José Luis Feijoo

Study of the hydrolytic degradation of polydioxanone PPDX

The hydrolytic degradation of bioabsorbable polydioxanone (PPDX) was studied in a phosphate buffer solution, pH=7.4, 37°C. The degradation was evaluated by analyzing the changes in the elastic modulus and morphological changes. After 10 weeks, the weight loss, pH and molecular weight changes suggested diffusion of low molecular weight chain segments into the reaction medium as a consequence of the breaking of ester bonds in the material. The crystallinity (Xc) increased with hydrolysis time, indicating the first step of the degradation process. Modulus was found to decrease, indicating that chain scission proceeded in two steps: the first occurring in the amorphous regions of microfibrils and intermicrofibrillar space; the second in the crystalline regions, resulting in a lower stress value for the chain segments when submitted to external forces. Surface morphological changes suggest a heterogeneous degradation mechanism by layers.

Crystallisation and morphology of neat and degraded poly(p-dioxanone)

The present work compares the morphology and crystallisation kinetics of neat and hydrolytically degraded poly(p-dioxanone), PPDX. The hydrolytic degradation was performed in a phosphate buffered saline solution (0.2 M, pH 7.4) at 37 °C. The spherulitic morphology and the isothermal spherulitic growth rate were studied by optical microscopy while the overall crystallisation rate was determined by DSC. A thermal fractionation procedure was applied to the samples using the successive self-nucleation and annealing (SSA) protocol. The results indicate that the peculiar spherulitic morphologies exhibited by neat PPDX change as a function of molecular weight loss, but this change can be qualitatively rationalised as a function of the apparent supercooling applied to the crystallising sample. A change in growth regime with supercooling (from regime II to regime III) was detected using Hoffmans kinetic crystallisation theory for neat and degraded PPDX. It was found that both the overall crystallisation rate and the spherulitic growth rate were substantially increased as the molecular weight was decreased by hydrolytic degradation. The SSA results indicate that degradation depletes the longest chains within the molecular weight distribution of neat PPDX first, a fact consistent with an initial attack of the amorphous regions of the sample.

Application of the SSA calorimetric technique to characterise an XLPE insulator aged under multiple stresses

In this work the thermal behaviour of crosslinked low density polyethylene (XLPE) used as an insulator for commercial underground high tension (15 kV) cables was studied. Three types of materials were selected: an uncrosslinked low density polyethylene (NXLPE) used as a control sample, an XLPE and an aged XLPE sample. The ageing conditions involved the application of multiple stresses: temperature, voltage and voltage impulses during 60 d under time and temperature cycles that are the most representative load of daily operation in Caracas, Venezuela. The effect of morphology segregation or thermal fractionation under multiple stresses conditions was analysed by measuring the percentage of crosslinking before and after the ageing tests were performed, and by investigating the thermal response of the material by conventional DSC and by the application of the successive self-nucleation and annealing (SSA) thermal fractionation technique. The degree of crosslinking was found to vary in the material depending on the distance from the conductor because a thermal gradient is generated radially during the curing reaction. Such differences did not significantly affect the usual DSC heating scans of the samples. However, when SSA was applied, a difference in the distribution of thermal fractions was detected as a function of the distance towards the conductor that could be correlated to the variations in the cross-linking degree. After the accelerated ageing the thermal response of XLPE changes as evidenced by the presence of multiple melting peaks in subsequent DSC heating scans. This multiple melting was interpreted, as a first approximation, as arising from thermal fractionation during ageing (ignoring the possible effects of the other stresses applied) and SSA was able to simulate a similar fractionation that was very accurate in the prediction of the exact temperatures of the melting peaks produced.

Caracterización mediante FTIR y DSC de la interacción colágeno — hidroxiapatita

In the present work, we studied the effect of the mineral content (hydroxyapatite), on the thermal stability of the bone collagen matrix. The demineralization process was followed measuring the percentage of remaining mass and characterized by Fourier Transform Infra Red Spectroscopy (FTIR) and Differential Scanning Calorimetry (DSC), using a Calcium chelant agent (EDTA). A 52 % loss of the initial weight was observed after 72 hours of treatment, associated with a decrease of the mineral content, indicated by changes in the intensity of the peaks correspondent to PO4-3 vibrations. The bone collagen’s denaturalization and degradation temperatures was followed by DSC, showing an endotherm in 166° C for the untreated bone and in 113°C for the 72 hours treated bone, correspondent both to the bone collagen denaturalization temperature. The thermogram for the 12 hours demineralized exhibited a small endotherm in 250°C, corresponding to slight degradation, for the 72 hours treated bone the endotherm decreased to 230° C, correspondent to the bone collagen degradation. The above results lead us to conclude that the bone collagen triple helix losses its thermal stability when the interactions with the surrounding hydroxyapatite are eliminated.

