Charge-based mechanism of aggregation. (A) In healthy physiological conditions, blood cells and proteins are suspended in a stable colloidal suspension in water, separated due to electrostatic repulsion (left). Upon introduction of an ion (Fe³⁺ (aq), Al³⁺ (aq)) that can bind directly (form inner sphere complexes) to negatively charged proteins on cell surfaces or in suspension, blood cells and proteins are arrested on endothelial surface and form aggregates (middle). Upon introduction of a kinetically inert ion (Cr³⁺), the ion binds directly to water molecules and thus only has transient interactions with blood cells and proteins, allowing the cells to maintain their colloidal suspension as in the control condition (right). (B) The introduction of AlCl₃ causes RBCs to aggregate at the fluidic interface in bare (left) and endothelialized (middle) microfluidics. AlCl₃ causes endothelial cell death, but the cells remain adherent (right) (green/live: calcein; red/dead: propidium iodide). (C) The introduction of CrCl₃ does not result in RBC aggregation in bare (left) or endothelialized (middle) microfluidics. CrCl₃ causes endothelial cell death, but the cells remain adherent (right) (green/live: calcein; red/dead: propidium iodide). All scale bars represent 50 μm; N = 3.