Rhéologie non linéaire et rupture de caillots sanguins formé sous écoulement

par Mohamad Alajami

Projet de thèse en MEP : Mécanique des fluides Energétique, Procédés

Sous la direction de Denis Roux, Benoît Polack et de François Caton.

Thèses en préparation à l'Université Grenoble Alpes , dans le cadre de École doctorale Ingénierie - matériaux mécanique énergétique environnement procédés production , en partenariat avec Laboratoire Rhéologie et Procédés (laboratoire) depuis le 01-10-2017 .


  • Résumé

    Les maladies actuelles parmi les plus graves que ce soit en termes humains ou économique sont les pathologies cardiovasculaires et plus particulièrement les maladies thromboemboliques. Une embolie correspond à l'obstruction d'une artère provoquée par un caillot s'étant développé de façon anormale (thrombus) dans la circulation. L'une des questions clefs de ces maladies concerne la façon dont, chez certains patients, ces thrombus se fragmentent et s'embolise, ceci suggérant une importance majeure des propriétés mécaniques du caillot. S'il existe depuis plus de 60 ans un instrument déterminant le module élastique du caillot en formation (thrombo-élastographe), cet outil ne présente pas d'intérêt dans le diagnostic ou le suivi des pathologies thromboemboliques [2,3]. Les résultats de la thèse de X. Garçia, soutenue fin 2016 [4] suggèrent que ce déficit vient justement du fait que la thromboélastographie mesure la viscoélasticité linéaire. Or, puisqu'une embolie correspond à la rupture et à la migration d'un caillot dans la circulation, provoquant ensuite l'obstruction d'une artère, une hypothèse vraisemblable est que ce sont les propriétés mécaniques fortement non-linéaires conduisant à la rupture du caillot qui sont importantes. Étonnement, dans toute la littérature aussi bien clinique que fondamentale, il n'existe qu'une seule étude, très succincte, ayant abordé cet aspect [1]. Nous proposons ici de combler ce manque.

  • Titre traduit

    Non linear rheology and failure of clots formed under flow


  • Résumé

    Currently, the costliest diseases, both in human and economic terms are cardiovascular ones and in particular thrombo-embolic diseases. One of the key questions in thrombosis concerns the way an abnormal clot transforms into an embolus, i.e. how it breaks free from the vascular wall, suggesting that mechanical properties play a key role. While an instrument exists to measure the linear elastic modulus of the forming clot (thromboelastography), this tool appears useless for the diagnosis or follow-up of thromboembolic diseases [2,3]. Results from X. Garcia PhD (2016) [4] suggest that this comes precisely from the fact that thromboelastography determines a linear mechanical property, while it should be the strongly non-linear mechanical properties that should be relevant for the rupture of the clot. Surprisingly, in the whole clinical and fundamental literature, only one study explored briefly this essential aspect of clot's mechanical properties [1]. We propose in this PhD to bridge this gap. This PhD proposal concerns the non-linear rheology and failure of clots formed under flow. Il will be co-tutored by Pr. B. Polack (CHU, Grenoble et TIMC TheREx), and will also be supported by current collaborations with Diagnostica Stago and le Laboratoire Français du Fractionnement et des Biotechnologies. While a number of world renowned teams (e.g. D. Weitz [5], Harvard University, F. Mackintosh [6], Amsterdam University…) have been investigating the rheology of biopolymer networks such actin, collagen or fibrin clots, those studies have taken the biology out of the equation. Indeed they work with highly purified materials from the same provider (Enzyme research Lab), which allows direct comparisons but limits the biological and medical scope of their results. Similarly, Xabel Garcia PhD's results concerning the rupture of fibrin networks –upon which this PhD proposal is based- were obtained on similar purified systems and therefore beg to be extended to more physiologically relevant systems. Furthermore all experimental systems used in these studies cannot be used in the clinical environnement. In consequence, three main aspects will be developed in this PhD. - First, physiology asks for measurements dealing with whole blood. In a first time the non-linear rheometrical properties of plasma clots formed in the presence and absence of flow will be investigated, adapting and refining the protocols developed in X. Garcia's PhD. The interest of plasma versus whole blood is that plasma can be deep-frozen while whole blood cannot. However, since red cells are thought to be of little importance in clots, extension to whole blood should be straightforward. - The second task is more technological in nature as it consists in developing a magnetic microrheometer applying forces up to ~10 nN on micronic particles (100-500μm), forces which will allow soliciting the clot in its non-linear domain (P. Allan, Leeds, 2012, [7]), while using very small volumes (<100μl). Measurements on this new instrument will be compared to those obtained on the standard research rheometer. The main interest is that such an approach as never been tried to investigate clotting. - The third tasks is more risky as it aims at measuring the local mechanical properties of a clot formed in a experimental microfluidic model of thrombosis which consist in a back-facing step. To this end , the plasma will be seeded with micronic magnetic beads (~5-10μm) which will be trapped by the clot during its formation. We will use the magnetic micro-rheometer to apply forces on the beads. Using video microscopy or speckle imaging the displacement of the beads will be evaluated, providing the non-linear rheology and the rupture mechanics of the clots. Those results will be compared to the one obtained in the standard rheometer.