Interfacial rheology of microcapsules and dynamics in flow

A capsule is a drop bounded by a thin solid membrane providing specific mechanical properties. It is used to control the spatio-temporal delivery of substances in numerous processes and also as a model system of cells. Its dynamics under flow depends on its membrane characteristics. Moreover, the delivery of encapsulated drugs is controlled by its deformation. The interfacial rheology of microcapsules can be tuned according to their formulation. We will focus on cross-linked membrane made with human serum albumin and chitosan assembled with a surfactant via electrostatic interactions. The interfacial rheological properties of these soft microparticles are deduced from their dynamics of deformation in elongation and shear flows. In elongation flow, the surface shear modulus of the membrane is measured and related to the kind of biopolymer used and to the main parameters of the process of fabrication. In the regime of large deformations, the microcapsules can present a non-linear elastic response or plastic deformations. Non-linear elastic constitutive law is deduced by comparison of the evolution of the shape of the microcapsule in the two main planes of deformation of the capsule with numerical simulations. In shear flow, the rotation of the membrane, i.e. the tank-treading, is visualised and quantified by decorating the membrane of microcapsules with particles. The tracking of the distance between two close microparticles showed membrane contraction at the tips and stretching on the sides. This dynamic of deformation induce viscous dissipation inside the membrane. The order of magnitude of membrane viscosity is determined by comparison with numerical simulations. Wrinkling instability is observed in extensional flow and studied by varying the interfacial properties of the microcapsules. In this way, the phase diagram of wrinkle instability for microcapsules has been deduced as the scaling law between the wrinkles wave-length and the membrane thickness. Finally, we have developed a set of tools to characterize the interfacial viscoelasticity of microcapsules, their bending modulus and their non-linear elastic properties. We conclude the talk with some results on break-up of microcapsules in flow. Please click Additional Files below to see the full abstract

Monodisperse microcapsules with controlled interfacial properties generated in microfluidic T-shape junction

Microcapsules are widely found in the nature (e.g. red blood cells and some bacteria), as well as in artificial products. They are generally well-considered as liquid drop bounded by an elastic membrane which is often used to protect the core materials from the external harsh environments. Capsules of biopolymers are exhibiting a large increase of promising applications in the encapsulation and release of medical drugs, food additives, and cosmetics[1-3]. Indeed, there is also a growing interest to model the dynamics of red blood cells (RBCs) motion in vessels or circulations using artificial microcapsules. Particularly, in some cases, it requires the homogeneous physic-chemical properties of capsules, such as uniform size, same shell structure and mechanical characteristics. Therefore, the most challenging work could be to develop a facile strategy to synthesis microcapsules with controlled properties-determined parameters. The preparation of monodisperse microcapsules involves emulsification of the disperse phase into the continuous phase which both are immiscible. There are several strategies been developed to fabricate capsules including batch methods (high-pressure valve homogenisers, static mixers, and rotor stator systems), electrospray techniques, and emulsification through membrane pores. These methods, however, require multistage emulsion processes, and capsules obtained with non-uniform properties and a largely wide distribution of sizes[4-5]. To overcome these problems, recently, microfluidic controlling techniques are introduced, by which monodisperse biopolymer capsules in micrometer size ranges are allowed to be generated in a single step. The main purpose of this study is to develop an approach of fabricating monodisperse biopolymer microcapsules with homogeneous properties on the base of microfluidic controlling components. Thereafter, the membrane properties of obtained capsules are proposed to be measured consisting of flowing a capsule suspension into an elongation flow. The deformation of capsules in the elongation flow can be divided into two regions: linear and non-linear zones. Surface shear elastic modulus of the shell in the linear region (small deformation) and membrane wrinkles instability or plastic deformation in the non-linear region are detected, respectively. Furthermore, thanks to the microfluidic techniques, the interfacial rheological properties of microcapsules are able to be modified via the synthetic procedures, such as the concentrations of chemicals and interfacial polymerization time. Results show that the physic-chemical properties of biopolymer capsules produced by the microfluidic route are very close for the same generating lot

Instabilités secondaires et structures cellulaires de canaux ioniques

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Plant VDAC Permeability: Molecular Basis and Role in Oxidative Stress

