# The buckling instability of aggregating red blood cells

## TL;DR

This study investigates the buckling instability in aggregating red blood cells, revealing how contact zone deformation occurs and identifying specific cell configurations through experiments and simulations, with implications for blood flow and tissue mechanics.

## Contribution

It provides the first detailed experimental and theoretical analysis of RBC aggregation morphology and the buckling instability at the contact zones.

## Key findings

- Identified three RBC contact zone shapes: parachute, male-female, and sigmoid.
- Demonstrated nonlinear increase in deformation with interaction energy.
- Predicted a buckling bifurcation leading to contact zone deformation.

## Abstract

Plasma proteins such as fibrinogen induce the aggregation of red blood cells (RBC) into rouleaux, which are responsible for the pronounced shear thinning behavior of blood, control the erythro- cyte sedimentation rate (ESR) a common hematological test and are involved in many situations of physiological relevance such as structuration of blood in the microcirculation or clot formation in pathological situations. Confocal microscopy is used to characterize the shape of RBCs within rouleaux at equilibrium as a function of macromolecular concentration, revealing the diversity of contact zone morphology. Three different configurations that have only been partly predicted before are identified, namely parachute, male-female and sigmoid shapes, and quantitatively recovered by numerical simulations. A detailed experimental and theoretical analysis of clusters of two cells shows that the deformation increases nonlinearly with the interaction energy. Models indicate a forward bifurcation in which the contacting membrane undergoes a buckling instability from a flat to a de- formed contact zone at a critical value of the interaction energy. These results are not only relevant for the understanding of the morphology and stability of RBC aggregates, but also for a whole class of interacting soft deformable objects such as vesicles, capsules or cells in tissues.