are explained and the general characteristics of the D-type F/O sensor are presented. Results indicate that the sensor’s responses in three different RI ranges can be improved by the polishing depth. A maximum sensitivity of around 2x105 for the D-type F/O sensors is demonstrated in the RI range of 1.44-1.46.