Darras group

• Dr. Alexis Darras

  Lecturer

Research Interests & Activities

Although being a physicist, my research interests are centred around human tissues. Specifically, I currently developed an expertise on red blood cells suspensions. My aim is to develop fundamental knowledge that can advance medical applications. I am therefore studying red blood cells collective behaviour, as a function of their single properties (a topic making use of statistical physics tools and concepts). Amongst others, I’m an expert of the erythrocyte sedimentation rate and density separation. My investigations are strongly influenced by my background in self-assembly and collective properties of superparamagnetic colloids, which I studied during my PhD thesis.

Erythrocyte Sedimentation Rate: Gravitational collapse of  weak gels

Red blood cells (or erythrocytes) sedimentation rate (ESR) is a physical parameter of blood which is often checked in medical diagnosis. It is indeed well known that in case of inflammation, the increase in fibrinogen and other proteins induces a higher ESR. A higher ESR is clinically established as a disease marker. Recently, we demonstrated that Red Blood Cells (RBCs), when left at rest and suspended at physiological volume fractions, form percolating aggregates as wide as the container.  It follows that they sediment following a so-called " gel collapse", governed by the geometry of the percolating aggregate acting as a porous material. By comparing physical models to experimental sedimentation curves, we have shown that this knowledge can help to quantify physically meaningful parameters that characterize the details of the collapse dynamics. Amongst others, we provide a dependency of the maximal sedimentation velocity as a function of the initial RBC volume fraction (i.e. the hematocrit), which was a long-sought correction for ESR measurements from anemic patients. Our current goal is to determine how the shape and rigidity distribution of the RBCs influence the dynamics of this process.

Erythrocyte Density Separation: Band Pattern formation from competing aggregation and net buoyant forces

Centrifugation of erythrocytes (aka Red Blood Cells, RBCs) in self-forming percoll gradient is a protocol often used as a way to sort RBCs by age. However, a pattern formation of discrete bands is systematically observed along the continuous density gradient. Although early studies mentioned that aggregation between cells might modify their spatial distribution, it was long debated whether a population with continuous density distribution can form discrete bands. We recently developed a continuous model, considering the aggregation of cells with a continuous density distribution, which describes the macroscopic evolution of RBCs concentration in a density gradient. Using numerical resolutions, we demonstrated that the competition between iso-density distribution and aggregation is sufficient to create band patterns. This model reproduces qualitatively the temporal evolution observed in the conventional experimental protocol, but also predicts several types of bifurcation-like behaviors for the steady-state patterns in constant gradients, when the volume fraction and aggregation energy of the cells are varied. The competition between aggregation and iso-density distribution is therefore a novel physical mechanism leading to a new and rich pattern formation system.

Current researchers and PhD students