Erythrocyte sedimentation: Collapse of a soft-particle gel

The erythrocyte sedimentation rate (ESR) is commonly viewed as a marker of cell aggregation and inflammation. While increased fibrinogen levels are known to accelerate erythrocyte sedimentation by promoting the formation of large aggregates, aspects of this process align more closely with the collapse of a particle gel than with the sedimentation of separate aggregates. Through experiments and numerical simulations at the cell level, we explore how ESR depends on fibrinogen concentration and its impact on the microstructure of gel-like erythrocyte suspensions. Our findings reveal that stronger attraction between cells results in a network with larger void spaces due to the anisotropic shape and deformability of erythrocytes. This geometric change in network structure enhances gel permeability, leading to faster sedimentation. Hoever, we also find that the ESR is very sensitive to the inital phase when cracks are formed and the system starts to fluidise that has a very different dynamic to the collapsing phase itself. These insights deepen our understanding of the relationship between ESR and the cellular structure of erythrocyte suspensions, lending support to the gel hypothesis in blood sedimentation interpretation.