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Contributions from mechanics to the understanding of living matter dynamics

The recent developments in living matter sciences show that multidisciplinary and multi-scale research is fundamental to tackle such problems, and require combined mechanical, physical and biological/biomedical approaches to make significant progress. Our research group associates scientists from different communities (mechanics, physics, biology, medicine). Another specificity of this group is its unique reunion of expertise in experimental mechanics at all scales, together with a very strong background in theoretical mechanics, associated with multiscale numerical simulation experts. We are thus in a good position to contribute tackling major public health issues : cell mechanics and motility (cancer), thrombosis, designing new medical devices or biomaterials.

The sequencing of the human genome and the recent advances in post-genomics have revolutionised research into human health and diseases. However gene-oriented approaches, while necessary, are not sufficient to tackle major health issues such as, for example, cancer or cardio vascular (i.e. acquired) diseases and obviously the design and elaboration of biomedical tools and materials (prostheses). Multi-disciplinary vertical approaches are required in order to understand the extraordinary complexity of biological processes. While it is nowadays a lieu commun to say that mechanics is an important topic in cell biology, its description and modelling together with the transfer of this knowledge into diseases diagnostic and monitoring requires a rare reunion of expertises in both experimental, modelling and numerical simulation of multiscale mechanics. Such a reunion, together with the existing intimate ties with both biologists, physicians and surgeons provides a unique opportunity to tackle three large scale health-related issues.

The understanding of cell mechanics in relation with its environment is of strategic importance first because cells form the basic units of all living organisms. While such a topic is highly competitive, the main originality of the fundamental approach proposed below is that it aims at simulating the whole behaviour of the cell so that quantitative predictions can be obtained. This predictive capability will then be used both in illnesses modelling (e.g. organ invasion : cancer) and in cure finding (e.g. tissue reconstruction). It must deal with the biology of living cells, the physico chemistry of biopolymers or the micromechanics of adhesion, as well as the development of ground breaking numerical simulation techniques, all of which are well developed in our consortium.

This core of skills will strongly support the investigation of the major "ingredient" of most cardio-vascular diseases : thrombosis whose secondary consequences (cerebral, heart and lung embolisms) are a major public health issue as they cause the death of several tens of thousands of persons per year in France alone. Similarly, while a thrombus is present in the majority of the abdominal aortic aneurysms (AAA), its impact on the potential rupture of AAA needs to be clarified. In spite of those clear societal incentives, thrombosis still eludes diagnosis today as its multiple origins are far from being understood, owing to the deep lack of understanding of clotting itself, associated with an obvious lack of adequate diagnostic instruments.

This knowledge production will be translated into new medical tools and materials, which require a strong engineering expertise since the usage properties of the tools and materials must be precisely tailored by the industrial process to possess, for example, the required durability, mechanical properties, biocompatibility or/and biodegradability as well as biological safety. The know-how to translate the above basic science into industrial products and processes is a strong specificity of the group as shown by the large number of industrial contracts supporting current research. Finally, such developments will considerably benefit from the existing close ties with biologists (Institut Albert Bonniot, Institut des Neurosciences, Laboratoire Physiopathologie des Maladies du Système nerveux Central, INSERM) and physicists (Centres Hospitaliers Universitaires de Grenoble et St Etienne).

Multi-scale mechanics of the cell

During their lifetime, cells are subjected to various environments, mostly solid (soft tissues, extra-cellular matrix) which exert mechanical stresses on them and vice-versa. One scientific issue is to observe, understand and model these couplings in order, in fine, to predict the cells behaviours. These couplings occur at different scales (Chauvière 2010) going from the smallest molecules (signalling molecules, proteins, actin), then to the intermediate scale where the cell behaviour as a whole is relevant (deformation, adhesion) and, finally, to the mesoscopic interactions with tissues or/and the extracellular matrix (migration). In order to predict the behaviour of the cells, three key aspects, actin polymerization, cell-substrate interactions, and numerical simulations, must be developed.

At the smallest scale, the motility of the cells is mainly due to the directional polymerization of actin. This process, in order to move simple objects (beads or bacterias) must show symmetry-breaking phenomena (John 2008). To build a realistic model of the cell membrane movement, those theoretical advances will be pursued and incorporated in larger scale simulations, using, to account for complex shapes, Level Set or Immersed Boundary Methods.

