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Universidade do Minho (2006)

Avaliação do processo de degradação de misturas termoplásticas à base de amido de milho

Araújo, M. Alberta

Titre : Avaliação do processo de degradação de misturas termoplásticas à base de amido de milho

Auteur : Araújo, M. Alberta

Université de soutenance : Universidade do Minho

Grade : Tese de doutoramento 2006

Résumé
Starch based thermoplastic blends and its composites have been used for several biomedical applications, namely for temporary bone replacements, due to their mechanical properties similar to the bone, biodegradability and bioactivity (promote bone bonding). These materials are known to be biocompatible, as shown in in vitro and in vivo studies. The main property of these materials is their ability to be enzimatically degraded ; the implant is metabolized and excreted by normal physiological mechanisms. Starch based blends ; contain natural monomers that reduce the problems associated with the materials and its degradation products (glucose, maltose), toxicity or the stimulation of chronic inflammatory reactions. Although in vitro and in vivo studies have been made in order to apply these materials in the biomedical field, the information regarding its degradability is scarce. The evaluation of the degradation products is of utmost important. They must be non toxic and excreted by any normal physiological mechanisms. In the use of temporary bone replacement, the functional viability of the implant is related to its degradation rate and its ability to accompany the bone growth. If a gradual load transfer from the implant to the bone is successful then a second surgery for implant removal can be avoided. The main objective of the work is the characterization of the degradation mechanisms, in physiological media of these blends, which should simulate the implant conditions. The effect of enzymes in the degradation process was evaluated by degradation studies in simulated body fluid, with and without α- amylase in a concentration similar to human blood plasma. The degradation behaviour was evaluated by quantification of the degradation products and plasticizer in solution and by the surface morphological modifications during immersion. Diffusion studies in the absence of enzymatic reaction, where the quantification of diffusion coefficients of glucose and glycerol in SEVA-C was achieved, were carried out in order to study the degradation kinetics of these materials. The structural limitations to degradation were studied using materials with different thickness (different exposure areas) and varying enzyme concentration solutions. In the absence of enzymes thermoplastic blend barely degrade in physiological media. In this case three phases can be identified : the first, in the initial 3 to 4 days, related both to the simple and rapid diffusion to the solution of plasticizer (glycerol) and low molecular weight polymeric chains and to the lubricating action of water. The next phase is a period of no significant weight loss (between 4 and 30 days). The third and last phase occurs for immersion periods longer than 60 days ; there is a stabilization of the degradation rate due to the attack of a lesser susceptible to degradation amylose- EVOH complex. SEM observations suggest surface morphological modifications, with an increase, with immersion time, in the pore size and number and in roughness, allowing a rising in the amount of water absorption and diffusion of degradation products to the solution. Glycerol, with a diffusion coefficient higher than glucose was the only compound released during the diffusion studies. Changes on the diffusion coefficient are directly related to the materials’ thickness. The correlation coefficient indicates that the calculated values are well in accordance with the model. Enzymatic degradation studies clearly show a strong enzymatic action in the degradation rate. In the absence of enzyme the amount of sugar released is 100% less. A structural model that describes the degradation process conditionings,associated to the specific microstructure/porosity of these materials and to the starch intern rearrangement (strongly bonded to the synthetic component), was conceived. This configuration limits the intern/extern hydrolytic diffusion and the enzyme bonds to the substrate ; the degradation rate stabilizes after 100 days and only one quarter of the amorphous phase is degraded. Never the less, the absorption of structural water increased, obtaining a good correlation between the hydration rate and the amount of total mass loss of released compound. A clear change in the amount of sugars in solution when using different thickness materials was shown. The enzymatic degradation depends on the exposed surface area. Materials with the same weight and different exposed surface area have different amount of saccharides in solution. However, the increase of film surface area did not result in a proportional increase in the saccharides amount due to the lack of active and enzymatic union sites. The number of enzyme/substrate bonds increases with the enzyme concentration, and enhances the initial degradation rate. The final degradation rate is similar for all enzymatic concentrations due to the structural limitations of the SEVA-C polymeric substrate. In accordance to the maximum degradation saturation limit in the films experiments, the critical thickness of these thermoplastic blends is lower than 0.5 mm. Starch degradation is influenced by a number of factors, namely : processing mechanism (especially surface), microstructure/porosity specificity, starch encapsulation rate and amount of exposed surface

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