Platelet stiffness correlates inversely with platelet volume during expansion and compression

The stiffness and the volume of human platelets change under various conditions, affecting their function and viability. Although the influence of platelet volume on platelet function in health and disease has been extensively studied, the relationship between volume and stiffness - in contrast to many other cell types - remains unexplored for platelets, probably due to the difficulty in measuring platelet mechanics as platelets tend to activate under stress. Here, we investigate the relationship between platelet volume and stiffness using scanning ion conductance microscopy (SICM). SICM can image the topography and therefore quantify the volume as well as measure the mechanical properties of living cells under physiological conditions with submicrometer resolution. We found a link between platelet stiffness and volume changes caused by water efflux/influx due to osmotic compression/expansion at the single cell level. With increasing platelet volume, the stiffness decreased and <i>vice versa</i>. We then confirmed this inverse relationship by measurements of platelets during two additional, physiologically highly relevant situations: The dynamic spreading of platelets on a surface and platelets subjected to a spatial confinement, where a decrease in volume was also accompanied by an increase in stiffness and platelets subjected to spatial confinement showed a significantly larger volume compared to unconfined platelets, with a correspondingly lower stiffness, respectively. In conclusion, our SICM analysis revealed a universal, inverse correlation between platelet stiffness and volume change, opening up new perspectives in platelet research.