The computer simulation of microscopic interactions of RBC aggregation based on the depletion model under pulsatile flow

In a blood vessel, the main scatterers of Ultrasound (US) waves are the red blood cells (RBCs) and their aggregation. The depletion model proposes RBC aggregation formed from osmotic attractive forces due to polymer depletion, which overcome electrostatic repulsion due to RBC surface charge. Previous studies of simulation model under steady flow elucidated the relationship between shear rate and RBC aggregation. But shear rate could not fully explain the cyclic variation of backscattered power from blood under pulsatile flow. In the current study, a two-dimensional particle model capable of RBC mimicking the main characteristics of RBC aggregation kinetics was proposed to elucidate the relationship between microscopic RBC interactions and macroscopic rheological behavior. The mechanical model of RBCs is a depletion model under pulsatile flow. There are 596 RBCs randomly placed in a vessel (0.1X1mm) in the model driven by hydrodynamic force, aggregation force and elastic force. The mean flow velocity at the center of the tube was 2cm/s with the variation 0.5~1.5cm/s and stoke rate was changed from 40 to 80 beats per minute (bpm). The results showed mean aggregated size (MAS) was increased as velocity amplitude variation was changed from 0.5 to 1.5cm/s. The maximum MAS happened when the aggregated number decreased. Another finding was that the time to reach the maximum of RBC MAS is shorter as stroke rate was increased from 40 to 80 bpm. In addition, MAS variation was related with mean velocity, and hematocrit. The simulated results are in good agreement with previous experimental results, showing `the Bright Collapsing Ring' phenomenon and supporting the combined effects of flow acceleration and shear rate on RBC aggregation under pulsatile Poiseuille flow.