International audienceThe mitochondrial voltage-dependent anion-selective channel (VDAC) is highly abundant in the mitochondrial outer membrane. It is permeable to molecules with a size up to about 5kDa and is the main pathway for the exchange of metabolites and ions between the mitochondrial intermembrane space and the cytosol. Experimental studies performed for plant VDAC have shown that the channel displays properties reported for VDACs of other eukaryotic organisms. Firstly, it transports compounds as diverse as inorganic ions (e.g., K + and Cl-), adenylates (e.g., ATP and AMP) and large macromolecules (tRNA and DNA). Secondly, despite its wide pore, the channel displays selectivity towards these compounds, i.e. it distinguishes between K + and Clbut also between ATP and AMP. The question of how VDAC can selectively transport these different compounds is addressed in this chapter based on data obtained for plant VDAC. It is well known that all organisms have at least one canonical VDAC isoform that shares similar electrophysiological properties and secondary structure with cognate VDAC of other organisms. For instance, this is the case of the mammalian VDAC1, the yeast Saccharomyces cerevisiae VDAC1 and the PcVDAC purified from the bean Phaseolus coccineus seeds. Consequently, Brownian dynamic simulations of monatomic ion permeation through the experimental threedimensional structure of the mammalian VDAC1 and the PcVDAC modeled structure predict fairly well conductance and selectivity of both the proteins. In addition, the data of molecular simulation studies performed on the mammalian VDAC1 agree with the experimental data obtained for PcVDAC, which suggests a similar permeation process for these VDAC proteins. Accordingly, both the experimental and theoretical studies indicate that the selectivity for inorganic ions is a consequence of the excess of positive charges and their distribution inside the pore and the absence of defined pathways for the permeation. In contrast, the permeation of metabolites involves a major binding site located at the N-terminal helix which folded into the pore lumen and occurs through a preferential pathway. The key residues forming the binding site are conserved in the PcVDAC pointing to the conserved permeation process. The process might be affected by VDAC-interaction with other proteins. For example, it is suggested that plant VDAC is involved in the oxidative stress response which includes cytosolic hexokinase and thioredoxin binding to VDAC. This in turn may influence the exchange of molecules between mitochondria and the cytosol

Dynamics of a deflated vesicle in bipolar pulsed electric field

submitted to Soft MatterGiant unilamellar vesicles (GUVs) under pulsed direct current (pulsed-DC) fields are promising biomimetic systems to investigate electroporation of cells and vesicles. A question of relevance is the shape deformation of a vesicle when a DC-pulse is applied. Previous theoretical studies have looked at vesicles in DC fields (which are not pulsed). However, a pulsed-DC field yields electric stresses that can push a long time prolate spheroidal shape into an oblate spheroid. In this work, we computationally investigate the deformation of a vesicle under unipolar, bipolar, and two-step unipolar pulses. Our study indicates that the transmembrane potential can be regulated using a bipolar pulsed-DC field. For the ratio of inner to outer fluid conductivity, $\sigma_\mathrm{r}$ = 10, the shape always remains prolate, including when the field is turned off. For $\sigma_\mathrm{r} = 0.1$ and the electric field strength $\beta$ (the ratio of electric to viscous force), $\beta \beta_c$, a metastable oblate equilibrium shape is predicted in pulsed-DC fields similar to that in the DC field. A prolate-to-oblate transition on turning off the field is an important characteristic of the dynamics in unipolar and bipolar pulsed-DC electric fields. When a two-step unipolar pulse (a combination of a strong and a weak subpulse) is applied, a vesicle can reach an oblate or a prolate final shape depending upon the relative durations of the two subpulses. The simulation results can be demonstrated in an experiment under typical experimental conditions

Membrane emulsification for the production of suspensions of uniform microcapsules with tunable mechanical properties

International audienceA way forward for high throughput fabrication of microcapsules with uniform size and mechanical properties was reported irrespective of the kinetic process of shell assembly. Microcapsules were produced using lab-scale emulsification equipment with a micro-engineered membrane in the size range 10-100 m. The shell of the microcapsules was assembled at the water-oil interface by complexation of polyelectrolytes or cross-linking of proteins providing two different kinetic processes. Elasticity of microcapsules was characterized with an automated extensional flow chamber. Process parameters were optimized to obtain suspensions with size variations of 15%. Some strategies were developed to obtain uniform elastic properties according to the kinetics of shell assembly. If kinetics is limited by diffusion, membrane emulsification and shell assembly have to be split into two steps. If kinetics is limited by the quantity of reactants encapsulated in the droplet, variations of elastic properties result only from size variations

Structural characterization of the interfacial self-assembly of chitosan with oppositely charged surfactant

International audienceControlling the assembly of colloids at liquid-liquid interfaces offers new ways to fabricate soft materials with specific physical properties. However, little is known of the relationships between the kinetics of interfacial assembly, structural and rheological properties of such interfaces. We studied the kinetics of the assembly of two oppositely charged polyelectrolytes using a multi-scale approach. Soft interfaces were formed from the complexation at water-oil interface of chitosan, a polysaccharide carrying positively charged groups, and a fatty acid exhibiting negative charges. The growth kinetics of the membrane was followed by interfacial rheometry and space- and time- resolved dynamic light scattering. This set of techniques revealed that the interfacial complexation was a multi-step process. At short time-scale, the interface was fluid and made of heterogeneous patches. At a gelation time, the surface elastic modulus and the correlation between speckles increased sharply meaning that the patches percolated. Confocal and electron microscopy confirmed this picture, and revealed that the basic brick of the membrane was sub-micrometric aggregates of polyelectrolytes