After a donation, plasma and white blood cells are separated from red blood cells, which are the most commonly transfused blood product. In most countries, red blood cells can be stored under refrigeration – as a liquid solution – for up to 42 days. Over time, red blood cells undergo structural changes: their membranes become weaker, and harmful metabolic by-products build up. It’s essentially a tiny stress test for blood cells: the faster they break, the less healthy the blood is. Without the acoustic vibrations, donor-specific differences in red blood cells were not possible to detect.