Advanced Materials Forum III

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Authors: Robert N. Mead, G. Mountjoy
Abstract: Enhanced bioactivity has been observed for amorphous CaO-SiO2 sol-gels with 30mol% CaO, and several structural techniques have recently been used to investigate the structural basis for this bioactivity. The current work presents the first detailed atomic model of (CaO)0.3(SiO2)0.7 solgel after heat treatment at 600°C, produced using molecular dynamics. The model contains 1056 atoms in cubic box with length 24.1Å, and specifically incorporates hydroxyl groups which are characteristic of the sol-gel. The model is in good agreement with experimental X-ray and neutron diffraction results. Inspection of the model shows a network of SiO4 tetrahedra with an average connectivity of approximately 3. Ca have coordination of NCaO=5.3, in agreement with experimental results. On average, each Ca is surrounded by 4 other Ca, and visual inspection shows several large clusters of Ca. These clusters should influence the dissolution of Ca, and hence the bioactivity of (CaO)0.3(SiO2)0.7 sol-gel.
Authors: Rui Igreja, C.J. Dias
Abstract: An interdigital chemocapacitor (IDC) sensor array has been developed to discriminate organic vapour samples with pure organics and mixtures. Sensors were coated with diverse polymers (PEUT, PDMS and modified PDMS). Dedicated signal conditioning circuits were developed with a reduced S/N ratio enabling measurement of capacitance changes as low as 0.07 fF. A new sampling technique was also developed to improve isobaric sampling conditions on the sensors chamber. Linear techniques such as Principal Component Analysis (PCA) for discrimination between the different samples were sucessfully applied. As far as we know, this is the first time that an array of interdigital chemocapacitors is used for organic vapour discrimination.
Authors: M. Sivakumar, Rui Vilar, Vitor Oliveira
Abstract: In the present paper, the influence of tubule orientation and areal density on the development of surface textures by excimer laser processing of dentin is analysed. Disks of dentin 2 mm thick were extracted from caries-free human teeth by cross-sectional cutting above the pulp cavity, polished and fixed using standard procedures. The samples were laser-processed using 100 laser pulses of 248 nm wavelength radiation at a fluence of 1 J/cm2, pulse duration of 30 ns and pulse frequency of 5 Hz. The surface texture after processing depends on the angle between the tubules and the laser beam. In inner dentin, where tubules are parallel to the laser beam, cone-like artefacts form, considerably increasing surface roughness. The cones are constituted by partially melted peritubular dentin and develop because the ablation rate of peritubular dentin is lower than the ablation rate of surrounding intertubular dentin. The areal density of cones is roughly identical to the areal density of tubules except when the tubule density is high enough to allow adjacent cones to coalesce. In outer dentin, where tubules are tilted with respect to the laser beam, the surface remains flat. The reason for this orientation dependence is that, when tubules are tilted towards the laser beam, preferential removal of intertubular dentin will expose an increased area of underlying peritubular dentin to laser radiation preventing cone development.
Authors: Carlos M. Queiroz, Maria Helena F.V. Fernandes, Jorge R. Frade
Abstract: Impedance spectroscopy was used to measure the changes of electrical properties during the isothermal crystallisation of the phosphate phase in bulk samples of a glass with nominal molar composition (3CaO.P2O5)20(SiO2)35(MgO)38.354(K2O)6.646, developed for biomedical applications. Crystallization studies were performed in isothermal conditions at 800°C and 860°C, and were monitored by impedance spectroscopy. XRD shows that a stable orthophosphate phase, Ca9MgK(PO4)7 crystallizes at about 860°C, after formation of earlier phosphate precursors formed at lower temperatures, namely oxyapatite (Ca5(PO4)3O) and tricalcium phosphate (Ca3(PO4)2). The conductivity of bulk glass samples decreases at 800°C, as the phosphate precursors phases crystallize. The opposite trend is observed on crystallizing the orthophosphate phase, at 860°C.
Authors: Carlos M. Queiroz, Maria Helena F.V. Fernandes, Jorge R. Frade
Abstract: Impedance spectroscopy was used to monitor the non-isothermal crystallisation of a phosphate phase in bulk samples of a glass with the molar composition 0.45SiO2–0.36MgO– 0.09K2O–0.1(3CaO⋅P2O5), intended for biomedical applications. The K+ alkali ions are probably the main charge carriers in glass of this composition [1]. Impedance spectra were obtained as a function of temperature, following non-isothermal heat treatments at 2°Cmin-1. It is shown that suitable analysis of impedance spectra can be used to evaluate the crystallization peak of a orthophosphate phase, Ca9MgK(PO4)7. Results obtained by this method are comparable to those obtained by differential thermal analysis (DTA) of the corresponding glass frits. Impedance spectroscopy is thus suitable to study the crystallization of bulk samples, and can be used both with variable temperature and under isothermal conditions.
