Papers by Author: M. Helena Gil

Abstract: This paper investigates the rheological behaviour of chitosan solutions crosslinked with different concentrations of genipin at body temperature and physiological pH. The effect of the crosslinker concentration on the rheological properties of hydrogels was evaluated. The oscillatory time sweep was used to analyze the dynamics of G’ during in situ gelation experiments enabling the determination of the gelation time. Additionally, the stress and frequency sweeps were employed to measure G’ of cured hydrogels. The solutions of chitosan crosslinked with genipin at physiological conditions were found to form relatively strong elastic gels for all the concentrations, when compared to pure chitosan. A significant reduction on gelation time was achieved. This behaviour shows that these formulations are able to be produced in situ and thus constitute promising matrices for cells and bioactive molecules encapsulation.

Abstract: Graft copolymerization induced by gamma radiation has being used as a tool to modify the surface properties of a range of polymeric substrates. This work describes the use of this technique for the grafting of 2-hydroxyethyl methacrylate (HEMA) branches onto low-density polyethylene (LDPE) surface, in order to improve its hydrophilic properties. Sample preparation protocols were selected from previous work in order to obtain films with high grafting yields. The obtained PE-g-HEMA films were characterized by thermal analysis techniques (DSC and TGA), and by Fourier transform infrared spectroscopy (FTIR). The results obtained show that, upon irradiation, there is some loss of cristalinity of the copolymer backbone, but also that the samples keep a good thermal stability. The water uptake of the samples was evaluated. Hydration levels up to near 95 % with a ydration/dehydration average ratio of 1:5 were obtained.

Abstract: Two materials with potential application in bone tissue repair have been developed: 1) a non-biodegradable composite based in a new methacrylic-co-acrylic matrix; and 2) a biodegradable composite based in a chitosan (Ch) matrix. Both matrices were reinforced with glass-ceramic particles of composition (mol%) 70 SiO2 – 30 CaO. The in vitro bioactivity of composites was assessed by soaking in simulated body fluid (SBF) for periods of up to 7 days at 37º C. X-ray diffraction (XRD) and scanning electron microscopy coupled with X-ray energy dispersive spectroscopy (SEM-EDS) were used for deposit identification after different soaking periods. Calcium phosphate particulate deposits were detected after 3 days of immersion, followed by growth and maturation towards apatite.

Abstract: Vinyl Chloride (VCM) based copolymers were synthesised by using Living Radical Polymerization. The obtained materials were characterized by determining their molecular weight, glass transition temperature and mechanical properties after processing in industrial equipments. Their chemical composition was evaluated by using NMR.

Abstract: The structural versatility of polyurethane based membranes is enhanced through the introduction of two soft segments to obtain new membranes with improved hemocompatibility. Crosslinked polyurethane/urea membranes with two soft segments were prepared by extending a polypropylene oxide-based tri-isocyanate terminated prepolymer (PU) with polybutadiene diol (PBDO). The ratio of PU to PBDO was 100/0 and 90/10. The membranes were characterised in terms of permeation towards O2 and CO2 and their hemocompatibility was evaluated through hemolysis and thrombosis experiments. It was found that the membrane with PBDO had the highest permeability to O2 and CO2 and it was non-hemolytic and less thrombogenic.

Abstract: The objective of the present work is the evaluation of the contents of inorganic particles in the mechanical and tribological behavior of polymeric matrix composites. In order to control easily the production of the specimens, a polyester resin was used as matrix and silica particles were added as inorganic filler. The volumetric particle content was ranged from 0 to 46%. In order to understand the influence of the inorganic load was evaluated the mechanical and tribological behaviors for several percentage of particle content was evaluated. There are several applications of inorganic fillers where their volume percentage is important, namely in dentistry. In posterior restorative resin materials, the particles percentage in volume goes up to 50 or more. In most cases spherical and irregular shaped fillers are dispersed randomly. In the studied composites the filler has irregular shape therefore the connection between the matrix and the particles is more effective. Function of the shape, concentration degree and particle size of the filler the composite mechanical properties vary greatly. All of these factors influence the mechanical properties of the particlereinforced composite, namely: wear resistance, hardness, flexural modulus, flexure strength and toughness The morphology of the failure surfaces was observed by scanning electron microscopy and the results were widely discussed.