Archives de la catégorie: Research

Publications Research

A new method to measure mechanics and dynamic assembly of branched actin networks

Pierre Bauer, Joseph Tavacoli, Thomas Pujol, Jessica Planade, Julien Heuvingh* and Olivia du Roure* (2017)

Scientific Reports to appear soon.

Publications Research

Microfluidic fabrication solutions for tailor-designed fiber suspensions

Hélène Berthet, Olivia du Roure and Anke Lindner

Fibers are widely used in different industrial processes, for example in paper manufacturing or lost circulation problems in the oil industry. Recently, interest towards the use of fibers at the microscale has grown, driven by research in bio-medical applications or drug delivery systems. Microfluidic systems are not only directly relevant for lab-on-chip applications, but have also proven to be good model systems to tackle fundamental questions about the flow of fiber suspensions. It has therefore become necessary to provide fiber-like particles with an excellent control of their properties. We present here two complementary in situ methods to fabricate controlled micro-fibers allowing for an embedded fabrication and flow-on-a-chip platform. The first one, based on a photo-lithography principle, can be used to make isolated fibers and dilute fiber suspensions at specific locations of interest inside a microchannel. The self-assembly property of super-paramagnetic colloids is the principle of the second fabrication method, which enables the fabrication of concentrated suspensions of more flexible fibers. We propose a flow gallery with several examples of fiber flow illustrating the two methods’ capabilities and a range of recent laminar flow results.

Publications Research

Solid microscopic rings formed via wetting and subsequent dewetting

Tavacoli, J., Brown, A., Bauër, P., du Roure, O., & Heuvingh, J.

RSC Advances, 6(67), 62624–62629.

SEM image of a spontaneously-formed ring. the diameter is 10 µm and the thickness is sub-micron.

SEM image of a spontaneously-formed ring. the diameter is 10 µm and the thickness is sub-micron.

We report the spontaneous formation of rings when a colloidal dispersion, containing silica-coated iron-oxide particles and the liquids ethanol and ethoxylated trimethylolpropane triacrylate, is deposited within micron-sized PDMS wells. Just after filling, the interface between air and the dispersion is a meniscus dictated by the dispersion’s contact angle on PDMS. Upon evaporation of ethanol the meniscus lowers and, if a critical volume is reached, a rupture process is initiated and the dispersion adopts a ring morphology. The final dispersion consists only of particles and ethoxylated trimethylolpro- pane triacrylate that can be reticulated to solidify the ring geometry. The colloidal particles within the dispersion are essential for the stability of the rings prior to the reticulation. Here, by using iron-oxide based colloidal particles we fabricated superparamagnetic rings, opening up newavenues for applications. The dimensions of the rings can be tuned by adjusting both the size of the PDMS wells and the amount of ethanol in the dispersion. In this manner it is possible to fabricate rings with annuli smaller than a micron – a scale below the lower limit of standard lithography. Calculations assuming an equilibrium contact angle of ethoxylated trimethylolpropane triacrylate on PDMS reproduce the experimental results strikingly well.

Publications Research

Force-Velocity Measurements of a Few Growing Actin Filaments.

Brangbour, C., du Roure, O., Helfer, E., Démoulin, D., Mazurier, A., Fermigier, M., Carlier, M.-F., Jérôme Bibette et Jean Baudry

PLoS Biology, 2011, 9(4), e1000613. doi :10.1371/journal.pbio.1000613

Polymerization of short actin filaments induce separation of the colloids.

Polymerization of short actin filaments induce separation of the colloids.

The polymerization of actin in filaments generates forces that play a pivotal role in many cellular processes. We introduce a novel technique to determine the force-velocity relation when a few independent anchored filaments grow between magnetic colloidal particles. When a magnetic field is applied, the colloidal particles assemble into chains under controlled loading or spacing. As the filaments elongate, the beads separate, allowing the force-velocity curve to be precisely measured. In the widely accepted Brownian ratchet model, the transduced force is associated with the slowing down of the on-rate polymerization. Unexpectedly, in our experiments, filaments are shown to grow at the same rate as when they are free in solution. However, as they elongate, filaments are more confined in the interspace between beads. Higher repulsive forces result from this higher confinement, which is associated with a lower entropy. In this mechanism, the production of force is not controlled by the polymerization rate, but is a consequence of the restriction of filaments’ orientational fluctuations at their attachment point.

