Katia Rubini

Microstructural investigation of hydroxyapatite–polyelectrolyte composites

Hydroxyapatite–poly-L-aspartic acid (HA-PASP) and hydroxyapatite–polyacrylic acid (HA-PAA) composite crystals have been prepared by direct synthesis in aqueous solution. The polyelectrolytes are quantitatively incorporated into the crystals up to about 8–9 wt%, as a function of their concentration in solution. The structural and morphological properties of the crystals vary as a function of polyelectrolyte content. TEM images show that the composite crystals display a greater length/width ratio with respect to the control HA crystals. The broadening of the X-ray diffraction reflections on increasing polyelectrolyte content was investigated using three different methods: (i) the Scherrer method, (ii) the Warren–Averbach approach, and (iii) a whole pattern analysis approach. Both polyelectrolytes induce a greater reduction of the mean crystallite size along a direction perpendicular to the c-axis direction, suggesting a preferential interaction of the polymers with the crystal faces parallel to the c-axis. PASP interaction with HA structure provokes a greater increase of strain in comparison to PAA. The data indicate that anionic polyelectrolytes can be usefully applied to modulate the structural and morphological properties of hydroxyapatite crystals.

Gas–solid reactions between the different polymorphic modifications of barbituric acid and amines

The two known crystalline polymorphic forms of barbituric acid (C4H4N2O3) (form I and form II) and the dihydrate form [(C4H4N2O3)·2H2O] have been reacted with vapours of ammonia, methylamine, and dimethylamine and the crystalline products investigated by means of single crystal and powder X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry and infrared spectroscopy. It has been shown that form I, II and the dihydrate form of barbituric acid react with ammonia leading to the same crystalline ammonium barbiturate salt NH4(C4H3N2O3) 1a, while the gas–solid reactions of form II with methylamine, and dimethylamine yield the corresponding crystalline salts, CH3NH3(C4H3N2O3) 1b and (CH3)2NH2(C4H3N2O3) 1c. Thermal desorption of the bases at ca. 200 °C leads to formation of a new crystal form of barbituric acid, form III, as confirmed by H1-NMR spectroscopy and by chemical behaviour. De-hydration of the dihydrate form has also been investigated showing that it releases water to yield exclusively crystals of form II.

Films of self-assembled purely helical type I collagen molecules

The structural, morphological and chemical–physical characterization of films constituted of self-assembled non-helical region free type I collagen molecules has been investigated. The results indicate the presence of information at the molecular level which allow collagen I molecules to self-assemble. In the fiber formation process the collagen molecules re-establish the greater part of the native intermolecular cross-links. The films, obtained by air drying the fiber suspension, are water insoluble and characterized by a high mechanical performance. The mechanical and thermal properties of the films increase strongly as a function of the collagen fiber orientation induced in the films by uniaxial stretching.

In vitro mineralization of gelatin-polyacrylic acid complex matrices

Gelatin-polyacrylic acid (gel-PAA) matrices were obtained by slow diffusion of polyacrylic acid into gelatin gels. The matrices were submitted to uniaxial stretching, which induces a preferential orientation of the collagen molecules, and used as biomimetic substrates for the nucleation of hydroxyapatite from simulated body fluid (SBF). The relative amount of hydroxyapatite deposited from 1.5SBF increases as a function of polyelectrolyte content in the matrices, up to about 30 wt%. In the absence of PAA, the inorganic phase is laid down on the surface of the gelatin matrices as hemispherical aggregates. At variance, hydroxyapatite deposition in the gel-PAA composite matrices at relatively low PAA content occurs preferentially in the spaces between the layers on the surface of the matrices and displays a tablet-like morphology. At high polyelectrolyte concentration, an almost uniform layer of hydroxyapatite covers the whole surface of the matrices. The preferential orientation of the (002) hydroxyapatite reflection indicates a close relationship between the inorganic crystals and the collagen molecules.