Looking at the cell as a whole, a major specificity is its ability to migrate through tissues and/or to invade new organs (cancer, etc.) by developing mechanical stresses. The precise coordination of their adhesive properties (focal adhesions), as well as their complex rheology – related to the cytoskeleton microstructure – is a key point to their motility. Current state of the art 2D experimental studies (Ambrosi 2009, Iordan 2010) must be extended to 3D, eventually anisotropic, extra-cellular matrices or scaffolds, combined with new advances in confocal microscopy available at the Biology platform (Iordan 2010), while original discrete element-based numerical simulations focused on cytoskeleton mechanics will be put forward.

Finally, at the tissue scale, it is now well established that tissue morphogenesis and healing are closely linked to cell division and growth, with a strong interaction with mechanical efforts. Modelling these couplings and their biomechanical consequences requires the use of multiscale numerical simulations methods incorporating both continuum ("large" scale) and discrete (sub-mesh) mechanics, which will include items like nutriment availability, cell signalling, apoptosis... This modelling effort should provide the prediction tools necessary to understand morphogenesis, healing, tumour growth and cell differentiation.

Topographie d'un fibroblaste adhérent sur une surface de verre

Thrombosis : physics and mechanics of blood clotting

Even though the different aspects of clotting are the subject of many scientific investigations (more than 2000 papers/year in WoS), most of them are clinic, statistical or biochemical studies, while a very significant part of the clotting process remains rather obscure, because of the multiscale, multiphysics, in other words extreme complexity of the process. While the biochemical aspects (nature, crystallographic structure of the numerous participating enzymes and proteins) are well described, the physics, solid and fluid mechanics of the three main steps of clotting remain essentially unexplored.

For instance, the white thrombus formation whereby platelets are recruited and activated to form a plug has only been discovered ten years ago. This complex phenomenon obviously occurs while a concentrated suspension ( 40% in volume) of deformable objects (erythrocytes) flows pulsatingly past and inside the wound. The understanding and modelling of this recruitment process (Kaoui 2009, Coupier 2008) is a challenge concerning both experimental and theoretical microfluidicsof concentrated multicomponent suspensions. The effect of flow will be in particular studied using a real endothelium monolayer (grown by culture of endothelial cells), under normal and microgravity conditions. Microgravity experiments will be used to differentiate between subtle effects (Podgorski 2010), such as that of the deformability of the cells (erythrocytes/sickle cells), or the existence of a plug flow, phenomena that are usually masked by the predominance of gravity. Those phenomena will be modelled using large scale numerical simulations based on boundary integral and level set methods.

The fibrin clot, which stabilizes and gives it strength to the clot, is, as shown very recently (Yeromonahos 2010) is a multifractal structure going continuously from the molecule to the clot, as the mesh (pores 20um) is made of fibers (persistence length 500nm), structured as a fractal of protofibrils made of fibrin monomers (diameter= 5nm). Time resolved spectrometry and preliminary SAXS results from our consortium show that the 1950’s standard fibrin polymerization model is fundamentally wrong. Investigations of the kinetics of this process at each important scale (nm->mm->mm) will be performed, using the numerous investigation techniques available in the Fed3G (fluorescence, light spectrometry, scatterings, multiwavelength confocal microscopy) together with multiscale mechanical testing (Traction Force Microscopy, Piezorheometry, Time Resolved Rheometry, Strain Controlled Extensional Rheometry, AFM) to improve upon existing steady state data. Simultaneously, dynamic molecular simulation tools will be used to compare the consequence of different polymerization scenarios to the experimental database, allowing the development of sensible models.

In vivo, the fibrin formation occurs after the formation of the white thrombus. The explosive generation of thrombin and the polymerization thus occurs around and inside a nanoscopic porous medium mainly made of activated platelets. The same problem exists for the lysis where plasmin must penetrate the thrombus in order to cleave peptide bounds. In this context, the size of the pores is at most micronic, so that confinement should play a major role in the reaction kinetics. This unexplored challenge will first be addressed on model materials using state of the art fluorescence techniques to obtain sub-wavelength resolution, and then onto animal models using two photon multi-wavelength confocal microscopy (an equipment already available at the Nanobio platform). Such experiments will be an invaluable tool to validate the coherence of the different models coming from the above tasks and shall lead to the development of a global model for the growth of the thrombus in realistic situations. Those activities will considerably improve the diagnostic tools currently under development in our group.

