M. Helena Gil

In Situ Forming Chitosan Hydrogels Prepared via Ionic/Covalent Co-Cross-Linking

In situ forming chitosan hydrogels have been prepared via coupled ionic and covalent cross-linking. Thus, different amounts of genipin (0.05, 0.10, 0.15, and 0.20% (w/w)), used as a chemical cross-linker, were added to a solution of chitosan that was previously neutralized with a glycerol-phosphate complex (ionic cross-linker). In this way, it was possible to overcome the pH barrier of the chitosan solution, to preserve its thermosensitive character, and to enhance the extent of cross-linking in the matrix simultaneously. To investigate the contributions of the ionic cross-linking and the chemical cross-linking, separately, we prepared the hydrogels without the addition of either genipin or the glycerol-phosphate complex. The addition of genipin to the neutralized solution disturbs the ionic cross-linking process and the chemical cross-linking becomes the dominant process. Moreover, the genipin concentration was used to modulate the network structure and performance. The more promising formulations were fully characterized, in a hydrated state, with respect to any equilibrium swelling, the development of internal structure, the occurrence of in vitro degradability and cytotoxicity, and the creation of in vivo injectability. Each of the hydrogel systems exhibited a notably high equilibrium water content, arising from the fact that their internal structure (examined by conventional SEM, and environmental SEM) was highly porous with interconnecting pores. The porosity and the pore size distribution were quantified by mercury intrusion porosimetry. Although all gels became degraded in the presence of lysozyme, their degradation rate greatly depended on the genipin load. Through in vitro viability tests, the hydrogel-based formulations were shown to be nontoxic. The in vivo injection of a co-cross-linking formulation revealed that the gel was rapidly formed and localized at the injection site, remaining in position for at least 1 week.

Novel polymer-modified electrodes for batch injection sensors and application to environmental analysis

Various polymer coatings have been investigated for the protection of mercury thin-film electrodes in the square wave anodic stripping voltammetry of environmental samples using batch injection analysis, with injection of untreated samples of volume 50 μl directly over the sensing electrode. Polymer coatings studied include those with controlled porosity, and cation-exchange polymers based on sulphonated polymers. Of the polymers tested, films of ca. 1 μm thickness made from Nafion® mixed with 5% poly(vinyl sulphonic acid) were found to give the best results in tests with the model surfactants Triton-X-100 detergent, sodium dodecyl sulphate and protein standard. The validity of the approach is demonstrated by application to real samples.

13C nuclear magnetic resonance studies of cellulose ester derivatives in solution, powder and membranes

A 13C nuclear magnetic resonance study (with cross polarization and magic angle spinning) is presented of commercially available cellulose acetate and cellulose propionate as powder samples and also in different membranes. Some physical properties of the films are interpreted in the light of data obtained for the principal components of the shielding tensor of 13C nuclei on carbonyl groups. In particular, it is shown that the shielding asymmetry factor, measured on samples with a regular shape and with a fixed orientation in the rotor, is a sensitive probe to ordered domains in the polymers.

Characterization and in vitro hemocompatibility of bi-soft segment, polycaprolactone-based poly(ester urethane urea) membranes.

In this study, surface, bulk, and hemocompatibility characteristics of crosslinked, bi-soft segment poly (ester urethane urea) membranes, prepared by extending a poly(propylene oxide)-based triisocyanate-terminated prepolymer (PU) with a polycaprolactone diol (PCL), were investigated. Variation of the ratio of PU to PCL diol content in the membrane formulation yielded alteration of surface energy, phase morphology both in the bulk and in the region near the surface, and it affected hemocompatibility. Nearly all membranes were nonhemolytic, with hemolysis degrees between 1 and 2.1% and, for short-time contact with blood (15 min), all membranes showed in vitro thrombosis degrees between 27 and 42%. The membranes prepared with 5 and 25% of PCL diol showed almost no adherent platelets. These two membranes had a higher hard segment aggregation in the region near the surface and mixing between the two soft segments in the bulk, but showed contrasting surface energy characteristics. The results obtained in this work give evidence that surface energy and its polar and dispersive components did not correlate with any of the hemocompatibility aspects studied. In contrast, the phase morphology in the region near the surface was a major influence on membrane hemocompatibility.