Publications Research

Impact of branching on the elasticity of actin networks.

Pujol, T., du Roure, O., Fermigier, M., and Heuvingh, J.

Proceedings of the National Academy of Sciences, 2012, 109(26), 10364–10369.

Chain of magnetic colloids (top, bright field). An actin network (bottom, fluorescent image) has grown from the colloids that have been covered by the activator of Arp2/3 machinery. Magnetic field is horizontal. Bead diameter is 4.4 µm.

Chain of magnetic colloids (top, bright field). An actin network (bottom, fluorescent image) has grown from the colloids that have been covered by the activator of Arp2/3 machinery. Magnetic field is horizontal. Bead diameter is 4.4 µm.

Actin filaments play a fundamental role in cell mechanics: assembled into networks by a large number of partners, they ensure cell integrity, deformability, and migration. Here we focus on the mechanics of the dense branched network found at the leading edge of a crawling cell. We develop a new technique based on the dipolar attraction between magnetic colloids to measure mechanical properties of branched actin gels assembled around the colloids. This technique allows us to probe a large number of gels and, through the study of different networks, to access fundamental relationships between their microscopic structure and their mechanical properties. We show that the architecture does regulate the elasticity of the network: increasing both capping and branching concentrations strongly stiffens the networks. These effects occur at protein concentrations that can be regulated by the cell. In addition, the dependence of the elastic modulus on the filaments’ flexibility and on increasing internal stress has been studied. Our overall results point toward an elastic regime dominated by enthalpic rather than entropic deformations. This result strongly differs from the elasticity of diluted cross-linked actin networks and can be explained by the dense dendritic structure of lamellipodium-like networks.

Publications Research

Bending of elastic fibres in viscous flows: the influence of confinement.

Wexler, J. S., Trinh, P. H., Berthet, H., Quennouz, N., du Roure, O., Huppert, H. E., Lindner, A. & Stone, H. A.

 Journal of Fluid Mechanics, (2013)  720, 517–544. doi:10.1017/jfm.2013.49

 

Photopolymerized fiber confined in a microfluidic channel and bent by a flow.

Photopolymerized fiber confined in a microfluidic channel and bent by a flow.

We present a mathematical model and corresponding series of microfluidic experiments examining the flow of a viscous fluid past an elastic fibre in a three-dimensional channel. The fibre’s axis lies perpendicular to the direction of flow and its base is clamped to one wall of the channel; the sidewalls of the channel are close to the fibre, confining the flow. Experiments show that there is a linear relationship between deflection and flow rate for highly confined fibres at low flow rates, which inspires an asymptotic treatment of the problem in this regime. The three-dimensional problem is reduced to a two-dimensional model, consisting of Hele-Shaw flow past a barrier, with boundary conditions at the barrier that allow for the effects of flexibility and three-dimensional leakage. The analysis yields insight into the competing effects of flexion and leakage, and an analytical solution is derived for the leading-order pressure field corresponding to a slit that partially blocks a two-dimensional channel. The predictions of our model show favourable agreement with experimental results, allowing measurement of the fibre’s elasticity and the flow rate in the channel.

Publications Research

The fabrication and directed self-assembly of micron-sized superparamagnetic non-spherical particles

 Tavacoli JW, Bauër P, Fermigier M, Bartolo D, Heuvingh J & du Roure O

Soft Matter, 2013,9, 9103-9110

 

We outline a simple techniq2013_Tavacoli_SoftMatterue to engineer monodisperse, superparamagnetic, micron-sized prisms of arbitrary cross-section and large magnetic susceptibility. The fabrication process allows pre-positioning of the particles that introduces another lever to guide self-assembly. In this method, a dispersion of magnetic colloids in a UV-curable monomer is molded in PDMS wells and subsequently reticulated. High homogeneous magnetic content is achieved by preventing colloidal aggregation through careful choice of the colloid and monomer. Additionally, on their removal from the PDMS molds, the relative position of the magnetic particles is conserved: they are extracted as arrays whose patterns are set by the PDMS mold. This novel method therefore offers unique control of the self-assembly of specific ‘higher order’ structures mediated by dipolar interactions and directed by the geometry and initial positioning of the particles. This is also a promising approach to develop devices with complex responses to external fields.

