Acide lactique | EPFL Graph Search

Les acides lactiques (acides hydroxypropanoïques) sont des acides organiques dont la chaîne est composée de trois atomes de carbone. Les ions lactate sont les bases conjuguées de ces acides (autrement dit, la forme ionisée des acides lactiques). Les acides lactiques doivent leur nom par référence au lait, d'où un des énantiomères (l'acide L-lactique, ou S) a été extrait pour la première fois par le chimiste suédois Carl Wilhelm Scheele, mais il est largement présent (souvent dissocié sous forme de lactate) dans les organismes vivants où il joue un rôle dans divers processus biochimiques (notamment la production d'énergie lors d'effort musculaire anaérobie), et dans des produits d'origine vivante, par exemple dans le vin, certains fruits et légumes, la choucroute et d'autres produits fermentés, etc. Les acides lactiques sont des acides α-hydroxylés, de formule brute . Leur structure se reflète dans leur nom systématique : acide 2-hydroxypropanoïque. Chimie L'atome d

Labor division in engineered cross-kingdom consortia: Consolidated bioprocessing of lignocellulosic biomass to carboxylic acids

Robert Lawrence Shahab

Lignocellulose is a renewable source of fixed carbon and a promising substrate for the production of chemicals and fuels in second generation biorefineries. In this thesis, we engineered artificial microbial consortia for consolidated bioprocessing (CBP) of lignocellulose to carboxylic acids and ethanol. Inspired by natural ecosystems following the principle of labor division, we distributed the required metabolic capabilities among different specialized microorganisms. As a vast majority of product-forming microorganisms is non-cellulolytic the fungus Trichoderma reesei was employed to produce cellulolytic enzymes. For the production of lactic acid from steam-pretreated beech wood, the aerobic fungus T. reesei was co-cultivated with the non-cellulolytic facultative anaerobic bacterium Lactobacillus pentosus. The known instability problem of such a cooperator-cheater community was overcome by enforced niche differentiation. To this end, a novel stirred tank bioreactor was engineered that was equipped with a continuously aerated, oxygen permeable membrane that locally provided oxygen for the aerobic fungus in an otherwise anaerobic bioreactor. The success of this strategy was shown by the formation of a fungal biofilm on the membrane's surface and a lactic acid concentration of 19.8 gL-1 (85.2 % of theoretical maximum). For the production of various carboxylic acids, obligate and strict anaerobic bacteria were added to the community of T. reesei and L. pentosus. The lactic acid bacterium metabolically funneled the heterogeneous lignocellulosic carbohydrates to lactate as central intermediate in synthetic food chains and with that, compensated for missing metabolic capabilities of the anaerobic secondary fermenters. The feasibility and modularity of the lactate platform was shown by the direct production of acetic, propionic, butyric, valeric and hexanoic acid from beech wood. Integration of the obligate anaerobes Clostridium tyrobutyricum or Veillonella criceti resulted in 196.5 kg butyric acid or 113.6 kg propionic and 133.3 kg acetic acid, respectively, per ton beech wood. For the production of mixtures of volatile fatty acids (VFAs) with up to six carbon atoms from beech wood the non-cellulolytic strict anaerobe Megasphaera elsdenii was integrated into the lactate platform and 220 kg total VFAs per ton beech wood were produced. By adding C. tyrobutyricum or V criceti to a three-member microbial community it was shown that the ratio of odd- and even-numbered VFAs can be tuned through intra-consortium competition. For the production of ethanol from lignocellulose, T. reesei and Saccharomyces cerevisiae were co-cultivated. They formed a two-layered biofilm on the aerated membrane. The in situ degradation of ethanol was dedicated to aerobic metabolism and strategies to tune the oxygen supply were provided. An advanced bioreactor was engineered and applied that provided multiple niches for the co-cultivation of microorganisms with highly diverse requirements for abiotic parameters (e.g. oxygen, light) to widen the product range. The demonstrated co-cultivation of T. reesei and the microalgae Chlamydomonas reinhardtii in a two-layered biofilm is an important milestone towards the direct production of lipids from lignocellulose. The results of this thesis illustrate the potential of community-based CBP of renewable lignocellulosic biomass to fuels and chemicals in second generation biorefineries in tomorrow's bioeconomy.

