A Comparison of Mechanical Properties of Different Hydroxyapatite (HAp) Based Nanocomposites: The Influence of Morphology and Preferential Orientation.

Three different types of hydroxyapatite (HAp) based porous ceramic materials were obtained through the modified gel casting method; one of them was made of commercial HAp particles and used as a reference in the mechanical characterization. Other type of ceramic was elaborated using HAp nanofibers, which were synthesized through the microwave assisted hydrothermal method and they possess a high crystallinity, purity and a preferential crystalline orientation in the [300], such were grown along the [001]. The third type of porous ceramic was elaborated using a combination of HAp nanofibers and particles. The HAp nanofibers and particles were previously analyzed by using X-ray diffraction to study their crystal structure, the topology and morphology of those HAp aggregates were observed with scanning electron microscopy (SEM); high-resolution transmission electron microscopy was useful to carry out a detailed crystallographic analysis. Afterwards, an organic phase made of gelatin was added to the porous ceramics in order to obtain nanocomposite materials. Two different concentrations of gelatin were used separately, and the combination of three types of porous ceramics and two concentrations of gelatin produced six different nanocomposite materials. All of these composite materials were observed through the SEM to see their topology and porosity and after that, they were probed under compression tests and their corresponding mechanical behavior was analyzed. All the composites showed mechanical properties similar to those observed in cellular materials. The Young modulus and ultimate strength were compared, finally, it was determined the contribution to the mechanical properties of the morphology, crystalline quality and preferential crystalline orientation in the HAp nanofibers. According to such properties, the composite material made of HAp nanofibers has bone tissue implant potential applications.

Interconnected porosity analysis by 3D X-ray microtomography and mechanical behavior of biomimetic organic-inorganic composite materials.

Hydroxyapatite-based materials have been used for dental and biomedical applications. They are commonly studied due to their favorable response presented when used for replacement of bone tissue. Those materials should be porous enough to allow cell penetration, internal tissue growth, vascular incursion and nutrient supply. Furthermore, their morphology should be designed to guide the growth of new bone tissue in anatomically applicable ways. In this work, the mechanical performance and 3D X-ray microtomography (X-ray µCT) study of a biomimetic, organic-inorganic composite material, based on hydroxyapatite, with physicochemical, structural, morphological and mechanical properties very similar to those of natural bone tissue is reported. Ceramic pieces in different shapes and several porous sizes were produced using a Modified Gel Casting Method. Pieces with a controlled and 3D hierarchical interconnected porous structure were molded by adding polymethylmethacrylate microspheres. Subsequently, they were subject to a thermal treatment to remove polymers and to promote a sinterization of the ceramic particles, obtaining a HAp scaffold with controlled porosity. Then, two different organic phases were used to generate an organic-inorganic composite material, so gelatin and collagen, which was extracted from bovine tail, were used. The biomimetic organic-inorganic composite material was characterized by Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy, X-ray Diffraction, Fourier Transform Infrared Spectroscopy and 3D X-ray microtomography techniques. Mechanical properties were characterized in compression tests, obtaining a dramatic and synergic increment in the mechanical properties due to the chemical and physical interactions between the two phases and to the open-cell cellular behavior of the final composite material; the maximum compressive strength obtained corresponds to about 3 times higher than that reported for natural cancellous bone. The pore size distribution obtained could be capable to allow cell penetration, internal tissue in-growth, vascular incursion and nutrient supply and this material has tremendous potential for use as a replacement of bone tissue or in the manufacture and molding of prosthesis with desired shapes.

Crystal Structure Determination of the S/TiO2 System and the Correlation with Its Photocatalytic Properties.

A sulfur doped anatase photocatalyst was synthesized through a microwave-assisted sol-gel method and posteriorly, it was characterized using X-ray diffraction by powders (XRD) and High Resolution Electron Microscopy (HREM). Morphology and average size of the crystallite were determined for the sample, preferential crystal orientation was also analyzed and all these structural features were related to the photocatalytic properties. Moreover, a pure TiO2 sample was also obtained by the same synthesis method and it was used as a reference, and its structure and photocatalytic properties were compared to those observed for the sulfur doped anatase. The structural analysis showed similar results for both sample types but, photocatalytic properties are rather different. A small difference in chemical composition had an impact in the catalytic properties.

Synthesis of hydroxyapatite nanostructures using microwave heating.

Hydroxyapatite (HAp) nanoplates and nanofibers have been synthesized using CaNO3, KOH and K2HPO4 as chemical precursors. The concentration of these precursors was kept constant in all experiments. Synthesis reactions were carried out inside of pressurized Teflon vessels. The energy required for the synthesis was supplied by microwaves. Most of the reactions were performed in the presence of glutamic acid. The concentration of this substance was the only difference in the formulation of the reacting mixture, and its effect on the morphology of nanostructures has been evaluated. The use of pressurized vessels and the heating by microwaves in the synthesis reactions made easier the obtaining of HAp crystals. The morphology of nanostructures was influenced through the differences in the concentration of glutamic acid. The variations on the reaction time were useful to obtain nanofibers with an adequate size. The nanoplates and nanofibers obtained will be used to synthesize an organic-inorganic composite which has potential application on medicine and odontology. X-ray diffraction and FTIR were performed to verify the obtaining of a hydroxyapatite phase. High-resolution electron microscopy was carried out for microstructure analyses. Energy-dispersive X-ray spectroscopy was used to evaluate the Ca/P ratio in all nanostructures.