Band pattern formation of erythrocytes in density gradients is due to competing aggregation and net buoyancy

Centrifugation of biological materials in density gradient solutions is a widely accepted technique for the separation of cell types or cellular components. This approach is also utilized to distinguish red blood cells (RBCs) according to their age, as RBCs progressively lose water and increase in density throughout their lifecycle. When employing Percoll to prepare the density gradient, distinct bands of RBCs are consistently observed along the gradient. Initial studies have posited that cell aggregation may play a role in shaping spatial distributions; however, the question of whether a continuous density population can yield discrete bands remains unresolved. We have developed a dynamical density functional theory (DDFT) that integrates cell aggregation to describe the macroscopic evolution of RBC volume fraction within a density gradient, accounting for the continuous distribution of RBC densities. Numerical analyses indicate that the interplay between net buoyancy at isodensity and cell aggregation is sufficient to generate banding patterns. Our model accurately replicates the temporal progression witnessed in experimental settings and additionally predicts multiple bifurcation-like behaviors for steady-state patterns, contingent upon RBC volume fraction and aggregation energy. These findings reveal that the dynamic interaction between RBC aggregation and net buoyancy across iso-density layers constitutes a novel mechanism underpinning pattern formation.