Engineering of medical materials and devices

A key strategy for improving health related issues is to invent, validate and routinely use new technologies and materials for the diagnostic, monitoring and prognosis of illnesses, as well as for the improvement of therapeutic approaches. Tools and methods are central in that activity. In particular, the usage properties of the devices and materials must be precisely tailored by the industrial process to possess the required durability, mechanical properties, bioresorbability and safety (as for example vascular endo-prosthesis). An important route for the repair of heavily damaged tissue, including that of the brain, is regenerative and mini-invasive methods which use and enhance the natural ability of the tissues to heal themselves. Obviously, the understanding of the cell mechanical behaviour and of fibrin construction will be major assets to optimize this tissue engineering since, in vivo, the fibroblasts move on a natural scaffold made of fibrin. Most tissues have a very limited capacity to regenerate themselves, this regeneration in case of severe trauma or pathologies remaining too slow for acceptable recovery. A strategy to facilitate the growth of tissues is to prepare not only biocompatible but bioresorbable and bioactive gel matrices incorporating ligands that promote the adhesion, growth and migration of appropriate cells. In the case of neural growth for axon reconstruction, the control of the anisometry of a scaffold made of resorbable porous polymer nanofibers which will act as a guide for the optimal growth of tissue, electrospinning being the most appropriated process to product such a structure. As for bone regeneration, the main concerns of surgeons using injectable “bioceramics” are to trim down surgical procedures, limit the post-operative infectious hazards and enhance the regeneration rate. This requires the design and control of a new generation of injectable, resorbable and bioactive composites with a tailored microstructure (micro and macroporous). The key aspect of this action is the central role played by the structure on cell migration and the tissue growth, the characterization of which will be inter-linked with the knowledge development of the theme "Multi scale mechanics of the cell", including the numerical simulations.

While tissue regeneration is a very promising concept, actual developments are rather on the five year scale before actual use in hospitals. On the other hand, new mini-invasive tools (Cinquin 2002) (or prothesis) are much closer to actual use, such as Intelligent Needles or Haemostatic Vacuum Devices. Those new tools offer to interventional physicians a new environment that will enable them to replace heavy surgical gestures with carefully monitored lighter ones. The development of these new devices clearly requires constant interactions with the surgeons and physicians of the Centres Hospitaliers Universitaires and also a precise characterization of (i) the new artificial materials they are made of (NiTi alloys, silicone matrices…), (ii) the actual living tissue of the patient and, even more importantly, (iii) their mechanical interactions, asking for the development of new in situ biomechanical tests and numerical modelling tools.

References

  1. Chauviere, L. Preziosi, C. Verdier Eds Cell mechanics. From single scale-based models to multiscale modelin, Chapman & Hall/CRC, Mathematical and Computational Biology Series, ISBN 978-1-4200-9454-1 (2010).
  2. John, Peyla, Kassner, Prost, and Misbah, Nonlinear Study of Symmetry Breaking in Actin Gels : Implications for Cellular Motility Phys Rev Letters, 100, 068101, (2008)
  3. Ambrosi, Duperray, Peschetola and Verdier C., Traction patterns of tumor cells, J Math Biol 58, 163-181 (2009).
  4. Iordan, Duperray, Gerard, Grichine, and Verdier., Breakdown of cell-collagen networks through collagen remodeling, Biorheology, 47(5-6), 277-295 (2010)
  5. Kaoui, Biros and Misbah (2009). Why Do Red Blood Cells Have Asymmetric Shapes Even in a Symmetric Flow ? Phys Rev Letters 103, 188101 (2009).
  6. Coupier, Kaoui, Podgorski and Misbah, Noninertial lateral migration of vesicles in bounded Poiseuille flow, Phys Fluids 20, 111702 (2008).
  7. Podgorski, Callens, Minetti, Coupier, Dubois and Misbah. ,Dynamics of Vesicle Suspensions in Shear Flow Between Walls, Microgravity Sci. Technol. In Press. (2010)
  8. Yeromonahos, Polack, Caton F., Nanostructure of the Fibrin Clot, BioPhys J., , 99, n°7, 2018 (2010)
  9. Cinquin Ph., Favier D., Orgéas L, Pécher M., Pujol S. Vascular Anastomosis Device, Patent WO/2003/007843 (2002).