Authors: Cláudia M.S. Ranito, Fernando A. Costa Oliveira, João P. Borges
Abstract: Bioactive dense HAp ceramics possess a unique set of properties, which make them suitable as bone substitute. However, both physical and mechanical properties of HAp have to be evaluated in order to produce new materials that match the bone stiffness. This paper highlights the influence of both porosity and grain size on the four-point flexural strength and the indentation fracture toughness of pure dense HAp blocks sintered at 1300°C. Both discs and rectangular bars were produced by uniaxial pressing at 40MPa and sintered in static air at temperatures between 1150 and 1325°C for 1 h in order to assess the densification behaviour of the P120S medical grade HAp powder used. After sintering, both the density and the open porosity were measured. In addition to FT-IR, XRD and SEM, the mechanical properties of the dense HAp blocks, including Young´s modulus, flexural strength, Vicker´s hardness and fracture toughness, were characterized and whenever possible these properties were compared to those reported for cortical bone. Pressureless sintering to full density at temperatures below 1300°C does not occur for the stoichiometric powder used. The results obtained underline the importance of full mechanical characterisation of dense HAp so that new implant materials can be developed. There is a need to improve the microstructure and thus enhance mechanical strength of HAp ceramics, as it was found that flexural strength is closely related to the micropores present in the sintered samples.
Authors: Cléber C. Silva, Manuel A. Valente, Manuel P.F. Graça, António S. B. Sombra
Abstract: The bioceramic hydroxiapatite [Ca10(PO4)6 (OH)2 – HAP] is the main mineral constituent of teeth and bones with excellent biocompatibility with hard and muscle tissues. These materials exhibit several problems of handling and fabrication, which can be overcome by mixing them with a suitable binder. The dry milling process of fabrication of HAP presents the advantage that melting is not necessary and the powder obtained is nanocrystalline. The HAP has been obtained from three different experimental procedures (HAPA: Ca(H2PO4)2 + Ca(OH)2; HAPB: Ca(H2PO4)2 + CaCO3; and HAPC: CaHPO4 + CaCO3). In the reactions HAPA and HAPB the hydroxiapatite phase was obtained after 5, 10 and 15 hours of milling and after 15 hours in the reaction HAPC. In order to improve the mechanical properties of HAP, calcium phosphate ceramics with titanium (CaP-Ti) has been prepared by dry ball milling (Ca(H2PO4)2 + TiO2). The calcium titanium phosphate phase, CaTi4P6O24, was obtained. The dielectric study in function of frequency, at constant temperature of the ceramics, was made using the Modulus (M*) formalism and a distribution of relaxation times was observed. The values of the dielectric constant of the ceramics measured at room temperature are between 5.04 (CaP-Ti_5H) and 13.70 (HAPA_10H). The structure of the samples was studied by X-Ray diffraction, infrared and Raman scattering spectroscopy.
Authors: Rosa Losada, José Luís Mier, Fernando Barbadillo, Ramón Artiaga, Angel Varela, Salvador Naya
Abstract: A diglycidyl-ether of bisphenol-A (DGEBA)/Triethylenetetramine (TETA) system was studied by non-isothermal differential scanning calorimetry (DSC) to establish its kinetics of cure. The DGEBA resin was Araldite GZ 601 X75 used in the marine coatings formulations. Previously, the optimum resin/hardener ratio was determined by the reaction heat measuring (.Hc) calculated from the curing exothermic peak. Tests at different heating rates (10, 15, 20, 25 and 30°C/min) under inert atmosphere were carried out in order to study the reaction kinetics. The activation energy of the cure (Ea) was obtained from these tests data by Borchardt-Daniels, autocatalytic, Duswalt and isoconversional Ozawa methods. Once the activation energy was determined, the master curves method was applied to find the kinetic model which best describes the measured DSC data. The Sestak-Berggren model SB (m,n) was found to be the most adequate for the system studied.
Authors: Ruthilene C. Lima da Silva, Janaina Silva de Santana, José F. Silva Jr, Ademir Oliveira da Silva, João Telésforo Nóbrega Medeiros
Abstract: Polyurethane is a biocompatible material and its use is increasing as human or animal implants. However, problems due to wear debris and their interaction with the host require a better understanding of wear mechanisms and the genesis of the debris formation. Some effects of the contact pressure and geometry were experimentally investigated to identify the response, obtained by contact temperature, mass variation, Scanning Electron Microscopy and EDS microanalysis, of polyurethane samples from sliding against metallic surfaces.
Authors: Catarina Santos, M.A. Martins, Margarida M. Almeida, Maria Elisabete V. Costa
Abstract: The adsorption of the drug 5-Flourouracil (5-FU) on Hydroxyapatite (HAP) particles having different surface conditions is studied at 37°C. Commercial HAP particles, either as received or soaked in citrate solutions are used as drug adsorbents and compared to a third category of HAP particles which consists of granules obtained by spray drying suspensions of HAP nanoparticles previously precipitated in citrate solutions. The adsorbed amount of 5-FU on the HAP particles surface is conditioned by the existing pre-adsorbed citrate ions which contribute to modify the particle surface charge and thus decrease the drug adsorption. However the obtained results also demonstrate that the particle surface charge is not the unique factor to govern the 5-FU adsorption on the HAP particles surface.

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