EPFL2019

Processing of fibre and porosity gradients in cellular thermoplastic composites

Manuel Bühler

The aim of this study has been to develop processing techniques for novel bioresorbable thermoplastic cellular composites, reinforced with continuous inorganic fibres, for use as biodegradable scaffolds for bone tissue engineering. As the implanted composite scaffold starts to degrade, it should be progressively replaced by natural bone tissue, which should consequently support increasing proportions of the applied mechanical loads. In order to simulate natural high performance porous structures such as bone or light wood, composites with simultaneous gradients in fibre content and porosity were studied. In order to carry out investigations over wide ranges of fibre content and porosity and to ensure biocompatibility and bioresorbability, it was decided to focus on foamable poly(L-lactic acid) (PLA) and continuous glass fibres. As well as improved mechanical reinforcement due to the high aspect ratio, the use of continuous glass fibres offers the possibility of achieving higher loadings than with short fibres or particles, increasing the potential range of mechanical properties. For processing the composites, a fibre winding technique developed in house was used to prepare unidirectional preforms with gradient structures tailored at the fibre bundle level. PLA fibre bundles, containing 36 monofilaments were prepared on a spinning line, and mingled with glass fibre bundles comprising 250 or 1000 monofilaments. During the winding process, the PLA fibres were intimately mixed with the reinforcing fibres, to ensure a good impregnation during subsequent processing steps. Moreover it was possible to maintain the relative positions of the fibres during pre-consolidation of graded composites. The resulting preforms had a porosity of 25% and could be further consolidated to decrease the porosity, or foamed in an autoclave to obtain cellular composites (foams) with porosities of up to 90%. It was observed that, independently of the applied foaming conditions, the presence of continuous fibres reduced the porosity by an amount equivalent to twice the fibre volume fraction. This processing method provided composites with a simultaneous gradient in fibre content and porosity. In order to predict the Young's modulus and the collapse strength of the cellular composites a model was proposed, which combines the buckling of composite columns within a porous structure with the property predictions for neat polymer foams. In neat foams with a porosity of 60%, the Young's modulus was calculated to be 0.25 GPa. By adding a fibre volume fraction of 14% the modulus reached 1.2 GPa. The respective collapse strengths were 5 MPa and 16 MPa. Experimental results were consistent with the predictions and validated the model. The model determines the fibre content required for composites with a given porosity and stiffness. Gradient cellular composites were obtained with a range of porosity between 50% to 90% and a range of stiffness between 100 and 1500 MPa. Furthermore the processing parameters can be adjusted to maintain a desired porosity when fibres are added. Thus, composites that fulfill the mechanical requirements for bone scaffolds can be achieved in a rational manner. The long term behaviour of the foams was tested under physiological conditions. Viscoelasticity tests provided evidence for the important role of the unidirectionally oriented fibres. Whereas the cells in neat foams collapsed after at most two days when loaded at 0.69 MPa, the fibre reinforced foams resisted longer and their creep strain was well described by a power law. The fatigue properties were compared to reported values of natural trabecular bone. None of the neat foams with a porosity of 74% resisted to the first load cycle of 7.3 MPa. With a fibre volume fraction of 11% and a comparable porosity of 72%, the composite scaffolds resisted for at least 11'000 loading cycles. To compare, trabecular human bone failed after 1909 cycles, when loaded with 8.5 MPa. It was concluded that the reinforced composites have the potential to replace natural bone. Preliminary in vitro results with human foetal osteoblasts and in vivo tests on femurs of rats were promising because the cells proliferated well on the composite substrates. Further investigations for applications in bone tissue engineering can be planned. Furthermore, the studied process and the obtained results are envisaged for other material systems to prepare cellular composites for novel lightweight applications in transportation and building industry.

EPFL2008

Effect of temporal onsets of mechanical loading on bone formation inside a tissue engineering scaffold combined with cell therapy

Tanja Cloé Hausherr, Dominique Pioletti

Several approaches to combine bone substitutes with biomolecules, cells or mechanical loading have been explored as an alternative to the limitation and risk-related bone auto- and allo-grafts. In particular, human bone progenitor cells seeded in porous poly(L-lactic acid)/tricalcium phosphate scaffolds have shown promising results. Furthermore, the application of mechanical loading has long been known to be a key player in the regulation of bone architecture and mechanical properties. Several in vivo studies have pointed out the importance of its temporal offset. When an early mechanical loading was applied a few days after scaffold implantation, it was ineffective on bone formation, whereas a delayed mechanical loading of several weeks was beneficial for bone tissue regeneration. No information is reported to date on the effectiveness of applying a mechanical loading in vivo on cell-seeded scaffold with respect to bone formation in a bone site. In our study, we were interested in human bone progenitor cells due to their low immunogenicity, sensitivity to mechanical loading and capacity to differentiate into osteogenic human bone progenitor cells. The latest capacity allowed us to test two different bone cell fates originating from the same cell type. Therefore, the general aim of this study was to assess the outcome on bone formation when human bone progenitor cells or pre-differentiated osteogenic human bone progenitor cells are combined with early and delayed mechanical loading inside bone tissue engineering scaffolds. Scaffolds without cells, named cell-free scaffold, were used as control. Surprisingly, we found that (1) the optimal solution for bone formation is the combination of cell-free scaffolds and delayed mechanical loading and that (2) the timing of the mechanical application is crucial and dependent on the cell type inside the implanted scaffolds.

2018