Responsive Giant Vesicles filled with Poly(N-isopropylacrylamide) Sols or Gels

4 pagesInternational audienceWe prepared giant unilamellar vesicles (GUVs) enclosing solutions or covalent gels of Poly(Nisopropylacrylamide) (PolyNipam). Concentrated suspensions of GUVs were prepared by applying an alternative field on a lipid film hydrated by a monomer solution containing N-isopropylacrylamide, crosslinker (N,N-methylene-bis-acrylamide), initiator and sucrose. Vesicle inner medium was polymerised and crosslinked by UV irradiation of the suspension, yielding viscous vesicles enclosing a solution of linear PolyNipam chains (when no bisacrylamide was used) or elastic vesicles filled with a covalent PolyNipam gel. We show that gel-filled vesicles are responsive systems triggered by the temperature: they shrink, reducing by a factor eight their volume below the critical temperature (32 ◦C in water, lower in glucose solution) and re-swell in a reversible and reproducible way upon decreasing temperature. In both cases, we show that the vesicle lipid membrane interacts with the internal polymer, resulting in an strong resistance of the vesicles to external mechanical stresses (enhanced tension of lysis)

Gel-phase vesicles buckle into specific shapes

International audienceOsmotic deflation of giant vesicles in the rippled gel-phase $P_{\beta '}$ gives rise to a large variety of novel faceted shapes. These shapes are also found from a numerical approach by using an elastic surface model. A shape diagram is proposed based on the model that accounts for the vesicle size and ratios of three mechanical constants: in-plane shear elasticity and compressibility (usually neglected) and out-of-plane bending of the membrane. The comparison between experimental and simulated vesicle morphologies reveals that they are governed by a typical elasticity length, of the order of one micron, and must be described with a large Poisson's ratio

Swinging of red blood cells under shear flow

We reveal that under moderate shear stress (of the order of 0.1 Pa) red blood cells present an oscillation of their inclination (swinging) superimposed to the long-observed steady tanktreading (TT) motion. A model based on a fluid ellipsoid surrounded by a visco-elastic membrane initially unstrained (shape memory) predicts all observed features of the motion: an increase of both swinging amplitude and period (1/2 the TT period) upon decreasing the shear stress, a shear stress-triggered transition towards a narrow shear stress-range intermittent regime of successive swinging and tumbling, and a pure tumbling motion at lower shear stress-values.Comment: 4 pages 5 figures submitted to Physical Review Letter

Milieux aleatoires silice-siloxane et reticulats polymeres : approche des proprietes semi-locales RMN, gonflement, etirement

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

CHAPTER 10. On the Importance of the Deformability of Red Blood Cells in Blood Flow

International audienc

vesicles and red blood cells in shear flow

highlightInternational audienceWe describe the similarities and the specificities of the behaviour of individual soft particles, 10 namely, drops, lipid vesicles and red blood cells subjected to a shear flow. We highlight that their motion depends in a non trivial way on the particle mechanical properties. We detail the effect of the presence of a wall with or without wall-particle attractive interaction from a biological perspective

Weak adhesion and dynamics of sedimentation of giant lipid vesicles

AIX-MARSEILLE2-BU Sci.Luminy (130552106) / SudocSudocFranceF

A simple model to understand the effect of membrane shear elasticity and stress-free shape on the motion of red blood cells in shear flow

International audienc

Application de contraintes sur des systèmes complexes artificiels ou vivants (dégonflement de liposomes fonctionnalisés et réorganisation mécanosensible du cytosquelette de cellules dictyostelium)

Dans la première approche de ce travail, j'ai quantifié le dégonflement osmotique de liposomes remplis d'un gel d'agarose. La fabrication de tels systèmes reconstitués vise à permettre de mimer le comportement de cellules soumises aux mêmes contraintes. En particulier, j'ai observé que ces liposomes fonctionnalisés acquièrent des morphologies crénelées lors de leur dégonflement pour une concentration du gel comprise entre 0.07 et 0.18 % en masse. Ces formes originales ressemblent à celles d'échinocytes parfois prises par les globules rouges. Le gel est responsable de l'apparition de ces formes, ne modifie pas les cinétiques de dégonflement mais sa pression élastique arrête précocement le dégonflement comparativement aux liposomes aqueux, mettant en évidence un phénomène de rétention d'eau. Dans la deuxième approche, j'ai étudié l'effet de contraintes hydrodynamiques sur des amibes Dictyostelium adhérentes à un substrat et ai quantifié la réorganisation mécanosensible du cytosquelette de ces cellules vivantes. Pour obtenir les cinétiques de relocalisation de protéines majeures du cytosquelette en réponse aux forces d'un flux, j'ai marqué l'actine et la myosine-II avec des protéines fluorescentes et ai fabriqué une chambre à flux permettant de changer rapidement la direction du flux. Les cellules étudiées s'orientent contre les forces du flux et se réorientent contre en inversant leur polarité après une inversion du flux: d'abord l'actine dépolymérise puis des protrusions sont émises contre les nouvelles forces mécaniques, et 15 sec plus tard, l'arrière rétracte en utilisant la myo-II. La contractilité du système actine-myosine n'est pas nécessaire pour sentir les forces.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Chaotic Dynamics of Red Blood Cells in a Sinusoidal Flow

4 pagesInternational audienceWe show that the motion of individual red blood cells in an oscillating moderate shear flow is described by a nonlinear system of three coupled oscillators. Our experiments reveal that the cell tank treads and tumbles either in a stable way with synchronized cell inclination, membrane rotation and hydrodynamic oscillations, or in an irregular way, very sensitively to initial conditions. By adapting our model described previously, we determine the theoretical diagram for the red cell motion in a sinusoidal flow close to physiological shear stresses and flow variation frequencies and reveal large domains of chaotic motions. Finally, fitting our observations allows a characterization of cell viscosity and membrane elasticity