M Biarnes-Pelicot, P Bongrand, P Robert, O Theodoly. Collaboration with  J Bico, ESPCI-Paris; JM Forel, L Papazian AP-HM.

Several characteristics of the blood microvasculature make it a special location for pathological events. First, capillaries are remarkably small as compared to cells in the blood stream, i.e. blood cells or cancer circulating cells, which have a typical size of approximately 10 µm. Circulating cells must therefore strongly deform to pass through the microvasculature, which puts their mechanical properties to the test. Second, trafficking cells in capillaries are in close contact with the microvascular endothelium during their passage, which favors cell-endothelium interaction and potentially adhesion. Third, the hydrodynamic shear stress in the bloodstream reaches its highest values in arterioles and capillaries, applying important stresses on endothelial and arrested cells, like immune cells or pathogens. Last, the intimate contact between the microvasculature and surrounding organ tissues favors the sensitivity of immune cells to inflammatory signals and the extravasation of immune cells toward inflammatory zones. This set of characteristics explains why certain pathological events occur specifically in the blood microcirculation.

Figure 1 : A microfluidic sorter of blood cells by stiffness. A-Picture of the microfluidic device B- Sketch of the simple system of injection using standard hospital tools C- zoom on the microfluidic gradual filter where stiff cells (red) are separated from soft cells (green) by a flow from left to right.

Microfluidic tools have been developed in LAI since 2007 to explore the passage of circulating leukocytes through narrow constrictions mimicking lung blood capillaries. We could reveal the importance of actin cytoskeleton organization on the rheological properties of leukocytes and hence on the time required to cross the microvasculature (Biophysical Journal 2009). A microfluidic device was then designed to achieve the first microfluidic rheometer yielding quantitative absolute measurements of cells loss modulus or apparent viscosity (Biomicrofluidics 2013). A microfluidic diagnostic tool was also build and tested at hospital to assess stiffening of leukocytes in whole blood samples (Lab Chip 2013).

Finally, an in depth medical study of adhesiveness and stiffness of leukocytes using serum samples of ARDS patients allowed identification of cytokines triggering a massive arrest of leukocytes in the lung upon early ARDS (Critical Care 2016).

Figure 2 : A microfluidic rheometer for medical investigation. A- Sketch of a microfluidic constriction allowing a quantitative measurement of cell rheological properties (the loss modulus) B,C- Image sequences showing the passage of leukocytes incubated in sera of a sane volunteer (B) and a ARDS patient (C), exemplifying the strong stiffening effect of ARDS patient sera.