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Research interests

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My current research interests focus on cell-surface interactions (adhesive or hydrodynamic) using biomimetic or artificial surfaces that need accurate characterization. This cross-disciplinary research area implies a number of collaborations playing a central role in each project listed below.

Glycocalyx and blood cells : how do they interact under flow ?

in collaboration with Ralf Richter, Lionel Bureau, Claude Verdier and Heather Davies

M2 internship of Nouha El Amri

Endothelial cells lining blood vessels are covered with a thick (100nm-1µm) layer of biopolymers called glycocalyx, whose biological role is still unclear. In this project, we focus on deciphering how the interplay between this layer and circulating cells regulates cell interaction with the vessel walls under flow.

Using a model glycocalyx and microbeads mimicking cells, one of our finding is that in the absence of biochemical interactions, a flow-induced repulsion force pushes cells away from the polymer brush surface : this is one of the first experimental evidence of the elastohydrodynamic force caused by the deformation of the soft brush below the bead. This study shows that this force may play a significant role in cell margination in blood vessels.

Figure : bead lift above a soft hyaluronan brush as a function of the shear rate. The three curves correspond to different brush thicknesses and elasticities, and solid lines are experimental predictions.

Publications : H. Davies et al., Elastohydrodynamic lift at a soft wall, submitted to PRL (arXiv preprint here)

Characterization of "smart" surfaces

in collaboration with Lionel Bureau

"Smart" polymer brushes are increasingly used to study and control cell adhesion but the characterization of such substrates is technically challenging. We have developed a method to study in situ the conformation polymer brushes of thickness 30nm-2µm relying on the measurement, on an optical microscope, of the spectral reflectance of the substrate in the visible range. This method permits calculating the density profile on a brush grafted on a transparent surface (e.g. microscope coverslip) with micrometric lateral resolution and non-destructively.

We have used this approach to characterize the conformation changes of poly(N-isopropylacrylamide) (PNIPAM) brushes as a function of temperature, confirming the theoretically predicted existence of an axial phase separation at the lower critical solution temperature (LCST). We have also shown that these measurements permit a full characterization of the brush in situ : grafting density, chain length, and polydispersity.

Figure : (a), reflectance spectra from a dense PNIPAM brush of dry thickness 108nm. (b), fitting the spectra (dotted lines in (a)) yields the density profiles vs temperature. Inset, swelling ratio of the brush vs temperature, showing a collapse around the LCST.

Publications : S. Varma et al., The Conformation of Thermoresponsive Polymer Brushes Probed by Optical Reflectivity, Langmuir 32 (13), 3152–3163 (arXiv version here)

How do brushes behave under flow ?

in collaboration with Lionel Bureau
M2 internship of Anthony Dupont

We are currently investigating the effect of flow on the structure of a polymer brush. Indeed, theoretical and numerical investigations have produced conficting predictions : beyond a critical shear rate, does the brush swell, collapse or keep the same structure ? This effect could be important to understand how the blood vessel wall glycocalyx behaves under flow.

Coupling our spectral reflectance setup with microfluidics, we are currently investigating experimentally the effect of the shear rate on a model (PNIPAM) brush.

Studying the first steps of bacterial adhesion

In collaboration with Sigolène Lecuyer

When do bacteria choose to attach to surfaces, switching from a motile to a sessile lifestyle ? How does the (mechanical) microenvironment influence their fate ? These questions are at the heart of numerous research topics, in particular those aiming to design anti-fouling substrates.

In collaboration with Sigolène Lecuyer, we are developing multimodal approaches to study the onset of bacterial adhesion by combining original optical microscopy approaches and microfluidics to gain insight about the initial steps of adhesion : reflection interference contrast microscopy (RICM) provides information about the conformation of bacteria with respect to the surface and can be combined with fluorescence imaging reporting phenotypic changes. These parameters can be monitored over time during surface colonization with the aim of relating the “strength” of surface adhesion to the changes in gene expression in individual bacteria.

Figure : Pseudomonas Aeruginosa colonizing a glass surface, imaged with RICM. Stripes reflect the angle between the bacteria and the surface.

And now for something completely different... Bone structure imaged with nonlinear microscopy

in construction...

past research : adaptive optics

in construction...

past research : tissue imaging with THG microscopy

in construction...