In Vitro Bioactivity in Glass-Ceramic / PMMA-co-EHA Composites

We developed new composites that consist of poly(methylmethacrylate )-co-( thyl-hexylacrylate) (PMMA-co-EHA) filled with a glass-ceramic (m ol%) (70 SiO2 – 30 CaO). The in vitro bioactivity was assessed by determining the changes in surface m orphology and composition after soaking in simulated body fluid (SBF) for periods of up to 21 days at 37o C. X-ray diffraction (XRD) and scanning electron microscopy coupled with X-ray energy dis persive spectroscopy (SEM-EDS) after different soaking periods confirmed the growth of apatite-like deposits after 3 days. The deposits consisted of spherical aggregates of acicular crystalli tes. Introduction Polymethylmethacrylate (PMMA) bone cement has been widely used i n orthopedic surgery for prosthetic fixation since it was introduced by Charnley. However, an unres olved problem with using PMMA bone cement is a thickening of the intervening fibrous tissue la yer that leads to aseptic loosening of the cement in some cases [1]. To improve fixation of PMM A with the host bone, various composites with bioactive materials have been developed and studie d [2, 3]. A number of attemps have been made at filling PMMA matrix with calcium phos ates [2] and with bioactive glass [3]. The short-term results obtained are encouraging and sugge st that this bioactive materials form an apatite surface layer in vivo [1] or in contact with simulated plasma (SBF) in vitro [4]. This layer is considered to be responsible for the bonding of bioactive ceram i s to the host bone [3]. The aim of this work is to follow the formation of the apatite layer on the surface of bioactive glassceramic/PMMA-co-EHA composites. Materials and Methods Preparation of composites Methylmethacrylate (MMA) and ethyl-hexyl-acrylate (EHA) were obtained from Aldrich Chemical Company. Bioactive glass with composit ion 70SiO2 – 30CaO (mol%) was obtained by a sol-gel route and calcined at 900o C, for 1 hour, in air, milled and sieved to less than 90 μm particle size. The resultant powder consists of a glass phase and crystalline pseudowollastonite. Benzoyl peroxide (PBO) was obtained from Merck. Only the MMA was purified, extraction of hydroquinone, all the other reagents were used as received. Glassceramic/PMMA-co-EHA composites were prepared by addition of the m ono ers to 50 wt% of the ceramic filler. Benzoyl peroxide was added to the monomer mixture in a ratio of 0,5 mol% as a Key Engineering Materials Online: 2003-12-15 ISSN: 1662-9795, Vols. 254-256, pp 581-584 doi:10.4028/www.scientific.net/KEM.254-256.581

Mechanical Properties of Particle Reinforced Resin Composites

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.

New LDPE Copolymeric Films with Enhanced Hydrophilic Properties Prepared by Gamma Irradiation

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.

Surface modification of an intraocular lens material by plasma-assisted grafting with 2-hydroxyethyl methacrylate (HEMA), for controlled release of moxifloxacin

ABSTRACT Endophthalmitis, an inflammation of the eye due to perioperative infection, may occur after cataract surgery. Intraocular lenses (IOLs) loaded with an antibiotic have been proposed as an alternative to the conventional postoperative endophthalmitis prophylaxis, since the antibiotic is delivered directly to the target site. In this work, an IOL‐based antibiotic releasing system was prepared from a copolymer used in the production of IOLs and a fluoroquinolone used in endophthalmitis prophylaxis (moxifloxacin, MFX). Argon plasma‐assisted grafting with 2‐hydroxyethyl methacrylate (HEMA) in the presence of MFX was the approach selected for surface modification, with MFX loaded both by entrapment in the grafted polyHEMA coating and by soaking. Surface and bulk properties were evaluated before and after surface modification and the MFX release profiles were obtained both in batch mode (sink conditions) and under hydrodynamic conditions, employing a purpose‐built microfluidic cell, which simulated the hydrodynamic conditions around the eye lens. The effect of storage on the release profile of the best system was also assessed. The best system released MFX for ca. 15 days above the minimum inhibitory concentration for Staphylococcus aureus and Staphylococcus epidermidis. The released MFX showed antimicrobial activity against these bacteria and was non‐cytotoxic against corneal endothelial cells.

Rheology of Chitosan and Genipin Solutions

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.

Comparison of quantification methods for the condensed tannin content of extracts of Pinus pinaster bark

Bark from Pinus Pinaster is an interesting source of polyphenolic natural compounds, that can be used successfully as total or partial replacement of conventional phenolic resins. These compounds, among other applications, are used as adhesives in the wood agglomerate industry. In this kind of application some problems remain to be solved in order to obtain a Pine extract of commercial value. It is necessary to optimise the extraction procedure and select a suitable method for the quantification of the tannin content of the bark. In order to study these problems, the tannin extraction from the Pine bark was tested with an alkaline solution (NaOH), and with a fractionation procedure based on a sequence of an organic (ethanol) and aqueous extraction. The phenolic content of each extract or fraction was evaluated by the FolinCiocalteu colorimetric assay for total phenols and two procedures using the Stiasny reaction: the gravimetric Stiasny method and the indirect colorimetric procedure that uses the Folin-Ciocalteu reagent to evaluate the total phenols present in the extract solution before and after it condenses with formaldehyde. The yield value when the alkaline extraction is used is substantially higher than the values obtained with organic or aqueous solutions. However, the selectivity of the process is low. In fact, it was found that the alkaline extract Formaldehyde Condensable Phenolic Material (FCPM) content represents 95-96 % of the total phenols content of the extract but this fraction is only ≈ 40 % of the total mass of extract. So, the alkaline extract is relatively poor in phenolic material, exhibiting a large variety of non-phenolic extractives. On the other end, ethanol provides a very rich phenolic extract, in which 96 % of total phenols are condensable with formaldehyde, but exhibits a relatively low extraction yield. The aqueous extract presents the lowest extraction yield with low content either in phenolic material as in FCPM, but, as most of the phenolics had already been extracted by the previous organic extraction, especially the low molecular weight fractions, this result was predictable.