In a heart-lung machine, blood flows through narrow tubing and past rapidly spinning pump blades. Sometimes red blood cells can't take it: their membranes rupture, and hemoglobin spills into the plasma. For a patient, this is like pouring sand into a car's fuel system — small damages accumulate and wreck the whole mechanism.
To know in advance how dangerous a new pump might be, engineers build computer models. But old models often miss the mark by orders of magnitude: they predict either near-total blood destruction or none at all, while reality is somewhere in between. The authors of this study took a different approach: instead of focusing only on shear stress, they described the red blood cell as a viscoelastic sphere that stretches and then slowly returns to shape. When stretching becomes too severe, pores open in the membrane, like cracks in dried clay, and hemoglobin leaks out.
They tested this model on two standard benchmarks: a pump and a nozzle developed by the U.S. Food and Drug Administration (FDA). The results matched experimental measurements within the scatter of the data. Old models deviated by several orders of magnitude. This means we can now test the safety of dialysis machines, artificial hearts, and transfusion equipment faster and cheaper.
The new model is like a precise weather forecast for blood: it accounts not only for the 'wind' (shear stress) but also for 'soil moisture' (membrane properties). This gives doctors a tool that predicts cell damage before a device ever reaches a patient.
A red blood cell passes through narrow capillaries about 200,000 times in its lifetime, and each time its membrane deforms like a balloon squeezed in a fist, but it doesn't burst.