Bioglass implant-coating interactions in synthetic physiological fluids with varying degrees of biomimicry.

Synthetic physiological fluids are currently used as a first in vitro bioactivity assessment for bone grafts. Our understanding about the interactions taking place at the fluid-implant interface has evolved remarkably during the last decade, and does not comply with the traditional International Organization for Standardization/final draft International Standard 23317 protocol in purely inorganic simulated body fluid. The advances in our knowledge point to the need of a true paradigm shift toward testing physiological fluids with enhanced biomimicry and a better understanding of the materials' structure-dissolution behavior. This will contribute to "upgrade" our vision of entire cascades of events taking place at the implant surfaces upon immersion in the testing media or after implantation. Starting from an osteoinductive bioglass composition with the ability to alleviate the oxidative stress, thin bioglass films with different degrees of polymerization were deposited onto titanium substrates. Their biomineralization activity in simulated body fluid and in a series of new inorganic-organic media with increasing biomimicry that more closely simulated the human intercellular environment was compared. A comprehensive range of advanced characterization tools (scanning electron microscopy; grazing-incidence X-ray diffraction; Fourier-transform infrared, micro-Raman, energy-dispersive, X-ray photoelectron, and surface-enhanced laser desorption/ionization time-of-flight mass spectroscopies; and cytocompatibility assays using mesenchymal stem cells) were used. The information gathered is very useful to biologists, biophysicists, clinicians, and material scientists with special interest in teaching and research. By combining all the analyses, we propose herein a step forward toward establishing an improved unified protocol for testing the bioactivity of implant materials.

Modification of Surface Charge Properties during Kaolinite to Halloysite-7Å Transformation.

The surface charge properties of well and poorly ordered kaolinite and halloysite-7Å, representing three different stages of kaolinite to halloysite-7Å transformation identified in the kaolin deposit of São Vicente de Pereira (Portugal), were studied. Mineralogical (X-ray diffraction, Fourier transformation-infrared) and chemical data (analytic electron microscopy) showed that the gradual transformation from kaolinite to halloysite-7Å minerals was accompanied by an increase in hydration and a decrease in Si/Al ratio. In particular, the replacement of Si(IV) by Al(III) in the tetrahedral layer caused an electrical charge unbalance and a modification of surface charge properties during the kaolinite to halloysite transformation. Accordingly, the cation exchange capacity (CEC) gradually increases when passing from well-ordered kaolinite to halloysite-7Å, attesting the direct correspondence existing among the structural order of the samples, number of tetrahedral substitutions, and CEC. Electrophoretic experiments provided further evidences of the origin of surface charge properties and the variation encountered during kaolinite to halloysite-7Å transformation. The curves of zeta-potential versus pH show the same pH dependency for the three minerals, but a gradual increase of zeta-potential when passing from well ordered kaolinite to halloysite-7Å through the poorly ordered kaolinite. The results can unambiguously be attributed to the increase of permanent charge due to the higher degree of isomorphic substitution, more than to the increase of structural and ionizable water. Copyright 1999 Academic Press.

Dispersion Properties of Silicon Nitride Powder Coated with Yttrium and Aluminium Precursors.

A coated silicon nitride (Si3N4) powder with yttria and alumina precursors as sintering additives was prepared by a heterogeneous precipitation method. The rheological and electrophoretic properties of the suspensions obtained from the coated (CO) powder were investigated and compared with those of pure Si3N4 powder and of the mechanically mixed (MM) powders of Al2O3, Si3N4, and Y2O3. The results showed that the CO powder calcined at 500 degreesC exhibited improved dispersion properties compared with the pure Si3N4 powders. The CO powder possessed the surface character of Al2O3 and Y2O3 particles, that made it easier to process in aqueous media, yielding a higher solid loading than the pure Si3N4 powder. These improvements were attributed to a change in the resultant interaction forces between particles from attractive (pure Si3N4, and MM powders) to repulsive in the case of the CO powder. A homogeneous distribution of sintering additives in the Si3N4 matrix was obtained. Copyright 1998 Academic Press.

Sol-Gel Preparation and Electrorheological Activity of SiO2-TiO2 Composite Powders

The SiO2-TiO2 composite powders with various Si/Ti ratios were prepared by the sol-gel processing, using tetraisopropylorthotitane (Ti(OPri)4) and tetraethoxysilane (Si(OEt)4) as starting materials. The effect of the powder's heat history on the electrorheological (ER) activity of the resulting suspensions, which were prepared by dispersing the powders into silicon oil, was investigated. The infrared spectra, scanning electron microscopy, X-ray diffraction, and zeta potential were used to characterize the structural evolution of the as-prepared gel powders upon heating. The results show a remarkable reduction in gelation time, relative to the single-component systems, due to the combined effects of a higher reactivity of Ti(OPri)4 over Si(OEt)4 and a nucleophilic attack of the Ti atom by the as-hydrolyzed species of Si precursor to form an environment similar to alkali catalysis. It was found that gel powders would gradually eliminate the -OH groups and become denser in structure upon heating, leading to a decrease of ER activity of the resulting suspensions. However, the initial Si/Ti composite compositions have little effect on ER activity behavior. Copyright 1997 Academic Press. Copyright 1997Academic Press