Publications Research

Human adipocyte function is impacted by mechanical cues.

Pellegrinelli V, Heuvingh J, du Roure O, Rouault C, Devulder A, Klein C, Lacasa M, Clément E, Lacasa D, Clément K.

J Pathol. 2014 Jun;233(2):183-95. doi: 10.1002/path.4347.

Fibrosis is a hallmark of human white adipose tissue (WAT) during obesity-induced chronic inflammation. The functional impact of increased interstitial fibrosis (peri-adipocyte fibrosis) on adjacent adipocytes remains unknown. Here we developed a novel in vitro 3D culture system in which human adipocytes and decellularized material of adipose tissue (dMAT) from obese subjects are embedded in a peptide hydrogel. When cultured with dMAT, adipocytes showed decreased lipolysis and adipokine secretion and increased expression/production of cytokines (IL-6, G-CSF) and fibrotic mediators (LOXL2 and the matricellular proteins THSB2 and CTGF). Moreover, some alterations including lipolytic activity and fibro-inflammation also occurred when the adipocyte/hydrogel culture was mechanically compressed. Notably, CTGF expression levels correlated with the amount of peri-adipocyte fibrosis in WAT from obese individuals. Moreover, dMAT-dependent CTGF promoter activity, which depends on β1-integrin/cytoskeleton pathways, was enhanced in the presence of YAP, a mechanosensitive co-activator of TEAD transcription factors. Mutation of TEAD binding sites abolished the dMAT-induced promoter activity. In conclusion, fibrosis may negatively affect human adipocyte function via mechanosensitive molecules, in part stimulated by cell deformation.

Publications Research

Microfluidic in situ mechanical testing of photopolymerized gels

Duprat, C., Berthet, H., Wexler, JS., du Roure, O., & Lindner, A.

Lab on a Chip (2015)  15 244

Photopolymerized fiber deformed by increasing flow rates.

Photopolymerized fiber deformed by increasing flow rates.

Gels are a functional template for micro-particle fabrication and microbiology experiments. The control and knowledge of their mechanical properties is critical in a number of applications, but no simple in situ method exists to determine these properties. We propose a novel microfluidic based method that directly measures the mechanical properties of the gel upon its fabrication. We measure the deformation of a gel beam under a controlled flow forcing, which gives us a direct access to the Young’s modulus of the material itself. We then use this method to determine the mechanical properties of poly(ethylene glycol) diacrylate (PEGDA) under various experimental conditions. The mechanical properties of the gel can be highly tuned, yielding two order of magnitude in the Young’s modulus. The method can be easily implemented to allow for an in situ direct measurement and control of Young’s moduli under various experimental conditions.

Publications Research

Deformation and shape of flexible, microscale helices in viscous flow

2015_Pham_PREPham, J.T., Morozov, A., Crosby, A.J., Lindner, A. & du Roure, O.

Physical Review E. (2015) 011004(R) 1-5

 

We examine experimentally the deformation of flexible, microscale helical ribbons with nanoscale thickness
subject to viscous flow in a microfluidic channel. Two aspects of flexible microhelices are quantified: the overall shape of the helix and the viscous frictional properties. The frictional coefficients determined by our experiments are consistent with calculated values in the context of resistive-force theory. The deformation of helices by viscous flow is well described by nonlinear finite extensibility. Under distributed loading, the pitch distribution is nonuniform, and from thiswe identify both linear and nonlinear behavior along the contour length of a single helix. Moreover, flexible helices are found to display reversible global to local helical transitions at a high flow rate.