Influencia de la piezoelectricidad del colágeno tipo I en la adhesión celular

Bone healing and growth are controlled by the rate of deposition of hydroxyapatite (HA). This process have been so far accredited to the work of osteoblasts, which are attracted by the electrical dipoles produced either by piezoelectricity, due to deformation of the bone, specially the collagen in it, or due to outside electrical stimuli. The main purpose of this work was to study the influence of the cortical bone collagen piezoelectricity effect, on the osteoblastic cells orientation. To evaluate the cellular adhesion on the cortical bone collagen subject to deformation, bone cells of newborn calvaria’s rats were extracted. The bone collagen was prepared and deformed following the specifications described in earlier studies. The results of this study shown that the piezoelectric phenomena of bone collagen promotes the cell’s adhesion on the compression side more than tension side compared with undeformed surface. Further studies ascertaining the osteoblastic activity due to the electric field are being advanced.

Electrochemical Influence of Collagen Piezoelectric Effect in Bone Healing

Bone healing and growth are controlled by the rate of deposition of hidroxiapatite (HA). This process have been so far accredited to the work of osteoblasts, which are attracted by the electrical dipoles produced either by piezoelectricity, due to deformation of the bone, specially the collagen in it, or due to outside electrical stimuli. The present work shows that even without osteoblasts present, the piezoelectric dipoles produced by deformed collagen, can produce the precipitation of HA by electrochemical means, without catalyzer as in biomimetic deposition. These findings could clarify the contribution of osteoblasts in bone growth as compared to the electrochemical action by itself. Further studies ascertaining the osteoblastic activity due to the electric field are being advanced.

TR-XLPE medium voltage cables subject to accelerated aging cycle under multiple stresses

In this article we report the most important results about the evaluation of TR-XLPE cables after an accelerated aging process under multiple stresses: temperature, AC voltage, impulse voltages and humidity. This work was developed at the Simon Bolivar University High Voltage Laboratory, Venezuela. 15 kV cables were subjected to thermal cycles between 55 - 130/spl deg/C, 26 kV AC voltage and (+/-) 88 kV impulse voltages applied periodically. The cables were aged in a duct filled with water. Dissipation factor and permittivity were evaluated periodically. Differential scanning calorimetry (DSC), crosslinked percentage, crystallinity and tree formations were evaluated periodically in a radial direction across the thickness of the samples and longitudinally along the cable. The most important observation was the increment in the dissipation factor after impulse applications and their corresponding reduction after thermal cycles. Important changes in DSC and considerable loss of crosslinked reticulation percentage, mainly in the region near the conductor and the termination zone, were detected. This research makes evident the nonuniformity of the properties according to the radial and longitudinal position. This is a result of different thermal-electric stress across the thickness and in the termination area. In consequence, at these points multiple stresses lead to premature degradation.

Estudio de Mezclas de Polietileno de Alta Densidad (PEAD) con colágeno/acetato de sodio e Hidroxiapatita (HA)

Blends and/or composites of HDPE, type I collagen gel extracted from rat tails and dried in presence of sodium acetate, poly(ethylene -co- acrylic acid) (a compatibilizg agent) and hydroxiapatite were studied. Differential Scanning Calorimetry (DSC) analyses demonstrated that the collagen gel was able to tolerate the processing conditions of HDPE since it kept its characteristic denaturalization temperature. Transmision Electronic Microscopy (TEM) showed that the fibrillar structure of collagen was kept and it finds itself dispersed on the HDPE matrix. Collagen supplies an important reinforcement to HDPE by increasing 2,7 times the HDPE Young’s modulus values. The compatibilizing agent did not show any important contribution to the Young’s modulus of the HDPE/collagen/copolymer blend in comparison with the blend where no compatibilizing was used. On the other hand, the HDPE/HA blends showed a Young’s modulus about two times that of pure HDPE, due to the HA crystals which confer stiffness to the whole system.